Changes with senescence in the fine structure of the granular convoluted tubule of the submandibular gland of the mouse

Cells of the granular convoluted tubules (GCTs) of the submandibular glands of senescent male mice show structural changes indicative of functional decline. In order to define the nature of these age-related changes more clearly, the fine structure of GCT cells of 12- and 28-month-old males was compared. In old mice, there was cell-to-cell variation in the extent of these changes, with some cells of senescent males appearing no different from those of young adults. In affected cells the most striking alterations were seen in secretion granules and lysosomal elements. Secretion granules varied greatly in size, with some GCT cells having only very fine apical granules. Secondary lysosomes and large lipofuscin granules were frequent in the basal cytoplasm. Very large dense bodies (3-5 μm) occurred in many cells. These possibly represent intracellular pools of released secretory materials, as they were occasionally seen in continuity with the luminal contents. Structures whose appearance was intermediate between the very large dense bodies and lipofuscin granules were common, suggesting crinophagic activity. There was an apparent decrease in numbers of polysomes and in the extent of the Golgi apparatus. These fine structural changes are consistent with impairments with advanced age in synthesis and posttranslational processing of secretory products by affected GCT cells. In addition to cell-to-cell variation in any one male, there was also interanimal variation in the degree and extent of these senescent changes.