Abstract
Song control nuclei have distinct sexual differences and thus are an ideal model to address how brain areas are sexually differentiated. Through a combination of histological analysis and electrical lesions, we first identified the ventricle site for HVC progenitor cells. We then found that there were significant sex differences in the cellular proliferation activity in the ventricular zone of the HVC, the number of migrating cells along the radial cells (positive immunoreactions to vimentin) and differentiation towards neurons. Through co-culturing of male and female slices containing the developing HVC in the same well, we found that the male slices could produce diffusible substances to masculinize the female HVC. By adding estrogen, an estrogen antagonist, brain-derived neurotrophic factor (BDNF) or its antibody into the culture medium, separately or in combination, we found that these diffusible substances may include estrogen and BDNF. Finally, we found that 1) estrogen-induced BDNF upregulation could be detected 48 hr after estrogen treatment and could not be blocked by a vascular endothelial growth factor (VEGF) receptor inhibitor and 2) the amount of VEGF mRNA expressed in the developing HVC and its adjacent area did not display any significant sex differences, as did the distribution of VEGF and laminin-expressing endothelial cells in the developing HVC. Because these findings are largely different from previous reports on the adult female HVC, it is suggested that our estrogen-induced BDNF up-regulation and the resultant sexual differentiation might not be mediated by VEGF and endothelial cells, but instead, may result from the direct effects of estrogen on BDNF.
Highlights
Dimorphic brain areas have been documented in a variety of vertebrates, of which song control nuclei in oscine species are the most obvious [1]
The developing male high vocal center (HVC) was obviously larger than the female HVC, they were both located in a similar brain position, and some labeled cells were observed in both sexes (Fig. 1A–C)
We showed that coculturing of male and female brain tissues could result in a significant ‘‘masculinizing’’ of female proliferation, and that the sex differences in cellular proliferation and differentiation were estrogen sensitive, mediated by BDNF
Summary
Dimorphic brain areas have been documented in a variety of vertebrates, of which song control nuclei in oscine species are the most obvious [1]. Because one to three weeks are needed for proliferated cells in the ventricular zone to migrate into the song control nuclei [10,11,12], sexually dimorphic proliferation and migration en route to their final destination or differentiation in their targets might affect sex differences following the net addition of neurons [13,14,15,16,17]. To our knowledge, it has not been established how sexual differences during the development of a song control nucleus, high vocal center (HVC), are affected by the aforementioned activities in juvenile birds, and the underlying mechanisms have not been established
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