Characteristic features of preleptotene spermatocytes in Xenopus laevis: increase in the nuclear volume and first appearance of flattened vesicles in these cells.

In Xenopus laevis, the spermatogenic cells derived from a primary spermatogonium divide synchronously and form a cyst surrounded by somatic Sertoli cells. To clarify how many times the spermatogonia divide mitotically before differentiating to primary spermatocytes, the number of spermatogenic cells in the cysts was counted. Each cyst which consisted of cells morphologically identical to secondary spermatogonia contained discrete numbers of cells such as 2(6), 2(7), or 2(8) cells. On the other hand, the number of the germ cells in a cyst of zygotene/pachytene primary spermatocytes was approximately 2(8). These results indicate that spermatogenic cells which had finished eight mitotic divisions and showed a similar morphology to secondary spermatogonia were preleptotene spermatocytes. The nuclei of the preleptotene spermatocytes increased in volume concomitantly with premeiotic DNA replication and differentiation into zygotene/pachytene primary spermatocytes. Therefore the cells morphologically indistinguishable from secondary spermatogonia but with larger nuclei than typical secondary spermatogonia were identified as preleptotene spermatocytes. Electron microscopic observations of the preleptotene spermatocytes identified as above showed that flattened vesicles, 0.1-0.5 micron in length, about 0.05 micron in width, and with more electron-dense membrane than typical endoplasmic reticulum, first appeared in the cytoplasmic area of preleptotene spermatocytes.