Abstract
Meiosis is an event that occurs prerequisitely and specifically in gametogenesis. However, the mechanisms of conversion from mitosis to meiosis are poorly understood. I will review the results so far obtained by us using newt testis as a model system, and discuss about the extrinsic mechanism(s) controlling the conversion from mitosis to meiosis. In the newt spermatogonia enter meiosis in the 8th generation after 7 mitotic divisions. We developed organ and reaggregate culture systems with a chemically defined medium in which porcine follicle-stimulating hormone (pFSH) promotes spermatogonial proliferation and differentiation into primary spermatocytes. Human recombinant stem cell factor (RhSCF) in vitro stimulates the spermatogonial proliferation and progression to the 7th generation, but not the differentiation into primary spermatocytes; instead they die of apoptosis. The reason why rhSCF does not stimulate meiosis entrance seems to be due to the low level expression of c-kit protein at the 7th generation of spermatogonia. Ovine PRL induces apoptosis in the 7th generation of spermatogonia in vivo and in vitro. Incubation of newts at low temperature causes spermatogonial apoptosis by the elevation of plasma PRL titer. In the absence of FSH in organ culture spermatogonia can progress until the 7th generation, but the 8th generation never appear due to the apoptosis. Altogether there seems to be a regulatory checkpoint for entrance into meiosis in the 7th generation. Spermatogonia could circumvent the checkpoint by the influence of some factor(s) produced by Sertoli cells upon activation by FSH. Trial to isolate factor(s) responsible for the meiosis-initiation is now underway.
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