The olfactory origin of luteinizing hormone-releasing hormone (LHRH) neurons. A new era in reproduction physiology.

Abstract

This paper reviews those studies which conceived the concept that the brain LHRH-synthesizing neurons originate in the nasal placode. LHRH isolated from mammalian hypothalamus in 1971 was first shown immunohistochemically two years later in the hypothalamic neurons which project processes to the median eminence, to release it into the portal capillaries in the guinea pig. At an early stage of development, the LHRH cells were found in the nasal placode but not in the hypothalamus as shown in in vivo and in vitro developmental studies. The cells arising in the brain were delayed. This discrepancy was solved in 1989-1990 by findings that the cells derived in the placode at an early stage left the site and migrated to the forebrain vesicles along the placode-derived terminal and vomeronasal nerve fibers, both of which were found to express immunoreactive cell adhesion molecules. The neurons, after reaching the surface of the forebrain vesicles, entered into the brain by the guidance of the cell adhesion molecule-positive fibers, and came to be distributed not only in the hypothalamus but also in the telencephalon cortex, midbrain, limbic brain, and main and accessory olfactory bulbs. The attention to these heterogeneties led to discussion of the possible neurobiological significance of this peculiar peripheral neurogenesis from an evolutionary viewpoint.