COMIGRATION OF TYROSINE HYDROXYLASE-RELEASING AND GONADOTROPIN-RELEASING HORMONE-IMMUNOREACTIVE NEURONS IN THE NASAL AREA OF HUMAN EMBRYOS

Tyrosine hydroxylase (TH) immunoreactive (IR) central catecholaminergi c neurons have been observed in human CNS from 4.5 gestational weeks ( g.w.) on [Verney, C., Zecevic, N. and Puelles, L., Eur. J. Neurosci, S uppl. 8 (1995) 7044; Zecevic, N. and Verney, C., J. Comp. Neurol., 351 (1995) 509-535]. We describe here a discrete TH-IR cell population lo calized in the rostral nasal region during embryonic development. Tyro sine hydroxylase-IR cells spread from the olfactory placode towards th e basal and medial telencephalon. They follow the same migration path as the gonadotropin-releasing hormone (GnRH)-IR hypothalamic neurons. Tyrosine hydroxylase-IR neurons are first detected at 4.5 g.w., while GnRH-IR cells are visualized later at 5.5 g.w. Double immunocytochemic al labeling reveals the presence of three neuronal populations comigra ting along the developing vomeronasal-nervus terminalis complex. These populations express either one or both TH and GnRH phenotypes dependi ng on their position in the migration route. At 6 g.w., most of the ne urons express TH immunoreactivity as they leave the vomeronasal organ whereas most of the GnRH-IR neurons are detected closer to the CNS and in the CNS itself. These results emphasize the early phenotypic heter ogeneity of the different migrating neuronal populations generated in the olfactory placode in humans. At later stages, very few TH-IR neuro ns are detected in the anterior forebrain suggesting a transient expre ssion of TH immunoreactivity within these neuronal populations.