Migration of LHRH neurons born in the olfactory placode into the forebrain in the chick embryo

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(1)The epithelial cells of the olfactory placode of the chick embryoswere labeled with a fluorescent carbocyanine dye DiI at embryonic days 3.5-4.0. The DiI-labeled cells were first detected in the olfactory epithelium and olfactory nerve one day after the application of DiI.Two or four days after the dye application, the labeled cells sequentially appeared in the rostral medial forebrain, and in the septo-preoptic area. The distribution pattern of DiI-labeled cells closely resembled that of LHRH neurons. Double staining for DiI and LHRH demonstrated that DiI-labeled cells co-expressed LHRH.These results provide evidence for the actual migration of LHRH neurons from the olfactory placode to the septo-preoptic area during the development.(2)In the embryos with a incomplete placodectomy, a small number of LHRH-immunoreactive (ir) cells expressing NCAM-H were detected in the olfactory epithelial fragments and in the NCAM-H-positive olfactory nerve remnants which ceased to extend their axons to the forebrain. The lack of the central projection of the olfactory nerve caused stagnation of LHRH-ir cells, no LHRH-ir cells being found in the forebrain. Furthermore, the migrating LHRH-ir cells deviated from the poorly developed olfactory nerve and migrated into the NCAM-H-positive medial nasal branch of the ophthalmic nerve of the trigeminal nerve. These results suggest the importance of the structual support for the migration of LHRH neurons, and that the migration route and target of LHRH neurons are not exactly programd in the precursor cells of LHRH neurons in the olfactory placode. Since the migrating LHRL-ir cells were never independent of NCAM-H-positive neural elements in vivo and in vitro, the possible interaction between the migration of LHRH neurons and NCAM-H may be needed for successful LHRH neuronal migration.