2 POPULATIONS OF LUTEINIZING-HORMONE-RELEASING HORMONE NEURONS IN THEFOREBRAIN OF THE RHESUS MACAQUE DURING EMBRYONIC-DEVELOPMENT

To investigate the possibility that a second luteinizing hormone-relea sing hormone (LHRH) population appears during development in primates, embryos and fetal brains of rhesus monkeys were immunostained with an tisera specific to different LHRH forms. Two LHRH cell populations wer e discernible by immunoreactivity to antisera LR-1 and GF-6. Because o ne LHRH cell type migrated out from the olfactory placode several days earlier than the other, they were referred to as ''early'' and ''late '' LHRH cells, respectively. Although late LHRH neurons were immunorea ctive to all anti-mammalian LHRH antisera tested, early LHRH neurons w ere only detected by antiserum GF-6. Early LHRH neurons (similar to 10 x 7 mu m) were smaller than late LHRH neurons (similar to 18 x 7 mu m ). Early LHRH neurons were first found around the olfactory placode, i n the nasal mesenchyme, and in the rostroventral forebrain on embryoni c day 30 (E30), whereas late LHRH neurons were first seen in the olfac tory pit on E32. Early LHRH cells were located throughout the basal fo rebrain on E32-E42, whereas late LHRH cells were found in the olfactor y pit and along the terminal nerve on E34-E36 and were not seen in the forebrain until E38. By E51-E62, late LHRH neurons reached into the b asal hypothalamus in a distribution resembling that in the older brain , while early LHRH neurons were found in the septum, preoptic region, stria terminalis, medial amygdala, claustrum, internal capsule, and gl obus pallidus. Based on the distribution pattern of immunopositive cel ls with antiserum LR-1, late LHRH cells are bona fide LHRH neurons tha t regulate the pituitary-gonadal axis. In contrast, the molecular form of early LHRH cells is unclear, although it is plausible that early L HRH cells may contain the molecule in which the C-terminal epitope of LHRH is modified or absent. It is concluded that in primates there is a second population of LHRH neurons that originates from the embryonic olfactory placode before the origin of mammalian LHRH-like neurons, a nd that these two populations of LHRH-immunopositive neurons have diff erent morphologic features and different final distributions in the br ain. (C) 1997 Wiley-Liss, Inc.