The dog pancreas isolated in situ was perfused with oxygenated dog blood and stimulated with pancreozymin, secretin, or both. There were no significant changes in the fine structure of the acinar, centroacinar, or duct cells attributable to the perfusion. Combined glutaraldehyde and osmium fixation gave good preservation of the secretory products of the acinar cell. Before stimulation, the lumen of the acini is filled with material similar in texture to the content of the zymogen granules, but of somewhat lower density. Release of secretion commonly takes place by coalescence of the limiting membrane of zymogen granules with the plasmalemma, but one granule opening at the surface may frequently be joined by others coalescing with its membrane and forming an interconnected series all with contents having the same texture as the released zymogen. Such a mechanism seems to permit a more rapid release of secretory product than discharge of individual granules. Pancreozymin stimulation caused marked depletion of zymogen granules, but no obvious changes in the Golgi apparatus. It is clear, therefore, that this hormone exerts its effect upon release of granules rather than upon their formation. Secretin stimulation of water and bicarbonate secretion caused a marked washing out of the luminal contents, but had little detectable effect on cellular structure.

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