Novel Exocytotic Features Occur In Rat Submandibular Granular Tubules

Exocytosis from granular tubule cells was studied electron microscopically at different times after the onset of secretion from submandibular glands of anaesthetised rats in response to the secretagogue cyclocytidine (75mg/kg i.p.) Ultrathin sections were stained by the periodic acid-thiocarbohydrazide-silver proteinate (PA-TCH-SP) method. Events were not synchronous in adjacent cells. At the onset of secretion the membranes of granules near Lumina showed thickening and increased PA-TCH-SP staining. The granules also altered shape and became aligned parallel to luminal membranes. Attachments appeared to occur over a length of opposing surfaces with the Luminal membrane developing a honeycomb-like appearance. A limited number of exocytotic profiles from single granules were observed in the initial phases. Thereafter the altered granule membranes developed microvesicles and this was associated with fusion of granules leading to the formation of large intracellular aggregates. Such aggregates then appeared to provide the main source far the secretion of granule proteins during ongoing secretion. Deeper aggregates sometimes showed exteriorization of their contents via neck-like connections. Blebbing of the apical surface of a number of granular tubule cells soon became evident and the shedding of blebs into lumina was seen, thereby creating an apocrine form of secretion. By the time fluid secretion had ceased, after 2-3 hours, extensive but not total degranulation had occurred.