Sequential events in the formation of collagen secretion granules with special reference to the development of segment-long-spacing-like aggregates

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Abstract
The formation and maturation of collagen secretion granules in periodontal ligament (PDL) fibroblasts of young male Balb-C mice were studied by electron microscopy and cytochemistry. The Golgi apparatus was composed of several dictyosomes, each consisting of a stack of five smooth-walled cisternae. Each cisterna was connected at both ends to a dilated saccule. The cisternae, with their associated pairs of saccules, underwent progressive maturational changes from the immature to mature face of each dictyosome. The most immature saccules (type 1) were large, spherical, and filled with loosely arranged short filamentous structures. These saccules were continuous with the first and second Golgi cisternae (counting from the immature side). Golgi saccules type 2 were ellipsoidal, associated with the third and fourth cisternae, and contained parallel, elongate filaments. The most mature saccules, type 3, were more rectangular and connected by a short fifth cisterna. They contained aggregated filamentous material in the form of eight segment-long-spacing (SLS)-like crystallites, one in the center and seven at the periphery, to form a basic secretory unit. As type 3 saccules matured, the interconnecting cisterna progressively shortened until the two saccules were nearly juxtaposed. At this time the shortened fifth cisterna split to give rise to two independent presecretory granules. By progressive condensation, presecretory granules matured into secretion granules that contained a densely packed SLS- like aggregate, within which individual crystallites were no longer discernable. Maturation of cisternae and saccules involved removal of membrane, apparently by the formation and detachment of coated vesicles. The staining reaction with silver methenamine and phosphotungstic acid increased over the procollagen as the saccules matured, indicating addition of carbohydrate moieties and possible crosslinkages. It is concluded that the formation and maturation of collagen secretion granules in PDL fibroblasts involves the packaging and further modification of eight SLS-like crystallites, which are secreted as a basic unit.