Exocytosis in human salivary glands visualized by high-resolution scanning electron microscopy

Abstract

The luminal membrane of salivary acinar cells creates a specialized cell surface area that accepts exocytosis and undergoes dynamic changes during secretion. These changes were visualized three-dimensionally from both the inside and outside of the cell in human parotid and submandibular glands, by application of in vitro secretory stimulation and then of OsO4 maceration to remove cytoplasmic organelles by varying degrees. In control glands treated without secretagogues, the luminal surface of serous acinar cells bore well-developed microvilli with only an occasional incidence of exocytotic profiles. Following treatment with the beta-adrenergic agonist, isoproterenol, considerable shortening and loss of microvilli occurred along the luminal membrane where, on its cytoplasmic side, many protuberances of sizes similar to or smaller than those of single secretory granules (approximately 1 micron in diameter) appeared. The cytoplasmic surface of these protuberances exhibited small vesicles (approximately 100-150 nm in diameter) that, by transmission electron microscopy, were shown to be coated pits or vesicles present on or around the exocytosed granule membranes. Treatment of tissues with the muscarinic agonist carbachol also caused a decrease of microvilli and the appearance of protrusions at the luminal membrane. However, unlike isoproterenol treatment, many of these protrusions were devoid of small pits or vesicles and were much larger than a single secretory granule. These results indicate that (1) secretory stimulation causes the dynamic transformation of microvilli at the luminal membrane, where granule docking and membrane fusion take place, and (2) after fusion, the exocytosed membranes are processed differently, by coated pit/vesicle mediated or non-mediated mechanisms, according to the autonomic receptor control.

Expression of plasmid DNA in the salivary gland epithelium: novel approaches to study dynamic cellular processes in live animals

“…We sought to determine whether the uptake of plasmid DNA into the acini could be enhanced. It has been suggested that, during exocytosis, membranes delivered during the release of the secretory granules are retrieved by compensatory endocytosis (3,13,36,42). To test whether this process occurs in rat SMGs, we Fig.…”

Spatiotemporal analysis of exocytosis in mouse parotid acinar cells

“…Thus it is tempting to speculate that the initial phase of exocytosis induced by Ca 2ϩ ramps identified in the present study reflects recruitment of the minor pathway. Indeed, ultrastructural studies of parotid and submandibular salivary glands have demonstrated the presence of small-diameter vesicles (Ͻ150 nm) at the subluminal face of apical membrane or on or around ZSGs (23,27). However, it is also possible that transient fusion of ZSGs, delayed fusion pore expansion, or retention of the phase-dense material underlies the initial phase reported by C m measurements.…”

Differential Regulation of the Apical Plasma Membrane Ca2+-ATPase by Protein Kinase A in Parotid Acinar Cells

“…Therefore, PKA-mediated regulation of the apical PMCA may give the cell further control over these important Ca 2ϩ -dependent effectors. This may be particularly important within the highly invaginated microvillar structures of the apical membrane, whereby the PMCA may be the only means for the clearance of local concentrations of Ca 2ϩ because of the absence of ER (37).…”