cell physiology Flashcards

Question

Epithelia Overview

Answer 1

Epithelia are the tissues that line body surfaces, body cavities, and the surfaces of internal tubes, ducts, and other spaces in organs. They comprise the main functional units of glands and several organs. In general, these tissues and their cells tend to share several properties: Epithelial cells are adherent to one another. The cells are arranged in one to several layers or sheets. Most epithelia are polarized (asymmetric): One side of an epithelium usually has a free outer or apical surface; plasma membranes of apical cells are directly exposed to fluids or the environment. The inner surface of an epithelium, called the basal or basolateral surface, is connected to underlying connective tissue. Many epithelial cells are themselves polarized, especially those in single-layer epithelia. A basal lamina (a sheet of extracellular material) lines and is attached to the basal surface and is also attached to elements of the underlying connective tissue. Epithelial tissues undergo turn-over or renewal: cells die and must be replaced by cell division and differentiation, processes that are driven by epithelial stem cells. Epithelial tissues are avascular (no direct blood supply). Nutrients and oxygen must diffuse through connective tissue and through the basal lamina to reach epithelial cells. Epithelial tissues are highly diverse; there are many different types. Moreover, within any given epithelium, there can be several different cell types.

Answer 2

A barrier that protects internal tissues against abrasion, infection, harmful chemicals or radiation (e.g. sunlight). Selective absorption and transport of various molecules from the environment. Selective secretion of various molecules and fluids. Movement of particles and mucous through passage ways. Biochemical modification of molecules (e.g. liver). Communication to (and from) other tissues and organs. Reception of sensory stimuli (e.g. smell, taste and touch). Different epithelia have distinct functions: Some carry out a very specific function (e.g. cells of the anterior pituitary gland secrete pituitary hormones) while some carry out many of the functions listed above (e.g. the intestinal epithelium protects, absorbs, secretes, and sends molecular signals to other organs/tissues).

Answer 3

A tissue that faces blood and lymph, made of endothelial cells. Like other epithelia, these tissues are made of tightly adherent cells, have a free surface facing blood and lymph (or body cavities), and (usually) rest on a basal lamina.

Answer 4

the sheets of cells that line the enclosed internal spaces of the body cavities

Answer 5

The most obvious epithelia at the gross level is the outer layer of the skin and hair (epidermis). However, epithelia are found in and around most organs and body structures. The moist linings of the internal passage ways (e.g. mouth, nose, GI tract, reproductive organs, etc.) are generally each called a mucosa (aka mucous membrane): The surface layer of all mucosae is an epithelium (more on this below). In addition, several organs are composed mostly of epithelial cells (e.g. liver, pancreas, kidney), where epithelia are the primary functional units.

Answer 6

Epithelia are derived from all three primary germ layers (ectoderm, mesoderm, endoderm), by a variety of mechanisms. During early development, many epithelia form, disassemble, and reform in new patterns until body morphology and organ morphogenesis are achieved. In particular, embryonic epithelia often disassemble and move into the mesenchymal (connective) tissues; there they may migrate to other locations to form new epithelia, or they may transform into distinct non-epithelial cell lineages that give rise to other tissues. This process is known as the epithelial to mesenchymal transition. Some embryonic epithelia serve as crucial signaling centers that organize pattern formation.

Answer 7

Epithelial tissues line all surfaces that face the outside world (e.g. the air and the passageways of the gastrointestinal tract) and they form the surfaces of internal tubes, ducts, and secretory tissues. Therefore, throughout the body, they have a common general relationship to each of the other major tissue types; connective tissues (CT), blood vessels (and blood), muscles, and nerves. The apical side faces the space or outside world, the basal side is attached to the basal lamina, which in turn is attached to the underlying connective tissue (CT) (all by a series of multiple protein:protein interactions; described below). Blood vessels run through (CT), as do many nerves. Because epithelia are avascular, they must get O2 and nutrients from and communicate with blood within blood vessels (and lymph in lymphatic vessels) that course through the CT. Therefore, the CT directly underlying epithelia often houses many of the smallest blood vessels (capillaries) and lymphatic vessels (discussed in blood vessel lecture). Capillaries are connected to larger vessels that are typically deeper (more internal) within the CT. Muscles (both striated and smooth) are also imbedded within connective tissue layers that are also typical deeper (more internal) relative to the epithelial surface, although some small muscle tissue can be very close to the epithelial surface. These other tissue types (muscle, blood vessels, nerves) typically are surrounded by their own basal laminae that attach to their own neighboring connective tissues. Therefore, epithelia directly attach to connective tissues, and are separated from but attach to blood vessels, muscles, and many nerve endings by these connective tissues and by the different basal laminae that surround each tissue type. Of course there are exceptions to this general orientation: For example, some specific sensory nerve cells make intimate contacts with specific epithelial cells specialized for sensory input (e.g. taste buds in the tongue). Other exceptions are specialized immune system cells called dendritic cells, which infiltrate epithelia and can migrate out (and in), enter the connective tissue, and get into blood or lymph.

Answer 8

many moist internal linings (mouth, nose, throat, GI tract, reproductive systems, etc.) that separate “outside” from “inside”. A mucosa is composed of two layers: (1) the outer epithelium and (2) the CT directly underneath, which is typically (unfortunately) called the lamina propria.

Answer 9

Lamina propriae typically contain alot of immune system cells and small blood vessels, because they survey and extract foreign or ingested materials, cells, and molecules from the environment and must efficiently transport and monitor. Deeper layers of CT are directly continuous with lamina propria CT, but will often have different properties and house other tissues (bigger vessels and muscles, nerve axon bundles, etc.). This deeper CT tissue is typically called the submucosa.

Answer 10

The external skin also has it’s own nomenclature, but reflects analagous relationships: the epithelium of skin

Answer 11

directly underlying CT; deeper CT is called hypodermis and like submucosae has distinct composition and function (more in Dermatology). However, many tubes and ducts emanate from (and to) all of these epithelial surfaces, and can penetrate deep into the deep layers of CT (submucosa or hypodermis) leading to epithelial glands and organs, including large internal organs like the liver and pancreas. The outer surface is DIRECTLY contiguous with the spaces of most of these tube and ducts, and ALL of tube duct surfaces are lined by epithelia. Thus, epithelial sheets exist in all of these layers. However, even within these deep lying organs and epithelial tissues, the basic relationships hold: Space(lumen)-Epithelia-epithelial basal lamina-CT-other CT embedded tissues (blood vessels, muscle, nerves.....all usually with their own basal laminae that connects them with the CT).

Answer 12

have all cells arranged in a single layer or sheet.

Answer 13

have more than one layer of cells in which cells of the outer layers do not directly contact the basal lamina. Stratified epithelia are named according to their outermost layer (e.g. stratified squamous epithelia have a squamous outer layer, though inner cells are often cuboidal). Further subclassification is sometimes used reflecting surface specializations of the outer layer of epithelial cells.

Answer 14

are a special case where some cells do not reach the free surface (giving a stratified appearance), but all directly rest on the basal lamina.

Answer 15

are flattened cells,

Answer 16

are cube-like,

Answer 17

are taller than they are wide.

Answer 18

(found around the bladder) are a special case: these epithelia are stratified, but when stretched change their shape from cuboidal to squamous, and appear to decrease the layering: this is indicative of a tightly adherent epithelium that is very resilient and stretchable.

Answer 19

(or the zonula occludens): provide a highly selective barrier that limits or prevents diffusion of substances between epithelial cells. These also limit/control diffusion of membrane proteins through the plasma membrane bilayer. Key core proteins of tight junctions are occludins and claudins: In some epithelia the “tightness” of this barrier is regulated. Tight barriers ensure that substances absorbed or secreted must pass through the epithelial cell by specific transport pathways.link neighboring epithelial cells together tightly, forming a seal that separates apical and basolateral components. Proteins that compose tight junctions include occludins and claudins. They limit the movement of molecules between cells (or block it all together). This forces transport to go through the epithelia instead of between them. They are also important for membrane polarity.

Answer 20

(zonula adherens): Promote attachment, but also polarity, morphological organization and stem cell behavior within the epithelial sheet. Adherence junctions contain specific cadherins that link to actin filaments and other adapter/signaling proteins in the cytoplasm. Cadherins are transmembrane proteins with extracellular domains that interact with each other, and cytoplasmic tails that bind adapters and actin filaments. Some cadherin-associated proteins (e.g. beta catenin and protein kinases) control various aspects of epithelial polarity, development, and function. Important for the structural integrity of epithelia. Also control physiology and biology of the cells through signalling.

Answer 21

(macula adherens): Promote mechanical strength and resist shearing forces and promote the structural organization of the epithelial sheet. Core components of these junctions include a different class of cadherins that link to intermediate filaments and other adapter proteins. a type of e-cadherin that interacts with intermediate filaments. They are prominent in skin epithelia

Answer 22

Promote rapid communication between epithelial cells, through diffusion of ions and small molecules. exist in cardiac muscle cells form channels that allow passage via diffusion of small molecules between cells.

Answer 23

Epithelia cells adhere to and communicate with one another through various cell junctions.Various types of cell junctions exist to mediate and control cell attachments. These cell junctions are present on the lateral surfaces of epithelial cells, generally towards the apical side, and are essential for several purposes. Most epithelial cells, particularly those in simple epithelia, are polarized containing distinct regions of plasma membrane and cytoplasm. Polarization leads to formation of an outer domain, the apical domain, that faces the free surface, and an “inner” basal or basolateral domain that faces the basal lamina.

Answer 24

There are two general but key aspects of polarity: (1). The plasma membrane composition is locally segregated into domains: The plasma membrane in the apical domain contains distinct membrane proteins and a distinct phospholipid content compared to the membrane in the basal domains. Key membrane proteins that localize to specific domains include: transporter enzymes, ion channels, receptors for exocytosis/endocytosis, signaling receptors and effectors, and proteins that mediate cell-cell and cell-lamina attachments. The tight junction complexes tend to reside near the apical surface in these cells. Therefore, the membrane surrounding the lateral and basal side is often called the basal-lateral (or basolateral) membrane because the protein and lipid content is similar on both the lateral and basal sides. However, in many epithelial cells (e.g. columnar and cuboidal cells), there are differences between the lateral and basal membranes too. (2) The cytoplasm is polarized: The cytoskeleton (particularly microtubules) is asymmetric or polar in orientation. Organelles are distributed in a precise polarized pattern; secretory vesicles in the apical domain are usually different from those in the basal domain, and they move in specific directions.

Answer 25

Epithelial cell polarity is crucial to allow unidirectional secretion and/or absorption of molecules to or from one side of the epithelium. Polarity is also necessary for “trans-epithelial” transport of ions and macromolecules from the apical to basal surfaces (or visa versa). The endocytosis of substances from one membrane region, followed by trans-cellular transport of the vesicles and their exocytosis from another membrane region is called transcytosis. In addition, polarity is important for localizing and orienting intercellular signaling either among epithelial cells, or between epithelial cells and other cell types.

