EXOCYTOSIS IN HUMAN SALIVARY-GLANDS VISUALIZED BY HIGH-RESOLUTION SCANNING ELECTRON-MICROSCOPY

The luminal membrane of salivary acinar cells creates a specialized ce ll surface area that accepts exocytosis and undergoes dynamic changes during secretion. These changes were visualized three-dimensionally fr om both the inside and outside of the cell in human parotid and subman dibular glands, by application of in vitro secretory stimulation and t hen of OsO4 maceration to remove cytoplasmic organelles by varying deg rees. In control glands treated without secretagogues, the luminal sur face of serous acinar cells bore well-developed microvilli with only a n occasional incidence of exocytotic profiles. Following treatment wit h the beta-adrenergic agonist, isoproterenol, considerable shortening and loss of microvilli occurred along the luminal membrane where, on i ts cytoplasmic side, many protuberances of sizes similar to or smaller than those of single secretory granules (similar to 1 mu m in diamete r) appeared. The cytoplasmic surface of these protuberances exhibited small vesicles (similar to 100-150 nm in diameter) that, by transmissi on electron microscopy, were shown to be coated pits or vesicles prese nt on or around the exocytosed granule membranes. Treatment of tissues with the muscarinic agonist carbachol also caused a decrease of micro villi and the appearance of protrusions at the luminal membrane. Howev er, unlike isoproterenol treatment, many of these protrusions were dev oid of small pits or vesicles and were much larger than a single secre tory granule. These results indicate that (1) secretory stimulation ca uses the dynamic transformation of microvilli at the luminal membrane, where granule docking and membrane fusion take place, and (2) after f usion, the exocytosed membranes are processed differently, by coated p it/vesicle mediated or non-mediated mechanisms, according to the auton omic receptor control.