5. Tissue Types: Structure and Function Flashcards
(190 cards)
1
Q
What are the different types of intercellular junctions and how do they work together?
A
- Anchoring junctions -> Hold the cells together and attach them to the basal lamina
- Tight junctions -> Add to the basic connection enabled by the anchoring junctions and make it tighter near the top of the cells
- Gap junctions -> Allow cells to communicate
2
Q
Describe the structure and function of anchoring junctions.
A
- Connect cells together at distinct points:
- Desmosomes + Adherens -> Cell to cell
- Hemidesmosomes + Focal adhesions -> Cell to basal lamina
- Desmosomes and hemidesmosomes connect to intermediate filament network
- Adherens and focal adhesions connect to actin
3
Q
Describe the structure of a desmosome.
A
- Intermediate filaments attach to desmosomes on inside of cells via anchor proteins
- Adhesion proteins (e.g. cadherins) hold the junction together
4
Q
What intermediate filament are desmosomes and hemidesmosomes connected to in skin?
A
Keratin
5
Q
Give an example of the clinical relevance of hemidesmosomes.
A
Epidermolysis bullosa, which leads to blistering of skin, can be caused by various defects in the hemidesmosomes, so cell-basement membrane adherence is lost.
6
Q
Describe the structure and function of tight junctions.
A
- Dynamic seal near the top of epithelial cells that completes the joining of the cells started by the anchoring junctions
- It has variable paracellular permeability
- Barrier to harmful luminal contents by allows uptake of nutrients
- Includes strands of tight-junction proteins, which appear as ridges
7
Q
Give an example of the clinical relevance of tight junctions.
A
Crohn’s disease can make tight junctions more permeable, so that uptake of harmful luminal contents can be dangerous.
8
Q
Are all tight junctions equally tight?
A
No, tightness can be varied between different tissues and can also be transiently regulated. For example:
- Small intestinal -> Less tight
- Blood-brain barrier -> More tight
9
Q
What do tight junctions do to the structure of cells?
A
They divide the cell into apical (top) and basolateral (bottom) domains, each with their distinctive proteins.
10
Q
In terms of glucose transport, describe the proteins at the apical and basolateral domains.
A
- Apical -> Active glucose transport
- Basolateral membranes -> Facilitated diffusion
11
Q
What are the two directions of substance transport across epithelial cells?
A
- Transcellular -> In the apical-basal axis (i.e. across the cell)
- Paracellular -> Between cells through tight junctions
12
Q
Describe the structure of gap junctions.
A
- Connexons are made from 6 connexins arranged in a ring
- Two connexons meet to form an aqeuous channel
13
Q
Describe the functions of a gap junction.
A
- Electric coupling of cells -> Synchronise activity of cardiac and smooth muscle
- Metabolic coupling of cells -> Co-ordinate activity of neighbouring cells and smooth out random fluctuations in metabolite concentration
14
Q
What can gap junctions exchange and what can they not exchange?
A
- Can exchange small molecules like sugars, amino acids and nucleotides
- Cannot exchange macromolecules like polysaccharides, proteins or nucleic acids
15
Q
Give some experimental evidence for the selective size permeability of gap junctions.
A
Dyes can be used to see the molecular weight that can pass through the gap junctions.
16
Q
Give some examples of clinical relevance of gap junctions.
A
Connexin-26 mutations lead to:
- Skin disease (Vohwinkel syndrome and palmoplanter keratoderma) -> Excessive formation of keratin since gap junctions are involved in differentiation of keratinocytes in epidermis
- Inherited deafness -> Disrupted flow of potassium ions from cell to cell in sensory epithelia in inner ear
17
Q
What is a junctional complex?
A
Symmetrical structures formed between adjacent cells and consist of three components:
- Band of tight junctions, forming an occluding zone in the top position
- Band of adherens junctions in the middle position
- Circle of desmosomes in the bottom position
It is the structure of this that determines the degree of trans-epithelial transport.
18
Q
In short, summarise the function of these:
- Desmosome
- Adherens junction
- Gap junction
A
- Desmosome -> Mechanically linking intermediate filaments of adjacent cells
- Adherens junction -> Linking actin filaments
- Gap junction -> Allowing intracellular communication by ions and small molecules (and electrical coupling)
19
Q
Before doing the connective tissue flashcards, remember to read your essay about connective tissues.
A
Do it.
20
Q
What is connective tissue?
A
- One of the 4 basic tissue types
- Connects or supports other tissue types
21
Q
What are some of the functions of connective tissues?
A
- Mechanical
- Metabolic
- Defence and repair
- Growth and morphogenesis
22
Q
Describe the basic structure of connective tissue.