Answer 26

The cell surface of epithelial cells, mostly the apical surface, can be highly modified in terms of structure and function. The modifications of interest include: apical surface modifications (Microvilli, Cilia). Basolateral surface modifications.

Answer 27

Cell surface extensions (protrusions) that contain actin bundles connected to cytoskeletal elements in the cell interior. The primary function of microvilli is to increase surface area, which greatly increases the rate/efficiency of membrane transport and secretion. The size and abundance of microvilli varies with cell type.

Answer 28

Microtubule-containing extensions (protrusions). There are at least three kinds of cilia: (1) A primary cilium is a single (one/cell) non-motile microtubule-based extension found on many different epithelial cell types. Primary cilia organize and promote signal transduction systems (receptors and effectors) that control epithelial cell division, fate (differentiated state), and function. (2) Motile cilia are related microtubule extensions that move, and are found only on specific epithelial cell types; these cilia wave like a boat oar to move mucous and other materials along passageways. Thus they tend to be found in epithelia of the respiratory tract (to move junk and mucous out of the airways), and in the oviduct (to assist in moving ova towards the uterus). (3) Some highly specialized non-motile sensory cilia are not motile and appear to function in sensory reception (e.g. in vestibular hair cells of the ear). These sensory cilia are likely specific variations of primary cilia, specialized to house sensory (touch, taste) systems that are connected to the central nervous system. Mutations in proteins common to cilia or their support structures result in a large set of diseases called ciliopathies.

Answer 29

infolds and outfolds of the basolateral membrane are also found in many epithelial cells, though they seem to lack the structural organization of the apical microvilli and cilia. Again, these folds increase surface area and are likely to be seen in cells that transport heavily to or from their basolateral surface.

Answer 30

An unusual type of microvillus, unfortunately called stereocilia, are found in the epididymis and in sensory cells in the ear. These are extremely long, actin-filled microvilli, and are not related to cilia. In the ear, stereocilia function in the reception of sound.

Answer 31

A basal lamina is a thin sheet of extracellular material that underlies the basal surface of each epithelial tissue. Basal laminae also surround many other cell and tissue types as well; including the endothelium of blood vessels, muscle and nerve tissue. Basal laminae are formed by a special type of network-forming collagen, a fibrous protein. This type of collagen forms sheets of thin fibers that are interwoven with a variety of other extracellular glycoproteins. Network-forming collagen (especially “type IV”) and some other glycoproteins including laminins and entactin are common to most basal laminae. However, many other glycoproteins are specific to different basal laminae. Thus, the molecular structures of basal laminae in different epithelial tissues are quite diverse. It is thought that most of the basal lamina components are actually synthesized and secreted by epithelial cells. However, some extracellular components of the connective tissue (made by fibroblasts) bind to and possibly organize elements in the basal lamina. Basal laminae both separate epithelial cells from the underlying connective tissue, and attach epithelial cells to the extracellular matrix of the connective tissue. Epithelial cells directly connect to basal laminae by attachment of hemidesmosomes and focal adhesions on the basal surface of the epithelial cell to components of the basal lamina. The key class of proteins that form these connections are the integrins (distinct proteins from the cadherins that mediate cell-cell linkage).

Answer 32

1. They mediate attachment of epithelia to the underlying connective tissue. 2. Basal laminae often contribute to selective filtration of substances diffusing to or from the epithelia. 3. They are necessary for the establishment and maintenance of epithelial cell polarity. 4. They can serve as specific “highways” for the migration of cells through connective tissue. 5. They provide a barrier to movement of invading microbes or cancerous cell into other tissues. 6. They control the gene expression of cells to affect their proliferation or development. 7. They control the development, morphogenesis, and organization of epithelial cells, providing a sort of “tissue scaffolding” function. Thus, they are critical to the repair of epithelial tissue following damage by injury or disease.

Answer 33

Anchors intermediate filaments in a cell to the basal lamina. core protein is integrin, interacts with intermediate filaments

Answer 34

Focal adhesions serve as the mechanical linkages to the ECM, and as a biochemical signaling hub to concentrate and direct numerous signaling proteins at sites of integrin binding and clustering. Focal adhesions regulate epithelial polarity and function through signaling mechanisms, and probably are particularly important during re-establishment of an epithelium following injury or disease, or during the normal process of cell replacement (turn-over). like adherins junctions. Organize signalling complexes controlling polarity, gene expression, transport, etc within epithelial cells

Answer 35

Integrins couple the ECM outside a cell to the cytoskeleton (in particular, the microfilaments) inside the cell. Which ligand in the ECM the integrin can bind to is defined by which α and β subunits the integrin is made of. Among the ligands of integrins are fibronectin, vitronectin, collagen, and laminin. The connection between the cell and the ECM may help the cell to endure pulling forces without being ripped out of the ECM. Specific integrins of hemidesmosomes connect to intermediate filaments in the epithelial cell and provide strength to the epithelial-connective tissue attachment. Different integrins of focal adhesions connect to actin filaments inside the epithelial cell.

Answer 36

Epithelial tissues undergo substantial turn-over throughout adult life. Consequently, epithelia generally contain stem cells that are capable of cell division, and that serve to (i) self renew: regeneration of stem cells with each division, and (ii) produce differentiated cell types specific to each epithelia. In essence, these are the same processes that occur during early development, and in fact many of the same regulatory pathways critical to development are re-used to control stem cell-driven tissue regulation. Like all stem cells, epithelial stem cells replace cells that die and can flexibly regulate (change) the form and function of tissues. Stem cells produce differentiated cell types by cell division coupled with specific pathways of cell specification imposed on some of their daughter cells. In most stem cell-tissue systems, cell division and specification goes through several steps.

Answer 37

Many stem cells produce daughters that also proliferate themselves (undergo cell division cycles), often at faster rates; these transitional intermediates are called transit amplifying cells. These transit amplifying cells then produce differentiated cells, either directly or through several steps of specification. At each step, specific changes in protein expression patterns and activities occur.

Answer 38

Many epithelia are composed of multiple cell types arranged in precise ways. The developmental state of stem cells determines the constellation of cell types, their relative proportions, and their arrangements. A specific stem cell type, its intermediate progeny, and their differentiated progeny are collectively called a cell lineage.

Answer 39

Both division and differentiation in stem cell lineages are tightly regulated by cell to cell communication and cell signaling pathways, which are activated or inhibited by physiological and developmental cues. Because of tight regulation, stem cells typically divide very slowly, or infrequently in some tissues. Consequently, they are usually much less abundant than their differentiated offspring. Loss of this regulation is generally at the heart of the many diseases of stem cell systems. A typical cell signaling pathway consists of (i) an extracellular ligand (signal) secreted by signaling cells, (ii) a receptor in receiving cells that binds and is activated/inactivated by the ligand, (iii) downstream effector proteins in the cytoplasm and nucleus, (iv) numerous modulator proteins that promote or suppress pathway components. Cell signaling of epithelial stem cells may be local involving ligands secreted by cells within the same epithelia or by cells in neighboring connective tissue. Epithelial stem cells and/or their progeny respond to these ligands, and may also secrete their own ligands. The structural organizations described above serve to constrain, spatially orient, and control these signaling events. In some cases, however, epithelial turn-over may also be controlled by long-distance signaling between tissues. Long distance signals are typically secreted from endocrine glands into the blood stream (described below), or may be produced by the nervous system or other tissues.

Answer 40

Some of the most important signaling systems that control epithelial stem cell renewal and differentiation are the same or very similar to the core group of signaling pathways that control development of many tissues during embryogenesis, fetal development, and childhood. These include the Wnt, Sonic Hedgehog, TGFβ, Notch, and receptor tyrosine kinase family (RTK; for example the EGF receptors), and FGF receptor pathways. Some key principles of these signaling pathways are important to understand: i. Each pathway is used by multiple, very distinct stem cell systems in different organs/tissues. Although some pathway proteins may be unique to certain stem cell subsets, most pathway components are shared by several distinct stem cell lineages (stem cell lineages=stem cells and all their daughters, granddaughters, etc). ii. A single signaling pathway often triggers different developmental outcomes in different stem cell lineages. These differences result from combinations of: (a) Different developmental histories of each stem cell type (cells can inherit and maintain unique differences in nuclear (chromatin) and cytoplasmic states that are generated earlier in development, which dramatically impact how a cell responds to a ligand) (b) Different environments stem cells reside in (or to put another way, the different combinations of signaling inputs a cell is exposed to). (c) Differences in the levels of the extracellular ligands, receptors, and/or downstream components. Defects in the regulation, expression, or structure of signaling components (ligands, receptors, effectors) lead to disease, particularly cancer. Thus, these pathways are prime targets for drug therapies. Indeed, drugs that specifically target several components of these pathways are now in use as anti-tumor agents (one example: Tarceva (erlotinib) inactivates the EGF receptor and is commonly used to treat lung and pancreatic cancers).

Answer 41

Glands of the body are derived from epithelial tissue. They are found in many sites of the body, often associated with the epithelia of body surfaces and cavities, and as components of specific organs. The primary function of glands is secretion of either specific bioactive molecules (e.g. hormones), complex fluids (e.g. mucous, sweat), or both. There are two major types of epithelial glands :Exocrine glands and Endocrine glands. Glands secrete their substances in one of two general ways. (1) Exocytosis: glands secreting by this familiar, and most common, mechanism are sometimes called merocrine or apocrine glands. (2) Total cell disintegration, which leads to the entire cellular contents becoming part of the secretion; glands of this sort are less common, and are sometimes called holocrine glands (examples include the secretion of oily sebum onto hair and skin).

Answer 42

secrete materials onto epithelia-lined surfaces or the outside world. Exocrine glands secrete materials onto the apical side of epithelial surfaces. They are generally multicellular. However, some epithelial cells in the lining of internal passageways function as unicellular glands secreting materials directly onto those surfaces (e.g. goblet cells). All multicellular exocrine glands have two main components: The Secretory units and Ducts. In general, exocrine glands of the body “tubes” (alimentary canal, respiratory, and urogenital systems) are of three general types: mucous: from which a viscous glycoprotein-rich fluid is produced, serous: from which a watery fluid, containing salts and some specific proteins, is produced, and mixed: from which both kinds of secretions are produced. The presence of mucous or serous secreting cells can be seen in histological sections. As might be expected, secretory epithelial cells of these glands secrete materials unidirectionally from their apical surface (with the exception of holocrine secretion).

Answer 43

secrete substances into the blood stream. Endocrine glands have no ducts and secrete substances to the blood stream. These glands produce mostly specific hormones that act over relatively long distances to control tissue function. Endocrine glands are generally organized as clumps or chords of cells that are embedded with and surrounded by connective tissue containing extensive capillary networks. Each clump is, of course, surrounded by a basal lamina. Blood capillaries are imbedded in connective tissue on the other side of the epithelial basal lamina. Therefore, hormone molecules must cross the basal surface and basal lamina of the epithelium (and finally the basal lamina and endothelial layer of the capillary) to reach the blood stream. For this reason, most endocrine cells secrete these molecules from the basolateral membrane.

