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MCB 2020 > 3.7 - 3.9 Integrating cells into tissues > Flashcards

3.7 - 3.9 Integrating cells into tissues Flashcards

1
Q

What are tissues?

A

Cells in most multicellualr organisms are arranged into tissues. Tissues are cooperative assemblies of cells and the extracellular matrix

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2
Q

What are organs?

A

Cooperative assemblies of tissues

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3
Q

What are the 5 major types of tissue?

A
  • Epithelial
  • Connective
  • Nervous
  • Muscle
  • Blood and lymphoid tissues
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4
Q

What are the key characteristics of epithelial tissue?

A
  • Cells intimately connected to each other (junctions)
  • Apico-basal polarity
  • Little extracellular matrix (basement membrane)
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5
Q

What are the key characteristics of connective tissue?

A
  • Cells have few contacts with each other
  • No apico-basal polarity
  • Large amount of extra cellular matrix
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6
Q

What is nervous tissue?

A

Specialised, electrochemical signalling

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7
Q

What is muscle tissue?

A

Specialised, contractile

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8
Q

How are all tissue types seen in an organ such as the gut?

A
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9
Q

What do the epithelia line?

A
  • External body surfaces
  • Internal body cavities
  • Tube organs that communicate with exterior (alimentary, genito-urinary and respiratory tracts)
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10
Q

What does the epithelia form?

A
  • Secretory parts of glands and their ducts
  • Receptors for centain sensory organs
  • Even the brain arises from an epithelium (neuroectoderm) in the embryo
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11
Q

Name these epithelia

A
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12
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A
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13
Q
A
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14
Q

What are the functions of the epithelia?

A
  • Protection from mechanical and environmental insults
    • Lining of internal tube organs (oviduct and respiratory tracts)
    • Example: respiratory epithelium line air conducting tubes to lungs (infection risk)
    • Secrete mucus
    • Cell specialisation
      • Cilia
      • Goblet cells
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15
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A
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16
Q
A
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17
Q

How is a thick keratinised layer produced in the epidermis?

A
  • Epidermis starts as a stem cell on the basal laminar
  • Proliferates and differentiate to form keratinocytes that produce keratin and intermediate filaments
  • Modified cell death where they lose their nucleus and are just bags of keratin protein
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18
Q

What happens in the respiratory epithelium if there is a genetic defect of cilia?

A
  • Genetic defect of cilia in dynein genes stops them from beating, as dynein are responsible for movement
  • Results in primary ciliary dyskinesia
  • Situs inversus, male infertility
  • Recurrent respiratory disease in children
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19
Q

What happens to the respiratory epithelium if there is a genetic defect of chloride channel?

A
  • Abnormal export from the ER
  • Causes cystic fibrosis
  • Mucus so thick that cilia can’t move it
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20
Q

What is the function of epithelia in the organs (intestine)?

A
  • Absorption from lumen of organs (intestinal tract and kidney tubules)
  • In the intestine there are villi
  • Further membrane specialisations in the microvilli (brush border)
  • Protect epithelium from acids by secreting mucus
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21
Q

What is the polarity of the gut epithelium which absorbs nutrient molecules?

A
  • It has apico-basal polarity and apical and basal membrane have different properties
  • Apical has active transport channel where there is increased glucose concentration in the cell
  • Basal has passive channel and transporter protein for diffusion of glucose to the blood
  • There are special cell-cell junctions that prevent backflow of molecules
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22
Q

How do the apical and basal membranes differ?

A
  • Apical is for absorption, secretion, specialisations (microvilli and cilia)
  • Basal is for adhesion to extracellular matrix and secretion into sub mucosa
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23
Q

What are the four different types of junctions for epithelial cells?

A
  • Tight junctions
  • Adherens junctions (desmosomes)
  • Gap juncitons
  • Focal contacts (hemidesmosomes)
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24
Q

How are cell junctions in epithelial cells usually aranged?

A
  • Cells may have more than one type of junction
  • Arranged in junctional complexes
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25
Q
A
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26
Q

What are tight junctions?

A
  • Usually near the apical surface
  • They are bands of plasma membrane proteins encircling the cell
  • Prevent leakage of molecules across the epithelium
  • Separate different membrane domains of epithelium, essential to maintain cell apico-basal polarity
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27
Q

What is the structure of tight junctions?