A
Consists of two main parts:
- Cells
- ECM secreted by the cells -> Made of ground substance and fibres
23
Q
sWhat accounts for the differences between different connective tissue types?
A
The cells within the connective tissues lay down different ECMs.
24
Q
Describe the types of connective tissues.
A
- Unspecialised connective tissues:
- Loose connective tissues
- Dense connective tissues
- Dense irregular
- Dense regular
- Specialised connective tissues:
- Cartilage
- Bone
- etc.
25
What are the main types of cells that make up connective tissues and what is each of their functions?
Fixed cells:
* Fibroblasts -\> Lay down ECM
* Adipocytes -\> Triglyceride storage
Migratory cells:
* Macrophages -\> Phagocytosis and degradation of ECM
* Mast cells -\> Histidine signalling in immune response
* Lymphocytes -\> Immume response
26
What is loose connective tissue in terms of where it is found, the properties and the composition?
* Component that surrounds many organs and structures, plus it is in the hypodermis of skin
* Delicate, flexible, well-vascularised, not very resistant to stress
* Cells: Fibroblasts and macrophages
* Fibres: Moderate amount of collagen, elastic and reticular fibres
27
What is dense IRREGULAR connective tissue in terms of where it is found, the properties and the composition?
* Found in areas that give resistance in all directions (e.g. dermis of skin)
* Less flexible, resistant to stress -\> Strong in all directions
* Cells: Few fibroblasts
* Fibres: Lots of collagen fibres arranged without predominant orientation
28
What is dense REGULAR connective tissue in terms of where it is found, the properties and the composition?
* Found in areas that give resistance in a given direction (e.g. tendons)
* Less flexible, resistant to stress -\> Strong in a given direction directions
* Cells: Few fibroblasts
* Fibres: Lots of collagen fibres arranged with a predominant orientation
29
What determines whether dense connective tissue is regular or irregular?
It depends on the orientation of the fibraoblasts, which lay down collagen fibres along their long axis.
30
Do fibroblasts lay down collagen?
No, technically they lay down procollagen, which is modified extracellularly to collagen.
31
What is the important property of collagen in connective tissues?
High tensile strength
32
If loose connective tissue does not have a high tensile strength, why is it good at supporting organs?
It is flexible so it can allow movement of tissues past each other.
33
Describe the connective tissues in the skin. What is the purpose of each?
* Epidermis -\> Epithelial tissue (not connective)
* Dermis -\> Dense irregular connective tissue -\> Anchors epithelium
* Hypodermis -\> Loose connective tissue and adipose tissue -\> Allows skin to move over underlying muscle
* Muscle -\> Muscle tissue (not connective)
34
What is elastic tissue in terms of where is it found, its properties and its composition?
* Found in areas that need to stretch (e.g. vocal chords)
* It is classified as an unspecialised connective tissue (connective tissue proper) -\> So it really isn't separate from dense or loose connective tissue
* It is characterised by a high content of elastin (elastin fibres are associated with a glycoprotein)
35
What are the two types of adipose tissue?
* White adipose tissue
* Brown adipose tissue
36
Is brown adipose tissue a connective tissue?
* It's not very clear
* Pam says it is not, because the cells are not fibroblast-derived, but a lot of sources say it is
37
What is white adipose tissue in terms of where is it found, its properties and its composition?
* Found in areas that require padding or insulation (e.g. subcutaneous fat)
* Used for energy storage, insulation and padding
* Contains many white adipocytes containing a single large drop of triglyceride fat and little cytoplasm
* It is classed as a loose connective tissue
38
What is brown adipose tissue in terms of where is it found, its properties and its composition?
* Found in fairly anatomically consistent areas (e.g. between the shoulder blades)
* Used for heat generation
* Contains many brown adipocytes containing a multiple small drops of triglyceride fat and many mitochondria
* It is difficult to class and is often not considered a connective tissue
39
What layers of the skin are actually considered the skin?
* Epidermis (a.k.a. epithelium)
* Dermis (dense irregular connective tissue)
The hypodermis (loose connective and fat tissue) and muscle below this are not considered skin.
40
What are the basement membrane and basal lamina?
* Basement membrane is the triple layer of ECM proteins and GAGs that connects the epidermis and the dermis
* It is composed of the lamina lucida, lamina densa and fibroreticular lamina
* Basal lamina is another name for the lamina lucida and lamina densa (meaning that the basal lamina is part of the basement membrane)
Note: The basement membrane also exists to join muscle cells and blood vessels to connective tissue
41
What is the experimental importance of the basement membrane?
It shows up in miscroscopy, showing the boundary between the epidermis and dermis.
42
What are some functions of the basement membrane?
* Cell adhesion (epithelial cells join via adhesion anchors and hemidesmosomes)
* Diffusion barrier
* Regulation of cell organisation
43
What are the resident cells in soft connective tissues?
