Developmental changes of sugar residues and secretory protein in mucous cells of the early postnatal rat parotid gland.

Mucous cells have been identified in the terminal portions of the early postnatal parotid gland in human and rat, although mature parotid gland acini are composed of serous cells or seromucous cells. Previously, Ikeda et al. demonstrated that mucous cells are present in the rat parotid gland on days 1 to 8 after birth and that the secretory granules within these mucous cells share some histochemical characteristics with mature serous cells. However, it is still not clear whether the mucous cells change into serous cells as the gland develops. The purpose of this study was to determine whether the mucous cells that appear in the early postnatal rat parotid gland change into serous cells. Parotid glands were obtained from male or female Wistar rats (aged 0-14 days and adults). Fixed tissue sections were reacted with soybean agglutinin (SBA) and wheat germ agglutinin (WGA) to detect glycoconjugates, or were stained using an anti-neonatal submandibular gland protein B1 (SMG-B1) antibody to identify serous acinar cells. The sections were observed by transmission electron microscopy. Electron microscopy revealed that cells with characteristics intermediate between those of mucous and serous cells (transitional cells) appeared around day 8 and that the nuclei of these cells did not show chromatin condensation, a characteristic of apoptotic cells. Lectin histochemistry showed that the mucous cells had the same sugar residues as the serous cells, which appeared after day 10. Immunohistochemistry with an anti-SMG-B1 antibody gave a positive reaction not only in the cells with highly electron-dense granules but also in the electron-dense cores of bipartite or tripartite granules in the transitional cells. Cells with morphological characteristics intermediate between those of mucous and serous cells (transitional cells) appearing in the early postnatal rat parotid gland begin to produce B1-immunoreactive protein common to serous acinar cells during development of the gland.