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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    40 Sexual development ment, but what? Early workers assumed that the mascuhnizing influence on the female twin was exerted by a hormone: this is probably still the most plausible theory, though repeated failures to achieve masculinization by the experimental administration of testosterone and related androgens suggest that some as yet unidentified hormone may be responsible. The discovery of secondary blood chimaerism led to the suggestion that the modification of phenotypic sex might result from the presence in the female twin of cells with a male chromosome constitution. However, it is hard to envisage XY blood cells exerting a masculinizing effect on the gonad; moreover, in a series of heterosexual cattle twin pairs examined during foetal life the degree of mascu¬ linization of the female proved in no way correlated with the proportion of XY cells present in her liver (Vigier, Prepin & Jost, 1972). Ohno et al. (1962) suggested that primordial germ cells as well as blood cells were exchanged between cattle twins, but the observations have yet to be conclusively con¬ firmed. Extensive progeny-testing of like-sexed cattle twins, as well as of bull calves born twin to freemartins, have failed to reveal any evidence of germ cell exchange. Thus the phenomenon of freemartinism, interesting though it is, has as yet shed no new light on the question of whether the phenotypic sex of a gonad derives from its germinal or its somatic elements. In marmosets, almost all pregnancies end in twin births, and blood chi¬ maerism occurs regularly, yet heterosexual twin pairs suffer no abnormalities of sexual development. Vascular anastomosis between the two placental circulations develops at a very early stage of pregnancy, so the failure to develop freemartinism is hard to understand. In Man too, opposite-sexed twins with XX/XY blood chimaerism (e.g. Miss Wa. and her brother, see Chapter 1) show normal sexual development. Secondary sex glands A striking example of cellular non-autonomy in the development of the reproductive tract was described by Mintz (1969^/) and Mintz, Domon, Hungerford & Morrow (1972). In a sterile XX/XY male chimaera of the strain combination CBA-T6T6 C57BL, electrophoretic analysis of IDH iso¬ zymes showed that not only the testis, but also the epididymis and seminal vesicles, appeared to consist entirely of the XX (= CBA) component. The technique would have recognized a minor component making up more than 5 % of the tissue. This suggests that XX cells in the foetus, despite their geno¬ type, were capable of responding to an androgenic stimulus by forming mor¬ phologically normal seminal vesicles. The analysis was extended from the morphological to the biochemical level. Seminal vesicle fluid contains characteristic proteins not found in any female tissues. Electrophoretic analysis has revealed that these proteins differ from one inbred strain to another. Mintz et al. (1972) identified an