* Fibroblasts
* Adipocytes
44
What are the migratory cells in soft connective tissues?
* Macrophages
* Mast cells
* Leukocytes (of which lymphocytes are the most common)
45
What is the function of fibroblasts?
Lay down most of the ECM (both fibres and ground substance), including:
* Collagen (as procollagen precursor)
* Elastin (as tropoelastin precursor)
* GAGs
* Proteoglycans
* Glycoproteins
* Growth factors
46
What are macrophages, what are they derived from and what is their function?
* Tissue phagocytes
* Derived from blood monocytes
* Functions: Phagocytose dead cells and invading organisms, Degrade ECM, Regulate inflammatory response, Recruit leukocytes
47
What are mast cells and what is their function?
* Cells containing bioactive molecules (e.g. histamine)
* Function: Mediate immune respones upon antigen recognition by releasing histamines
48
What are leukocytes and what is their function?
* White blood cells
* Function: Release pharmacological compounds (e.g. histamine), Control mast cells, Inflammation, Phagocytosis
49
What is the appearance of mast cells?
They appear granulated due to the granules of histidine then store.
50
What is the appearance, derivation and function of white adipocytes?
* Contain a single large triglyceride droplet
* Derived from a fibroblast-like precursor cell
* Used for energy storage, padding and insulation
51
What is the appearance, derivation and function of brown adipocytes?
* Contain multiple much smaller lipid droplets and numeruous mitochondria with good blood supply. Also receive sympathetuic innervation.
* Derived from the same cells that muscles are derived from
* Function: Generate heat when stimulated
52
What are the two components of the ECM in soft connective tissues?
* Fibres
* Ground substance
53
What are the main connective tissue fibre types in the ECM?
* Collagen
* Reticular fibres (very thin type III collagen)
* Elastic fibres
54
What is the most common type of collagen in the human body?
* Collagen I accounts for 90% of body collagen
* It is fibril-forming
* Found in bone, skin, tendons, ligaments, cornea, internal organs
55
Describe the structure of collagen.
* Made up of 3 left-handed polypeptide strands that are coiled into a right-handed collagen helix
* Each strand contains lots of glycine because it is the only amino acid small enough to fit into the crowded interior of the triple helix
* Genral structure: -Gly-X-Y-Gly-X-Y-
* X: typically proline
* Y: typically hydroxyproline
* Collagen molecules are assembled parallel to each other but are staggered, forming a long fibril
* Fibril assembles into a collagen fibre
56
In collagen, what is the effect of a mutation in a single glycine codon?
A kink is produced in the pro-collagen molecules.
57
Name some connective tissue disorders.
* Osteogenesis imperfecta (collagen)
* Scurvy (collagen)
* Marfan's syndrome (fibrillin)
* Ehlers-Danlos syndrome (collagen)
58
What are the causes and symptoms of osteogenesis imperfecta?
* Causes: Defects in collagen type I, which is found in bones (among other tissues)
* Symptoms: Blue sclerae (whites of the eyes), Bone deformities and brittle bones
59
What are the causes and symptoms of scurvy?
* Causes: Lack of ascorbic acid (vitamin C) means that there is not enough to hydroxylate proline and lysine in collagen, which is necessary for cross-linking
* Symptoms: Poor wound healing, Deficient growth, Capillary weakness
60
What are the causes and symptoms of Ehlers-Danlos syndrome?
* Causes: Different types, all caused by defects in collagen, usually genetic
* Symptoms: Stretchy skin, Joint hyperflexibility, Weak and easily bruised skin
61
What are reticular fibres, what is their function and where are they found?
* Reticulin: Fibres of Type III collagen -\> Short and thin
* Form the delicate meshwork holding tissue elements together
* Highly evident in haemopoietic tissue -\> Lymph nodes, spleen, bone marrow
62
Describe the structure of elastic fibres.
* Have a core of elastin, on the surface of which there are microfibrils of fibrillin (a glycoprotein)
* Elastin fibres are often branched and cross-linked
* When relaxed, the fibres appear random and coiled, but when stretched they straighten
63
What is fibrillin?
A glycoprotein on the surface of elastin (in elastic fibres).
64
What are the causes and symptoms of Marfan's syndrome?
* Causes: Fibrillin abnormalities, which weaken elastic fibres
* Symptoms: Tall and thin stature, Felxible joints, Scoliosis, Heart and lung problems
65
What is ground substance secreted by?
Fibroblasts
66
What is ground substance?
Gel-like substance in the extracellular space that contains all components of the extracellular matrix (ECM) except for fibrous materials such as collagen and elastin.
67
Describe the structure of the ground substance and the function of each component.
