Luteinizing hormone-releasing hormone (LHRH) and neural cell adhesion molecule (NCAM)-immunoreactivity in development of the forebrain and reproductive system.

The origin and migration of LHRH neurons (detected by immunocytochemical procedures) is preceded by a migration of NCAM-immunoreactive cells from the olfactory epithelium, and the formation of an NCAM-immunoreactive cellular aggregate between the olfactory epithelium and the developing forebrain. The central processes of the olfactory nerves grow into the lateral parts of this aggregate and the terminal and vomeronasal nerves grow into the medial parts. No nerve fibers of the main or accessory olfactory systems grow directly into the forebrain. The LHRH neurons, following the course of the terminal and vomeronasal nerves, traverse the medial edge of the NCAM-immunoreactive cellular aggregate before they enter the medial forebrain caudal to the developing olfactory bulbs. The LHRH neurons do not migrate through the olfactory bulbs. After formation of the olfactory bulbs, the cellular aggregate disappears and is replaced by the olfactory nerve layer of the olfactory bulb. The NCAM and LHRH-immunoreactive cells on the medial side appear to the retained in the ganglion terminale of the terminal nerve. The fate of the NCAM-immunoreactive cells that formed the aggregate could not be determined by the methods used in these studies. The early-appearing NCAM-immunoreactive cells may function to separate and direct axons of the olfactory, vomeronasal and terminal nerves (and the LHRH neurons) to their respective targets in the forebrain. The development and migration of neurons from both the lateral and medial parts of the olfactory placode appears to be essential for the normal development of the forebrain and reproductive system.(ABSTRACT TRUNCATED AT 250 WORDS)