Granule types and their morphological changes in terminal cluster and acinar cells in the late pre- and early postnatal rat sublingual gland

The developmental characteristics of serous cells appearing in the rat sublingual gland from the late prenatal to the early postnatal period were investigated in this study. Particular attention was paid to the morphological changes observed in the secretory granules at the histochemical and ultrastructural level. On prenatal day 18, granules with homogeneous high electron density (Type I granules), and mottled granules (Type II granules) with heterogeneous electron density appeared in the narrow luminar cytoplasm of cells constituting the terminal clusters. On prenatal day 19, these granules decreased in number and were replaced by bipartite granules (Type III granules) composed of a highly electron-dense core and a more electron-lucent rim. Pronase treatment almost completely digested the Type I and II granules and the electron-dense core of the Type III granules, although some of the Type I and II granules in serous demilunes at a later stage were insufficiently digested. On prenatal day 19.5, homogeneous granules of low electron density (Type IV granules) appeared in the terminal clusters and acini, and increased in number daily, making up 92.8% of the total granules on postnatal day 28. The granule morphology on electron microscopy, Alcian blue, and periodic acid-Schiff staining strongly suggested that Type I and II granules were serous granules, Type IV granules were mucous granules, and Type III granules were transforming-type granules. None of the secretory cells showed chromatin condensation, which is a characteristic of apoptosis. These findings suggest that the developing rat sublingual gland from the late prenatal to early postnatal period has numerous serous granules in the terminal clusters and acini, and that the majority of granules are replaced by mucous granules through transforming-type granules. In addition, because apoptotic figures of secretory cells could not be detected, it appears that most of the serous cells in the developing rat sublingual gland might have changed to mucous cells.