* GAGs (glycosaminoglycans) -\> Long unbranched polysaccharide chains that are negatively charged -\> Attract water for resistance to compression
* Proteoglycans -\> These are made of several GAGs joined to a protein -\> Attract water also and form molecular sieves
* Glycoproteins
* Water
68
What does GAG stand for?
Glycosaminoglycan
69
Give an example of a GAG.
Hyaluronic acid -\> Water attraction!
70
What are some functions of epithelial tissues?
* Barrier
* Protection
* Absorption (trans-epithelial transport)
* Secretion
* Movement over the surface (e.g. cilia)
71
What are the two broad types of epithelia?
* Covering and lining epithelium -\> Sheets that cover the body on the internal and external surface
* Glandular / Secretory epithelium -\> Originated from invaginated epithelial cells arranged as 3D secretory units
72
What are the different ways in which epithelia can be classified?
* Shape (e.g. cuboidal)
* Stratification (e.g. stratified)
* Function (e.g. absorptive)
* Specialisation (e.g. cilia - movement of particles)
73
What are the different shape types of epithelia? What is the function of each?
* Squamous
* Flat, plate-like
* Function (mostly): Transport across it
* Cuboidal
* Height and width similar
* Function (mostly): Absorption/Secretion
* Columnar
* Height 2-5 times greater than width
* Function (mostly): Absorption/Secretion, Protection, Lubrication
74
What are the different stratification types of epithelia?
* Simple -\> Single layer
* Stratified -\> Multiple layers (may be keratinised)
* Pseudostratified -\> Appear like several layers, but really only one
* Transitional -\> Several layers that can change shape
75
Name some types of specialisations of epithelial tissues.
* Cilia -\> Movement of particles along the surface (e.g. airway)
* Microvilli -\> Increase absorption area (e.g. gut)
76
What are all of the different types of epithelium you need to know?
* Simple squamous
* Simple cuboidal
* Simple columnar
* Stratified squamous
* Stratified cuboidal
* Pseudostratified columnar
* Transitional
77
What is another name for transitional epithelium and why?
Urothelium, because it is the epithelial tissue that lines the lower urinary tract.
78
Which is the only type of epithelium that may be keratinised?
Stratified squamous
79
Describe the location, structure and function of simple squamous epithelium.
Include a diagram.
Location:
* Blood and lympathic vessels (endothelium)
* Loop of Henle
* Alveoli of lungs
* Lining of the heart
Structure:
* Single layer of cells, often hexagonal
* In cross-section, the nuclei appear as bumps because the cells are so flat
Function:
* Diffusion and filtration / Absorption
* Secretion
80
Describe the location, structure and function of simple cuboidal epithelium.
Include a diagram.
Location:
* Kidney tubules (microvilli)
* Glands and their ducts
* Ovaries
* Bronchioles of the lungs (ciliated)
Structure:
* Single layer of cube-shaped cells
* Some have microvilli (kidney tubules) or cila (bronchioles)
Function:
* Absorption
* Secretionn
* Movement (by ciliated cells)
81
Describe the location, structure and function of simple columnar epithelium.
Include a diagram.
Location:
* Glands and ducts
* Uterine tubes and uterus (cilliated)
* Brochioles (ciliated)
* Small intestine (microvilli)
* Digestive tract
* Bladder
Structure:
* Single layer of tall, narrow cells
* Some cells are ciliated (bronchioles, uterine tubes, uterus)
* Some cells have microvilli (intestine)
Function:
* Absorption
* Secretion
* Movement of particles along surface
82
Describe the location, structure and function of stratified squamous epithelium.
Include a diagram.
Location:
* Moist -\> Mouth, oesophagus and vagina
* Keratinised -\> Skin
Structure:
* Multiple layers of cells that are cuboidal at the basal layer and are gradually more flattened towards the surface
* Can be moist or keratinised
Function:
* Protection against abrasion and infection
83
Why is the name "stratified squamous epithelium" misleading?
The cells in the more basal layers are actually cuboidal, but the layers become gradually more squamous towards the surface.
84
What are the two types of stratified squamous epithelium and what makes them different?
* Moist -\> Surface cells retain a nucleus and cytoplasm
* Keratinised -\> Surface cells are dead and the cytoplasm is replaced by keratin
85
Describe the location, structure and function of stratified cuboidal epithelium.
Include a diagram.
Location:
* Sweat glands
* Salivary glands
* Mammary glands
Structure:
* Multiple layers of somewhat cube-shaped cells
Function:
* Protection against infection
86
Describe the location, structure and function of pseudostratified columnar epithelium.
Include a diagram.
