Mammalian chimaeras / Anne McLaren.

  • McLaren, Anne.
Date:
[1976]
Catalogue details

Licence: Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)

Credit: Mammalian chimaeras / Anne McLaren. Source: Wellcome Collection.

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    Germ cell differentiation 43 not even to be found among primary spermatocytes (Mystkowska & Tar- kowski, 1968). A possible clue to their fate came from the observation by Mystkowska & Tarkowski (1970) that the testes of male XX/XY chimaeras on the 16th to 17th day of foetal development contained germ cells in meiotic prophase, mostly in pachytene. In the normal female mouse the germ cells enter meiosis before birth, on the 14th or 15th day of gestation. In the normal male, on the other hand, the germ cells do not enter meiosis until several days after birth, so that the foetal testis contains large numbers of spermatogonia undergoing mitosis, but no meiotic stages. In a later study, germ cells in the early stages of meiosis were found in the testes of foetal chimaeras up to 17^ days p.c. (Fig. 16), but they failed to reach diakinesis and by 18^ days they were degenerating (McLaren et ai, 1972). Many oocytes in the normal ovary degenerate at a corresponding stage of foetal development. The obvious explanation of these meiotic cells is that they are XX germ cells, behaving autonomously and entering meiosis at the time laid down by their genotype. An alternative possibility (see Fig. 17) is that the somatic tissue of the gonad determines the time of entry of germ cells into meiosis, so that those germ cells of either sex in the XX/XY testis that happen to be surrounded by a large enough patch of XX somatic tissue are driven into meiosis, while the remainder (XX and XY) continue to divide mitotically under the influence of XY somatic tissue. This explanation, favoured by Tarkowski (1969), is more easily reconciled with the low proportion (about 5 %) of germ cells observed in meiosis in the XX/XY testis: although XX and XY germ cells are presumably present in more or less equal proportions, the effect of XX somatic tissue could involve a threshold phenomenon such that only the occasional patch of unmixed XX cells was effective. The possibility that the cells in meiosis include XY germ cells is not supported by the observations of McLaren et al. (1972). Injection of pregnant females with tritiated thymidine showed that the meiotic germ cells in chimaeric testes replicated their DNA on the same day as did those in normal foetal ovaries, and developed at the same rate. No sign was seen of the striking late-labelling pattern characteristic of meiotic prophase in male germ cells (Kofman-Alfaro & Chandley, 1970), nor of the prominent sex vesicle that contains the XY bivalent. It therefore seems that the cells in meiosis are XX rather than XY germ cells. Their relative scarcity, and the differences reported in their incidence in the two testes of the same foetus, might suggest that some influence of XX somatic tissue is also required if meiosis is to begin (i.e. a combination of the two hypotheses illustrated in Fig. 17). The degeneration of meiotic germ cells before birth in the mouse is pre¬ sumably due to the adverse hormonal environment of the testis, and the absence of the layer of follicle cells that normally surrounds the oocyte at this time. Some may survive into the postnatal period, since Mystkowska & Tarkowski (1968) found growing oocytes in one area of the testis of a male 4 M M с