The ontogeny of gonadotropin-releasing hormone neurons in the chick

The origin of gonadotropin-releasing hormone (GnRH) neurons in the olfactory placode and their subsequent migration into the central nervous system (CNS) has been described in several species. The current study was undertaken to delineate the time course and route of GnRH neuronal migration in the chick using antisera generated against both the mammalian form of GnRH (LR1) and a specific chicken GnRH (cGnRH I). Chicks aged E4.5 through day of hatching were processed for GnRH immunocytochemistry. Under the incubation conditions employed in this study, GnRH neurons first appeared on E4.5 in the olfactory epithelium. GnRH neurons exit from the olfactory epithelium and follow the extracranial course of the olfactory nerve to the anterior pole of the developing forebrain. Within the nerve, cells are in close apposition to each other. By E5, cells had reached the surface of the telencephalon. Upon entering the CNS, the GnRH neurons dispersed within the neuropil and continued their migration as individuals. As development progressed, GnRH neurons migrated dorsally and caudally within the brain and reached their adult positions by E12. GnRH axons were detected in the vicinity of GnRH cell bodies with the cGnRH I antiserum as early as E4.5; however, GnRH axons did not reach the median eminence until E14. These results extend earlier reports in mammal and chick on the extracranial origin of GnRH neurons and their path of entry into and dispersion within the CNS.