Age-related morphological changes in the testis of the BDF1 mouse were studied by light and transmission electron microscopy. No apparent changes were detected until 12 months of age. After 18 months of age, vacuoles firstly appeared in the somniferous epithelium. These vacuoles were gradually increased in number and showed a tendency to cluster with each other in accordance with age. While, germ cells were decreased in number. The sloughing of germ cells caused a thin seminiferous epithelium. In the tubule with a thin epithelium, spermatogenesis was severely interrupted. After 30 months of age, extremely thin seminiferous epithelia were observed. In these epithelia, most of spermatids and spermatocytes disappeared, and most of Sertoli cells lost their polarity to be flattened. On the other hand, in the interstitial region, PAS-positive cells (mononuclear phagocytes) tended to increase in number after 24 months of age. PAS-positive extracellular matrix newly appeared at 27 months of age. In the cytoplasm of Leydig cells, a whorl of sER was frequently found. Degeneration of testes proceeded with age. The regressive tubules occupied only 2.2% at 18 months of age, but extended to 63.0% at 33 months.
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