A
  • Bands of membrane proteins, claudin and occludin, in adjoining cells
  • Proteins form very strong links, non covalent hydrogen bonds
  • More bands means more impermeability
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28
Q

How permeable are tight junctions?

A
  • Experiments with tracer molecules show how effective tight junctions are at preventing movement of molecules between cells. Permeability varies in different cell types
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29
Q

What is the role of adherens junctions?

A
  • They link epithelial cells to each other
  • Link with cell cytoskeleton rather than intermediate filaments through actin
  • They involve homophillic interactions between cell adhesion molecules called cadherins
  • E, P, N cadherins (classical)
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30
Q

What are the interactions between adherens junctions?

A
  • Cadherins are transmembrane proteins, made up of flexible extracellular domain
  • The flexibility is stabilised if you have Ca2+ so it extends into the extra-cellular space and adheres to each other via N-terminal cadherin repeat
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31
Q

What other proteins are there in adherens junctions?

A
  • Links to the actin cytoskeleton via linker/adaptor proteins
    • beta-catenin
    • alpha-catenin
    • p120 catenin
    • gamma-catenin
    • vinculin
  • Beta catenin also functions in growth factor signalling
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32
Q

What is the funciton of adherens junctions in movement?

A
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33
Q

What is a prominent example of epithelial folding in development?

A
  • Formation of neural tube
  • Notochord releases factors that stimulate infolding in neural tube
  • Change in cadherin as cells change their fate
  • Shows how cells change patterns of cadherin expression during tissue morphogenesis
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34
Q

How do cadherins influence cell sorting?

A
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35
Q

How is cadherin used in embryogenesis?

A
  • Cells from different layers of an early amphibian embryo will sort according to their origins,
  • mesoderm (green), neural plate (blue), and epidermis (red) sort into a structure that resembles and embryo with a neural tube in the centre
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36
Q

What do desmosomes do?

A
  • they spot weld cells together
  • Distribute tensile forces
  • Inter-connect intermediate filaments of adjacent cells
  • Found in tissues subject to high mechanical stress such as the heart, muscle and epidermis
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37
Q

What molecules interact in desmosomes?

A
  • Link epithelial cells to each other via homophillic interactions between cell adhesion molecules of the cadherin family (desmoglein, desmocollin)
  • Non classical cadherins
  • Link with cytoskeleton (intermediate filaments) via adaptor proteins (plakoglobin, plakophillin, desmoplakin)
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38
Q

How do desmosomes compare to adherens junctions with reference to the actin filaments?

A
  • Actin filaments terminates while intermediate continutes to pass through
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39
Q
A
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40
Q

What are the characteristics of intermediate filaments?

A
  • Criss cross cell cytoplasm
  • Confer tensile strength
  • Intermediate filaments are often cells specific (desmin in the heart, keratin in the skin and other epithelia)
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41
Q

How do desmosomes relate to autoimmune disease?

A
  • Pemphigus vulgaris due to antibodies to desdemonal proteins which distrupt the desmosome
  • Causes skin and mucosal blistering as epidermis separates from dermis
  • Potentially fatal
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42
Q

How do desmosomes relate to congenital disease?

A
  • Epidermolysis bullosa simplex due to mutations in keratins
  • Cuases skin and mucosal blistering
  • Potentially fatal
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43
Q

What linkages are involved in a focal linkage?

A
  • Transmembrane adhesion proteins called integrins which bind extracellular matrix proteins
  • Link to actin cytoskeleton via adaptor proteins (vinculin, talin)
  • Involved in cell movements and attachment (myotendinous junction)
44
Q

How do focal adhesions generate cell traction forces?

A
  • Interaction of integrins with their substrates plays an important role in cell motility via generation of cell traction forces (CTF) in combination with actin assembly and disassembly
  • Myosin interactions at the rear of the cell
45
Q

What are focal adhesions sites of anchorage for?

A
  • Focal adhesions are sites of anchorage for intracellular actin filaments and extracellular ECM molecules
  • Also concentrate other signalling molecules, where phosphotyrosines at the integrin junction is phosphorylated for cell signalling
46
Q

How is integrins an example of inside out signalling?

A
  • Cells regulate the activity of cell surface integrins
  • Integrins will not engage extracellular matrix ligand unless activated
  • Example : growth factor receptor signal activates integrins via intracellular signalling
  • Talin curls up on itself, vinculin binding sites are open with permits actin to engage with integrins, talin also bind to beta subunit
47
Q

How does outside-in signalling occur in integrins?