Location:
* Trachea (ciliated)
* Much of upper respiratory tract (ciliated)
Structure:
* Single layer of cells, some of which reach the free surface
* Cells usually ciliated and associated with golbet cells that secrete mucus
Function:
* Synthesis and secretion of mucus
* Movement over surface
87
Describe the location, structure and function of transitional epithelium.
Include a diagram.
Location:
* Bladder
* Ureters
* Superior urethra
Structure:
* Stratified cells that appear cuboidal when not stretched, but when the organ is stretched they appear squamous
Function:
* Protection
* Allows organs to expand and stretch
88
Draw a summary table of all of the different types of epithelia.
89
What is another name for transitional epithelium?
Urothelium (because it covers the bladder, urethra and ureters)
90
Compare the location, structure and function of cilia and microvilli.
Cilia:
* Location: Respiratory tract
* Structure: Microtubule core -\> Enable movement
* Function: Movement of substances (e.g. mucus) along surface
Microvilli:
* Location: Small intestine
* Structure: Actin microfilament core
* Function: Increase SA for absorption
91
Compare exocrine and endocrine epithelia.
Exocrine:
* Retain contact with surface through duct
* Secrete products on to internal or external epithelial surface
Endocrine:
* No duct (lost their connection to originating epithelium
* Secrete products into blood or lymphatic vessels
92
Describe goblet cells in terms of their location, structure and function.
* Location: Pseudostratified columnar epithelia, between epithelial cells
* Structure: Goblet-shaped, Filled with membrane-bound secretory droplets
* Function: Secrete mucus to lubricate cell surface
93
Name this epithelia type.
Stratified squamous
94
Name this epithelia type.
Simple columnar
95
Name this epithelia type.
Pseudostratified columnar
96
Name this epithelia type.
Simple squamous
97
Name this epithelia type.
Stratified cuboidal
98
Name this epithelia type.
Simple cuboidal
99
Name this epithelia type.
Transitional
100
What type of tissue is epithelia derived from?
* Ectoderm -\> Stratified or pseudostratified
* Endoderm -\> Simple
* Mesoderm -\> Simple
101
Is the basement membrane make out of two or three parts?
It can be said to be made up of the lamina lucida, lamina densa and fibroreticular lamina, but the lamina lucida and lamina densa together make up the basal lamina. Therefore, the basement membrane can be said to be made out of two or three parts.
102
Where is the basement membrane found?
Between the lowest layer of epithelial cells and the connective tissue below them.
103
Compare the basal lamina with the fibroreticular lamina.
Basal lamina:
* Secreted by epithelial cells above
* Contains anchoring fibrils of collagen
Fibroreticular lamina:
* Secreted by connective tissues below
* Contains fibres and collagen bundles around which fibrils loop
104
Summarise simply the function of the basement membrane.
Anchoring the epithelial cells to the connective tissues below.
105
Give some clinical relevance of junctional complexes in epithelia.
Pemphigus
106
Describe the causes and symptoms of pemphigus.
Causes:
* Autoimmune disease that affects a desomosome glycoprotein (desmoglein) in skin and mucous membranes
* Desmoglein is a cadherin that usually binds desmosome to keratin
* Junctions break down
Symptoms:
* Blistering
107
What is cancer of epithelial cells called?
Carcinoma (usually)
108
What are some of the functions of skin?
On syllabus:
* Protective (water, infection, UV)
* Sensory
* Thermoregulation
Other:
* Interactive (e.g. friction for grip)
* Immune surveillance
* Synthetic
* Major sense organ
* Absorptive
109
What are the three layers of skin?
* Epidermis
* Dermis
* Hypodermis (subcutis)
110
What type of epithelium is the epidermis of the skin?
Stratified squamous epithelium
111
What are the 5 layers of the epidermis of the skin?
* Stratum corneum (dead cells)
* Stratum lucidum (dead cells)
* Stratum granulosum
* Stratum spinosum
* Stratum basale
112
Describe the stratum basale in terms of depth, function and cell types.
Depth:
* Deepest of the 5 layers of the epidermis
Function:
* Renews remainder of epidermis every 25-30 days
* Attaches cell to underlying basement membrane
* Contains stem cells and daughter cells that will divide further to produce keratinocytes and other epidermal cell types
* Low columnar or cuboidal cells (remember that the membrane becomes more squamous as you go up) -\> Express specific keratin isoforms that aggregate to form tonofilaments (making up the intermediate filaments that join to desmosomes and hemidesmosomes)
* Other cell types -\> Melanocytes (pigment), Merkel cells (sensory) and Langerhans' cells (immune)
113
What is another name for the stratum spinosum and why?
Prickle cell layer, because the cells have cellular projections that permit attachment to neighbouring cells via desmosomes so that the tissue has this appearance upon fixation.
114
Describe the stratum spinosum in terms of depth, function and cell types.
