Supportive Role of Cellular Bridge of Neurons Expressing a Highly Polysialylated Form of NCAM (NCAM-H) at the Initial Stage of Migration of LHRH Neurons

Abstract

The early stage of cell migration from the olfactory placode to the forebrain was studied immunohistochemically in chick embryos to investigate the nature of early migrated cells and the role of these cells in the sebsequent migration of luteinizing hormone releasing hormone (LHRH) neurons. The initial cells migrating from the olfactory placode stained strongly positive for the highly polysialylated neural cell adhesion molecule (NCAM-H), but were negative for LHRH. These migrating NCAM-H immunoreactive cells were observed on embryonic day (ED) 2.5 as a bulge with a few cells from the base of the placode. By ED 3, they formed a wide cellular strand and developed into a cellular bridge between the olfactory placode and the ventro-rostral surface of the forebrain. These migrating cells are neurons because they stained positive for growth associated protein-43 (GAP-43), microtubule associated protein (MAP) 2, and MAP 5. Some of these cells seemed to migrate caudally along the ventro-lateral surface of the forebrain. LHRH-immunoreactive cells were not detected in the olfactory epithelium until after ED 3.5, which was one day after formation of the cellular bridge. Then, LHRH-immunoreactive cells appeared and began to cross the cellular bridge. The outgrowth of the olfactory nerve axon bundles from the olfactory epithelium was detected at around the same time when LHRH-immunoreactive cells first appeared. These olfactory nerve axons expressed NCAM-H, GAP-43, and MAP 5, as assessed by immunochemistry. After bundle formation, the olfactory nerve appeared to provide the migratory routes for LHRH neurons. These results suggest that the cellular bridge formed by the NCAM-H-immunoreactive neurons plays an important supportive role for LHRH neurons at the initial stage of their migration in chick embryos.