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.