Electron microscope studies of collagen fibril formation in cornea, skin and tendon: Implications for collagen fibril assembly and structure in other tissues

Abstract

Collagen is one of the most widespread structural proteins in animals, and more than 23 genetically-distinct types of collagen are found in man (for review see [1]). Collagens comprise three polypeptide chains in which glycine (the smallest amino acid) occurs at every third residue position. The repeating Gly-X-Y motif (in which X and Y can be any amino acid and is often proline and hydroxyproline amino acids) is required for three polypeptide chains to assemble into a triple helix. The most abundant collagens are the fibril-forming types I, II, III, V and XI, which contain three polypeptide chains, each containing ~1000 residues, wound into an uninterrupted triple helix of ~295 nm in length (for review see [2]). These collagens occur in the extracellular matrix as D-periodic fibrils (where D = ~ 67 nm, the axial periodicity), which are indeterminate in length [3], and have a near-uniform diameter in the range 12-500 nm depending on tissue and stage of development (see Fig.1). The fibrils are heterotypic and contain more than one genetic type of collagen. For example, collagen fibrils in cartilage comprise type II collagen and minor quantities of type XI collagen and type IX collagen. The type IX collagen is an example of a fibril-associated collagen with interrupted triple helices (FACIT). Fibrils in other tissues contain type I collagen with minor amounts of type III and V collagen. The fibrils are stabilized by interchain covalent crosslinks, which require oxidative deamination of specific lysyl and hydroxylysyl residues by lysyl oxidase(s) (for review see [4]). The fibrils have binding sites on their surfaces for small leucine rich proteoglycans (SLRPs) [5].