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First Class Lecture notes Dynamic Cell Module
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Extracellular matrix lecture notes
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ECMWhat is the extracellular matrix?
Definition: the extracellular matrix (ECM) is any substance produced by cells and excreted
to the extracellular space within tissues with a structural role.
Why is it important?
1. Structural - ECM provides physical support for cells and a linkage between
different cells or tissues and directly influences function. In plants the ECM
provides general support, but in animals it has more varied supportive roles:
contributes to the transparency of cornea, the rope length strength of the
tendon, the hard resilience of bone and impact resistance of cartilage.
2. Cell Behaviour – ECM forms a substrate on which cells can move and
provides cues that govern motility (influences where and when cells migrate)
and fate decisions, such as differentiation and proliferation (through
association with growth factors – constrains diffusion of molecules such as
EGF).
CONNECTIVE TISSUES: What are the major
components of the ECM and what are their
functions?
Connective tissues are predominantly ECM – consist of scattered cells (usually
fibroblasts, but in certain specialised types of connective tissues may be specific
members of the fibroblast family such as chondroblasts which form cartilage and
osteoblasts which form bone) that secrete and organise the fibres of the ECM.
The connective tissue contains a variety of other cells (e.g. macrophages which
degrade and move through the ECM) and ECM components (2 main classes of
macromolecules):
1. Glycosaminoglycans (GAGs): specialised linear polysaccharide chains of
repeating disaccharides (high osmotic pressure), often covalently linked to
protein… Proteoglycans: glyocoproteins with covalently attached GAGs
Form a highly hydrated gel-like substance in which fibrous proteins are embedded.
Resists compressive forces while permitting rapid diffusion of nutrients, metabolites
and hormones between blood and tissue cells.
2. Fibrous proteins:
, Collagens: trimeric molecules often bundled into fibres
and crosslinked by accessory proteins, strengthen and
help organise the matrix.
Elastin: protein forming core of elastic fibres – give the
matrix resilience.
Also 3. Multi-adhesive linker proteins: large multi-
repeat domain proteins that crosslink and scaffold, have
binding sites for different components.
COLLAGEN
Major fibrous proteins of the ECM,
convey high tensile strength and
stress resistance to the matrix.
Very abundant: 25% of total
protein mass of body (mostly in the form of fibrils – 80-90% FIBRILLAR type I,
II or III)
42 different collagen genes in mammals-but only c 40 collagen types
observed due to:
tissue specific expression (certain gene combinations expressed in certain
tissues) which restricts from 1000s of potential combinations and because
only some combinations are structurally possible (incompatibilities between
different collagens)
Typical structure of collagen:
Primary feature is long, stiff triple-stranded
helical structure in which 3 collagen
polypeptide chains (called α chains) are
wound around one another in a ropelike
superhelix.
A single collagen alpha chain is composed
of a series of triplet Gly-X-Y amino acid
sequences, in which X and Y can be any
amino acid (although X us commonly proline
and Y is commonly hydroxyproline or
hydroxylysine).
Three alpha chains are wrapped around one another to form a triple-stranded
helical rod. Glycine and Proline amino acids are important for packing and
stability of this self-association.
Triple-helical collagens self-assemble into fibrils, and these fibril strands form
collagen fibres. These form organised arrangements: e.g. in the connective
tissue of skin, fibrils are organised into bundles that run at right angles to one
another, so some are oriented longitudinally and some are seen in cross
section (at right angles to the fibroblasts).
Synthesis and secretion of the collagen fibril in fibroblasts:
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