Fine structural changes in the aging mouse thyroid.

Age related changes of the thyroid gland of CF1 mice ranging in age from 15 days to 24 months were studied morphologically and biochemically. The mean diameter of follicles becomes larger with advancing age due to fusion of adjacent follicles, and there is also an increasing variability of follicle diameter. The volumetric density of lysosomes in the cytoplasm of epithelial cells lining follicles increases with age. In the younger (seven days to 1.5-month-old) mice, most lysosomes are small and homogeneously dense; these are considered to be the primary lysosomes necessary for the hydrolysis of reabsorbed colloid droplets. The heterogeneously dense, large lysosomes containing lipid-like droplets increase in number with age, especially in mice over 15 months. The specific activities of acid phosphatase, beta-glucuronidase and beta-galactosidase show a relative decrease with age after six months; although there may be an absolute increase. Some of the lysosomes observed in old animals may be residual bodies.