Answer 44

clumps of secretory epithelial cells, which produce and secrete the bulk of the secretion. Secretory units can be organized into bowl or flask-shaped lobules called alveoli or acini, and are called alveolar or acinar glands, or they may be organized into tubes, and are called tubular glands. Some glands have both tubular and alveolar character and are called tubuloalveolar. Some glands can have a single secretory unit or they may have multiple branched units.

Answer 45

tubular structures that emanate from the secretory units. Ducts function as passageways to conduct secretions to their destinations, although their epithelia can also modify the secretion content by secretory and ion transport properties of duct cells. Glands that possess a single duct are classified as simple glands, whereas those with multiple branched ducts are called compound glands. Further subclassifications exist reflecting whether the glands is coiled, and whether the secretory units are multilobed (branched) or not.

Answer 46

The secretions from both exocrine and endocrine glands are generally regulated by the autonomic nervous system (i.e. by direct neuronal stimulation), by hormones from blood, or by both. Exocrine glands often secrete continuously at a low rate but can be triggered to greatly increase the volumes of secretion by these mechanisms. For example, the sight or smell of food stimulates serous secretions from salivary glands, and mucous and serous secretions from gastrointestinal glands. Endocrine secretions tend to be tightly regulated by hormonal or neuronal stimulation. Defects in this regulation lead to disease.

Answer 47

Epithelial tissues are targets (direct or indirect) of a large variety of diseases. Just a few general examples include diabetes, various liver and kidney diseases, lung diseases (e.g. emphysema, cystic fibrosis), GI disease (e.g. gastric ulcers, inflammatory bowel diseases), skin diseases, hearing diseases, and ciliopathies. Examples in the gastrointestinal tract include ulcerative colitis, in which extensive ulceration and destruction of the absorptive epithelium occurs. A disease called pemphigus is an autoimmune disease in which antibodies are produced against components of desmosomes in skin, leading to extensive blistering. This broad clinical importance reflects the essential roles of epithelia in organ/tissue function, their relatively high rate of turn-over requiring stem cell-driven renewal, and their exposure to external damage (UV light, pathogens, chemicals). Epithelia are also important to consider in wound repair. Injuries to skin epithelia that do not extensively damage the basal epithelial stem cells and basal lamina can often be repaired by intrinsic mechanisms, but if the basal skin layers and basal laminae are severely compromised they are replaced by scar tissue; for example severe burns may require skin grafts to replace a damaged basal lamina and restore a population of basal skin epithelial stem cells.

Answer 48

Cancers of epithelial origin. The most common cancers by far are derived from epithelial tissue, probably because of damage exposure and their high stem-cell driven turnover rates. Tumors usually first develop within an epithelial sheet, but they can become invasive and spread (metastasize) to other parts of the body. Often, carcinomas retain some properties of their tissue of origin. Thus, the diagnosis and treatment of different carcinomas is aided by histological appearance of the tumor and knowledge of epithelial biology. It is now believed that carcinomas result from defects in regulatory pathways that control epithelial stem cells or their progeny during tissue development and maintenance. Targeted regulatory pathways include signaling systems that directly control development (e.g. Wnt, EGF or Notch signaling systems), internal cell cycle control factors, and factors that control DNA damage repair and apoptosis (programmed cell death).

Answer 49

cancers derived from glandular epithelium

Answer 50

an autosomal recessive condition that results in a syndrome of chronic sinopulmonary infections and nutritional abnormalities secondary to malabsorption. It is one of the most common lethal genetic diseases in the United States, with an incidence of approximately 1:3000 among Caucasians and 1:9,000 in Hispanic populations. Lung disease is the major cause of morbidity and mortality. Most individuals with CF develop obstructive lung disease associated with chronic infection that leads to progressive loss of pulmonary function and ultimately death from respiratory failure. Currently, the median life expectancy for people living with CF in the US is 37 years. CF is caused by a defect in an ATP-binding cassette transporter gene on chromosome 7 that encodes for the CF Transmembrane Conductance Regulator (CFTR) protein. The CFTR protein is an epithelial chloride channel that also has signaling effects on other membrane channels. The most common mutation is the F508del, although approximately 1500 other disease-causing mutations in the CF gene have been identified. Gene mutations lead to defects or deficiencies in CFTR, causing problems in salt and water movement across cell membranes, resulting in abnormally thick secretions in various organ systems and critically altering host defense in the lung. CF is now included in the newborn screening program for all states in the US, and many countries world-wide.

Answer 51

Greasy, bulky, malodorous stools; failure to thrive due to exocrine pancreatic insufficiency, Recurrent respiratory infections with opportunistic bacteria (e.g. Staphylococcus aureus, Pseudomonas aeruginosa and Burkholderia cepacia), Chronic sinus infections, Digital clubbing on examination. Bronchiectasis, Sweat chloride > 60 mmol/L.

Answer 52

Due to widespread adoption of CF newborn screening in the past decade most children in the US with CF are identified soon after birth. However, older children born prior to newborn screening, and those missed by the screen (false negative rate ranges from 1-5%) may present with clinical symptoms later in life. The most common presentation of CF is failure to thrive. Children typically fail to gain weight despite good appetite and have frequent, bulky, foul-smelling, oily stools. These symptoms are the result of severe exocrine pancreatic insufficiency, the failure of the pancreas to produce sufficient digestive enzymes to allow breakdown and absorption of fats and protein. Pancreatic insufficiency occurs in more than 85% of persons with CF. Infants with undiagnosed CF may also present with hypoproteinemia with or without edema, anemia, and deficiency of the fat-soluble vitamins A, D, E, and K, because of ongoing steatorrhea. CF should also be considered in infants and children who present with severe dehydration and hypochloremic metabolic alkalosis. Other findings that should prompt a diagnostic evaluation for CF include unexplained bronchiectasis, rectal prolapse, nasal polyps, chronic sinusitis, and unexplained pancreatitis or cirrhosis. Approximately 15% of newborns with CF present at birth with meconium ileus, a severe intestinal obstruction resulting from inspissation of tenacious meconium in the terminal ileum. Infants with meconium ileus should be treated presumptively as having CF until a sweat test or genotyping can be obtained. From a respiratory standpoint, clinical manifestations include productive cough, wheezing, chronic bronchitis and recurrent pneumonias, progressive obstructive airways disease, exercise intolerance, dyspnea, and hemoptysis. Chronic airway infection with bacteria, including S aureus and H influenzae, often begins in the first few months of life, even in asymptomatic infants. Eventually, Pseudomonas aeruginosa becomes the predominant pathogen. Acquisition of the characteristic mucoid Pseudomonas is associated with a more rapid decline in pulmonary function. Chronic infection leads to airflow obstruction and progressive airway and lung destruction resulting in bronchiectasis. An acute change in respiratory signs and symptoms from the subject’s baseline is generically termed a pulmonary exacerbation. Clinically, an exacerbation is typically manifested by increased cough and sputum production, decreased exercise tolerance, malaise, and anorexia. These symptoms are usually associated with decreased measures of lung function, new chest radiographic findings, or both. Treatment for pulmonary exacerbations generally consists of antibiotics and augmented airway clearance and often requires hospitalization for 1-2 weeks.

Answer 53

Management of exocrine pancreatic insufficiency with enzyme supplementation, high calorie, high protein, and high fat diet, and fat soluble vitamin supplements. Daily salt supplement due to salt losses in sweat. Airway clearance treatment to loosen and remove thick mucus from airways: Daily percussive therapy (Chest PT, vest therapy); Recombinant human DNAse (Pulmozyme), an inhaled mucolytic agent; Inhaled hypertonic saline; and Bronchodilators. Antibiotic therapy targeting common CF related bacteria such as P. aeruginosa: Inhaled tobramycin (TOBI); Inhaled aztreonam (Cayston); and Oral or intravenous antibiotics. Anti-inflammatory treatments: Ibuprofen (high dose) and Chronic azithromycin. CFTR modulators: ivacaftor and lumacaftor

Answer 54

Only approved for people age 6 years or older with at least one G551D mutation or gating mutation

Answer 55

(CFTR corrector/potentiator) for people ages 12 years or older with 2 copies of the F508del mutation is currently undergoing review with the FDA.

Answer 56

A few decades ago, CF was fatal in early childhood. Now the median life expectancy is around 35 years of age. The rate of lung disease progression usually determines survival. Lung transplantation may be performed in those with end-stage lung disease. In addition, new treatments, including gene therapy trials and agents that modulate CFTR protein function, are being developed based on improved understanding of the disease at the cellular and molecular levels.

Answer 57

Sinuses – Sinusitis, nasal polyps. Lung – Endobronchitis, bronchiectasis. Pancreas – Exocrine Insufficiency CF Related Diabetes. Intestine – Meconium ileus Constipation/DIOS. Liver – Focal sclerosis. Vas Deferens – failure to develop. Sweat gland – salt-losing dehydration

Answer 58

I. no synthesis (nonsense, frameshift, splice junction). 2. block in processing (AA deletion, this includes deltaF508). 3. Block in regulation (missense and gating mutations). 4. Altered conductance (missense). 5. Reduced synthesis (missense and alternative splicing).

Answer 59

Member of ATP-binding cassette (ABC) superfamily. Expressed in epithelial cells lining exocrine glands. 2 six-span membrane spanning domains (MSD). 2 nucleotide-binding domains (NBD). Unique, highly charged regulatory (R) domain

Answer 60

An estimated 1,000 proteins are coordinated to form the cilium. These polypeptides form the major domains that build the cilium. These domains include the centriole/basal body, axoneme, transition zone, ciliary membrane, and intraflagellar transport (IFT) machinery. Each of these domains is essential for ciliary formation and function.

Answer 61

the core anchors from which cilia are formed. Basal bodies are microtubule rich cylinder shaped structures formed from nine triplet microtubules (A-B-C tubules). Basal bodies are typically 150-200 nm in diameter and 500 nm in length. This polarized structure is formed beginning at the proximal-end and the distal-end is responsible for nucleating the cilium. centrioles migrate to the plasma membrane to organize the cilliary axiallary. Basal bodies are derived from centrioles through a largely mysterious process. They are structurally the same, each containing a microtubule triplet 9*3 + 1 helicoidal configuration forming a hollow cylinder.

