The purpose of this investigation was to study the microscopic anatomy of the living pancreas i i i s i t u . The results indicate that: (1) it is possible to study cellular detail in the living pancreas with resolution closely approaching the limit of resolution of the light microscope; (2) blood flow through individual capillaries in both acinar and islet tissue is intermittent; (3) local blood flow through acinar capillaries is regulated by smooth muscle precapillary sphincters and by endothelial sphincters, while flow through capillaries in islets is regulated locally by endothelial sphincters only; (4) insular-acinar capillary anastomoses exist but are not frequent; (5) secretory canaliculi between adjacent acinar cells exist in life and pass between centro-acinar cells to reach the lumen of the ducts; (6) processes of beta cells may pass between two adjacent cells to provide additional surface area for transcapillary exchange; (7) the formation and release of zymogen granules occurs within 45-90 minutes in acinar cells stimulated with pancreozymin and the formation and release of beta granules occurs within 20-60 minutes in beta cells stimulated with tolbutamide; and (8) cytological changes during the secretory cycle in acinar and beta cells are consistent with the current ultrastructural theory of secretion

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