Dynamic Changes in the Ultrastructure of the Acinar Cell of the Rat Parotid Gland During the Secretory Cycle

Abstract

Synchronization of the secretory cycle in vivo was obtained by injecting isoprenaline as an inducer of secretion . A quantitative correlation between enzyme release, its subsequent reaccumulation, and the sequence of ultrastructural changes was found . At the ultrastructural level secretion was paralleled by depletion of zymogen granules through fusion of the granule membrane with the lumen membrane and discharge of the content . Each zymogen granule membrane, once connected with the lumen, acted as a lumen membrane . Fusion was thus sequential and resulted in a dramatic enlargement of the lumen space . During the entire process the passage between the lumen and the intercellular space remained blocked by the tight junctions, as shown by their impenetrability to ferritin . Reduction of the lumen size following enzyme discharge seemed to be achieved by withdrawal of lumen membrane in the form of small smooth vesicles which appeared mostly in the apical part of the cell . At the same time, the cell retracted towards the lumen, the whole process being completed within 2 hr from onset of secretion . Disappearance of the smooth vesicle followed, concomitant with formation of many condensing vacuoles and appearance of mature zymogen granules . The fate of the zymogen granule membrane, including its fusion with the lumen membrane, resorption in the form of small smooth vesicles, and its eventual reutilization mediated by the Golgi system, is discussed .

Fusion of secretory vesicles isolated from rat liver

“…Exocytosis requires the fusion of the vesicle membrane with the plasma membrane. However, as already shown in several cells, the stimulus for secretion not only induces the fusion of the plasma membrane with the vesicular membrane but also of secretory vesicles among themselves [2,3,6,18,29,39,53,65]. Thus the exocytotic process appears to involve both types of fusions.…”

“…Upon increasing the concentration of Mg z+, Sr 2+, or Ba 2+ to 10 -4 M in the incubation medium, vesicles became attached to each other and MAPs were aggregated. 2 The same changes in the distribution of vesicles in the suspension and in the array of MAPs could be observed if C a 2 + (final concentration 10 -4 M) was added to the incubation medium. However, in addition to the morphological changes described above for a series of cations, The numbers of fused vesicles here reported are minimum values.…”

“…8a). The twinned structure of completely fused vesicles does not differ from that found in stimulated secretory cells [2,3,6,12,18,29,39,53,65]. Magnification of Fig.…”

“…Intervesicular fusion of secretory vesicles, as well as the interaction between the cell membrane and the vesicle membrane, was observed in a variety of secretory cells [2,3,6,18,29,39,53,65]. Douglas [20] showed that in "compound exocytosis" by mast cells an external stimulus initiates fusion between secretory vesicle membranes and the 0022-2631/78/0040-0343 $4.40 9 Springer-Verlag New York Inc.…”

Serous Cells in the Parotid Glands of Two Species of Tamarins: Polarized Secretory Granules

“…As a result, during release of the secretory product and briefly thereafter, the luminal membrane has a mosaic arrangement. After (and even during) exocytosis, the excess luminal membranes are retrieved by endocytosis (Amsterdam et al, 1969) by a calcium-dependent mechanism (Koike and Meldolesi, 1981); this reuptake is highly specific for those patches originating from the exocytosed granules (Meldolesi and Ceccarelli, 1981). The fate of these recaptured membranes still is controversial.…”