Integrating Cells Into Tissue Flashcards
(26 cards)
Squamous epithelium
Composed of flattened, tightly apposed, polygonal cells. This type of epithelium is described as tessellated when the cells have complex, interlocking borders rather than straight boundaries. Because it is so thin, simple
squamous epithelium allows rapid diffusion of gases and water across its surface; it may also engage in active transport, as indicated by the presence of numerous endocytic vesicles in these cells. Tight junctions
(occluding junctions, zonulae adherentes) between adjacent cells ensure that materials pass primarily through cells, rather than between them.
Cuboidal and columnar epithelia
Cuboidal and columnar epithelia consist of regular rows of cylindrical cells. Cuboidal cells are approximately square in vertical section, whereas columnar cells are taller than their diameter. Commonly, microvilli are found on their free surfaces, which considerably increases the absorptive area, e.g. in the epithelia of the small intestine (columnar cells with a striated border of very regular microvilli), the gallbladder (columnar cells with a brush border of microvilli); proximal convoluted tubules of the kidney (large cuboidal to low columnar cells with brush borders); and the epididymis (columnar cells with extremely long microvilli, erroneously termed stereocilia).
Ciliated columnar epithelium
Ciliated columnar epithelium lines most of the respiratory tract, except for the lower pharynx and vocal folds, and it is pseudostratified as far as the larger bronchioles; it also lines some of the tympanic cavity and auditory tube; the uterine tube; and the efferent ductules of the testis.
Pseudostratified epithelium
Pseudostratified epithelium is a single-layered (simple) columnar epithelium in which nuclei lie at different levels in a vertical section. All cells are in contact with the basal lamina throughout their lifespan, but not all cells extend through the entire thickness of the epithelium. Much of the ciliated lining of the respiratory tract is of the pseudostratified type, and so is the sensory epithelium of the olfactory area.
Sensory epithelia
Sensory epithelia are found in special sense organs of the olfactory,
gustatory and vestibulocochlear receptor systems. All of these contain
sensory cells surrounded by supportive non-receptor cells.
Stratified squamous epithelia
Stratified squamous epithelia are multilayered tissues in which the formation, maturation and loss of cells is continuous, although the rates of these processes can change, e.g. after injury. New cells are
formed in the most basal layers by the mitotic division of stem cells and transit (or transient) amplifying cells. The daughter cells move more superficially, changing gradually from a cuboidal shape to a more flattened form, and are eventually shed from the surface as a highly flattened squame. Typically, the cells are held together by numerous desmosomes to form strong, contiguous cellular sheets that provide protection to the underlying tissues against mechanical, microbial and chemical damage. Stratified squamous epithelia may be broadly subdivided into keratinized and non-keratinized types.
Keratinized epithelium
Found at surfaces that are subject to drying or mechanical stresses, or are exposed to high levels of abrasion. These include parts of the oral lining (gingivae, hard palate and filiform papillae on the anterior part of the dorsal surface of the tongue). A distinguishing feature of keratinized epithelia is that cells of the superficial layer, the stratum corneum, are anucleate, dead, flattened squames that eventually flake off from the surface. This unusual combination of strongly coherent layers of living cells and more superficial strata made of plates of inert, mechanically robust protein complexes, interleaved with water-resistant lipid, makes this type of epithelium an efficient barrier against different types of injury, microbial invasion and water loss.
Non-keratinized epithelium
Non-keratinized epithelium is present at surfaces that are subject to abrasion but protected from drying. These include: the buccal cavity; oropharynx and laryngopharynx; oesophagus; part of the anal canal; vagina; distal uterine cervix; distal urethra; cornea; inner surfaces of the eyelids; and the vestibule of the nasal cavities. Cells go through the same transitions in
general shape as are seen in the keratinized type, but they do not fill
completely with keratin or secrete glycolipid, and they retain their nuclei until they desquamate at the surface. In sites where considerable abrasion occurs, e.g. parts of the buccal cavity, the epithelium is thicker and its most superficial cells may partly keratinize, so that it is referred to as parakeratinized, in contrast to the orthokeratinized state of fully keratinized epithelium.
Stratified cuboidal and columnar epithelia
Two or more layers of cuboidal or low columnar cells are typical of the walls of the larger ducts of some exocrine glands, e.g. the pancreas, salivary glands and the ducts of sweat glands, and they presumably provide more strength than a single layer. The layers are not continually replaced by basal mitoses and there is no progression
of form from base to surface, but they can repair themselves if damaged.
Glands
Glands may be subdivided into exocrine glands and endocrine glands.
Exocrine glands secrete, usually via a duct, on to surfaces that are continuous with the exterior of the body, including the alimentary tract, respiratory system, urinary and genital ducts and their derivatives, and the skin.
