Straying phenomenon of migrating LHRH neurons and highly polysialylated NCAM in the chick embryo.

The present study on unilaterally incompletely placodectomized chick embryos revealed that arrival of migrating LHRH neurons into the forebrain area is dependent on the presence of a central projection of the olfactory nerve. When a fragment of the lateral and medial olfactory epithelium was spared the damage, a small number of LHRH-ir cells were found to migrate into the forebrain along a thin NCAM-H-expressing fiber bundle of the olfactory nerve. In embryos with large lesions which destroyed the lateral olfactory epithelium, the poorly developed NCAM-H-positive olfactory nerve fibers were arrested where they meet the NCAM-positive medial nasal branch of the ophthalmic nerve and frequently fused with this branch fiber bundle. In these embryos, no LHRH-ir cells were detected in the forebrain area. They were deviated from their regular migratory course to the NCAM-H-positive medial nasal branch fiber bundle. This straying phenomenon indicates that some structural support is apparently needed for the migration of LHRH-ir cells, but their migratory route is not completely programmed in their precursor cells in the olfactory placode. In cultures of placodal tissues, the coexistence of migrating LHRH-ir cells with NCAM-H-expressing neural elements was always confirmed. It is suggested therefore that not only the structural support for the migration but also the interaction between LHRH-ir cells and NCAM-H-expressing neural elements is a prerequisite for successful LHRH neuronal migration.