Freeze-fracture and thin-section study of condensing vacuoles in rat pancreatic acinar cells

Abstract

Early and late developmental forms of condensing vacuoles are prominent in the relatively low rate secreting acinar cells of suckling rat pancreas. These vacuoles, when studied in freeze-fracture replicas and ultrathin sections under standard processing conditions, showed a biphasic evolution. During the first stage the condensing vacuoles (referred to as CVj) enlarge, accumulating contents of rather low electron density. Fracture faces with irregular patterns, possibly the result of fusion (pinching off) of microvesicles with (from) the condensing vacuoles, were occasionally encountered. The infrequency of such images indicates that fusion-fission during the growth stage must be a very rapid event. One common type of surface irregularity is gibbosities (or convexities) in the P fracture face with complementary images in the E fracture face. The significance of these irregularities, which are in apparent discordance with the theory of microvesicular transport, is unclear. By the end of the growing period the condensing vacuoles are large and smooth-surfaced (referred to as CV₂) with contents of intermediate electron density (between that of the initial growing stage and that of the mature zymogen granule). The number of intercalated particles on both the large irregularly surfacec CV_1·) and large smooth-surfaced condensing vacuoles (CV₂) membranes is high and comparable to that of the Golgi saccule and endoplasmic reticulum membranes. During the second stage, the smooth-surfaced condensing vacuoles undergo volume reduction associated with progressive increase in the electron density of their contents, thus becoming zymogen granules. Concomitant with size reduction, the number of intercalated particles in the membranes with CV₂ diminishes markedly. The process of membrane retrieval appears to be accomplished selectively by pinching off coated microvesicles heavily studded with intercalated particles.

Freeze-fracture cytochemistry: localization of wheat-germ agglutinin and concanavalin A binding sites on freeze-fractured pancreatic cells.

“…In exocrine pancreas, a significant minority of the vesicles had an exoplasmic face with a coarse structure and dense labeling, whereas the majority had an unusually smooth exoplasmic face and much sparser label. These results parallel those derived from conventional freeze-fracture preparations which show that zymogen granule membranes have a remarkably low density of membrane particles compared to the endoplasmic reticulum, Golgi, or immature granule (condensing vacuole) membranes (21) . It appears, in consequence, that the low density of both WGA label and membrane particles might reflect a relative scarcity of intramembrane components in mature zymogen granules.…”

Resting (Basal) Secretion of Proteins Is Provided by the Minor Regulated and Constitutive-like Pathways and Not Granule Exocytosis in Parotid Acinar Cells

“…Exocrine granule maturation entails progressive reduction in the surface density of intramembranous particles as viewed by freeze fracture (47), suggesting that vesicles emanating from maturing granules may engage in selective membrane protein removal. SNARE and associated proteins have been implicated in nucleating membrane protein sorting during vesicle formation (48 -50), and one or more SNAREs that eventually support exocytosis of vesicles derived from immature granules may also function in forming the vesicles.…”

Defective cytoplasmic granule formation

“…In pancreatic acinar cells and in many other cell types, the newly formed Golgi vesicles (i.e., progranules) fuse to form immature granules, which are also termed condensing vacuoles. Both qualitative observations in transmission electron micrographs Palade 1967, 1971;Helander 1978;Sesso et al 1980;Cope 1983;Bendayan 1984;Saito et al 1987;Rambourg et al 1988) and an ultrastructural morphometric analysis of the pancreatic acinar cells of normal mice (Lew et al 1994) indicate that condensing vacuoles can decrease in size over time as they form mature, electron-dense secretory granules.…”