Endocrine glands are ductless and secrete hormones directly into interstitial fluid and thence the circulatory system, which conveys them throughout the body to affect the activities of other cells.
Exocrine glands: merocrine secretion
In merocrine secretion, which is by far the most common secretory mechanism, vesicle membranes fuse with the plasma membrane to release their contents to the exterior.
Glands such as the simple sweat glands of the skin, where ions and water are actively transported from plasma as an exudate, were once classified as eccrine glands. They are now known to synthesize and secrete small amounts of protein by a merocrine mechanism, and have been reclassified as merocrine glands.
Exocrine glands: apocrine secretion
In apocrine glands, some of the apical cytoplasm is pinched off with the contained secretions, which are stored in the cell as membrane-free droplets. The best-understood example of this is the secretion of milk fat by mammary gland cells, in which a small amount of cytoplasm is incorporated into the plasma membrane-bound lipid
globule as it is released from the cell. Larger amounts of cytoplasm are included in secretions by specialized apocrine sweat glands in the axilla and anogenital regions of the body. In some tissues there is a combination of different types of secretion, e.g. mammary gland cells secrete milk fat by apocrine secretion and milk
protein, casein, by merocrine secretion
Exocrine gland: Holocrine secretion
In holocrine glands, e.g. sebaceous glands in the skin, the cells first fill with secretory products (lipid droplets or sebum, in this instance), after which the entire cell disintegrates to liberate the accumulated mass of secretion into the adjacent duct or, more usually, hair follicle.
Endocrine Gland
Endocrine glands secrete directly into connective tissue interstitial fluid and thence the circulation. Their cells are grouped around beds of capillaries or sinusoids, which typically are lined by fenestrated endothelia
to allow the rapid passage of macromolecules through their walls. In addition to the cells of
specialized ductless endocrine glands (e.g. pituitary, pineal, thyroid and parathyroid), hormone-producing cells also form components of other organ systems. These include: the cells of the pancreatic islets; thymic
epithelial cells; renin-secreting cells of the kidney juxtaglomerular apparatus; erythropoietin-secreting cells of the kidney. Some cardiac myocytes, particularly in the walls of the atria, also have endocrine functions.
Fibroblasts
Fibroblasts are usually the most numerous resident cells. Fibroblasts synthesize most of the extracellular matrix of connective tissue; accordingly, they have all the features typical of cells active in the synthesis and secretion of proteins. Possess prominent nucleoli.
Fibroblasts are usually adherent to the fibres of the matrix (collagen and elastin), which they lay down. In some highly cellular structures, e.g. liver, kidney and spleen, and in most lymphoid tissue, fibroblasts and delicate collagenous fibres (type III collagen; reticular fibres) form fibrocellular networks, which are often called reticular tissue. The fibroblasts may then be termed reticular cells or reticulocytes.
Fibroblasts are particularly active during wound repair following traumatic injury or inflammation, when tissue mass is lost through cell death. They proliferate and lay down a fibrous matrix that becomes
invaded by numerous blood vessels (granulation tissue). Contraction of wounds is, at least in part, caused by the shortening of myofibroblasts, specialized contractile fibroblast-like cells.
Fibroblast activity is influenced by various factors such as steroid hormone concentration, dietary content and prevalent mechanical stresses. Collagen formation is impaired in vitamin C deficiency.
Adipocytes (lipocytes, fat cells)
Adipocytes occur singly or in groups in many, but not all, connective tissues. They are numerous in adipose tissue. Individually, the cells are oval or spherical in shape, but when packed together they
are polygonal. Each cell consists of a peripheral rim of cytoplasm, in which the nucleus is embedded, surrounding a single large central globule of fat, which consists of glycerol esters of oleic, palmitic and stearic acids.
Another form of adipose tissue, brown fat. Brown fat is characterized by the presence of large cells, each
of which contains several separate droplets of fat (multilocular adipose tissue) rather than a single globule (typical of unilocular adipose tissue). White fat cells are specialized to store chemical energy, whereas the physiological role of brown adipose tissue (BAT) cells is to metabolize fatty acids and generate heat.
Mesenchymal stem cells
Mesenchymal stem cells are normally inconspicuous cells in connective tissues. They are derived from embryonic mesenchyme and are able to differentiate into the mature cells of connective tissue during normal
growth and development, in the turnover of cells throughout life and, most conspicuously, in the repair of damaged tissues in wound healing. There is emerging evidence that, even in mature tissues, mesenchymal
stem cells remain pluripotent and able to give rise to all the resident
cells of connective tissues in response to local signals and cues.
