Birth of neural progenitors during the embryonic period of sexual differentiation in the Japanese quail brain.

Several brain areas in the diencephalon are involved in the activation and expression of sexual behavior, including in quail the medial preoptic nucleus (POM). However, the ontogeny of these diencephalic brain nuclei has not to this date been examined in detail. We investigated the ontogeny of POM and other steroid-sensitive brain regions by injecting quail eggs with 5-bromo-2-deoxyuridine (BrdU) at various stages between embryonic day (E)3 and E16 and killing animals at postnatal (PN) days 3 or 56. In the POM, large numbers of BrdU-positive cells were observed in subjects injected from E3-E10, the numbers of these cells was intermediate in birds injected on E12, and most cells were postmitotic in both sexes on E14-E16. Injections on E3-E4 labeled large numbers of Hu-positive cells in POM. In contrast, injections performed at a later stage labeled cells that do not express aromatase nor neuronal markers such as Hu or NeuN in the POM and other steroid-sensitive nuclei and thus do not have a neuronal phenotype in these locations, contrary to what is observed in the telencephalon and cerebellum. No evidence could also be collected to demonstrate that these cells have a glial nature. Converging data, including the facts that these cells divide in the brain mantle and express proliferating cell nuclear antigen (PCNA), a cell cycling marker, indicate that cells labeled by BrdU during the second half of embryonic life are slow-cycling progenitors born and residing in the brain mantle. Future research should now identify their functional significance.