Morphometric evaluation of zymogen granule membrane transfer to Golgi cisternae following exocytosis in pancreatic acinar cells from suckling newborn rats.

Pancreatic acinar cells from unfed, newborn rats and sucking for 4, 8 and 16 h were studied morphometrically in semi- and ultrathin sections. In the cells of the unfed, newborn rats, numerical and volume densities of the zymogen granules (ZG) and volume of the Golgi apparatus are respectively the highest and lowest observed during peri- and postnatal life. Cisternae of the rough endoplasmic reticulum (RER) appear irregularly disposed among the ZG. Once feeding starts, cytoplasmic volume becomes progressively reduced until the 16th hour owing to sustained exocytosis of ZG contents. The decline in numerical density of ZG between 0 and 4 h revealed the minimum number of ZG exocytosed in the first 240 min. The sum of the membrane surfaces measured in the various subcellular compartments [RER, condensing vacuoles (CV), Golgi cisternae (GC), Golgi apparatus-associated microvesicles (GM), 'other structures', apical and basolateral plasmalemmae and mitochondria] did not vary significantly in the various groups of rats. After 4 and 8 h, the net amount of cellular ZG membrane surface internalized represents 10% and 15% respectively of the total measured cell membrane. These quantities are sufficient to account for the expressive increase in membrane surfaces occurring at these times in CV, GC, and GM. The curves showing membrane surface decrease in ZG and increase in the Golgi appear to express a precursor----product relationship. The results of topochemical reactions are consistent with the interpretation that part of the ZG membrane internalized after exocytosis induced by alimentary stimulus is reused to expand and/or form trans (thiamine pyrophosphatase positive) and trans-most (acid phosphatase positive) GC.