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 CV1·) 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
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