COLLAGEN FIBRIL FORMATION

Collagen is most abundant in animal tissues as very long fibrils with a characteristic axial periodic structure. The fibrils provide the maj or biomechanical scaffold for cell attachment and anchorage of macromo lecules, allowing the shape and form of tissues to be defined and main tained, How the fibrils are formed from their monomeric precursors is the primary concern of this review. Collagen fibril formation is basic ally a self-assembly process (i.e. one which is to a large extent dete rmined by the intrinsic properties of the collagen molecules themselve s) but it is also sensitive to cell-mediated regulation, particularly in young or healing tissues. Recent attention has been focused on 'ear ly fibrils' or 'fibril segments' of similar to 10 mu m in length which appear to be intermediates in the formation of mature fibrils that ca n grow to be hundreds of micrometres in length. Data from several labo ratories indicate that these early fibrils can be unipolar (with all m olecules pointing in the same direction) or bipolar (in which the orie ntation of collagen molecules reverses at a single location along the fibril). The occurrence of such early fibrils has major implications f or tissue morphogenesis and repair. In this article we review the curr ent understanding of the origin of unipolar and bipolar fibrils, and h ow mature fibrils are assembled from early fibrils. We include prelimi nary evidence from invertebrates which suggests that the principles fo r bipolar fibril assembly were established at least 500 million years ago.