LUTEINIZING-HORMONE-RELEASING HORMONE (LHRH) AND NEURAL CELL-ADHESIONMOLECULE (NCAM)-IMMUNOREACTIVITY IN DEVELOPMENT OF THE FOREBRAIN AND REPRODUCTIVE-SYSTEM

The origin and migration of LNRH neurons (detected by immunocytochemic al procedures) is preceded by a migration of NCAM-immunoreactive cells from the olfactory epithelium, and the formation of an NCAM-immunorea ctive cellular aggregate between the olfactory epithelium and the deve loping forebrain. The central processes of the olfactory nerves grow i nto the lateral parts of this aggregate and the terminal and vomeronas al nerves grow into the medial parts. No nerve fibers of the main or a ccessory 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 olfac tory bulbs. The LHRH neurons do not migrate through the olfactory bulb s. After formation of the olfactory bulbs, the cellular aggregate disa ppears and is replaced by the olfactory nerve layer of the olfactory b ulb. The NCAM and LHRH-immunoreactive cells on the medial side appear to the retained in the ganglion terminale of the terminal nerve. The f ate of the NCAM-immunoreactive cells that formed the aggregate could n ot be determined by the methods used in these studies. The early-appea ring NCAM-immunoreactive cells may function to separate and direct axo ns of the olfactory, vomeronasal and terminal nerves (and the LHRH neu rons) to their respective targets in the forebrain. The development an d migration of neurons from both the lateral and medial parts of the o lfactory placode appears to be essential for the normal development of the forebrain and reproductive system. The lateral and medial parts o f the olfactory placode may develop independently in response to speci fic cues. In support of this notion, an individual with a failure of d evelopment of the lateral part of the olfactory placode may show an ab sence of the olfactory bulbs and still demonstrate LHRH-immunoreactivt y in the brain and a normally developed reproductive tract. Alternativ ely, a failure of the medial olfactory placode, while it may result in an absence of LHRH in the brain and a concomitant hypogonadism, does not necessarily undermine the development of the olfactory bulbs.