The First and The Second Mitotic Phases of Spermatogonial Stage in Xenopus laevis: Secondary Spermatogonia Which Have Differentiated after Completion of The First Mitotic Phase Acquire an Ability of Mitosis to Meiosis Conversion

Abstract

In Xenopus laevis, the spermatogenic cells derived from a primary spermatogonium divide synchronously and form a cyst surrounded by Sertoli cells. Therefore, it is easy to know how many times the spermatogenic cells divide from the primary spermatogonium by counting the cells in a cyst. In the present report, the distinctive feature of mitotic divisions during the spermatogonial stage in Xenopus laevis is described as having two phases. The first phase took place during five mitotic divisions. During this phase, the nucleus 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 spermatogonia, 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 observation of the cysts that consisted of approximately 25, 26, 27, or 28 primary spermatocytes, it was further suggested that the secondary spermatogonia at the second phase could also enter a meiotic phase. All primary spermatocytes were believed to have the ability to undergo the meiotic phase-specific gene expression and two meiotic divisions because the round spermatids in the cyst consisting of approximately 27, 28, 29, or 210 cells expressed SP4 mRNA which was transcribed in the spermatogenic cells at mid-pachytene stage and thereafter. These observations suggested that after the fifth mitotic division, the spermatogenic cells differentiated into the secondary spermatogonia and acquired an ability for mitosis to meiosis conversion.