Mj. Burek et al., ONTOGENY OF SEX-DIFFERENCES AMONG NEWLY-GENERATED NEURONS OF THE JUVENILE AVIAN BRAIN, Developmental brain research, 78(1), 1994, pp. 57-64
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.