Answer 62

the structural skeleton of the cilium. It is formed from doublet microtubules (A-B tubules) that assemble from the A- and B-tubules of the basal body. Microtubules are polar polymers and the plus-ends reside at the ciliary tip. Cilium lengths range from less than a micron to tens of microns in the case of sperm flagella. In addition to their scaffolding role, axonemes provide the tracks for movement within cilia. the axoneme of a motile cilium has two central microtubules in addition to the nine outer doublets (called a 9*2+2 axoneme). The axonemal cytoskeleton acts as a scaffolding for various protein complexes and provides binding sites for molecular motor proteins such as kinesin II, that help carry proteins up and down the microtubules. The protofilaments form triplet rings with some for the protofilaments shared in between. Primary cilia (non-motile/ sensory) have a 9+0 arrangement.

Answer 63

links the basal body to the axoneme and to the ciliary membrane. This is also considered the “gatekeeper” because it limits the diffusion of membrane and soluble proteins into and out of the cilium. This domain ensures that the ciliary membrane is a distinct compartment for cellular signaling. Many proteins of the transition zone, when absent or defective, are associated with human ciliary diseases (ciliopathies).

Answer 64

continuous with the cellular plasma membrane. However, this membrane is compartmentalized (by the transition zone) so that it is a compositionally distinct membrane with unique phospholipids and receptor molecules.

Answer 65

Axonemes function as the highways or tracks within cilia. Cargo, important for the assembly and maintenance of the cilium and for the movement of signaling components within the cilium, is transported along the axoneme by a mechanism called intraflagellar transport (IFT). Transport is bidirectional with kinesin motors (Kinesin-2 family) and the IFT-B protein complex directing movement to the ciliary tip (anterograde transport). Conversely, retrograde transport occurs by cytoplasmic dynein 2 motor driven transport with the IFT-A protein complex. Both transport mechanisms are required for cilium formation and function.

Answer 66

Ciliogenesis can be separated into two phases. First, centrioles/basal bodies are assembled and this is followed by the formation of the cilium. Basal bodies are derived from centrioles, cellular structures that organize the centrosome. Thus, centrioles interchange between their function at centrosomes to organize the cellular array of cytoplasmic microtubules during interphase and mitosis. Upon ciliogenesis, the older of the two centrioles (the mother centriole) functions as the basal body or anchor. Centriole duplication occurs during the G1 to S-phase cell cycle boundary coincident with DNA synthesis. Many of the same molecular components required to initiate DNA replication also function in centriole replication. Also analogous to DNA replication, the process is tightly regulated to limit replication to once-and-only-once during the cell cycle. A new centriole is assembled adjacent to the existing centriole. Of the two centrioles, the older centriole will become the mother basal body in the subsequent G1 phase of the cell cycle. Ciliogenesis normally occurs during G1 (or G0) of the cell cycle by assembling from the mother centriole (basal body) of the centriole pair. This begins when the distal end of the basal body is capped by a “ciliary vesicle”. Microtubule doublets then assemble into the ciliary vesicle before the entire structure fuses with the plasma membrane of the cell. The above description describes the tightly linked processes of centriole duplication and primary cilia formation. However, terminally differentiated cells nucleate many (sometimes 100s) of cilia per cell. Under these conditions, additional mechanisms are required to form the many basal bodies that are required for ciliogenesis in each cell. Basal body assembly is uncoupled from the cell cycle and replication is amplified to facilitate the multiple basal bodies required for ciliogenesis in multi-ciliated epithelial cells.

Answer 67

Motile and sensory cilia are two generalized classes of cilia. Importantly, most motile cilia also possess sensory functions. Motile cilia are required for the movement of fluid in the respiratory, neural, and reproductive tracts. Motility is produced by axonemal dynein dependent sliding motion between the doublet microtubules of the ciliary axoneme. Motile cilia typically have a 9+2 microtubule arrangement with 9 doublet microtubules organized around a central pair of singlet microtubules. Not all motile cilia have the central pair microtubules (e.g. Nodal Cilia possess a 9+0 arrangement but still beat). The distinguishing factor between motile and immotile cilia is the presence of axonemal dynein arms between the doublet microtubules. Non-motile, sensory or primary cilia possess a 9+0 microtubule arrangement and lack axonemal dynein arms. These cilia normally perform signaling functions described below.

