SEX DIFFERENCE AMONG NONNEURONAL CELLS PRECEDES SEXUALLY DIMORPHIC NEURON GROWTH AND SURVIVAL IN AN AVIAN SONG CONTROL NUCLEUS

In zebra finches only males sing, and several song control nuclei cont ain more neurons in adult males than in females. In the robust nucleus of the archistriatum (RA), this ses difference in neuron number arise s because neuron survival is greater in young males than in females. T he events initiating this set difference in neuron survival are not kn own, but in earlier studies we observed that during sexual differentia tion the proliferation and/or survival of RA cells exhibiting glial mo rphology is greater in males than in females. Because glia and glia-de rived molecules are known to exert trophic effects on developing neuro ns, we wanted to determine when the seu difference in RA glia develops relative to the sexually dimorphic growth and survival of RA neurons. Male and female zebra finches were injected twice daily with (3)[H]th ymidine for 2 days beginning either on day 15 or 27. Two days later (d ay 18 or 30) sections through the RA were processed for autoradiograph y. Virtually all of the (3)[H]thymidine labeled cells within the RA ex hibited morphological features characteristic of glia and were not imm unoreactive for the neuron-specific antigen, Hu. The number of these ( 3)[H]thymidine labeled cells nas measured, as were the number and soma size of RA neurons. Sex differences in RA neuron number and soma size were not evident at day 18, but emerged by day 30. However, at both a ges the density of (3)[H]thymidine labeled RA cells and their total nu mber/RA neuron were significantly greater in males than in females. No such sexual dimorphism in the density of (3)[H]thymidine labeled cell s was evident in the archistriatum lateral to the RA. or within the RA of adult birds. These data indicate that sexually dimorphic gliogenes is is an early event in the sexual differentiation of the RA, precedin g ses differences in RA neuron growth and survival. The possibility th at glia (or glia-derived substances) may contribute to the neurotrophi c effects of masculinization within the RA is discussed. (C) 1996 John Wiley & Sons, Inc.