Depth:
* Second deepest layer of the epidermis
Function:
* Provides strength and flexibility
* High levels of keratin expression
115
Describe the stratum granulosum in terms of depth and function.
Depth:
* Third deepest layer of the epidermis
Function:
* High levels of keratin synthesis (replaces inside of cells)
* Cells produce numerous basophilic granules with specialised proteins (giving the granulated appearance) and small keratinosomes containing lipids -\> These are used to synthesis the mature keratin under the keratinocyte plasma membrane
116
Describe the stratum lucidum in terms of depth and appearance.
Depth:
* Fourth deepest layer of epidermis
Appearance:
* Clear -\> Because cells are dying due to distance the keratinocytes find themselves from the rich blood supply
* It may often barely be visible
117
Describe the stratum corneum in terms of location and function.
Location:
* Most superficial
Function:
* Cornified cells provide protection
118
What are the different cell types in the epidermis of skin and what is each of their functions?
* Keratinocytes -\> Protection by forming a barrier
* Melanocytes -\> Pigment is protection against UV
* Langerhan's cells -\> Immune response
* Markel cells -\> Mechanoreceptors
119
Describe melanocyte development and function.
* Melanocytes are derivatives of the neural crest
* Melanin is made specifically in melanocytes
* Melanin forms melanosomes, which may be transferred to other cells, such as keratinocytes
* Two types: pheomelanosomes (lighter) and eumelanosomes (darker)
120
Describe some clinical relevance of melanocytes.
* Defects in melanin production pathway results in albinism
* Defects in neural crest development and migration produce pigmentation defect (e.g. dominant piebald trait)
121
Describe how nails relate to skin.
* They are a specialised form of the cornified layer of the epidermis
* They are keratin-filled squames in layers
* Nail appearance is an important diagnostic tool
122
Describe the sub-types of keratin.
* Keratins may be of either Type I (acidic, low weight) or Type II (basic, high weight)
* Keratins can also either be epithelial (about 20 types) or trichocytic (about 13 types, making up mostly hair, but also nails, etc.)
* Both epithelial and trichocytic keratins include some Type I and II keratins
123
Describe the maturation of keratinocytes.
* Keratinocyte stem cells reside in the basal layer of the epidermis, which is the lowest layer of the stratified epithelia.
* These cells divide to give rise to transient amplifying cells which divide further, and differentiate, as they move upwards in the epidermis.
* The differentiating cells produce compounds and other proteins which are critical to the integrity of the outermost layer of the skin, the stratum corneum.
* The keratinocytes in the stratum corneum are dead squamous cells that are no longer multiplying.
* Once keratinocytes reach the corneum, they are said to be keratinazed, or cornified, creating the tough outer layer of skin.
124
Describe the structure of the dermis, including the composition and role of each layer.
* Upper layer = Papillary dermis
* Loose (areolar) connective tissue -\> Collagen and some elastin
* Extensive blood vessels -\> Provide keratinocytes above with nutrients and involved in thermoregulation
* Nerve endings
* Lower layer = Reticular dermis
* Dense irregular connective tissue -\> Dense collagen and long thick elastin fibres -\> Provides strength and elasticity
* Hair follicles
* Sebaceous glands
* Sweat glands
Note that the reticular dermis is thicker than the papillary dermis, which is why the dermis is frequently considered to just be a dense connective tissue.
125
Describe the location and types of sweat gland in the skin.
* Types:
* Eccrine -\> Found all over the body -\> Contribute to cooling down
* Apocrine -\> Armpits and groin only -\> Contribute to body odour
* Location: In the lower dermis (reticular dermis) and upper hypodermis
126
Describe the location and function of sebaceous glands.
* Location: In the mid-dermis, next to hairs
* Function: Secrete an oily sebum that moisturises and waterproofs the skin
127
What tissues are hair, sebaceous glands and sweat glands derived from?
From the epidermis
128
Give some clinical relevance of sebaceous glands.
Androgen stimulation can lead to follicles becoming clogged with sebum, which causes acne.
129
What is a hair follicle?
* It is an epidermal downgrowth that extends into the dermis and hypodermis.
* It forms the space where the hair grows.
130
Describe the structure and location of hair in the skin.
* Hair follicle anchors hair into the skin.
* The hair bulb is the base of the hair follicle and contains actively dividing epithelial cells and melanocytes
* The base of the hair tends to be in the dermis or even hypodermis
* The hair bulb surrounds the dermal papilla, which is the very bottom of the hair itself
* The dermal papilla has a blood supply and innervation
131
What is a pilosebaceous unit?
The hair follicle, shaft, arrector pili muscle and sebaceous gland.
132
What is the function of the arrector pili muscle?
Are under sympathetic control to erect the hair and provide thermoinsulation.
