This ultrastructural analysis of pancreas development under in vivo and in vitro conditions complements earlier biochemical studies. The endocrine and exocrine tissue develops similarly in vitro. The early pancreatic epithelial cells, like those of the gut, form a single layer of cells facing the lumen, and are linked together by junctional complexes. This cellular polarization implies a constraint for the axis of division of exocrine cells, and demands that the endocrine cells loose their connection with the exocrine cells in order to form islets. The presence of mitotic figures in well differentiated acinar cells suggests that cell multiplication is not incompatible with cell differentiation. The first zymogen granules that can be observed look similar to the granules present later in development. This observation suggests that the “prozymogen granules” which were reported in earlier papers were probably the result of fixation artifacts, and did not represent a stage in granule formation between the condensing vacuoles in the Golgi and the definitive zymogen granules.

The morphological observations of this study are consistent with the previously proposed biphasic model of differentiation for the exocrine and endocrine B cells. The development of specific organelles correlates with the pattern of accumulation of the specific exocrine proteins and insulin. On the other hand, differentiated A cells are present when the pancreatic bud first forms. This early appearance of differentiated A cells suggests a regulatory role for this hormone in early development.