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Spermatogenesis in mature and regressed testes of the vole (Microtus agrestis).

Eight stages of spermatogenesis, each with a characteristic frequency and germ cell association could be recognized in the vole. There was no difference between laboratory bred and field animals in the frequency of the stages. Counts of the different types of germ cell showed that there was considerable cell loss during spermatogonial mitotic and spermatocytic meiotic divisions. Only 60% of germ cells became spermatozoa in sexually mature animals, and 19% in the regressing testes of voles exposed to short photoperiods. Animals with regressed testes probably have lowered circulating levels of gonadotrophins and testicular hormones, so that the greater germ loss suggests the importance of these hormones in the regulation of germ cell wastage. From the cell counts in mature animals, a scheme of cell divisions has been suggested by which spermatogonia produce progressively more highly differentiated germ cells while continuing to perpetuate stem cells.