THE MIGRATION OF LUTEINIZING-HORMONE-RELEASING HORMONE NEURONS IN THEDEVELOPING RAT IS ASSOCIATED WITH A TRANSIENT, CAUDAL PROJECTION OF THE VOMERONASAL NERVE

Luteinizing hormone-releasing hormone (LHRH) neurons originate in the olfactory placode and vomeronasal organ and migrate to the brain from embryonic day 14 (E14) in the rat. We investigated the development of the vomeronasal nerve and its role as a guide for migrating LHRH neuro ns. Using fluorescent, lipophilic dye tracing methods, we observed axo ns that emerge from the vomeronasal organ and cross the nasal septum a s several large fascicles. At E14-15, these fascicles converge as they enter the region of the cribriform plate and subsequently disperse, p rojecting dorsally and caudally across the olfactory bulb and rostral forebrain. At E16, the dorsal branch of the vomeronasal nerve forms a more tightly fasciculated projection; the caudal fibers remain dispers ed, extending along the medial forebrain. The number of caudally direc ted axons decreases during development, leaving four or five present a t postnatal day 4 (P4). Immunohistochemical studies indicate that the vomeronasal nerve can be divided into four spatially distinct subpopul ations of fibers. One subset! composed of caudal fibers that terminate in the lamina terminalis, selectively expresses TAG-1, a transient ax onal surface glycoprotein and PSA-N-CAM, a highly polysialated form of neural cell adhesion molecule. The extension and subsequent retractio n of this branch of the vomeronasal nerve corresponds spatially and te mporally with the migration of LHRH neurons from the nasal cavity to t he brain. Our studies show that between E14 and E18, LHRH neurons migr ate in contact with the TAG-1(+), PSA-N-CAM(+) caudal branch of the vo meronasal nerve.