To obtain insight into the development of the heterogeneous intracerebral populations of luteinizing hormone-releasing hormone (LHRH) neurons, their spatiotemporal appearance was examined at different stages in normal rat embryos, in nasal epithelial explants in vitro, and in intrauterine nasal-operated embryos. Following the appearance of nerve cell adhesion molecule in the nasal placode at embryonic day (E) 12.5, LHRH neurons, generated in the nasal placode at E13.5, penetrated the forebrain vesicle (FV) by E14.5-15.5. After E16.5, as the FV elongated to form the olfactory bulb, the migrating neurons traversed posteriorly through the interhemispheric space to penetrate the septopreoptic (S-P) area. By E18.5, LHRH neurons were detected in the preoptic-diagonal band (P-D) area as well as in the S-P region, along with some scattered extrahypothalamic LHRH neurons. To determine the source of these neurons, we separately cultured dissected parts of E12.5 nasal pit epithelium. Neuronal generation was predominantly from the medial wall epithelium (NAP), but some LHRH neurons originated in the roof epithelium. Cocultures of the NAP (E12.5) with the FV, median eminence-arcuate complex, Rathke's pouch, mesencephalon, or medulla oblongata from E14.5 embryos revealed the ability of LHRH cells to penetrate all of these tissues. Uni- or bilateral nasal destruction was conducted at E16.5 or E15.5, respectively, and examined at E18.5 and E21.5. In the operated embryos, most LHRH neurons were present in the P-D system and some in the S-P area. This finding suggests that the neurons generated before E15.5 are primarily predisposed to form the P-D system, whereas those derived afterward form the S-P system.

Using the olfactory placode of 12.5-and 14.5-day-old (E12.5, E14.5) rat embryos, we examined the migration of LHRH neurons by in vivo intraventricular transplantation and in vitro organotypic culture systems. In the transplantation, the olfactory placode of E12.5 embryos was co-transplanted with the cerebral cortex and also with medial basal hypothalamus (MBH). LHRH neurons that had migrated into the co-trans

This study was designed to gain insight into the underlying mechanisms of the centripetal migration of developing LHRH neurons. The medial wall of the nasal pit (NAP) of 12.5-day-old rat embryos (E12.5) was cultured singly or together with the E12.5 medial-basal wall of the forebrain vesicles (mFV) or with the E14.5 median eminence-arcuate complex (ME-Arc). Further, the NAP was cultured with the mFV and ME-Arc or with the mFV and nasal mesenchyme (NM), which lay between the mFV and the NAP, of E12.5 embryos (triple culture). The NAP gave rise first to fibers labeled with anti-neural cell adhesion molecules (NCAM) and then to LHRH neurons. In co-cultures, NAP-and brainderived NCAM fibers connected the NAP and brain cultures, and frequently linked with each other to form knots at the periphery. LHRH neurons migrating along the NAP-derived fibers directly or indirectly entered brain cultures. In the latter case, the cells strayed along the way from the NAP-derived fibers to the brainderived fibers at the knots and migrated retrogradely along the latter fibers to enter into the brain tissues; this occurred most frequently into the E14.5 ME-Arc. In triple cultures, abundant NCAM fibers emerging from the NAP were only found when the NM lay between the NAP and mFV; the fibers converged further to the mFV. These findings help elucidate the mechanisms underlying the centripetal LHRH cell migration from the NAP to the hypothalamus.

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