ORIGINS OF THE EXTRACELLULAR-MATRIX

Extracellular matrices of the vertebrates contain several families of proteins : adhesive glycoproteins (fibronectin, laminin, tenascin...), interacting with cells via specific receptors (including integrins); proteoglycans, which control the water contents of tissues and the migration of various substances (amongst which growth factors) ; and structural proteins such as fibrillin, elastin and collagens. The collagens themselves constitute a large family of approximately twenty members, some of them being present in all multicellular animals. The fibrillar collagens and the basement membrane collagens are the most characteristic and constant elements. The Porifera possess at least three collagen types. Among them, the fibrillar collagens appear highly homologous to the fibrillar collagens of vertebrates. This homology is even found at the level of gene organization and supports the concept of a monophyletic origin of metazoans. The non-fibrillar collagens in sponges are probably at the origin of basement membrane collagens, cuticular collagens of nematodes and fibril-associated collagens. The origin of collagens has yet to be found in the Protozoa, as membrane proteins similar to the transmembrane collagen have recently been described in vertebrates.