Sexually differentiated cell proliferation in the medial preoptic nucleus of Japanese quail

Abstract

The androgen-depend sequence of male copulatory behaviors is sexually differentiated in Japanese quail like in many other vertebrates. This behavior disappears within a week after castration of males and is rapidly restored following treatment with testosterone. In contrast, ovariectomized females treated with testosterone never show these behaviors. This sex-specific responsiveness to testosterone results from organizational effects of embryonic ovarian estrogens. The behavioral phenotype of male and female quail is completely reversed by treatment, before embryonic day 12 (E12), of male embryos with estrogens or of female embryos with an inhibitor of aromatase, the enzyme converting testosterone into estradiol. In the quail brain, the medial preoptic nucleus (POM) is a necessary and sufficient site for the activation of sexual behavior by testosterone. It can be identified by a dense population of aromatase-immunoreactive neurons and, importantly, aromatase activity in this region is sexually differentiated (males>females) and this difference is maintained even when birds are treated with a same dose of testosterone. Aromatase is thus, like sexual behavior, a sex-specific feature regulated by testosterone in adult quail and presumably organized in early life by steroid action. The cellular basis of these organizational processes have not been identified. We analyzed the ontogeny of POM cells by injecting 5-bromo-2-deoxyuridine (BrdU) in eggs at different embryonic (E) stages (E8, E10, E12, E14 and E16) and quantifying BrdU-labeled cells at postnatal (PN) day 56. Large numbers of BrdU-positive cells were observed throughout the POM of males and females injected on E8-E10 but most cells were post-mitotic in both sexes on E14-E16. E12 injections resulted in a larger number of BrdU cells in females than in males at PN56. However, male and female embryos injected on E12 and killed on E13, PN1 or PN15 had similar numbers of BrdU positive cells. Furthermore, BrdU injections on E14 labeled very few cells at PN 56 suggesting that the POM is essentially post-mitotic at that age. Together these data suggest that a sex-specific apoptosis must occur between PN15 and PN56. Double-label immunohistochemistry for BrdU and for the neuron-specific marker Hu indicated that BrdU-positive cells born between E8 and E16 are not neurons and are thus presumably glial cells. This sex-specific cell proliferation occurring around the end of the critical period of sexual differentiation may have an important impact on brain and behavior differentiation. The phenotype of these cells and the mechanisms mediating their differential development are currently under investigation