The development of a species difference in the local distribution of brain estrogen receptive cells

Estrogen receptors (ER) are crucial for estrogen-dependent brain differentiation and for estrogen-dependent neural functions of vertebrates. ER are expressed in a area-specific pattern in the vertebrate brain. The mechanisms that lead to the area-specific expression of ER are unknown. Here we use a species difference in the ER distribution between quail and chick and isochronic and isotopic quail to chick transplants to investigate mechanisms underlying the development of area-specific expression of ER. The entire neural tube rostral to the 6th somite (n = 2) or rostral to the otic capsules (n = 4) were transplanted at the second day of incubation (E2). In immunocytochemical stainings with the ER antibody H222Sp gamma, there is a defined cluster of intensely immunostained cells in the ventrolateral preoptic area of adult quails (the QERN). At the time of grafting, the entire brain primordium lacks ER. The QERN expresses ER as early as E13. The homologous area of the chicken brain does not differentiate cells that contain ER at any stage after ER are first detectable in the chicken brain at E11. In contrast, quail-chick chimeras develop the QERN phenotype in the ventrolateral preoptic area similar to quails. This implies that some signal which commits cells to the QERN phenotype (ER expression) is present in the quail brain primordium rostral to the otic capsules by embryonic day 2, and that this signal is unaffected by subsequent exposure to the global chick environment.