Mj. Burek et al., SEXUALLY DIMORPHIC NEURON ADDITION TO AN AVIAN SONG-CONTROL REGION ISNOT ACCOUNTED FOR BY SEX-DIFFERENCES IN CELL-DEATH, Journal of neurobiology, 33(1), 1997, pp. 61-71
Only male zebra finches sing, and several brain regions implicated in
song behavior exhibit marked sex differences in neuron number. In one
region, the high vocal center (HVC), this dimorphism develops because
the incorporation of new neurons is greater in males than in females d
uring the first several weeks after hatching. Although estrogen (E-2)
exposure stimulates neuron addition in females, it is not known where
(E-2) acts, or to what extent sexual differentiation influences the pr
oduction, specification, or survival of HVC neurons. In the present st
udy we first reassessed sex and (E-2)-induced differences in cell dege
neration within the HVC using the TUNEL technique to identify cells un
dergoing DNA fragmentation indicative of apoptosis. HVC neuron number,
as well as the density and number of TUNEL-labeled and pyknotic cells
within the HVC were measured in normal 20- and 30-day-old males and f
emales, and in 30-day-old females implanted with E-2 on posthatch day
18. Although HVC neuron number was greater in males than in females, a
nd was masculinized in E-2 females, no group differences were evident
in the absolute number of dying cells. These results indicate that sex
differences in cell survival within the HVC do not entirely account f
or sexually dimorphic neuron addition to this region. Rather, sexual d
ifferentiation acts on some HVC neurons before they complete their mig
ration and/or early differentiation. Although the migratory route of H
VC neurons is not known, a large number of E-2 receptor-containing cel
ls (ER cells) reside just ventromedial to the HVC and adjacent to the
proliferative ventricular zone. Next, we investigated whether these ER
cells contribute to early-arising sex differences in HVC neuron addit
ion. By combining [H-3]thymidine autoradiography with immunocytochemis
try for ERs, we first established that ER-expressing cells are not gen
erated during posthatch sexually dimorphic HVC neuron addition, and th
us are not young HVC neurons that transiently express ERs during their
migration. Furthermore, in 25-day-old birds we found no sex differenc
e in the density of pyknotic cells among this group of ER cells, sugge
sting that these cells do not promote the differential survival of HVC
neuronal precursors migrating through this region. Rather, ER cells o
r other cell populations may establish sex differences in HVC neuron n
umber by creating dimorphisms in cellular specification. (C) 1997 John
Wiley & Sons, Inc.