Removal of olfactory placode prevents the development of LHRH neurons in the forebrain of the chick embryo: possible interaction between migrating LHRH neurons and highly polysialylated form of neural cell adhesion molecule (NCAM-H).

A unilateral olfactory placodectomy prevented the development of LHRH- and NCAM-H-expressing neurons as well as NCAM-H expressing olfactory nerve on the operated side, whereas the development of LHRH- and NCAM-H-expressing neurons and olfactory nerve was not disturbed. In the embryos with an incomplete olfactory 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 remants 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. These results clearly suggest the importance of the structural support for the migration of LHRH-ir cells. Furthermore, the migrating LHRH-ir cells deviated from the poorly developed olfactory nerve and migrated not only to the central direction but also to the peripheral direction of the NCAM-H-positive medial nasal branch of the ophthalmic nerve of the trigeminal nerve. This suggests that the migration route of the LHRH neurons is not programmed in detail in the precursor cells of LHRH neurons in the olfactory placode. Since the migrating LHRH-ir cells were never independent of NCAM-H-positive neural elements in the operated embryos, it is suggested that not only the structural support for the migration but also the possible interaction between the migration of LHRH-ir cells and NCAM-H and/or other unknown factors may be needed for successful LHRH neuronal migration.