Answer 68

``` Cilia are often described as the cell’s antennae. Several reasons may exist for why cilia are used for cell signaling. The cilium concentrates the signal with a high receptor surface to volume ratio, the signal is localized and polarized within discrete domains of the cell, the receptors are positioned away from interfering cellular domains, and the cilium can function as a mechanical detector of flow. Cilia can sense physical stimuli (mechanical strain, temperature, osmolality, and gravity), light, and chemical stimuli (hormones, chemokines, growth factors, and morphogens). All of these can produce a diverse array of downstream events including cellular proliferation, cellular motility, polarity, growth, differentiation, and tissue maintenance. The hedgehog (Hh) signaling pathway is well established to signal through cilia. Activation and repression of the target of the Hh paracrine signaling pathway (Gli (glioma tumor) transcriptional activator) requires cilia. Additional signaling pathways (e.g. Wnt, PDGF, FGF, and others that remain to be discovered) function through cilia. ```

Answer 69

Both motile and immotile cilia are important for development and tissue homeostasis. In the case of Hh signaling, downstream targets facilitate: limb formation, bone foration and homeostasis, and neurogenesis. Both ciliary-based motility and signaling are required for the vast list of developmental and tissue homeostasis functions that include: Tissue and cellular polarity, Tissue patterning (neural and limb pattern), Cell fate specification, Left-right axis determination (laterality), Neural tube formation, Tubule formation (kidneys, liver, pancreas), Organogenesis, Bone formation, Eye development, Craniofacial development, Retinal degeneration (Degenerative), Renal cystic disease (Degenerative). An example of the coordinated efforts of both motile and sensory cilia function during development is the ciliary node that establishes the left-right asymmetry of the body plan (laterality). The ciliary node is an invagination of ciliated cells that forms during gastrulation on the midplate after anterior-posterior positioning is already established. Nodal cilia possess a 9+0 organization yet they beat in a rotary fashion (at 600 rpm!) to produce a net leftward flow of signaling molecules / morphogens. The signals are then detected by non-motile sensory cilia that reside near the periphery of the ciliary node to signal organogenesis. This signaling is localized specifically to the future left axis of the body.

Answer 70

Cystic kidneys – Multilumen formation and growth of fluid filled cysts. Nephronophthisis – Progressive renal damage. Obesity – Too much body fat. Polydactyly – Postaxial polydactyly with an extra digit on the ulnar side of hand or foot. Retinal degeneration – Deterioration of retina. Amnosia – Loss in olfaction. Cancer / Tumorigensis – Formation of tumors. Urinary tract malformation – Deformities in urinary tract. Cognitive impairment – Both developmental impairment and decline of cognition. Diabetes mellitus – Type 2 diabetes develops from obesity. Infertility – Disrupted female reproductive tract and male sperm motility. Occipital meningoencephalocele – Neural tube defects with hernial protrusion of the brain material. Microphthalmia – Small eye developmental disorder. Lung hypoplasia – Incomplete lung development. Renal hypodysplasia or dysplasia – Abnormal or missing kidney formation. Bile-duct dilatation – Progressive dilation of the bile duct. situs inversus – Left-right laterality defects to randomize organ position. Many syndromes have been described that are now recognized as ciliopathies. Several characteristics can be attributed to these diseases. Rare. On average, the incidence is approximately 1:100,000. While each syndrome is rare the combined incidence of ciliopathies may be more prevalent and similar syndromes may be combined: Pleiotropic. Each ciliopathy is characterized by different clinical outcomes. Overlapping. Most of the ciliopathies have overlapping phenotypes and genetic mutations. Structural. Mutations often affect core cilium structure and function. Diverse. ~50 genes are known to be mutated in ciliopathies to date. Genetically complex. Mutations in the same gene (Cep290) can produce four different clinical outcomes suggesting that second site modifier mutations and genetic background is important to the clinical manifestations of ciliopathies.

Answer 71

Autosomal recessive. Symptoms: Photoreceptor degeneration, Anosmia, Mental retardation / Developmental delay, Neural tube defects, Obesity, Hypogonadism, Kidney defects, Polydactyly, Diabetes, situs inversus.

Answer 72

Autosomal dominant and recessive forms (ADPKD and ARPKD, respectively). ARPKD is caused by fibrocystin mutations (incidence = 1:20,000). ADPKD is caused by mutations in polycystin-1 and polycystin-2 (PKD1 and PKD2; incidence = 1:1,000). Genes encode channel proteins that are responsible for calcium signaling. These channels sense mechanical flow of urine in the kidney lumen to transduce calcium signaling. However, the mechanism for how this forms cystic kidneys remains to be worked out. Symptoms: Renal cysts, Liver and pancreas cysts, Intracranial aneurysms

Answer 73

a proteinous inter-doublet linkage that prevents microtubules in the outer layer of axonemes from movement with respect to each other. Otherwise vesicular transport proteins such as dynein would dissolve the whole structure.

Answer 74

dividing cells do not want cilia because these components are important for dividing DNA. Centrioles/ centrosomes must also be duplicated and segregate during cell division. Steps: 1) centriole formation. 2) Celium formation

Answer 75

New centrioles become basal bodies for ciliogenesis. Centriole duplication maintains the basal bodies for future cells. The mother centriole becomes the basal body.

Answer 76

Ciliogenesis occurs through an ordered set of steps. First, the basal bodies from centrioles must migrate to the surface of the cell and attach to the cortex. Along the way, the basal bodies attach to membrane vesicles and the basal body/membrane vesicle complex fuses with the plasma membrane of the cell. The transition zone and doublets assemble within the ciliary vesicle. Fusion with the plasma membrane is likely what forms the membrane of the cilia. The alignment of the forming cilia is determined by the original positioning and orientation of the basal bodies. Once the alignment is determined, axonemal microtubules extend from the basal body and go beneath the developing ciliary membrane, forming the cilia.

Answer 77

TF cause multiple cilia to form needs lots of centrioles, many duplications during differentiation duterosome makes lots of centrioles they then dock at apical side

Answer 78

Concentration of signal: microenvironment for signaling with high surface (receptor) to volume ratio. Localized: localization to a signaling domain. Polarized: signals are polarized within the cell. Fluid mechanics: fluid mixing at surfaces is limited and cilium protrudes beyond surface. Charge disruption: at lipid bilayer creates a 40 A of altered ions and proteins from charged regions. Flow sensing: mechanical bending to sense fluid flow.

Answer 79

receptors are concentrated in cilia to detect: physical stimuli (mechanical strain, temperature, osmolarity and gravity), life, chemical stimuli (hormones, chemokines, growth factors, morphogens (Hh, Wnt)). The readout is cellular proliferation, cellular motility, polarity, growth, differentiation, tissue maintenance

Answer 80

When SHH reaches its target cell, it binds to the Patched-1 (PTCH1) receptor. In the absence of ligand, PTCH1 inhibits Smoothened (SMO), a downstream protein in the pathway. It has been suggested that SMO is regulated by a small molecule, the cellular localization of which is controlled by PTCH. PTCH1 has homology to Niemann-Pick disease, type C1 (NPC1) that is known to transport lipophilic molecules across a membrane. PTCH1 has a sterol sensing domain (SSD), which has been shown to be essential for suppression of SMO activity.Upon binding of a Hh protein or a mutation in the SSD of PTCH, SMO accumulates. The binding of SHH relieves SMO inhibition, leading to activation of the GLI transcription factor: the activators Gli1 and Gli2 and the repressor Gli3. The sequence of molecular events that connect SMO to GLIs is poorly understood. Activated GLI accumulates in the nucleus and controls the transcription of hedgehog target genes. PTCH1 has recently been reported to repress transcription of hedgehog target genes through a mechanism independent of Smoothened.

Answer 81

1. Protection for critical organs (e.g. the ribs for the heart and lungs, the skull for the brain) 2. Mechanical support for locomotion; by supporting and providing attachments for muscles and joints for flexible movement. 3. Calcium and phosphate homeostasis: Bone is a tightly regulated reservoir of calcium for the entire body. 4. To house, protect, and regulate the stem cell precursors of blood cells (the hematopoietic system) (a function of bone).

Answer 82

are specialized connective tissues. Together with ligaments and skeletal coverings, bone and cartilage comprise the skeletal system. Like all connective tissues, bone and cartilage are composed of collagen-containing extracellular matrix secreted by cells of the connective tissue family. However, these tissues contain specialized cell types that lead to precise structural organizations and produce specialized ECMs. In particular, the matrix of bone is mineralized or calcified, i.e. it contains crystals of calcium (Ca2+) and phosphate (PO4). Bone is a highly dynamic tissue. It is associated with numerous blood vessels (vascularized) and is constantly turned over and remodeled even in adults. Bone maintains a population of precursor cells that are capable of cell division and differentiation into new bone cells. This flexibility is required to adjust blood calcium and phosphate levels, to allow repair of damage, and allow changes in structure in response to various stimuli. Cartilage is much less dynamic; it is avascular in its matrix and is very limited in its ability for repair in adults. However, cartilage grows dramatically in the fetus and in children and serves, in part, as a template for conversion into bone. In adults, cartilage retains the capacity to convert into bone, which can cause problems as people age.

Answer 83

(1) to provide a resilient but pliable support structure. (2) to direct the formation and growth of bone.

Answer 84

make cartilage matrix and tissue. Chondrocytes arise initially from primitive mesenchymal cells during fetal development. Once cartilage structures are formed, chondrocytes can also arise from an external layer of connective tissue that surrounds the cartilage called the perichondrium. During cartilage growth, chondrocytes are proliferative and secrete the components of the cartilage extracellular matrix (ECM). As they secrete and surround themselves with matrix they become isolated from other cells, coming to reside in an isolated compartment called a lacuna. Proliferative chondrocytes are sometimes called chondroblasts, but in essence they are the same cells engaged in the cell division cycle. After growth is completed chondrocytes withdraw from the cell cycle, but retain the capacity to secrete cartilage matrix, albeit at lower rates.

Answer 85

a layer of dense irregular connective tissue that surrounds the cartilage of developing bone. undifferentiated mesenchymal cells or fibroblasts of the perichondrium generate new chondrocytes during growth.

Answer 86

a small space containing an osteocyte in bone or chondrocyte in cartilage.

Answer 87

Chondrocytes secrete a special extracellular matrix. There are three different kinds of cartilage which are distinguished by the characteristics of the matrix they contain. These include hylaine, elastic, and fibrocartilage. Cartilage tissue is not vascularized (avascular): i.e. it contains no blood vessels within the matrix. Therefore, all of the nutrients and metabolites that are needed and produced by chondrocytes must diffuse within the matrix to and from the perichondrium. This is facilitated by the large amounts of hydrated glycosaminoglycans.

Answer 88

contains collagen that forms relatively thin fibrils that are generally arranged in an irregular three dimensional pattern. The ground substance of hyaline cartilage is rich in proteoglycans and the free glycosaminoglycan hyaluronic acid, which promotes hydration and flexibility. This structural properties of this ECM: Allows metabolites to readily diffuse through the tissue. Promotes resiliency to compression forces during joint movement. Allows growth of chondrocytes and matrix from within the matrix. During growth, it can calcify and attract cell that initiate bone formation.

Answer 89

also contains thin collagen fibrils and proteoglycans, but is distinguished by abundant elastic fibers and interconnecting sheets (lamellae) of elastic material. Elastic cartilage is found in the external ear, in the epiglottis, and the larynx. This matrix is designed for elasticity and flexibility, and, at least under normal circumstances, does not calcify.

Answer 90

contains large bundles of regularly arranged collagen that is very similar to dense regular connective tissue. Indeed it is found as a continuation of dense regular connective tissue where tendons attach to bones, and also in the intervertebral discs. This ECM is designed to resist compression and sheer forces.

Answer 91

At a gross level, there are two general types of bone: the flat bones (e.g. the skull, mandible) and the long bones (e.g. tibia, femur, humerus). A long bone consists of a central shaft, the diaphysis, and two expanded ends, each called an epiphysis. If one makes a longitudinal section through a bone, two distinct areas. The outer, or cortical region, is solid and is composed of compact bone. The inner portion contains spongy bone which has thin anastomosing spicules called trabeculae (spongy bone is also called cancellous or trabecular bone). Compact and trabecular bone contain the same types of cells and the same matrix but different relative activities of bone cells lead to different structural arrangements. These arrangements lead to functional differences; compact bone provides most of the strength of the bone for support, while the trabeculae provide extensive surface area for metabolism. This overall structure results in two distinct surfaces of bone; periosteum and endosteum

Answer 92

Within the spaces between the trabeculae of the inner spongy bone is soft tissue called bone marrow. Bone marrow consists of either hematopoietic tissue (red bone marrow) or adipose cells (white bone marrow) surrounded by loose connective tissue containing numerous blood vessels.

Answer 93

The outer surface covering the bone called the periosteum, which contains dense connective tissue containing fibroblasts, bone precursors and bone cells.

Answer 94

The inner surface where trabeculae contact internal soft tissue is called the endosteum, which is where most calcium mobilization and storage occurs. In addition, there are tunnel-like channels through bone that also contain small amounts of soft connective tissue, blood vessels, and nerves.

Answer 95

