MIGRATION OF GNRH-IMMUNOREACTIVE NEURONS FROM THE OLFACTORY PLACODE TO THE BRAIN - A STUDY USING AVIAN EMBRYONIC CHIMERAS

Previous studies suggest that gonadotropin-releasing hormone (GnRH) ne urons appear in the olfactory placode and subsequently so migrate into the brain during embryonic development, The aim of the present study was to obtain direct evidence for migration of GnRH neurons from the o lfactory placode into the brain, Olfactory placodes from quail embryos were transplanted isotopically and isochronically, to replace the uni laterally ablated olfactory placodes of chick embryos. The chimeric em bryos were allowed to develop for several days until they reached the embryonic stages when GnRH neurons are seen in the brain in normal emb ryos, Quail olfactory epithelia were formed in the host chick embryos. Quail olfactory nerves were also formed and reached the olfactory bul b or primordial olfactory bulb, GnRH-immunoreactive cells of quail ori gin revealed by a triple staining method were observed in the quail ol factory epithelium, quail olfactory nerve, chick olfactory bulb, and c hick septo-preoptic area, These results indicate that GnRH neurons ori ginate in the olfactory placode and migrate into the telencephalon inc luding the septo-preoptic area. A migratory route of GnRH neurons was well documented by the use of a quail neuron-specific antibody, QN. Th e migratory route in the brain is discussed with special reference to the terminal nerve, A GnRH-immunoreactive neuronal group of chick orig in appeared in the diencephalon of chimeric embryos, These diencephali c neurons may be of non-placodal origin. FMRFamide-immunoreactive neur ons of quail origin were also found in the quail olfactory nerve and t he host olfactory bulb, suggesting that FMRFamide neurons also origina te in the olfactory placode and migrate into the brain.