ABSTRACT
The name collagen designates a family of structural proteins of the extracellular matrix-forming supramolecular aggregates that involve lateral interactions between characteristic triple-helical domains. The cartilage collagen fibrils are made up of, in large part, type II collagen molecules. It has recently been demonstrated that cartilage collagen fibrils are actually mixtures of several collagen types, namely types II, IX, and XI. These three types are characteristic of cartilaginous tissues, although they have been observed at low levels in some other extracellular matrices. Type X collagen is also cartilage specific, but its expression is restricted to the hypertrophic chondrocyte. Type II collagen is a genetically distinct member of the fibril-forming collagens and the gene is present in a single copy in the genome. Type V collagen is distributed in various connective tissues is associated with type I collagen in heterotypic fibrils. Type XI collagen can form fibrils in vitro. The interactions between proteoglycans and type XI collagen are strongly inhibited by heparin.