are stem cells that are capable of cell division to generate the osteoblasts and osteocytes that comprise most of the cells of bone. Osteoprogenitors are present in both the periosteal and endosteal surfaces, and in the soft connective tissue of the channels.

Answer 96

are cells with single nuclei that synthesize bone. line the inner layers of both periosteal and endosteal surfaces where bone growth or remodeling is occurring. These cells actively secrete the initial un-mineralized extracellular matrix of bone, which is called osteoid. Osteoblasts also pinch off membrane-enclosed vesicles, the matrix vesicles, which contain enzymes that initiate bone calcification (mineralization) (see below). Osteoblasts are connected to each other and to nearby osteocytes by gap junctions. Osteoblasts are also capable of cell division.

Answer 97

the unmineralized, organic portion of the bone matrix that forms prior to the maturation of bone tissue secreted by osteoblasts

Answer 98

are directly derived from osteoblasts. They form as they become surrounded and encased by bone matrix in a lacuna. However, unlike chondrocytes, they do not divide (they arrest in Go). Further, osteocytes extend long processes through tiny channels, called canaliculi, in the calcified matrix; these processes form gap junctions with processes from other osteocytes. Thus, the cells of bone form a living interconnected lattice of cells, which can communicate directly, and are surrounded by abundant calcified matrix. Osteocytes apparently retain a limited ability to modify bone matrix, but they do not secrete much matrix material; the osteoblasts are the secretory engines that make bone matrix. The role of osteocyte activity in bone maintenance is not clear, but they probably send signals to each other and to osteoblasts at the surface through their gap junctions.

Answer 99

microscopic canals between the lacunae of ossified bone. The radiating processes of the osteocytes (called filopodia) project into these canals. These cytoplasmic processes are joined together by gap junctions. Osteocytes do not entirely fill up the canaliculi. The remaining space is known as the periosteocytic space, which is filled with periosteocytic fluid. This fluid contains substances too large to be transported through the gap junctions that connect the osteocytes, including calcium and phosphate ions.

Answer 100

are not related by lineage to the above bone cells and have a unique function. Osteoclasts are derived from monocytes in the blood, which themselves originate from hematopoietic stem cells in the bone marrow. They resemble and are related to macrophages. Like macrophages, osteoclasts are phagocytic cells. However, osteoclasts specifically degrade bone or cartilage matrix. These cells play several critical roles in bone dynamics: (1) they degrade cartilage or bone matrix to allow inward growth of blood vessels during bone formation (discussed below). (2) they function to resorb already made bone to promote remodeling of the bone matrix. (3) They resorb bone for the purpose of mobilizing Ca2+ into the bloodstream (critical for maintaining proper Ca2+ concentrations in blood).

Answer 101

Most extracellular matrix in bone is calcified and packed with dense parallel collagen fibers. Like all connective tissues, this matrix also contains negatively charged proteoglycans and other glycoproteins, but several of these are specific to bone. Unlike most connective tissues, bone matrix is unique in containing large amounts of a crystallized form of Ca2+ and PO4 called hydroxyapatite [Ca10(PO4)6(OH)2], the crystals of which are found on collagen fibers, and in the ground substance; i.e. the matrix is mineralized or calcified. Bone is vascular and innervated. Threading throughout the matrix of bone tissue are a large number of channels that contain both blood vessels and nerves. Thus, relatively short distances of diffusion are needed for nutrients to reach osteocytes within the bone matrix, and for transport of mobilized Ca2+ from the matrix to the bloodstream.

Answer 102

a naturally occurring mineral form of calcium apatite with the formula Ca5(PO4)3(OH), but is usually written Ca10(PO4)6(OH)2 to denote that the crystal unit cell comprises two entities. Hydroxylapatite can be found in teeth and bones within the human body.

Answer 103

In long bones, channels that traverse the long axis through compact bone are called Haversian canals. Bone lamellae tend to surround a Haversian canal in concentric rings (like the rings of a tree); these lamellae and their canal are frequently referred to as a unit called the osteon.

Answer 104

link Haversian canals to each other and to the periosteum at the bone surface.

Answer 105

the fundamental functional unit of much compact bone. Each osteon consists of concentric layers, or lamellae, of compact bone tissue that surround a central canal, the haversian canal. The haversian canal contains the bone's nerve and blood supplies.

Answer 106

The formation of cartilage and bone tissue are highly regulated processes that integrate cell proliferation and differentiation with the synthesis and transformation of extracellular matrix. Most of the important diseases of bone affect these processes. In addition, new approaches to manipulate the formation of cartilage and bone are being developed to repair damaged or defective structures (such as knee joints). Because formation of long bones is directly tied to formation of cartilage, the growth and formation of cartilage and bone will be discussed together. Both cartilage and bone formation begins in embryogenesis. Some complete bones are formed before birth. Other bone tissue, particularly in the long bones of the extremities continue to be formed during post-natal growth. Upon reaching puberty, net bone growth stops. However, formation of new bone tissue continues through the remodeling of existing bone, and can be stimulated following severe injury (bone fracture). There are two general ways that bone as a tissue and organ is formed: intramembranous and endochondral ossification

Answer 107

In the absence of a pre-made cartilage tissue: Within a sheet of connective tissue, groups of mesenchymal cells come together (a process called condensation), transform into osteoprogenitors, which then differentiate into osteoblasts. These islands of osteoblasts secrete osteoid. More cells at the periphery of these islands convert into other osteoblasts adding to the mass of the developing bone; eventually “bone islands” join. Calcification of the matrix occurs, but is delayed relative to its initial formation. The result, initially, is a trabecular network of bone. Blood vessels grow in within this network. Initially, the bone matrix is of woven bone, but soon remodeling by osteoclasts and new bone deposition by osteoblasts converts some of this bone to a compact lamellar form. The flat bones all form by intramembranous ossification.

Answer 108

Within previously made cartilage tissue (a cartilage model): In the embryo and fetus, cartilage tissue generates various structures of the skeleton that will become bone. Subsequently, some cells within the perichondrium are converted to osteoprogenitors. This sets in motion a series of events that cause the replacement of cartilage by bone. This process includes both formation of the cartilage model and replacement of cartilage by bone.

Answer 109

Mesenchymal cells undergo division and differentiate to generate a group of chondrocytes. Chondrocytes begin to secrete the matrix typical of hyaline cartilage. As they secrete their matrix individual chondrocytes become encased in their lacuna. Eventually an elongate structure in the relative proportion of a long bone is formed. This cartilage model then continues to grow in two different ways: (1) Appositional growth (growth at the surface): In the perichondrium at the cartilage surface, mesenchymal and/or fibroblast-like cells proliferate and differentiate into more chondrocytes, which secrete more hyaline matrix. (2) Interstitial Growth (growth from within): Chondrocytes embedded in matrix continue to proliferate within their lacunae and secrete the ECM, leading to internal growth of the tissue. Groups of chondroctyes within a lacuna or still close together are actually clones of cells derived from mitosis and are called isogenous groups (of clonal origin).

Answer 110

a cluster of chondrocytes, all formed through division of a single progenitor cell, found in hyaline cartilage, elastic cartilage, and fibrocartilage, growing by interstitial growth

Answer 111

On the surface of the diaphyseal shaft of the cartilage model (i.e. within the perichondrium), a collar of bone is formed. Mesenchymal cells develop into osteoprogenitors, which in turn become osteoblasts. Thus, in this region the perichondrium is transformed into a periosteum. Nearby chondrocytes enlarge (hypertrophy) and the cartilage matrix become calcified (thus, cartilage matrix is mineralized prior to replacement by bone). Osteoclasts are recruited to the region and begin to degrade the calcified cartilage matrix. Blood vessels grow into the degraded regions, bringing with them their sheath of connective tissue carrying more osteoprogenitors. Some of these osteoclast-led blood vessels penetrate deep into the diaphysis to form a center of ossification (the primary or diaphysial ossification center). There, intense osteoclast activity and osteoblast proliferation replace the cartilage with bone; the zone of ossification continues to expand outwards and towards both ends of the model. Later, two new ossification centers are established within both epiphyseal ends. At birth, the only region of proliferative cartilage that remains is beneath these epiphyseal ossification regions, and is called the epiphyseal plate.

Answer 112

a hyaline cartilage plate in the metaphysis at each end of a long bone. The plate is found in children and adolescents; in adults, who have stopped growing, the plate is replaced by an epiphyseal line.

Answer 113

After birth, long bones must grow substantially both in length and in diameter. The remaining region of proliferative cartilage at the epiphyseal plate, also called the growth plate, is maintained until the full length of the bone is achieved. Thus, this cartilage is the crucial region of continued growth in the length of the bone, promoting continued endochondral ossification. This is ensured by the continued interstitial growth of chondrocytes in their lacunae in the direction of the long axis of the bone. In turn, new chondrocytes deposit new hyaline matrix in the cartilagenous region. Simultaneously, osteoclasts, capillaries, and osteoblasts encroach on this cartilage from the diaphyseal side causing a continued wave of bone formation towards the epiphyseal end of the bone. Bone growth stops when proliferation of the cartilage stops; thus there must be some important control over the cell cycle of chondrocytes that ultimately determines bone length. Once growth stops, only a sheath of non-proliferative cartilage remains at the very end (the articular surface) of the epiphysis; this is called articular cartilage. Growth in the diameter of the bone occurs on the outer surface of the bone in the periosteum; i.e. it occurs only by appositional growth. This requires continued proliferation of osteoprogenitors in the periosteum, their differentiation into osteoblasts and deposition and mineralization of new bone matrix on the surface of old bone matrix. Net growth in bone diameter occurs as long as bone resorption is significantly less than deposition in this region. Note the bone cannot grow by interstitial growth as does cartilage, probably because the bone matrix is more rigid and can not readily expand from within.

Answer 114

Once bone is formed, it is then continually remodeled to ultimately convert it into its mature structure. Remodeling also contributes to the reshaping of bones as they grow and develop. Finally, remodeling activity is also important in the continual turn-over of bone matrix in mature fully grown bones. In the general remodeling sequence, osteoclasts become activated in specific regions, presumably by some signaling event. They then begin bone resorption of older calcified bone in specific areas. Activated osteoclasts then signal to activate osteoblast migration and secretion. Osteoclast activation also stimulates osteoprogenitors to generate new osteoblasts. After some delay, the osteoblasts initiate mineralization of the osteoid. Clearly, where and when the sequence is initiated, as well as the temporal sequence itself, must be carefully controlled; how this occurs is not well understood. In mature adult bone, most of the resorptive activity occurs at the endosteal surface. It is critical that resorption is directly and carefully coupled to formation. It is also important that formation is coupled to calcification. In normal circumstances, the amount of bone resorbed is equivalent to the amount of bone formed. Thus, even if bone resorption is stimulated, there is no net loss (or gain) of bone. Many diseases of bone affect one or the other of these couplings. Osteoporosis leads to decrease in bone mass due to defects in resorption/formation coupling. Osteopetrosis causes defective resorption and increased bone mass. Osteomalacia Rickets causes abnormal increase in uncalcified osteoid, by interfering with mineralization.

Answer 115

Osteoblasts initiate mineralization of the osteoid by secreting matrix vesicles. This involves a pinching off of membrane vesicles from the plasma membrane surface. Matrix vesicles contain high levels of calcium, phosphate that is linked to other molecules, and alkaline phosphatase enzymes. Alkaline phosphatases inside the secreted vesicles somehow become activated to generate free phosphate, which forms precipitates with calcium (hydroxyapatite). These precipitates grow and rupture the vesicles, and then act as nucleation sites to trigger a cascading mineralization outside in the matrix. The details of this process remain a bit mysterious.

Answer 116

The process of cartilage and bone formation, growth, and remodeling must be precisely regulated in space and time to generate, maintain and modify specific bone structures. Bone and cartilage regulation is mediated by: short range signals, long range signals, mechanical stress, and neuronal stimulation.

Answer 117

produced in the local bone environment. Among these are the Bone Morphogenetic Proteins (BMPs). BMPs are proteins secreted by cells, bind surface receptors and trigger intracellular protein phosphorylation that alters gene expression, which in turn promotes specific patterns of differentiation. There are many BMPs encoded by different genes. Some of these have been shown to act on connective tissue precursors to stimulate either chondrogenesis (differentiation of chondrocytes and the formation of cartilage) or osteogenesis (the differentiation of osteoblasts and formation of bone). BMPs control bone development and probably remodeling and turnover. Additional types of signaling systems also control bone formation during fetal development, and may also contribute to regulation after birth; these include FGF, Notch, and Wnt pathways.

Answer 118

from endocrine glands. These include steroid hormones (e.g. estrogen), and calcium regulation hormones

Answer 119

Bone remodeling patterns are altered by muscular movements and other mechanical strains. The mechanism of how this occurs is poorly understood.

