GNRH NEURONS IN THE FETAL LAMB HYPOTHALAMUS ARE SIMILAR IN MALES AND FEMALES

The critical period for sexual differentiation of the brain of the dev eloping lamb occurs before birth. Exposure to steroids during this sen sitive period in midgestation alters the control of GnRH secretion aft er birth. The present study examined neurons immunolabeled for GnRH in male and female lambs during the critical period for sexual different iation to determine if these neurons are sexually dimorphic. Neuron nu mber, anatomical distribution, and the number of neuronal processes of GnRH-containing neurons from midgestation (85 days) male and female f etuses were compared (n = 5 each). Immunoreactive cells were labelled using LR-1 as the primary antiserum, followed by a biotinylated second ary antibody and the formation of an avidin-biotinhorseradish peroxida se (HRP) complex. The HRP was visualized histochemically using diamino benzidine as the chromagen. GnRH neurons were localized in 60-mu-m cor onal sections from the area of the diagonal band of Broca to the rostr al mammillary bodies. The neurons were classified as unipolar, bipolar , or multipolar, according to the number of neuronal processes. The nu mber of neuronal processes, the pattern of distribution, and the estim ated total number of GnRH neurons in male and female fetuses was simil ar (p > 0.05). Furthermore, these parameters were equivalent to those reported for the adult female. These data indicate the GnRH neurosecre tory system matures prior to midgestation in the sheep, and in a simil ar manner in males and females.