H-Y antigen negative XOSxrb mice, like their H-Y positive XOSxra counterparts, have testes; but, in contrast to XOSxra males, XOSxrb testes almost totally lack meiotic and postmeiotic stages of spermatogenesis. The quantitative analysis of the testes of XOSxrb males and their XY +/− Sxrb sibs, described in the present study, identified two distinct steps in this spermatogenic failure. First, there was a reduction in mitotic activity among T1 prospermatogonia, so that approximately half the normal number of T2 prospermatogonia were produced. Second, there was a dramatic decrease in the number of A3 and A4 spermatogonia and no Intermediate or B spermatogonia. These reductions were also largely due to decreased mitotic activity, there being a shortage of A1 and A2 spermatogonial divisions and no divisions among A3 or A4 spermatogonia. Mitotic activity among the T2 prospermatogonia and the undifferentiated A spermatogonia was normal. This means that the spermatogonial stem cells, which are a subset of the undifferentiated A spermatogonia, are unaffected in XOSxrb mice. Sxrb is now known to have arisen by deletion of DNA from Sxra. It is clear from the present findings that a gene (or genes) present in the deleted DNA plays a major role in the survival and proliferation of the differentiating A spermatogonia.

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