Collagen and glycosaminoglycans

Collagen structure

• Collagen is the most abundant protein in the body. About 28 types exist; types I, II, III, V and XI are fibrillar, while type IV forms a non-fibrillar mesh in basement membranes.
• Each molecule is a triple helix of three α chains. Every chain repeats Gly–X–Y: X is often proline, Y often hydroxyproline or hydroxylysine.
• Glycine sits at every third position because only the smallest amino acid (side chain a single hydrogen) fits in the crowded centre of the helix. A single glycine substitution kinks the helix, which is why glycine mutations cause severe osteogenesis imperfecta.
• Hydroxyproline forms hydrogen bonds that stabilise the helix at body temperature; hydroxylysine provides sites for glycosylation and crosslinks.
• On H&E, collagen bundles stain pink (eosinophilic); they are clearer with van Gieson (red) or Masson trichrome (blue/green). Fine reticular fibres (type III) need silver stains.

Synthesis: inside and outside the fibroblast

• Fibroblasts also secrete the amorphous ground substance (proteoglycans, glycoproteins, water) in which fibres lie.

Collagen types

• Skin dermis contains types I and III; fibrocartilage is rich in type I; hyaline cartilage in type II.
• The cornea is transparent because its type I fibrils are of uniform small diameter and evenly spaced; the sclera is opaque because its collagen bundles are irregular in size and arrangement.

Remember Vitamin C hydroxylates, copper crosslinks: scurvy is a hydroxylation failure, Menkes a crosslinking failure.

Collagen in wound healing

• Granulation tissue lays down type III collagen first; during remodelling it is replaced by type I, which gives the final tensile strength.
• Strength is about 10% of normal when sutures come out at one week and reaches only 70–80% by three months; it never returns to 100%.
• Wound contraction is driven by myofibroblasts (fibroblasts with smooth-muscle actin), not by collagen itself.
• Matrix metalloproteinases (collagenases) remodel scar; they need zinc.
• Healing is delayed by infection (the commonest local cause), ischaemia, steroids, diabetes, malnutrition and deficiency of vitamin C, zinc and protein.
• Hypertrophic scar stays within the wound margin and often regresses; keloid grows beyond the wound margin, is commoner in dark skin and recurs after excision.

Exam trap Type III collagen comes first in a healing wound; type I gives the final strength. Do not pick type III for "final tensile strength".

Inherited and acquired collagen disorders

• Scurvy (vitamin C deficiency): under-hydroxylated collagen cannot form a stable helix, so vessels and gums fail. Perifollicular haemorrhages, corkscrew hairs, swollen bleeding gums, petechiae, bruising and poor wound healing; in children, subperiosteal bleeds cause painful legs and pseudoparalysis. Typical patients: alcoholics, elderly people on tea and toast, infants on boiled milk without fruit.
• Osteogenesis imperfecta: usually autosomal dominant mutation in COL1A1 or COL1A2 (type I). Fractures with minimal trauma, blue sclerae (thin sclera shows the choroid), conductive deafness (ossicles), dentinogenesis imperfecta, wormian bones. Can be mistaken for non-accidental injury.
• Ehlers–Danlos syndrome: hyperextensible skin, hypermobile joints, easy bruising, wide papyraceous scars. The vascular type (type III collagen, COL3A1) causes arterial, bowel and uterine rupture.
• Alport syndrome: type IV collagen, usually X-linked: haematuria progressing to renal failure, sensorineural deafness and anterior lenticonus.
• Menkes disease: X-linked defect of copper transport (ATP7A), so lysyl oxidase lacks copper: kinky, brittle hair, hypotonia, growth failure, neurodegeneration, tortuous arteries.

Elastin and fibrillin

• Elastin is rich in glycine and proline, poor in hydroxyproline and has no hydroxylysine. Its unique desmosine crosslinks (made by lysyl oxidase) let it stretch and recoil. It is found in large arteries, lung, skin and ligamenta flava.
• Elastin is digested by neutrophil elastase, which is held in check by α1-antitrypsin; deficiency leads to panacinar emphysema and liver cirrhosis.
• Fibrillin-1 (FBN1, chromosome 15) forms microfibrils that scaffold elastin and bind TGF-β.
• Marfan syndrome (autosomal dominant): tall, arm span greater than height, arachnodactyly, joint laxity, pectus deformity, upward (superotemporal) lens dislocation, mitral valve prolapse (myxomatous valve) and aortic root dilatation causing aortic regurgitation and dissection (commonest cause of death). Cystic medial degeneration is seen in the aorta. β-blockers and losartan slow dilatation.

Marfan syndrome versus homocystinuria

• Homocystinuria looks Marfanoid but is a metabolic disease: cystathionine β-synthase deficiency (autosomal recessive) blocks conversion of homocysteine to cystathionine, so homocysteine and methionine build up.
• Homocysteine damages endothelium and fibrillin, giving thrombosis (DVT, stroke), downward (inferonasal) lens dislocation, intellectual disability and osteoporosis.
• Treatment: pyridoxine (B6) in responsive cases, methionine-restricted diet with added cysteine, betaine, folate and B12.

Exam trap A Marfanoid patient with a thrombosis, learning difficulty or a downward lens has homocystinuria, not Marfan syndrome.

Glycosaminoglycans and proteoglycans

• GAGs are long unbranched chains of repeating disaccharides (amino sugar plus uronic acid). They are strongly negatively charged and trap water, so they resist compression.

• The vitreous is about 99% water, held as a gel by hyaluronic acid and a sparse type II collagen network.
• Proteoglycans are a core protein carrying many GAG chains (aggrecan in cartilage binds hyaluronic acid). They are the main component of ground substance.
• Mucopolysaccharidoses are lysosomal defects of GAG breakdown: Hurler (α-L-iduronidase, autosomal recessive, corneal clouding) and Hunter (iduronate-2-sulphatase, X-linked, no corneal clouding).

Adhesion and other matrix proteins

• Fibronectin links cells to collagen and fibrin and guides cell migration in healing.
• Laminin is the main non-collagenous glycoprotein of the basement membrane.
• Integrins are transmembrane receptors that connect the extracellular matrix to the cytoskeleton (actin) and signal across the membrane; cadherins are calcium-dependent cell-to-cell adhesion molecules (adherens junctions, desmosomes).
• Intermediate filaments are tissue specific: keratin (epithelium), desmin (muscle), vimentin (mesenchyme), GFAP (astrocytes), neurofilaments (neurons).

How FCPS asks about this

• Vitamin needed for collagen hydroxylation: vitamin C (ascorbic acid) for prolyl and lysyl hydroxylase; copper is for lysyl oxidase crosslinking.
• Alcoholic with perifollicular haemorrhages, bleeding gums and poor healing: scurvy; defective collagen hydroxylation.
• Collagen giving final wound strength: type I (type III is laid down first); contraction is by myofibroblasts.
• Collagen in skin / basement membrane / cartilage: types I and III / type IV / type II.
• Defective protein in Marfan syndrome: fibrillin-1; aortic dissection and upward lens dislocation.
• Tall child with downward lens dislocation, thrombosis and high urinary homocysteine: cystathionine β-synthase deficiency (homocystinuria).
• Protein linking the cytoskeleton to the extracellular matrix: integrins; cadherins join cell to cell.