Spermatogenesis in the grey squirrel (Sciurus carolinensis) and changes during sexual regression.

In the testes of sexually active grey squirrels 8 stages of spermatogenesis were recognized each with a characteristic association of germ cells. The low numbers of spermatozoa in squirrel testes are the result of fewer spermatogonial divisions compared with those of most other mammals as well as a low efficiency of spermatogenesis, with only 42% of the germ cells becoming spermatozoa. A period of testicular regression which may be prolonged follows the breeding season in June/July. In fully regressed testes no stages of spermatogenesis could be recognized, the lumen of the seminiferous tubules was occluded and no stages later than primary spermatocytes were seen.