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In the chick, the implantation of a testis graft from a 13-day-old male donor embryo into the extra-embryonic coelom of 3-day-old female embryos induces the total regression of their Müllerian ducts because of the anti-Müllerian hormone (AMH or MIS) secreted by the implant. Pre-treatment of the donors with estradiol (E2), between day 12 and day 13, counteracts in a significant way the Müllero-regressive activity of the implant. Co-treatment of donors at the same stage with both Tamoxifen (TAM) and E2 restores the initially observed activity, thus demonstrating the presence of Tamoxifen-sensitive estrogen receptors at the late stage of treatment in the Sertoli cells responsible for AMH secretion. The treatment of 3-day-old male donor embryos with E2 causes the differentiation of their left gonad into an ovotestis which provides implants totally devoid of Müllero-regressive activity. The additional treatment with TAM of the grafted host embryos, does not modify the results obtained when E2-treated male gonads are grafted to host embryos not treated with TAM. This shows that the lack of Müllero-regressive activity exhibited by the E2-treated male gonads does not depend on the estrogens they may secrete during the time of the assay, i.e., it cannot be attributed to a protecting action of estrogens on the MDs of the host. Our results therefore favor the idea that E2 down-regulates AMH. The relevance of such a regulation to the phenomenon of Müllerian duct maintenance, either in the E2-feminized male or in the female chick embryo, is discussed.

The sex differentiation of the female chick embryo can be totally inverted toward the male sex by an early extraembryonic testis grafting. This sex reversal remains permanent, as shown by three adult fowls described in this paper. They possess two testes associated with normally differentiated male excretory ducts and their Müllerian ducts have regressed. The development of male sex characteristics such as external features, behavior and complete spermatogenesis is evidence that these cocks have endocrine and exocrine capabilities similar to those of normal cocks. Although these cocks were able to mate with female fowls, they were sterile. A mechanism is discussed by which grafted testes induce such modifications in females. Hypotheses considering the heterogametic sex (female in birds) as exerting a dominant influence on the phenotypic sexual differentiation can be discarded in light of our results because a homogametic testis provokes the definitive sex inversion of a female.

Embryonic testes grafted in the extraembryonic coelom of 3-day-old genetically female chick embryos may induce total and definitive reversal of gonadal sex differentiation. In this experimental condition, the left gonad becomes a testis instead of an ovary. This makes it possible to compare testicular and ovarian morphogenesis in animals having the same genetic sex and to discount what is due to differences in the genetic determination between male and female. The morphogenesis of such testes is marked by a disappearance of the cortical germinal epithelium. The medullary sex cords keep a narrow lumen instead of becoming large lacunae. The germ cells remain few in the sex cords and do not become meiotic. Furthermore, interstitial cell development is known to be very slow. As a consequence the gross size of the gonad is much smaller than that of an ovary. All these morphogenetic phenomena are unlike those observed during normal ovarian differentiation and evidence an inhibiting influence of the grafted testes. Since inhibition and masculinization are concomitant, inhibition appears to be the mechanism responsible for gonadal sex reversal. The extraembryonic situation of the grafted testes and their relation with the embryo only via the blood stream demonstrates the role of a secreted substance or substances still to be exactly identified. Previous data suggest that this could be the anti-Müllerian-hormone (AMH). Furthermore, previous and present results show that testis differentiation can be actively induced in a bird. This does not agree with the hypothesis that the gonads of the homogametic sex, i.e., the testes in birds, do not need any inducer in order to differentiate.(ABSTRACT TRUNCATED AT 250 WORDS)

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