Macrophages
Macrophages are numerous in connective tissues, where they are either attached to matrix fibres or are motile and migratory. They are relatively large cells, with indented and relatively heterochromatic nuclei and a prominent nucleolus. Their cytoplasm is slightly basophilic, contains many lysosomes and typically has a foamy appearance under the light microscope. Macrophages are important phagocytes and form part of the mononuclear phagocyte system. They can engulf and digest particulate organic materials, such as bacteria, and are able to clear dead or damaged cells from a tissue too. They are also the source of a number of secreted cytokines that
have profound effects on many other cell types. Macrophages are able to proliferate in connective tissues to a limited extent, but are derived and replaced primarily from haemopoietic stem cells in the bone marrow, which circulate in the blood as monocytes before migrating through vessel walls into connective tissues, where they differentiate.
Properties include: circulating monocytes, from which they are derived; alveolar macrophages in the lungs, which take up inhaled particles not cleared by the mucociliary rejection current; phagocytic cells in the lymph nodes, spleen and bone marrow; Kupffer cells of the liver sinusoids; and microglial cells of the
central nervous system.
Lymphocytes
Lymphocytes are normally present in small numbers; they are numerous in general connective tissue only in pathological states, when they migrate in from adjacent lymphoid tissue or from the circulation. Highly heterochromatic nuclei but they enlarge when stimulated. Two major functional classes exist,
termed B and T lymphocytes. B lymphocytes originate in the bone marrow, then migrate to various lymphoid tissues, where they proliferate. When antigenically stimulated, they undergo further mitotic divisions, then enlarge as they mature, commonly in general connective tissues, to form plasma cells that synthesize and secrete antibodies (immunoglobulins).
T lymphocytes originate from precursors in bone marrow haemopoietic tissue but later migrate to the thymus, where they develop T-cell identity, before passing into the peripheral lymphoid system, where
they continue to multiply. When antigenically stimulated, T cells enlarge. The functions of T lymphocytes are numerous: different subsets recognize and destroy virus-infected cells, tissue and organ grafts, or interact
with B lymphocytes and several other defensive cell types.
Mast cells
Mast cells are important defensive cells. They occur particularly in loose connective tissues and in the fibrous capsules of certain organs such as the liver, and are numerous around blood vessels.
The consequences of granule release include alteration of capillary permeability, smooth muscle contraction, and activation and attraction to the locality of various other defensive cells. Responses to mast cell degranulation may be localized, e.g. urticaria, or there may occasionally be a generalized response to the release of large amounts of histamine into the circulation (anaphylactic shock).
Granulocytes (polymorphonuclear leukocytes)
Neutrophil and eosinophil granulocytes are immigrant cells from the circulation. Relatively infrequent in normal connective tissues, their numbers may increase dramatically in infected tissues, where they are
important components of cellular defence. Neutrophils are highly phagocytic, especially towards bacteria. The functions of eosinophils are less well understood.
EXTRACELLULAR MATRIX
The term extracellular matrix is applied collectively to the extracellular components of connective and supporting tissues. Essentially, it consists of a system of insoluble protein fibres, adhesive glycoproteins and soluble complexes composed of carbohydrate polymers linked to protein molecules (proteoglycans and glycosaminoglycans), which bind water. The extracellular matrix distributes the mechanical stresses
on tissues and also provides the structural environment of the cells embedded in it, forming a framework to which they adhere and on which they can move. With the exception of bone matrix, it provides
a highly hydrated medium, through which metabolites, gases and nutrients can diffuse freely between cells and the blood vessels traversing it or, in the case of cartilage, passing nearby.
Collagens
Collagens make up a very large proportion (approximately 30%) of all the proteins of the body. They consist of a wide range of related molecules that have various roles in the organization and properties of connective (and some other) tissues. The first collagen to be characterized was type I, the most abundant of all the collagens and a constituent of the dermis, fasciae, bone, tendon, ligaments, blood vessels and the sclera of the eyeball. The characteristic collagen of cartilage and the vitreous body of the eye, with a slightly different chemical composition, is type II, whereas type III is present in several tissues, including the dermis and blood vessels, and type IV is in basal lamina. Biochemically, all collagens have a number of features in common. Functionally, collagens are structural proteins with considerable mechanical strength.
Adhesive glycoproteins
These proteins include molecules that mediate adhesion between cells and the extracellular matrix, often in association with collagens, proteoglycans or other matrix components. All of them are glycosylated and they are, therefore, glycoproteins. General connective tissue contains the well-known families of fibronectins (and osteonectin in bone), laminins and tenascins; there is a rapidly growing list of other glycoproteins associated with extracellular adhesion
