A rope of three protein strands twisted into a helix, with glycine as every third amino acid
Collagen — a triple helix with Gly-X-Y repeats.
Anatomy · Cells & tissues
Collagen
The most abundant protein in the body
Connective tissue · extracellular protein fibre
The main structural protein of the body, making up about a quarter of all the protein in a mammal. It is made outside the cell by fibroblasts and other cells from long triple-helical molecules that assemble into tough, rope-like fibres. It gives strength to skin, tendons, ligaments, bones, cartilage, blood vessels and the cornea. In electron micrographs, its fibrils show a characteristic cross-banding repeating every 67 nanometres.
How to spot them
- ▸Unbranched, pink-staining wavy bundles in H&E, or blue in Masson's trichrome
- ▸Electron micrographs show fibrils with a regular cross-banded pattern with a repeat about 67 nm
- ▸Bundles densely packed in parallel in tendons; woven in a basket-weave in the dermis
- ▸Cells scattered among the bundles are flattened fibroblasts with thin, dark nuclei
- ▸Not a cell at all, but a fibrous protein product of cells
What to know
Making it requires vitamin C, without which gums bleed and old wounds reopen, in scurvy
Collagen — vitamin C is needed to hydroxylate proline and lysine.
Its name comes from the Greek word for glue, because boiling animal hides and bones yields gelatin and glue
Collagen — from kolla, glue.
The commonest type, type I, forms the fibres of skin, tendon and bone; type II is in cartilage; type IV forms the meshwork of basement membranes
Collagen — the main types.
A fault in the type I version causes brittle bones, and faults in others cause the hypermobile, fragile-skinned Ehlers–Danlos syndromes
Collagen — osteogenesis imperfecta and Ehlers–Danlos syndrome.
A repeating banding of 67 nanometres seen on electron micrographs of its fibrils
Collagen — the D-period arises from the staggered arrangement of molecules.
Its loss in the dermis with age and sun damage is a main cause of wrinkles
Collagen — dermal collagen thins and fragments.
Key ideas
- Fibroblasts secrete procollagen; enzymes outside the cell trim its ends so the rod-like tropocollagen molecules can assemble side by side into staggered fibrils, then cross-link into fibres.
- Hydroxyproline stabilises the helix, which is why vitamin C deficiency, which blocks hydroxylation, produces weak collagen and scurvy.
Quick-fire facts
- •Collagen makes up roughly 25–35% of the total protein in the human body
- •There are around 28 recognised types; types I–IV are the most important
- •Type I is about 90% of the body's collagen, found in skin, tendon, ligament, bone and cornea
- •The structure of the triple helix was worked out in the mid-1950s by G. N. Ramachandran and Gopinath Kartha in India, and by Alexander Rich and Francis Crick
- •Boiling collagen partly unwinds it into gelatin, used in food and photography
- •Scurvy, the disease of sailors without fresh fruit, is a collagen failure caused by vitamin C deficiency
- •Collagen molecules are about 300 nm long and 1.5 nm wide
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