Journal of Neuroscience, Vol 9, 3176-3187, Copyright © 1989 by Society for Neuroscience
Genesis and death of vocal control neurons during sexual differentiation in the zebra finch
JR Kirn and TJ DeVoogd
Department of Psychology, Cornell University, Ithaca, New York 14853.
Several song-related regions in the adult zebra finch brain have
substantially more neurons in males than in females. Such differences
appear to arise from sex differences in circulating steroids during early
posthatch life. In the present study, developmental mechanisms involved in
the production of sex differences are explored by examinations of the
normal time course of posthatch neurogenesis and cell death in vocal
control circuits. As a first step toward determining whether rates of
neuron production may be different in males and females, tritiated
thymidine, a marker of cell division, was administered to zebra finches at
various times during the first month after hatching. Birds were sacrificed
at 60 d. The number of cells formed after hatching and present at 60 d was
then evaluated in 3 vocal control regions--HVc (hyperstriatum ventralis
pars caudalis) and its 2 principal targets, RA (robust nucleus of the
archistriatum) and Area X. Cell death was quantified by counts of normal
and pyknotic, degenerating cells made in these nuclei in additional,
untreated birds of both sexes at 5 d intervals from 5 to 45 d of age. The
combined results of these experiments suggest that differential cell death
is a major factor in the development of sex differences in the song control
system and provide the first direct evidence for sex differences in cell
death in the developing telencephalon. Although developmental time tables
differ among the 3 brain areas examined, at specific ages significantly
higher numbers of pyknotic cells were observed in HVc, RA, and presumptive
Area X in females compared to males. Peak levels of cell death in RA occur
4-6 weeks after hatching. This is about 3 weeks after the onset of sex
differences in steroid levels that, in turn, lead to differential
organization of song system nuclei. This pattern of results suggests that
designation for death and actual cell loss are temporally dissociated in
this system. Neuron proliferation for HVc and Area X, but not RA, continues
throughout the first 30 d after hatching, and a significant sex difference
was found in the number of cells present in HVc at 60 d that were formed
after hatching. Comparisons of the timing of cell death and cell
incorporation suggest that this difference may be best accounted for by
differential survival of neurons formed after hatching rather than
differential rates of neuron production. Neither differential neurogenesis
nor differential neuron death can fully account for the apparent extreme
sexual dimorphism in the number of neurons in Area X.(ABSTRACT TRUNCATED AT
400 WORDS)
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