The fetal germ cell DDX3Y expression suggests a role in early spermatogonial proliferation and implies that, in men with AZFa deletion, germ cell depletion may begin prenatally. The strong expression of DDX3Y in CIS cells, but not in gonocytes, indicates phenotypic plasticity of CIS cells and suggests partial maturation to spermatogonia, likely due to their postpubertal microenvironment.

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The female germ line (germ cell lineage, Keimbahn) is provided with only one proliferation wave, the oogenic, whereas male gametogenesis involves two successive waves: prespermatogenic, which corresponds to the female proliferation wave, and spermatogenesis, which is responsible for the immense number of male gametes produced in mature testes. Both male proliferation systems are linked by the transitional or T prospermatogonia. Using the reverse percentage of labelled metaphases method, it has been shown that the first differences between female and male germ cells can be identified by the end of the first wave, when oogonia and multiplying or M prospermatogonia are proliferating. This prenatal first wave of proliferation of male germ cells was also demonstrated in man and ceases around the 22nd week of pregnancy. Spermatogenesis involves a stock of stem cells (stem spermatogonia), a flexibly reacting pool of undifferentiated spermatogonia and several generations of differentiating spermatogonia, which proliferate almost exponentially. Furthermore, it consists of spermatocytes and haploid spermatids transforming into spermatozoa. The oocytes pass through the preleptotene stage, synthesizing DNA, and thereafter traverse the meiotic prophase up to the diplotene stage. In mammals they act as 'pre-embryos' in a similar but to a lesser degree than oocytes of amphibia and insects. The maternal chromosomes are largely responsible for the development of the embryo, the paternal genome for the development of the extra-embryonic tissue. The synthesis of transgenic animals is a powerful weapon in the armoury of geneticists, as has recently been demonstrated: a 14 kb genomic DNA fragment (Sry) is sufficient to induce testis differentiation and subsequent male development when introduced into chromosomally female mouse embryos.

Adult male rats were subjected to local testicular irradiation, plasma hormone levels and testicular histology being quantified at intervals up to 52 days thereafter. LH and FSH increased coincidently with spermatid but not with spermatocyte or spermatogonia depletion. Testosterone levels seemed to decrease but this effect was not significant. Oestradiol levels showed no significant changes. From the correlations between the various parameters it was concluded that the lack of inhibin was the main cause of the increase in both LH and FSH and that spermatids provide the signal for production of this non-steroidal inhibitor. The site of inhibin production was not definitively established but the results would be consistent with production of inhibin by the Sertoli cells in association with spermatids.

The aim of this study was the comparison between the mitoses of oogonia and the initial stages of oocyte meiosis. The structural alterations that the germ cell chromatin undergoes during the oogonial mitosis have been compared with those occurring during the G1- and S-phase just before meiosis. Using plastic embedded 1-microm sections of fetal rat ovaries (embryonic days = ED 14-20) labeled with 3H-thymidine and re-embedded for electron microscopy, a study of the structural conditions of the nuclear chromatin has been combined with a kinetic analysis of the oogonial cell cycle and the transitional period into the meiotic prophase. After ovarian differentiation (ED 14) the oogonia show a non-clonal, but strong proliferation. On ED 16, proliferation changes to a clonal pattern and decreases during ED 17. A final increase in 3H-thymidine incorporation on ED 18 characterizes the meiotic S-phase. On ED 19 the nuclear labeling drops to zero. The mitotic cycle of the oogonia lasts 16.5 hr and can be divided into 11 stages according to the concept of El-Alfy and Leblond [(1988) Am. J. Anat., 183:45-56] on the basis of the chromatin pattern. The S-phase (10.0 hours) extends from the telophase-interphase transition through the interphase to early prophase. The postmitotic G1- and S-phases show a more extensive duration, respectively 10 and 11.5 hours, and differ from their oogonial counterparts by the spherical shape of the nuclei from the very beginning. The chromatin pattern is similar until the end of the S-phase and lacks any prophase-like, preleptotenal chromatin condensation before the oocytes exhibit (pre-) leptotenal structures. Once the germ cell has completed a sequence of clonal mitotic divisions, it irrevocably progresses into meiosis. During an extended postmitotic period, the structural characteristics of meiosis emerge stepwise.

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