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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    Determination of phenotypic sex 39 chromosome, than in the chimaeras of Mintz (1969fl) and Ford et al. (1974), where strong selective forces are known to be operating. Ford et a!. (1974) find a strong correlation between right and left ovaries in the relative proportions of the two chimaera components among dividing follicle cells. They argue that the follicle cells in the two ovaries must therefore have a common origin. If these cells, contrary to common belief, enter the gonadal ridges secondarily, by migration, they need have no influence on sex determination and the 98 % of XY follicle cells in the XX/XY ovary requires no further explanation. If, as seems more likely, the left-right correlation implies that the gonadal ridges themselves have a common (pre¬ sumably dorsal mid-line) origin, the 98 % of XY cells would indicate either that gonadal sex is not determined by the chromosome constitution of the somatic cells or that, as suggested above, strong differential proliferation has taken place between the embryo and the adult, perhaps during follicular maturation. Milet et al. (1972) examined four XX/XY chimaeras, and found no rela¬ tion between phenotypic sex and the proportion of XX cells in skin, spleen, kidney, bone marrow or cornea. They postulate that the sex of the gonad is determined jointly by the somatic tissue and the primordial germ cells, but they bring forward no direct evidence in support of their hypothesis. In conclusion, the finding in experimental mouse chimaeras of XX/XY constitution that the somatic tissue of the adult gonad may be largely com¬ posed of cells discordant with its phenotypic sex does not necessarily con¬ tradict the widely held opinion that the sex of the gonad is determined by its somatic component, since it may be that the cellular make-up of the adult gonads bears little relation to that of the genital ridges in the embryo. Freemartinism The determination of phenotypic sex can be affected also by secondary chimaerism. In cattle twins, which nearly always share a common placental circulation, the female member of an opposite-sexed twin pair develops as a sterile freemartin. The fact that such sterile females are born co-twin to a male calf was first noted by John Hunter in the eighteenth century. The external genitalia of the freemartin are female, but uterus and oviducts are poorly developed or even absent and some male secondary sex organs (semi¬ nal vesicles, vasa deferentia, epididymides) are present. The gonads are small and usually contain testicular tissue; this is populated by germ cells during foetal life, but few if any germ cells survive beyond birth. The male partner is also affected, since bulls born co-twin to freemartins often become more or less sterile in later life. For a review of the consequences of secondary chimaer¬ ism for sexual development, see Short (1970). Obviously something passes from one twin to the other to affect develop-