Procollagen segment-long-spacing crystallites: Their role in collagen fibrillogenesis* (macromolecular aggregates/exocytosis/cell culture/tissue homogenates/electron microscopy)

Naturally occurring segment-long-s acing crystallites of procollagen or collagen have been foun Tin 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 col- lagen-synthesizing fibroblasts and chondrocytes. These obser- vations suggest that fibroblasts and chondrocytes secrete pro- collagen assemblies that are stable in the extracellular envi- ronment. 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. Morphological and biochemical studies on collagen-secreting cells indicate that newly synthesized procollagen associates within cell vacuoles to form laterally arrayed aggregates that are released by exocytosis into the extracellular compartment. Peptide extensions at both ends of the molecule are enzymati- cally removed and the collagen is assembled as native fibrils (2-10). The state of aggregation of procollagen from the mo- ment of release to the appearance of fibrils remains unresolved. Two mechanisms are usually considered: (i) procollagen is se- creted and enzymatically processed, and the dispersed mono- mers self-assemble to form native fibrils (11, 12); and (ii) pro- collagen is secreted as small molecular aggregates that persist during and after processing and then assemble into fibrous arrays and undergo intermolecular rearrangement to form native fibrils (13, 14). This paper presents evidence for the existence of extracellular segment-long-spacing (SLS) aggregates that apparently were secreted as such and that appear to assemble in linear arrays.