133
Describe the blood supply of the skin.
* Subpapillary plexus -\> In the upper epidermis, supplying the upper appendages (e.g. arrector pili muscles)
* Cutaneous plexus -\> On the hypodermis/dermis junction
* Arteriovenous shunts -\> Blood vessel connections controlled by glomus bodies that are used to divert blood away from the surface when it is cold
134
Describe the different nerve endings in skin and their locations.
* Merkel cells -\> Epidermal basal layer -\> Touch
* Free nerve endings -\> Dermis -\> Pain, itch and temperature
* Meissner's corpuscles -\> Dermis -\> Light touch
* Pacinian corpusclees -\> Deep in dermis -\> Coarse touch, vibration and tension
135
What is psoriasis?
* An autoimmune disease characterised by red, dry patches of skin
* It is due to a very excessive growth of the epidermis and rapid turnover of cells
136
Describe the different types of epidermolysis bullosa.
* EB simplex
* Keratin disorder
* Causes blistering due to the inability of the skin to resist stresses
* Junctional EB
* Hemidesmosome disorder
* Mutation in laminin causes weakened hemidesmosomes that lead to blistering
137
Name the different types of skin cancer.
* Melanoma -\> Cancer of the melanocytes
* Non-melanoma skin cancer
* Basal cell carcinoma -\> Starts in the cells lining the bottom of the epidermis
* Squamous cell carcinoma -\> Starts in the cells lining the top of the epidermis
138
Describe the germ layers from which each part of the skin develops.
* Epidermis -\> Ectoderm
* Dermis -\> Mesoderm
139
What type of connective tissue are bone and cartilage?
Dense connective tissue
140
What are the three types of cartilage?
* Hyaline cartilage
* Fibrocartilage
* Elastic cartilage
141
Where is hyaline cartilage found in the body?
* Template for formation of bony skeleton
* Articular surfaces in synovial joints
* Flexible skeleton of parts of rib cage, trachea, bronchi, nose
142
Where is fibrocartilage found in the body?
* Intervertebral discs
* Intra-articular discs in synovial joints
* Tendon attachment to bone ‘enthesis’
143
Where is elastic cartilage found in the body?
* External ear
* Epiglottis
144
Describe the histological divisions of bone.
Bone is divided into woven (immature) and lammellar (mature):
* Woven (or immature) bone is the first bone to develop when the skeleton forms, and it is also found in bone repair and tumours
* Lammellar (or mature) bone is what makes up all normal adult bones, and is divided into compact and spongy/trabecular bone:
* Compact bone -\> Dense bone found around the external surfaces of the bone, including the shaft
* Trabecular bone -\> Lightweight bone found in the ends of long bones, short bones and in the bodies of vertebrae
145
What are some of the properties of hyaline cartilage?
* Very low friction
* Slightly deformable
146
Can hyaline cartilage be seen by X-ray/CT or MRI?
* X-ray/CT -\> Can't be seen
* MRI -\> Can be seen
147
What are the different cell types in cartilage? What is the function of each?
* Chondroblasts -\> Stem cells for chondrocytes and lay down the ECM
* Chondrocytes -\> Maintain the ECM
148
What are lacunae in cartilage?
* Chondrocytes are contained in cavities in the matrix, called cartilage lacunae
* Around these, the matrix is arranged in concentric lines as if it had been formed in successive portions around the cartilage cells.
149
What is perichondrium?
A layer of dense irregular connective tissue that surrounds the cartilage of developing bone.
150
Label this cartilage.
151
What the two types of growth of cartilage?
152
Describe the ECM of cartilage. [EXTRA?]
153
Describe what can be seen in this image.
It is an epiphysial plate with the epiphysis at the top and diaphysis at the bottom.
154
Label the different zones of this epiphyseal plate.
155
Compare the ECM of hyaline cartilage and fibrocartilage. [EXTRA]
Fibrocartilage has much higher type I collagen content.
156
What stain can be used to view elastic cartilage?
Orcein
157
What two things is bone an important store of?
* Calcium
* Phosphate
158
What are the types of cell found in bone?
* Osteoblast
* Osteocyte
* Osteoclast
159
Draw the osteons in compact bone.
160
What are osteons and what is their structure?
* They are cylindrical structures that run along the length of bone, like individual units of bone
* Each osteon consists of lamellae (compact bone tissue) that surround a central canal, the haversian canal.
* The haversian canal contains the bone's blood supplies.
161
What are the three important components of bone ECM?
* Collagen
* Hydroxyapatite
* Proteoglycans
162
Describe the arrangement of calcium hydroxyapatite in bone.
163
What are osteoblasts derived from?
Fibroblast-like precursors that can also give rise to adipocytes, myocytes and chondrocytes.
