ONTOGENY OF SEX-DIFFERENCES AMONG NEWLY-GENERATED NEURONS OF THE JUVENILE AVIAN BRAIN

In zebra finches, only males sing and brain regions controlling song e xhibit sex differences in neuron number that stem from actions of estr ogen during a critical developmental period. In certain song nuclei, t hese dimorphisms emerge long after neurogenesis and migration are comp lete, and estrogen promotes masculinization by preventing the death of well differentiated neurons. But in another region, the higher vocal center (HVC), cellular mechanisms underlying sex differences in neuron number are not so well understood. In the HVC, neurogenesis continues throughout the post-hatch period of sexual differentiation, and sex d ifferences arise during this time because neuron number increases in m ales but not females. We used [H-3]thymidine autoradiography to establ ish when sex differences in neuron number first develop among a small group of HVC neuronal cohorts. We report that HVC neurons labeled by [ H-3]thymidine on days 15 and 16 after hatching are sexually dimorphic in number within 10 days of their birth, even before all cells in this cohort complete their migration and/or differentiation. This suggests that the cellular mechanisms contributing to sex differences in neuro n number in the HVC may differ from those in other sexually dimorphic neural regions of the vertebrate nervous system. In addition, we found that although many thymidine-labeled HVC neurons ultimately project t o the robust nucleus of the archistriatum (RA), a sexually dimorphic t arget, sex differences in their number develop before this efferent pr ojection is established. These results have important implications reg arding the site(s) of hormone action, since they suggest that sexual d ifferentiation acts on certain HVC neurons before they establish their efferent projections, and perhaps even before they arrive within the HVC.