Procollagen segment-long-spacing crystallites: their role in collagen fibrillogenesis

Naturally occurring segment-long-spacing crystallites of procollagen collagen have been found in the culture medium of fibroblasts from chick embryo tendon, human skin, and dermatosparaxic calf skin; in whole-mount preparations of cultured human skin fibroblasts; in homogenates of lathyritic chick embryo tendon, cartilage, and cornea; and in a partially purified preparation of procollagen. Bundles of similar aggregates occurred within secretory vacuoles of collagen-synthesizing fibroblasts and chondrocytes. These observations suggest that fibroblasts and chondrocytes secrete procollagen assemblies that are stable in the extracellular environment. We propose that subsequent enzymatic processing is accompanied by direct incorporation of such structures into the assembling fibril, which then may be considered as an n X 67 nm staggered array of segment-long-spacing crystallites.