Fine structure of transforming-type granules in mucous cells in the early postnatal rat parotid gland when processed by rapid freezing followed by freeze-substitution fixation

The present study was designed to clarify the more precise ultrastructural feature of granules, especially mucous granules in the early postnatal rat parotid gland by using rapid freezing followed by freeze-substitution fixat ion. The parotid gland of Wistar rats (aged 0-10 days) was removed under an esthesia and immediately underwent cryofixation followed by substitution wi th osmium tetroxide. After fixation, the samples were embedded in Epon-Aral dite, cut into ultrathin section, and then examined by transmission electro n microscopy. Electron microscopy showed that the mucous granules of sample s treated by freeze-substitution fixation had low electron density and were almost spherical in shape with a clear limiting membrane. By Day 5, granul es that were a little more electron dense than mucous granules, granules wi th a more electron dense portion at their periphery, and heterogeneous gran ules with an internal highly electron dense portion were found. Moreover, c ells containing both homogeneous granules with a high electron density simi lar to that of mature serous granules and heterogeneous granules were obser ved. These findings demonstrated that the morphology of the transforming-ty pe mucous granules by chemical fixation in the previous study was an artifa ct and, as a result, strongly suggested that on the sequential morphologica l changes of transitional mucous/serous granules by rapid freezing method i n the present study, the mucous cells in the early postnatal rat parotid gl and undergo transformation to serous cells. Anat Rec 260:387-391, 2000. (C) 2000 Wiley-Liss, Inc.