A
  • Activation by inside out signals allows binding to extra cellular matrix ligand
  • Extra cellular matrix binding recruits intracellular protein kinases, vinculin, talin, filamin, alpha-actin
  • Multiple integrins recruited to focal adhesions gives signals into the cell
48
Q

How is talin a tension sensor at cell-matrix junctions?

A
  • It has multiple binding sites (vinculin, actin, integrins)
  • Tension stress from actin filaments reveals cryptic binding sites
  • stabilises vinculin and integrins in focal adhesions
  • Promotes signalling at focal adhesions
  • Tension stretches the extra cellular matrix causing growth factor release
49
Q

What does this experiment show?

A
  • It has an actin binding domain and vinculin binding domain
  • Fix talin to a glass slide via N terminal domain
  • Magnetic bead at the C terminal which stretches it out
  • Vinculin binding site exposed and vinculin binds
  • Putting tension on actin filaments
50
Q

How is cell proliferation dependent on traction and distribution of growth factors?

A
  • Integrins may act as mechanoreceptors
  • Tension on the cytoskeleton may combine with growth factor signalling to promote G1 progression
  • Cell with one spot dies as it cannot spread out and establish focal adhesions to set up signalling centre
51
Q

How does outside in signalling with integrins require other kinases?

A
  • Clusters of intergrins permit extracellular matrix signals to be transmitted but integrins have no kinase domains
  • Signalling events require other kinases such as focal adhesion kinase (FAK)
  • FAK activates the Ras Map kinase pathway
  • Integrins don’t have kinase activity but by forming focal adhesion you can can activate signalling
52
Q

What are hemidesmosomes?

53
Q

What integrins are involved in the skin in hemidesmosomes?

A
  • alpha6beta4 integrins in hemidesmosomes adhere cells to the basement membrane
  • Link to intermediate filaments
54
Q

What happens if there is gene knockout of hemidesmosome integrins in mice?

A
  • Deletion of beta4 integrins means there are skin blisters
  • Failure of keratinocytes to adhere to basement membrane
  • Skin not attached to the dermis and separates with tension
  • Fatal resulting in neonatal death
55
Q

Summary table

56
Q

What can pass through a gap junction?

A
  • Adjacent cell membranes are 2-4 nm apart
  • Allow small molecules such as ions, sugars, nucleotides, vitamins, singalling mediators (Ca2+, cAMP, IP3) to pass through cells with pore size 1.5 nm
  • Excludes macromolecules > 1000 daltons (proteins, nucleic acids and polysaccharides)
57
Q

What are the channels between a gap junction?

A
  • Channels (connexons) consist of 6 membrane spanning proteins (connexins)
  • Channels/connexons in register between two cells form the gap junction
60
Q

What are the functions of gap juncitons in these tissues?

61
Q

What do connexin mutations lead to?

A
  • Cx26 mutations commonest cause of congenital deafness
62
Q

Where is the connective tissue here?

63
Q

What is the extracellular matrix and its components?

A
  • spaces between cells composed of a complex array of molecules
  • composed of tough fibrous proteins embedded in a polysaccharide gel-like material (ground substance)
64
Q

How does the extracellular matrix vary in conenctive tissues?

A
  • Variety in connective tissues due to differences in composition and arrangement of extracellular matrix
  • Tendon has numberous fibrous proteins, little ground substance
  • Bone has calcified ground substance and fibrils
  • Cartilage has large amount of polysaccharide gel
65
Q

What are the roles of the extracellular matrix?

A
  • Structural support
  • Regulation of cellular activities
  • Cell survival
  • Cell migration
  • Cell proliferation
  • cell shape
  • cell function
66
Q

What produces the extra cellular matrix?

A
  • Extracellular matrix is produced by cells within it
  • Cells produce and secrete organic components of the extracellular matrix
  • Cells organise the extracellular matrix
67
Q

How do cells organise collagen in the extracellular matrix?

A
  • Collagen fibres must be correctly aligned within tissues
  • Cells deposit and organise collagen matrices
  • Alternate layers of longitudinal and transversly sections fibres
  • Fibroblast organise and arrange that extracellular matrix
68
Q

Which cells produce extracellular matrix and in what tissues?