164
What are osteoblasts, osteocytes and osteoclasts? What are their functions?
* Osteoblasts -\> Cells that lay down the ECM of bone
* Osteocytes -\> Cells derived from osteoblasts, involved in the turnover of bone
* Osteoclasts -\> Cell that breaks down bone tissue, necessary for maintenance, repair, and remodelling of bones.
165
What do osteoblasts secrete?
Osteoid -\> Unmineralised organic part of bone matrix
166
What are some glycoproteins that are secreted by osteoblasts?
167
Which vitamins are involved in osteoblast gene expression?
Vitamins D and E
168
What are osteocytes situated in?
Lacunae
169
What are osteocytes connected by and what is the function of this?
* They are connected by processes in canaliculi, which link by gap junctions
* Can sense pressures or cracks in the bone
* They are connected to osteoblasts and help to direct osteoclasts to dissolve damaged bone
170
What are osteoclasts derived from?
Blood-borne monocyte precursors
171
How are osteoclasts attached to the bone ECM?
Attach via integrins to arg-gly-asp (RGD) sequences in matrix protein osteopontin.
172
Are osteoclasts found in lacunae?
No, but they form a ruffled edge facing a Howship's lacuna.
173
What do osteoclasts secrete?
Acid and proteases
174
What cell type is this? Label the diagram.
Osteoclast
175
Describe how osteoclasts work.
1. Integrins in osteoclast membrane seal it onto bone (forming a Howship’s lacuna) -\> Ruffled border develops at the contact zone
2. The osteoclast starts to produce and secrete hydrogen ions via a proton pump
3. Carbonic anhydrase produces more protons via the production of carbonic acid
4. Chloride is exchanged for the bicarbonate produced by a membrane exchanger
5. A chloride channel allows chloride to pass into the lacuna to produce hydrochloric acid
6. Proteases are secreted to break down the collagen and other proteins within the bone tissue
7. Calcium and signalling peptides are released from the degraded bone by the proteases – the signalling peptides can stimulate osteoblasts to fill in the holes created
176
Describe the appearance of osteoblasts.
* One nucleus
* Not many distinctive features
177
Describe the appearance of osteocytes.
* One nucleus
* Have long processes in canaliculi that connect to other osteocytes and osteoblasts
178
Describe the appearance of osteoclasts.
* Multinuclear
* Have a ruffled edge on the side of the lacuna
179
What controls the activity of osteoclasts?
Osteoblast products control osteoclasts:
* ODF (a.k.a. RANK-Ligand) -\> Stimulates osteoclast activity by binding to RANK receptor
* OPG (osteoprotegerin) -\> Inhibits the action of RANK-L by acting as a decoy receptor. It therefore inhibits bone breakdown.
The balance of ODF and OPG determines the rate of bone turnover.
180
Describe how bone remodelling works.
* Stimulus to remodel = Forces perceived by osteocytes and periosteal osteoblasts
* Osteoclasts remove unwanted bone, then die
* Osteoblasts are recruited, form new bone, then become inactive bone-lining cells
181
Draw a diagram to show how bone acts as an endocrine tissue. [EXTRA]
182
What are two things that control the growth of long bones at epiphyseal plates?
* Growth hormone
* IGF-1
183
What receptor mutation may be involved in achondroplasia?
FGF3 receptor
184
Describe how bone is involved in calcium homeostasis. What hormones are involved? [EXTRA]
* Calcitriol stimulates clacium storage in bone
* Parathyroid hormone (PTH) stimulates calcium mobilisation from bone
* Calcitonin inhibits calcium mobilisation from bone
185
Describe the causes and symptoms of Paget's disease. [EXTRA?]
186
What are the causes and symptoms of osteogenesis imperfecta?
Causes:
* In 90% of cases -\> Dominant mutation in the type 1 collagen gene.
* Remaining cases -\> Recessive mutation in a protein CRTAP (a non-enzymatic member of the prolyl-3-hydroxylase family) causes most of the remaining cases
Symptoms:
* This leads to very weakened bones which break very easily, leading to gross deformity.
187
What are the causes and symptoms of osteoporosis?
Causes:
* Imbalance: bone breakdown \> bone formation.
* Due either to too much osteoclast activity or too little osteoblast activity.
* Risk factors:
* Nutrition
* Smoking
* Exercise
* Alcohol
* Genetic
* Ageing
Symptoms:
* Weak, brittle bones
188
Which sex is more predisposed to osteoporosis?
Women
189
What are the causes and symptoms of rickets?
Causes:
* Lack of vitamin D3 or its action
* This leads to failure to absorb calcium, so bones are deficient in calcium
Symptoms:
* Softens growth plates, leading to bone deformities -\> e.g. Bowed legs or knock knees
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