Answer 120

Recent studies suggest that elements of the central and peripheral nervous system can control bone metabolism. Precisely how this control is achieved in not yet well understood.

Answer 121

Calcium mobilization from bone, and hence blood calcium homeostasis, is also under tight regulation by endocrine hormones and dietary intake of calcium. Parathyroid hormone stimulates calcium liberation (bone resorption), while calcitonin stimulates calcium uptake into bone. The details of the physiology of these hormones and how they control blood calcium and bone-turnover will be discussed in future lectures. However, for this class know that these hormones function (directly or indirectly) to control the activities of osteoclasts and osteoblasts. In addition, Vitamin D is important for systems that promote calcium uptake from the intestine.

Answer 122

The functions of connective tissues (CT) are many; they function: 1. To provide mechanical strength and support for the specialized tissues of organs. 2. To conduct and control the exchange of nutrients, metabolites, and signaling ligands between different cell types of organs, and between organ cell types and blood vessels. 3. To directly control the behavior and functions of cells that contact the connective tissue matrix (the ECM). Known or suspected regulatory functions of this matrix include: a. The control of epithelial cell polarization and shape. b. The guidance and regulation of cell migration through the matrix. c. The control of cell proliferation, differentiation, and metabolism. d. Defense against infectious agents (viruses and bacteria). e. Control of tissue formation, organization, and modification of tissue structure upon physiological stimulation and disease. f. The control of inflammation and repair due to injury. Connective tissues are very dynamic. CT cells can be stimulated to proliferate and to differentiate throughout life. The extracellular matrix is secreted and continually modified by these cells. Similarly, changes in the ECM can modify the function, development and metabolism of cells.

Answer 123

connect and regulate other tissue types in every organ in the body. They are found under the basal surface of epithelia, and around muscles and nerves. Blood vessels course through various connective tissues (except for cartilage). In essence, the connective tissues form a huge continuous compartment separated from other tissue types by various basal laminae. However, connective tissues in different locations vary enormously in composition and properties. Connective tissues share several structural and functional features. The most notable is that they produce a prominent extracellular matrix (ECM) of structural fibers, glycoproteins, and polysaccharides secreted by a relatively small number of cell types. The similarities among connective tissues reflect the common origin of these cells from similar precursors. Thus the core cell types of many connective tissues are similar to each other (though not identical), and in some cases may be capable of deriving from each other. However, although all connective tissues are related, they are highly diverse due to not only cell type diversity but especially variations in the ECM. In this first lecture, we will discuss the general cellular and extracellular components of connective tissues and their functions. In the following lectures, we will specifically examine the biology of cartilage and bone. The cells found in connective tissues are of two general categories: Core “resident” cells of the CT family and Immigrant blood-derived cells

Answer 124

Loose and dense connective tissues surround and permeate all of the organs of the body in diverse forms. The connective tissues near the body surface (directly below the skin) form a nearly continuous compartment of relatively loose and easily dissected tissue called superficial fascia. However, superficial fascia actually contains several distinct layers of connective tissue. Deeper to this material is the deep fascia, which in general is a much tougher region of dense connective tissue at the gross level. Again, the deep fascia consists of multiple distinct connective tissue elements, which includes the prominent thick epimysium (outer covering) of the muscles. Several dense connective structures are organized into specific functional units; these include the ligaments of joints that attach bone to bone, the tendons that attach muscle to bone, and various capsules and coverings (such as the knee capsule). Finally, all bones and cartilage structures are made of specialized connective tissue; these are discussed more thoroughly in the Bone and Cartilage handout.

Answer 125

Polarity is both at the membrane and intracellular level. The cytoskeleton plays an important role in this, as do pumps and secretory pathways, and membrane composition.

Answer 126

Made of collagen fibers, laminin, multidomain proteins. Is porous to small molecules. Form an attachment for epithelial cells, is deep to them, superficial to the tissues. They are specific to the tissue that they are attached to. They are just as diverse as the epithelia. Important functions: connects various tissues, serve as a barrier to cell movement. Some promote filtration of some small molecules, particularly in the kidney. Control polarity, development, function of epithelial cells. Provide an information-containing scaffold that dictate how the epithelial cells attach and the dynamics of their transport systems

Answer 127

Epithelia are derived from all three primary germ layers (ectoderm, mesoderm, endoderm), by a variety of mechanisms. During early development, many epithelia form, disassemble, and reform in new patterns until body morphology and organ morphogenesis are achieved. In particular, embryonic epithelia often disassemble and move into the mesenchymal (connective) tissues; there they may migrate to other locations to form new epithelia, or they may transform into distinct non-epithelial cell lineages that give rise to other tissues. This process is known as the epithelial to mesenchymal transition. Some embryonic epithelia serve as crucial signaling centers that organize pattern formation.

Answer 128

Throughout adult life, many epithelia are continuously renewed. Stem cells are proliferative, self renewing cells that are capable of cell division. They can produce different cell types. Stem cells live at the bottom of the crypt, and are very tightly regulated and divide slowly. Orientation of division is also tightly controlled. Skin epithelium has a different orientation of division.

Answer 129

Stem cells are a common target of disease, especially cancer. Epithelial cancer is the most common type, called carcinomas. Exocrine carcinomas are called adenocarcinomas. Many of the same pathways used in embryogenesis are used in cancer (TGF beta, wnt signalling, sonic hedgehog, tyrosine kinases, notch). The developmental pathways have different outcomes in different cell types because of the different developmental results. This is very important clinically. For example, WNT signaling in colon cancer (FAP specifically). When APC is mutated, Beta catenin increases proliferation and de-differentiation.

Answer 130

Section material from biopsy, put it on a slide, look at it under a microscope. Fix the tissue with a chemical fixative, then slice it very thinly. Most slides are washed with general stains- either bind to acidic or basic compounds. You see what is stained, and you don’t see what is not stained. Nuclei are usually dark. Mucus fails to bind dyes usually and can appear white. Unless you use a stain that loves carbs! If something is not fixed, it can leach out and you won’t see it either. The main thing you should do is find the nuclei (usually round and dark, but can vary in structure). Compare and contrast what is in the slide. Immunohistology: Immunohistochemistry and immunofluoresence.

Answer 131

is a disease state defined by localized,irreversible dilation of part of the bronchial tree. It is classified as an obstructive lung disease, along with emphysema, bronchitis and cystic fibrosis. Involved bronchi are dilated, inflamed, and easily collapsible, resulting in airflow obstruction and impaired clearance of secretions. Bronchiectasis is associated with a wide range of disorders, but it usually results from necrotizing bacterial infections, such as infections caused by the Staphylococcus or Klebsiella species or Bordetella pertussis.

Answer 132

Common test of lung function.

Answer 133

Give pancreatic enzymes, vitamins, salt, mucus clearance and nebulizer clearance, inhaled antibiotics, mucolytic agent. These treat the symptoms of CF.

Answer 134

These cells produce and secrete the components of the extracellular matrix (ECM), and many can proliferate to produce new connective tissue. All of these cells have a common developmental origin and are closely related. These cells include: Mesenchymal cells, Fibroblasts, Myofibroblasts, Adipocytes, Osteoblasts and osteocytes, Chondrocytes, Some smooth muscle cells

Answer 135

are precursors to all of the connective tissue family members; they primarily function in embryogenesis, but small numbers of them may persist through adulthood to function as stem cells for generation of new connective tissues.

Answer 136

are the pre-eminent cells of most connective tissues in the body. Fibroblasts are the central CT cell type that makes the components of the extracellular matrix of most connective tissues.

Answer 137

these derivatives of fibroblasts are capable of smooth muscle-like function, and are found in connective tissues that require a contractile function. These cells are often generated at the site of wounds where their contractile function contributes to retraction and shrinkage of scar tissue.

Answer 138

are derivatives of fibroblasts and/or primitive mesenchymal cells. The main type of adipocyte found in adults functions to store fat as energy for other cell types; tissue containing these cells is called “white fat”. A distinct type of adipocyte is prominent in newborns and children; it contains many mitochondria that convert fatty acid into heat. Tissue with these cells is called “brown fat”.

Answer 139

Some smooth muscle cells, particularly those in the walls of blood vessels, make some of the extracellular matrix components in which they are imbedded. These (but possibly not all) smooth muscle cells derive from the same types of precursors as other connective tissue cells, which would explain their ability to synthesize and secrete similar types of ECM components.

Answer 140

These are white blood cells that are produced from blood cell precursors (hematopoietic cells) in the bone marrow, and migrate from blood into connective tissue. Many of these migrants are part of the immune system and thus are important for defense against infection by viruses and microrganisms. They are also important for responses to tissue damage, allergic hypersensitivities, and other specific functions. These cells include: Lymphocytes, macrophages, neutrophils and eosinophils, mast cells, and osteoclasts. All of these cells come from precursors or differentiated cells that circulate in the blood. Thus, precursors or differentiated blood cells must traverse the endothelium of blood vessels to enter connective tissues. Many of these blood cells actually function and spend most of their lives in CT. Macrophages and osteoclasts derive and differentiate from blood monocytes. Mast cells differentiate from blood basophils. Lymphocytes, neutrophils and eosinophils come from similar if not identical differentiated cell types in the blood.

Answer 141

are central to acquired immunity to foreign organisms/viruses/materials.

Answer 142

are large “engulfing” cells that phagocytose (eat by endocytosis) cells, ECM, and other non-cellular material. Macrophages are also critical regulatory cells that secrete and respond to numerous extracellular signals. Macrophage-produced signals have numerous functions that include: stimulation of enodothelial cells to induce blood vessel growth and formation (blood vessel formation=angiogenesis), immune cell migration and function, fibroblast activation, blood vessel permeability, among others. Macrophages have several important physiological functions: (i) engulf (phagocytose) invading microorganisms, (ii) promote blood vessel formation (angiogenesis), (iii) remodel damaged tissue, (iv) remodel normal developing tissue and organs as part of their morphogenesis.

Answer 143

important for defense against microorganisms.

Answer 144

are secretory cells that, when stimulated by immune responses, release various substances, including vasodilators that promote swelling in connective tissue; thus mast cells are important in edema and allergic hypersensitivities.

Answer 145

are phagocytic cells that appear to be derived from blood monocytes and are very similar to macrophages. However they specifically function in bone resorption and remodeling, not general phagocytosis.

Answer 146

Fibroblasts are secretory machines that produce the fibrous proteins, proteoglycans, and other components of the extracellular matrix (the ECM). Fibroblasts are capable of cell division to produce new fibroblasts, and possibly other connective tissue types. The secretory activity and proliferation of fibroblasts are highly regulated. A common response to a laceration or other tissue injury is a stimulation of proliferation of fibroblasts, and a stimulation of ECM production. In situations where injury is severe, scarring of tissues results from the hypertrophy (increased growth) of fibroblast-dependent connective tissue. Fibroblasts are actually not a single cell type, but comprise a diverse collection of closely related cell types. Recent evidence shows that fibroblasts in different CT’s express different sets of markers and regulatory proteins. Even within a single CT region, several fibroblast “types” have been observed. Immunohistological methods are required to detect these differences; by standard histological staining (e.g. H&E) most fibroblasts look very similar throughout the body. The different fibroblast states are likely responsible for the enormous variation in ECM structure and composition. Fibroblasts are developmentally flexible: They can transform (differentiate) into other connective tissue cell types such as adipocytes, smooth muscle cells, chondrocytes, and osteoprogenitor cells. However, most of the evidence for this comes from fibroblasts in tissue culture, not in living tissue. There is now good evidence that small numbers “less determined” mesenchymal stem cells (MSCs) remain in many adult connective tissues, and these cells may be the primary precursors for these different cell types in vivo. These MSCs may originate in the bone marrow, like blood cells, but they produce different cell lineages from the hematopoietic stem cells; MSCs produce CT family cells, at least under normal conditions. Because MSCs cannot be easily distinguished from fibroblasts by standard histology, it remains an open question what this cells grown in culture were. At a minimum, it seems likely that fibroblasts or MSCs differ in their developmental potential, given the apparent diversity of fibroblasts.

Answer 147