A
  • Fibroblasts (loose connective tissue)
  • Osteoblasts(bone)
  • Chondroblasts(cartilage)
  • Epithelial cells (basement membrane in epithelia)
69
Q

What is the extracellular matrix broadly composed of?

A
  • Proteins
    • Fibrous/structural such as collagen and elastin
    • Adhesive such as laminin
  • Proteoglycans which are carbohydrate modified proteins, very large molecules which attract water
70
Q

What are the characteristics of collagen in the extracellular matrix?

A
  • About 25% of total protein mass in mammals
  • 42 different collagen genes with different properties
  • Provides tensile strength, can withstand stretching
  • Rich in proline and glycine arranged in repeats as an alpha helical polypeptide chain
71
Q

What is the structure of collagen?

A
  • Collagen alpha chains composed of a series of Gly-X-Y triples where X is any amino acid, usually proline or lysine and Y is any amino acid usually hydroxy-proline or hydroxy-lysine
  • Collagen molecules consist of three alpha chains arranged in a super helix
  • Alpha chain is a left handed helix due to dihedral angle of prolines
72
Q

How are collagens arranged in fibrils?

A
  • Many collagen molecules assemble together (covalent cross linking) to form collagen fibrils (10-300 nm in diameter)
  • Regular packing of collagen molecules leads to cross striations (67 nm periodicity)
73
Q

How do collagen fibres arrange themselves into fibrils?

74
Q

Which collagens form fibres?

A
  • Only collagens I, II, III, V form fibrilar collagens
  • Collagen IV forms sheets
  • Collagens VI, IX, XII are fibril associated collagens which decorate fibrilar collagens and mediate fibril interactions
75
Q

What is the structure of fibril associated collagens?

A
  • Triple helix is interrupted by non helical domains giving flexibility
  • They are not cleaved after secretion so they retain polypeptides
  • They do no aggregate to form fibrils
  • Bind periodicially to other collagen fibrils
    • Type IX binds to II
    • Type XII to type I
76
Q

What are the steps of collagen synthesis before secretion?

A
  1. Collagen precursors (procollagens) are synthesised into ER lumen
  2. N and C terminal ends have propeptide
  3. Intrachain disulfide bonds between N and C terminal propeptide sequences align chains to form triple helix in ER
  4. Procollagen is modified in ER and Golgi (hydroxylated and glycosylated) and secreted by exocytosis
77
Q

What are the steps of collagen synthesis after secretion?

A
  • After processing and assembly of 3 pro-alpha-chains, type I procollagen is secreted into the extracellular space
  • Extracellular enzymes (procollagen peptidases) remove N and C terminal propeptides so collagen self polymerises into fibrils
  • Collagen pro-peptides prevent premature assembly of collagen in side cells
78
Q

How does cross linking occur in collagen synthesis?

A
  • After secretion covalent bonds (lysine residues) cross-link the collagen molecules, particularly in non-helical ends
  • Extent of cross-linking affects tensile strength.
  • Inhibition of cross-linking reduces tensile strength
  • Highest level of cross-linking occurs in tendon collagen (high tensile strength)
79
Q

What happens if there is a defect in collagen I?

A
  • Osteogenesis imperfect (brittle bones)
  • Variety of mutations (col1A1, col1A2)
  • Glycine substitutions failure to form triple helices
80
Q

What happens if there is a defect in collagen II?

A
  • Achondrogenesis (col2A1, GMAP210)
  • Failure in collagen synthesis or transport
  • Abnormal cartilage gives abnormal bone and joint formation which is severe
81
Q

What happens if there is a defect in collagen III?

A
  • Ehlers Danlos syndrome
  • Fragile skin, blood vessels and hypermobile joints, elastic skin
82
Q

How is scurvy a defect in collagen synthesis?

A

Failure to hydroxylate prolines and lysines in fibrilar collagen due to reduced levels of ascorbate (vitamin C)

83
Q

What does elastin do?

A
  • It provides elasticity to tissues (lungs, blood vessels)
  • It is the major protein in arteries
  • Forms covalently corss linked network of elastin molecules
  • With fibrillin it forms elastic fibres
84
Q

What happens in Williams-Beuren syndrome?

A
  • Elastin is mutated giving supravalvular arotic stenosis, mental retardation, facial dysmorphoa
85
Q

How does the artery have arrangements of elastin and collagen?

