Stereological estimates of postnatal structural differentiation in a sexually dimorphic hypothalamic nucleus involved in vocal control

Display of a specific, courtship vocalization and other masculine functions in the adult gerbil, is associated with a sexually differentiated hypothalamic nucleus, the Sexually Dimorphic Area pars compacta (SDApc). Total SDApc volume and vocal function differentiate neonatally. Since total volume is a rudimentary measure of brain nucleus differentiation, we examined the more detailed cytoarchitectural parameters behind SDApc development in gerbils, cell number, density per nucleus and individual nuclear (soma) volume. Unbiased stereological estimates were made on thick (20-40 microm) brain sections from postnatal days 1 (D1), 3 (133), 6 (DO, 16 (D16), 40 (D40) and 60 (D60) animals. Sex differences in stereological parameters were not apparent on D1 but from D3, SDApc growth patterns widely differed between the sexes. Significant differences in (i) cell number, and (ii) nuclear volume were found at D3 and D60, respectively. In males, cell number increased between D1-D6 but subsequently decreased from the D6 value by approximately 80% to reach the value of D16 which remained constant. Cell density paralleled the decrease in cell number between D6-D16 in males, whereas a progressive expansion in nuclear volume occurred between D1-D40. Male total SDApc volume enlarged between D1-D3 and D40-D60. Conversely in females, cell number and density declined between D1-D3 and D1-D40, respectively, and then remained at these low values. Cell volume, however, increased up to D40 and then significantly decreased. The resulting change to female total SDApc volume was a reduction immediately after birth, D1-D3, to a constant low value. We conclude that first, the association between various stereological measures and total SDApc volume was minimal, suggesting independent mechanisms of sexual differentiation for each cytoarchitectonic parameter. Second, the neonatal peak in SDApc cell number indicates cell migration taking place contemporaneously with cell death in males. Third, the effect of changes in cytoarchitectural components between D6-D16 and D40-D60 in males is probably due to SDApc dendritic volume expansion, suggesting that the male SDApc retains plasticity until at least puberty. Fourth, the decrease in the number of cells in females early in neonatal life, suggests programmed cell death.