The dominant feature of connective tissue is that most of the tissue components are extracellular. These components are organized into an extracellular matrix (ECM) that is distinct for each type of connective tissue. It is these ECM components that largely determine the structure and function of each tissue. These components include: Structural fibers which provide mechanical strength and resiliency. A hydrated gelatinous material, called the ground substance, in which the structural fibers are enmeshed. Numerous other extracellular macromolecules embedded within or diffusing through the ECM.

Answer 148

are the most abundant structural fibers of the ECM and are composed of a large family of closely related proteins. The collagens are fibrous proteins of very similar primary sequence to each other; collagen proteins aggregate to form fibers of varying sizes and organizations. Each collagen molecule is composed of three intertwined polypeptide chains that form a fairly rigid rope-like triple helix. Each polypeptide is called an α chain. There are at least 25 different α chains encoded by different genes in humans. Assembly of these α chains in different combination leads to the formation of multimeric collagens of at least 17 types. Each collagen molecule can be aligned and cross-linked with other collagen molecules to form higher order fibrous structures. The many different forms of collagen lead to diversity of structural fibers. It is not important to memorize which types of tissues contain which types of collagen. It is more relevant to know the basic types of collage fibers and the kinds of fibers found in different CTs. The types of collage structures include: fibrillar, fibril-associated, and network-forming collagen. The different collagens, and differences in their assembly, leads to a number of different higher order packing arrangements. These arrangements together with the nature and abundance of ground substance and cells, leads to distinct classes of connective tissue with unique properties: Loose and dense connective tissues

Answer 149

Some collagen molecules assemble in large bundles, called fibrils. To form these bundles collagen molecules are aligned both head to tail, to generate length (up to several hundred um long), and they are stacked to generate fibril thickness. These rope-like structures can have great strength to resist tensile stresses in tissues. In the electron microscope collagen fibrils display a characteristic banding pattern. Collagens that form these structures are the most abundant in the body (especially Type I). Collagen fibrils can vary enormously in thickness. Collagen Type I is an abundant component of fibrillar collagen.

Answer 150

These collagens decorate the surfaces of collagen fibrils, and are thought to link collagen fibrils to each other, or to link collage fibrils to other tissue components.

Answer 151

These collagens form very thin fibers (perhaps a few molecules thick) and assemble into interlaced networks that form porous sheets. Such collagens are found in the basal laminae, and also as anchoring fibers that attach basal lamina and cells to the extracellular matrix. Some of these networks function as important filtration barriers (as in the kidney). Collagen Type IV is a common component of the network-forming sheets in basal laminae.

Answer 152

contain thin collagen fibrils that are relatively sparse, and are arranged in irregular lattices. Cell densities and ground substance components are relatively high in loose connective tissues. Blood and lymph capillaries, as well as nerves, are typically abundant in loose connective tissue.

Answer 153

contain thick collagen fibrils that are very abundant relative to ground substance, and have a low number of cells. These tissues can have collagen bundles that are arranged in various “irregular” orientations. Alternatively, the collagen bundles can be arranged in parallel-organized sheets, such as in ligaments and tendons. These tissues also have great strength, and are found where tissues must resist strong shear forces in particular directions, such as in tendons and ligaments.

Answer 154

Collagen is synthesized and modified intracellularly, and then secreted and further modified extracellularly. Collagen is made and secreted by cells of the connective tissue family. The final extracellular product is highly modified in a number of ways. Some of these modifications occur intracellularly in the secretory pathway, while others occur extracellularly following secretion

Answer 155

Collagen polypeptides are synthesized on the ER and translocated during synthesis to the ER lumen. Collagens are post-translationally modified (they are glycosylated and hydroxylated on selected amino acid residues). The individual polypeptides are assembled into a triple helix

Answer 156

The N- and C-termini of collagen are cleaved by specific proteases. The N-terminal fragments that are generated are called N-telo peptides. The N-telo peptides are clinically important because their levels in urine and blood are used to diagnose important connective tissue and bone disease. Proteolytic release of these fragments is important to initiate: Formation of bundles and end-to-end polymers of the collagen fibrils; Enzymes catalyze chemical cross-links between collagen molecules. These covalent cross-links can increase the tensile strength of the bundles

Answer 157

are found in connective tissues that require distensibility and resiliency. Elastic fibers contain the proteins elastin and fibrillin that assemble into stretchable and resilient fibers and sheets. Elastin is a filamentous protein that exists in a predominantly random coil conformation (i.e. largely unstructured), that can be stretched upon exertion of force. Elastin monomers, like collagen, are secreted by fibroblasts (in some cases, smooth muscle cells). Extracellularly, they form filaments and sheets where the molecules are highly cross-linked with each other to generate an extensive network. These elastin networks are interwoven with another filamentous protein, fibrillin, which appears to help organize the elastin elements in the fiber, and to organize the elastic fibers with the other components of the extracellular matrix. These elastic fibers can stretch and then recoil like a rubber band.

Answer 158

The structural fibers of the extracellular matrix are enmeshed in hydrated aqueous gelatinous material called the ground substance. The major components of ground substance include: proteoglycans, other secreted proteins and glycoproteins, inorganic and small organic solutes, and water

Answer 159

Proteoglycans contain a protein core attached to very large acidic polysaccharides, called glycosaminoglycans (GAGs). GAGs are long polymers of carbohydrate molecules (polysaccharides). These molecules typically contain hundreds to thousands of sugar residues. There are several different GAGs; most are covalently attached to proteins, but some like hyaluronic acid are not attached to a protein core. Proteoglycans are different from other glycoproteins because the GAG chains can be up to 95% of their mass. Three properties of GAGs are relevant to their function: (1) they are highly negatively charged. Consequently they are very hydrophilic (they attract lots of water). (2) Their rigid extended structure causes them to readily form gels. Therefore, GAGs promote hydration of the ground substance to allow diffusion of small metabolites but tend to inhibit the movement of large structures (like bacteria). Thus, these molecules can function as selective sieves by forming gels of varying pore sizes. Hydration is crucial to facilitate diffusion of important molecules. It also creates a high swelling (turgor) pressure that allows the ECM to resist large compression forces (in contrast to collagen which functions to resist stretching forces). Consider the value of this for the cartilage in your knee joints. (3) Some proteoglycans can also bind to and inactivate or activate other proteins, particularly growth factors and ECM modifying enzymes.

Answer 160

These include proteases that process collagen and other types of proteins, and a variety of other extracellular enzymes. These also include growth factors and other polypeptide ligands involved in cell signaling, some of which will bind to and be regulated by proteoglycans in the ECM.

Answer 161

including ions, carbohydrates, lipid vesicles or aggregates, non-protein signaling ligands.

Answer 162

In addition to water from blood filtrate/plasma, GAGs help to maintain hydration of the ECM.

Answer 163

Most connective tissues are subject to dynamic regulation, and also dynamically control development and repair of other tissues (epithelia, muscle, blood vessels). One important example is the remarkable and important role loose CT plays in inflammation and wound repair. At the heart of CT dynamics is cell communication: CT cells communicate with each other, with epithelial cells, with muscle and nerve cells, with endothelial cells, and with blood cells using a complex network of secreted ligands, cell receptors, and intracellular signaling systems. When a wound occurs, a general, overlapping sequence of events typically occurs: inflammation and blood clotting, new tissue formation, and tissue remodeling.

Answer 164

Rupture of tissue and blood vessels releases blood platelets into CT and activates them to produce blood clots that temporarily seal the wound (discussed in a later course on Blood system). CT fibroblasts, mast cells, and other blood cell derivatives like macrophages release a plethora of signaling compounds that (i) increase water permeability of capillary endothelia leading to swelling, (ii) increase cellular permeability of endothelia, to promote migration of monocytes, lymphcytes and other blood cells into the C.T., (iii) attract migration of white cells to the site of the wound (chemotaxis), (iv) stimulate proliferation of fibroblasts and differentiation of monocytes into macrophages. Histamine secreted by mast cells is thought to promote endothelial permeabilization, and numerous cytokines secreted by white blood cell derivatives and by fibroblasts promote several processes listed above, and also can signal long distance to hematopoietic tissue to stimulate productions of more white blood cells. Other signaling molecules are also important.

Answer 165

Fibroblasts are stimulated to divide and secrete ECM components. In addition, other specific signals trigger division and differentiation of epithelial stem cells and other stem cells of muscle. Cytokines and other protein growth factors also play important roles in this phase. Importantly, signals from multiple cell types, particularly macrophages, trigger new blood vessel growth (angiogenesis), repair, and remodeling.

Answer 166

The ECM, cellular composition, and overall structure of the CT, epithelium, and other tissues is altered to different extents, depending on wound location and severity. Typically, the cellularity (density of cells) is reduced, and ECM becomes thinner and altered in organization. If tissue damage is extensive, remodeled tissue is imperfect leading to formation of scar tissues (essentially disorganized epithelial/CT tissues lacking differentiated structural features of original tissue).

Answer 167

Several important diseases are tied to this inflammation process, or rather dysfunction in inflammation control. Chronic inflammation is a hallmark of diseases such as ulcerative colitis or Crohn’s disease (inflammatory bowel diseases), rheumatoid arthritis, stomach ulcers, and several skin disorders. Convincing new evidence has linked chronic inflammation under some epithelia to development and/or promotion of malignant carcinomas, including colon cancer and other GI cancers. Moreover, malignant metastatic tumors are believed to co-opt inflammatory processes to promote their continued growth and vascularization.

Answer 168

Connective tissues are always found next to the basal surfaces of epithelia, and they surround muscle and nerves in distinct patterns. C.T. surrounds all blood vessels, and surrounds and courses through all organs.

Answer 169

The precise composition of fibroblast-made ECM varies enormously in different parts of the body. Mature fibroblasts in different places look very similar; they are generally flat spindle shaped cells in most connective tissues. But, given the diversity of connective tissue, they are likely to be a collection of distinct but related cell types.

Answer 170

At the heart of CT dynamics is cell communication: CT cells communicate with each other, with epithelial cells, with muscle and nerve cells, with endothelial cells, and with blood cells using a complex network of secreted ligands, cell receptors, and intracellular signaling systems.

Answer 171

In the fetus, large segments of the skeleton are composed of cartilage. This is replaced by bone, so that by adulthood only the articular surfaces of bones (where bones come together at joints) retain cartilage tissue. Appositional (at the perichondrium) and interstitial (in the middle of bones)

Answer 172

Once bone is formed, it is then continually remodeled to ultimately convert it into its mature structure. Remodeling also contributes to the reshaping of bones as they grow and develop. Finally, remodeling activity is also important in the continual turn-over of bone matrix in mature fully grown bones. In the general remodeling sequence, osteoclasts become activated in specific regions, presumably by some signaling event. They then begin bone resorption of older calcified bone in specific areas. Activated osteoclasts then signal to activate osteoblast migration and secretion. Osteoclast activation also stimulates osteoprogenitors to generate new osteoblasts. After some delay, the osteoblasts initiate mineralization of the osteoid. Clearly, where and when the sequence is initiated, as well as the temporal sequence itself, must be carefully controlled; how this occurs is not well understood. In mature adult bone, most of the resorptive activity occurs at the endosteal surface. It is critical that resorption is directly and carefully coupled to formation. It is also important that formation is coupled to calcification. In normal circumstances, the amount of bone resorbed is equivalent to the amount of bone formed. Thus, even if bone resorption is stimulated, there is no net loss (or gain) of bone.

Answer 173

Many diseases of bone affect bone formation or resorption. Osteoporosis leads to decrease in bone mass due to defects in resorption/formation coupling. Osteopetrosis causes defective resorption and increased bone mass. Osteomalacia Rickets causes abnormal increase in uncalcified osteoid, by interfering with mineralization.

Answer 174

Calcium mobilization from bone, and hence blood calcium homeostasis, is also under tight regulation by endocrine hormones and dietary intake of calcium. Parathyroid hormone stimulates calcium liberation (bone resorption), while calcitonin stimulates calcium uptake into bone. The details of the physiology of these hormones and how they control blood calcium and bone-turnover will be discussed in future lectures. However, for this class know that these hormones function (directly or indirectly) to control the activities of osteoclasts and osteoblasts. In addition, Vitamin D is important for systems that promote calcium uptake from the intestine.