Gonadotropin-releasing hormone containing neurons and olfactory fibers during development: from lamprey to mammals.

Gonadotropin releasing-hormone (GnRH) regulates the hypothalamo-pituitary-gonadal axis in all vertebrates. The vast majority of GnRH neurons are thought to be derived from progenitor cells in medial olfactory placodes. Several antibodies and lectins that recognize cell surface carbohydrates have been useful for delineating the migratory pathway from the olfactory placodes and vomeronasal organ, through the nasal compartment, and across the cribriform plate into the brain. In rats, alpha-galactosyl-linked glycoconjugates (immunoreactive with the CC2 monoclonal antibody) are expressed on fibers along the GnRH migration pathway and approximately 10% of the GnRH neuronal population. In lamprey, the alpha-galactosyl binding lectin, Grifonia simplicifolia-I (GS-1), identifies cells and fibers of the developing olfactory system. In contrast to the CC2 immunoreactive GnRH neurons in rats, the GS-1 does not label a subpopulation of presumptive GnRH neurons in lamprey. Results from these and other experiments suggest that GnRH neurons in developing lamprey do not originate within the olfactory placode, but rather within proliferative zones of the diencephalon. However, the overlap of olfactory- and GnRH-containing fibers from prolarval stages to metamorphosis, suggest that olfactory stimuli may play a major role in the regulation of GnRH secretion in lamprey throughout life. By contrast, olfactory fibers are directly relevant to the migration of GnRH neurons from the olfactory placodes in mammalian species. Primary interactions between olfactory fibers and GnRH neurons are likely transient in mammals, and so in later life olfactory modulation of GnRH secretion is likely to be indirect.