Parotid glands of litters of rats at age intervals from 20 days in utero to 100 days were assayed for DNA content and examined by light- and electron-microscopy. The age differences in total DNA and DNA concentration indicated that there was a rapid rate of proliferation of parenchymal cells until 25 postnatal days, after which the rate declined rapidly, and that there was a rapid increase in cell size between 18 and 25 days. These findings were substantiated by histologic observations, such as the presence of numerous mitotic figures until 25 days of age, and the rapid maturation of the acinar cells between 18 and 25 days. These data suggest that the acinar cells of the rat parotid gland comprise an expanding cell population. Light- and electron-microscopic observations consistently indicated that cells with mitotic figures were about as well differentiated as other parenchymal cells at all stages of gland development, including mature acinar cells observed at ages 23 and 25 days. These observations support the view that the division of cells in advanced stages of differentiation may be important in the growth of certain organs and tissues.

The probable shape, size, and orientation of desmosomes of the cells comprising the secretory tubules in rat submaxillary gland was determined by statistical and algebraic methods applied to electron micrographs. It was concluded that these desmosomes are discrete ellipsoidal discs whose principal axes are in the order of 4100 and 2500 angstrom units, and that they are preferentially oriented with their long axis more or less parallel to the base-apex axis of the cell. Densitometric interpretation agrees with the statistically based reconstruction of desmosomal shape. By densitometric analysis it was also determined that the peak to peak distances between layers within these desmosomes are in essential agreement with other reported findings. The approach described may have general applications to problems in the analysis of submicroscopic morphology.