A
  • Elastic arteries such as the aorta undergo high pressure fluctuations
  • Need elastic fibres (elastin and fibrillin) for recoil of vessel wall
  • Collagen provides tensile strength and eleastin elasticity
86
Q

What happens if you have a detect in fibrillin I?

A
  • Marfan syndrome
  • Large blood vessels with dilation of pulmonary artery and aorta resulting in aneurysms
  • Skeletal defects causing scoliosis from elastic fibres in elastic cartilage and growth plates
  • Dislocated lens as fibrils that hold lens are rich in elastin
  • Heart has mitral valve insufficiency (valves usually rich in elastic fibres)
87
Q

What are the two ways that cells can attach to the extra cellular matrix?

A
  • Transiently and weakly. (Eg. migration)
  • Irreversibly and strongly. (Eg. Muscle and tendons).
88
Q

What are the 4 extracellular matrix proteins that mediate adhesion?

A
  • Laminin
  • Fibronectin
  • Tenascin
  • Collagen
90
Q

What are the two forms of fibronectin?

A
  • It is either a soluble dimer in plasma
  • Or insoluble cell-associated dimer with disulfide bonds. It requires integrins (RGD) or actin to form fibrillar fibronectin
  • Tension reveals cryptic binding sites to permit association between fibronectin molecules to form fibrils at adhesions
91
Q

What happens if you lose fibronectin via gene knockout?

A
  • Embryonic lethal, no formation of blood vessels as endothelial cells fail to migrate or attach to basement membranes
  • Antibodies to RGD domains or RGD peptides inhibit cell attachmetn and migration
  • If you lose RGP peptides you inhibit cell from attaching as it will compete with RGD in fibronectin
92
Q

What is laminin?

A
  • An adhesive extracellular matrix protein
  • Has 3 chains (alpha, beta, gamma) held together by disulphide bonds
  • Multiple different forms (5alpha, 3beta, 3 gamma)
  • Multiple binding domains to bind to cell (integrins) or to bind ot other extracellular matrix components
93
Q

Where are laminins found?

A
  • Common in basement membranes
  • Interact with collagens, nidogen and perlecan
  • Interact with integrins on cell surfac
94
Q

What happens if you knockout laminin gamma1 chain/ other mutations?

A
  • Knockout of laminin gamma1 chain is embryonic lethal as you fail to form a basement membrane
  • Mutations include;
  • Epidermolysis bullosa (LAMC2, LAMA3)
  • Muscular dystrophy (LAMA2)
  • Neuromuscular disorder (LAMB2)
95
Q

What are basement membranes?

A
  • They are specialised extracellular matrix
  • Underlies epithelium cells and tubular structures, separting epithelia and connective tissue
  • Synthesised by cells that rest upon it
96
Q

Is the basement membrane inert?

A
  • Not inert
  • Contains growth factors
  • Can modulate cell behaviours
97
Q

What is the structure of proteoglycans?

A
  • large protein core linked to negatively charged chains of polysaccharides (due to carboxyl and sulfate groups)
  • Glycosaminoglycans
  • Consist of disaccharide repeats
98
Q

What do proteoglycans do in the extracellular matrix?

A
  • Serve complimentary function to collagens
  • Form gel-like ground substance
  • Resist compressive forces
  • Permit diffusion of nutrients, metabolites, hormones and growth factors
99
Q

How are proteoglycans linked?

A
  • Multiple GAG chains are linked to a core protein (serine residues) via linker tetrasaccharides.
  • Need to connect to polypeptide chain of core protein
  • Link tetrasaccharide links to disaccharide repeat via serine amino acid
100
Q

What is the size of proteoglycans and how does it compare to glycoprotein?

102
Q

What are the very large aggregates of proteoglycans known as?

103
Q

How do proteoglycans form gels?

A
  • negative charge attracts cations such as Na+
  • Forms gels due to water following ion concentration
  • Osmotic pressure gives resistance to compression with balances tensile strength of colalgen
  • Form <10% of dry weight of fibrous protein but may occupy 90% of space.
104
Q

What is the role of proteoglycans in signalling?

A
  • Can bind and regulate activity of secreted proteins such as growth factors
  • Sequester from cells (inhibit signalling)
  • Present to cells (enhance signalling)
  • Increase diffusion capacity of growth factors (enhance signalling)

MCB 2020 (38 decks)

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