LHRH neuronal migration: Heterotypic transplantation analysis of guidance cues

During embryonic development, the olfactory placode (OP) differentiates int o the olfactory epithelium (OE), Luteinizing hormone-releasing hormone (LHR H) neurons migrate out of the OE in close association with the olfactory ne rve (ON) to the telencephalon, LHRH neuronal migration and ON extension to the telencephalon may be independent events which are correlated but do not represent a causal relationship. However, we hypothesize that LHRH neurons are dependent on ON axons to migrate to the brain. To test this hypothesis , we ablated the right trigeminal placode and replaced it with an OP from a nother chick embryo. After several days' additional incubation, the embryos were fixed, sectioned, and immunostained with antibodies against LHRH or N -CAM. The ectopic OPs were well integrated into the host and developed into relatively normal appearing OEs. The ONs extended from the OE to several d ifferent sites: the lateral rectus of the eye, the ciliary ganglion, and th e trigeminal ganglion. In all cases, LHRH neurons were found in the OE and ON, regardless of where the ON terminated. When the ON extended to the trig eminal ganglion, LHRH neurons could clearly be seen entering the metencepha lon. Our results support the idea that LHRH neurons are dependent on the ON for guidance as they appear to follow the nerve even when it extends away from the brain. The cues which direct the ON and LHRH neurons to the telenc ephalon do not appear to be unique to this brain region. (C) 2000 John Wile y & Sons, Inc.