SEXUALLY DIMORPHIC NEURON ADDITION TO AN AVIAN SONG-CONTROL REGION ISNOT ACCOUNTED FOR BY SEX-DIFFERENCES IN CELL-DEATH

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.