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Abstract:This study was carried out to observe morphologically and quantitatively the membrane fusion of secretory granules and exocytotic figures during secretion while keeping alive the submandibular gland acinar secretory granules of male rats . The results showed that in comparison with the secretory granules of stimulated and control rats by quick freezing with OsO4. acetone and paraformaldehyde•acetone, (a) secretory granules apposed and fused with each other in a convex and concave form, and filamentous structure (90-100 A) and actin-like filaments (40 A) were intercalated between the two closely apposed granules, (b) part of the two fused outer leaflets of the granule disappeared in the initial stages of the two fused granule membrane and a fused, double thickness lipid bilayer was observed. After the contents of one granule were poured into the other fused granule, association of the two fused membranes was observed; and (c) appearance of a convex shaped granule membrane facing toward the plasma membrane and depression of the plasma membrane facing the cytoplasm represented the initial stage of exocytosis. On the other hand, filamentous structures intercalated between the plasma membrane and the granule membrane were detected. Lastly, (d) the width of the tubular structures in the control groups was narrower than that in the stimulated gland at discharge after the fusion of the two membranes.From these results it was concluded that the concave and convex form in the granule membrane fusion and the association of plasmalemma-granules with tubular structures in the exocytosis may be intrinsic phenomena of the double lipid bilayer in secretory granule fusion, thus confirming the exclusion of intramembranous particles.

In order to clarify the membrane retrieval process after exocytotic secretion of submandi-a substitution agent. Jpn. J. Oral Biol. 25: 781-794,1983. 14) Odajima, M.: Studies on the formation, fusion and discharge of rat submandibular gland acinar secretory granules-observations by freeze substitution fixation-. Jpn. J. Oral Biol.

In the rat submandibular gland , the ultrastructure and carbohydrate cytochemistry of striated duct cells were studied following chemical fixation with aldehyde or quick freeze , freeze substitution fixation with OsO4•Eacetone and paraformaldehyde-acetone. The contents of granules in the clear cells were found to be PA-TSC-SP reactive in specimens prepared by paraformaldehyd e•Eacetone f reeze substitution fixation, but to be negative in those prepared by chemical fix ation with aldehyde. I n the specimens prepared by the former method , thus, glycoprotein was detectable, and the cytoplasmic filaments and contents of secretory granules were unusually well preserved , as compared with those in the specimens subjected to chemical fixation .