The development of the submandibular gland of the rat was studied between days 2 and 55 of the postpartum. Besides confirming previous ultrastructural findings we observed at the electron microscope that the architecturally complex terminal tubules, proacinar, acinar and striated duct cells retained during mitosis the major ultrastructural features they exhibited in interphase. The ultrastructure of cells that were in the S period of the proliferative cycle was evaluated in high resolution autoradiographs from rats injected with thymidine-H³. All cell types were thus studied and no obvious cytoarchitectural modifications could be detected in these cells preparing for division. We obtained ultrastructural evidence that between days 20 and 30 some terminal tubule cells undergo transformation into acinar cells.
The intercalated duct cells showed the highest rate of proliferation and the lowest daily increment in number. This suggests that cells from the intercalated ducts migrate to the neighboring morphological compartments. Quantitative data on the rate of cell proliferation and accumulation in the striated duct indicated that cells from the intercalated ducts should differentiate into striated duct cells. Differentiation of intercalated duct cells into the terminal tubule cells was observed with the electron microscope.