Development of acinar cells in the rat submandibular gland.

Three types of cells - terminal tubule cells, proacinar cells and acinar cells - have been observed during the development of acini in the rat submandibular gland. The terminal tubules are lined by terminal tubule cells which show PAS-positive, dark granules. From the terminal tubules pouches develop which are lined by similar cells to those lining the terminal tubules. These pouches give rise to the next generation of terminal tubules. The last generation of terminal tubules, whose cells lose their dark, PAS-positive granules, develop into secondary tubules lined by proacinar cells. At 14 days of age large coarse granules appear in the cytoplasm of the proacinar cells lining the secondary tubules as well as in the cells of intercalated ducts. These granules are discharged into the lumen of the tubule and the proacinar cells become foamy-looking acinar cells, while the secondary tubule becomes an acinus. Thus the terminal tubule cells give rise to proacinar cells which become acinar cells and cells to the intercalated duct cells. The development of acini in the rat submandibular gland thus takes place in three stages: (1) development of terminal tubules by continuing pouch formation up to 12 days of age, (2) conversion of terminal tubules into secondary tubules tined by proacinar cells from 6 to 12 days of age, and (3) development of large coarse granules in proacinar cells and their excretion into the lumen of tubules, resulting in the transformation of proacinar cells into acinar cells between 14 and 21 days.