The perinatal ontogeny of estrogen receptor-immunoreactivity in the developing male and female rat hypothalamus.

Developmental expression of the estrogen receptor (ER) in rat hypothalamus was examined using immunohistochemistry. In the medial preoptic nucleus and ventromedial nucleus ER-immunoreactivity was detected as early as E17, whereas ER protein expression in the periventricular preoptic nucleus and arcuate nucleus was delayed until E19. These results show that following a region specific onset of the ER protein expression sex differences in ER levels are already detectable during the perinatal period.