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The extracellular matrix contains fibres made out of protein. The fibres can vary in diameter, and in the way in which they are ‘woven’ into a network. For example, in tendons, the fibres run parallel to resist forces in the direction in which the muscle produces force. In bone they are arranged in alternating parallel layers. Whereas GAGs are good at resisting compressive forces, fibres are good at resisting tensile forces. There are two main types of fibre; collagen, and elastin and fibronectin. You may also come across Fibronectin which is a non collagenous fibrous connective protein that helps cells to stick to the extracellular matrix.

A diagram of a collagen molecule.

Collagen is the most common fibrous protein in the ECM, and it isi important for resisting tensile forces. It is the most abundant protein in mammals 25% of total protein mass! It is a protein composed primarily of the amino acids glycine proline, and it also contains sugar groups. It is acidophilic and stains with eosin in H (to look pink in sections). It is secreted by fibroblasts as procollagen, and amino acid residues at the ends of the protein are cleaved off outside the cell to form collagen, which then aggregates into fibres, etc. This prevents formation of large fibres in the cell (would not be good for the cells!). Collagen consists of three protein chains which wrap around each other in a helical arrangement. The collagen molecules are often crosslinked, for extra strength tensile strength.

About 15 kinds of collagen are known. Type I is the most common 90% of all collagens are type I. This type is found in bone, skin, tendon, ligaments, cornea etc. Types I, II, III, V and IX are fibrillar, which means that the protein chains assemble into fibrils, 10 300nm in diameter. Types V VI form networks in the basal lamina. Type VI helps to anchor basal lamina of skin to underlying connective tissue.

5 times more extensible than an elastic band! Mixing up collagenwhich are made up of a mixture of elastin and glycoproteins,sydrome, which leads to fragile skin, blood vesels and hypermobile.
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