Collagen fibrillogenesis: intermediate aggregates and suprafibrillar order.

Polymerization of collagen in vitro has been studied with the electron microscope at early time points of fibril assembly. We have found morphologically distinct stages of aggregation, which we suggest represent successive steps in fibril formation. Linear growth of the fibril appears to occur by the tandem addition of aggregates to each other and subsequently to the ends of a subfibril; lateral growth occurs by the entwining, like a rope, of these subfibrils. Fibrillogenesis is also accompanied by extensive development of suprafibrillar order in which various patterns of parallel, spiral, and orthogonal sets of fibrils were frequently observed.