Localized regions of increased cellular proliferation within the ventricular zone (VZ) of juvenile male songbirds may contain progenitor cells that give rise to song-control neurons and, thereby, contribute to the construction of brain areas important for song learning. The purpose of this study was to examine levels of cell division throughout the telencephalic VZ of juvenile birds. A single pulse of [(3)H]thymidine was administered to 30-day male and female zebra finches, and the birds were killed 2 hours later. The VZ was divided into segments throughout the entire anterior-posterior and dorsal-ventral neuraxes, and levels of thymidine labeling were measured within each subdivision. By subdividing the VZ into segments, we were able to construct a "map" of proliferation throughout the telencephalic VZ, thereby allowing us to compare levels of mitotic activity within corresponding locations of the VZ between males and females. Our map revealed two major findings: (1) proliferation in both juvenile males and females was spatially differentiated throughout the VZ, suggesting that mitotic activity is differentially regulated across the neuraxis; (2) sex differences in proliferation were present in 30-day-old birds, but were highly restricted. The most robust sexual dimorphism occurred within the ventral aspect of the VZ at rostral levels of the song-control nucleus Area X, with males demonstrating an increased number of dividing cells compared with females. This result raises the possibility that Area X neurons in males are derived from committed progenitors within the adjacent VZ in close proximity to this nucleus.
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