Development of immunoreactive lhrh neurons in the fetal rat hypothalamus

The ontogenesis of immunoreactive (ir) LHRH neurons was investigated in rats applying the double-bridge PAP method of Vacca et al. (J. Histochem. Cytochem. 28, 297-307, 1980). Ir LHRH was first evident in the cell bodies confined in the ventromedial surface of the anterior part of the forebrain vesicles on day 16.5 of gestation. Only one or two cells, if any, were found in the brains examined; the cells are oval showing ir brown granules within the perikarya and also within the processes extended from both poles. On day 17.5 of gestation, a few ir cells appeared in the olfactory cortex, medial septum, medial preoptic area, diagonal band of Broca, and ventro-lateral surface of the anterior hypothalamus, and, with development, their numbers increased gradually. During the prenatal period, no ir cell exists in the arcuate nucleus or in the medial-basal hypothalamus. Beaded ir fibers appeared in the organum vasculosum of the lamina terminalis (OVLT) and in the external layer of the median eminence on 18.5 and 19.5 days of gestation, respectively. The present findings ascertain that the hypothalamic regulation of hypophysial gonadotrophic function exists during fetal period in rats.