The first and the second mitotic phases of spermatogonial stage in Xenopuslaevis: Secondary spermatogonia which have differentiated after completionof the first mitotic phase acquire an ability of mitosis to meiosis conversion

In Xenopus laevis, the spermatogenic cells derived from a primary spermatog onium divide synchronously and form a cyst surrounded by Sertoli cells. The refore, it is easy to know how many times the spermatogenic cells divide fr om the primary spermatogonium by counting the cells in a cyst. In the prese nt report, the distinctive feature of mitotic divisions during the spermato gonial stage in Xenopus laevis is described as having two phases. The first phase took place during five mitotic divisions. During this phase, the nuc leus of primary spermatogonium, which was identified by its large size and lobulated shape with weak stainability, progressively decreased in size and became round with heterochromatin patches. In the second phase, the sperma togonia, that had completed the fifth mitotic division and morphologically differentiated into the secondary spermatogonia, could undergo up to three additional mitotic divisions without any morphological change. From the obs ervation of the cysts that consisted of approximately 2(5), 2(6), 2(7), or 2(8) primary spermatocytes, it was further suggested that the secondary spe rmatogonia at the second phase could also enter a meiotic phase. All primar y spermatocytes were believed to have the ability to undergo the meiotic ph ase-specific gene expression and two meiotic divisions because the round sp ermatids in the cyst consisting of approximately 2(7), 2(8), 2(9), or 2(10) cells expressed SP4 mRNA which was transcribed in the spermatogenic cells at mid-pachytene stage and thereafter. These observations suggested that af ter the fifth mitotic division, the spermatogenic cells differentiated into the secondary spermatogonia and acquired an ability for mitosis to meiosis conversion.