Morphology and kinetics of spermatogenesis in Xenopus laevis

The light microscopic characteristics of spermatogenic stages of the germ cell line in the anuran, Xenopus laevis, have been described as they appear in both nuclear squash preparations and plastic embedded thick sections of intact testes. Tritiated thymidine autoradiography was employed to unequivocally identify stages. Using this methodology, it was determined that the premeiotic DNA synthetic period occurs in a cell which is morphologically indistinguishable from a late secondary spermatogonial cell. In addition, kinetic studies with tritiated thymidine were employed to determine the duration of meiotic prophase and spermiogenesis in Xenopus. Results indicate that at 18 degrees C, the most rapidly matururing cells in the testis spend four days in leptotene, six days in zygotene, twelve days in pachytene, one day in diplotene, one day in meiotic division, and twelve days in spermiogenesis. An estimate for the duration of the premeiotic S stage was indirectly calculated from combined data to yield a value of six to seven days. Pooled spermatocyte counts measuring the frequency of occurrence of individual stages produced results which correlated closely with estimates obtained by tritiated thymidine labelling. Individual counts, however, show wide variations between testes from single animals and between testes from different animals, whether sacrificed together or at different times of the year. Nevertheless, in all cases, both morphology and labelling patterns indicate that spermatogenesis is continuously active. The variation observed in individual testes appears to be the result of waves of non-random entry of spermatogonial stem cells into the population of cells irreversibly committed to differentiation.