Morphological and histochemical changes in the secretory granules of mucous cells in the early postnatal mouse parotid gland

It has previously been known that the developing parotid glands in humans a nd rats contain mucous cells in their terminal clusters and acini, but thes e cells disappear within a short period of time. Using rat parotid glands, IKEDA and AIYAMA (1997, 1999) suggested that the mucous cells might change into serous cells in the early postnatal period, but it is uncertain whethe r mucous cells appear only in the developing parotid gland of a few species such as humans and rats, or whether the cell transformation actually occur s. To clarify these points, the present study investigated the developing m ouse parotid glands. Light microscopy showed cells with secretory granules that stained extensiv ely with PAS and alcian blue in the terminal clusters of a 1-day-old mouse parotid gland. Mucous cell numbers in the terminal clusters and the acini r eached a peak on day 5 and decreased on day 7. By day 10, the mucous cells had disappeared altogether. Thus, the presence of mucous cells in the devel oping mouse parotid gland was confirmed. Electron microscopy showed granules of low electron density and bipartite g ranules in the mucous cells. Bipartite granules and highly electron-dense g ranules sometimes co-existed in a single cell. Immuno-electron microscopy r evealed a positive reaction for amylase to the low-electron-density granule s and the low-electron-density portions of the bipartite granules, in addit ion to the highly electron-dense granules and the electron-dense cores of t he bipartite granules. No mucous cells with nuclei displaying characteristi cs of apoptosis were recognizable. Lectin histochemistry both at the light and electron microscopic levels sho wed that the secretory granules in the mouse parotid gland mucous cells had sugar residues similar to those of the mature serous granules. These findings demonstrate that mucous cells appear in the early postnatal mouse parotid gland, and that almost all of these cells may be converted in to serous.