Ontogeny of GnRH systems.

In all vertebrate species studied, the main central population of GnRH neurones, which produces the final messages regulating reproduction, originates outside the brain. Early during fetal life, they appear in the olfactory placode epithelium and then migrate toward the base of the telencephalon in close association with the nervus terminalis, penetrate the brain within the nervus terminalis roots, reach their final locations and eventually grow axons toward their targets. Only part of this process is documented in ruminants. In the sheep fetus, the olfactory placode develops between day 22 and day 26 of gestation, but the first GnRH-immunoreactive neurones have been detected only at day 35, associated with the extracerebral part of the nervus terminalis. During the next 30-40 days, the GnRH neuronal systems progressively invade the brain. In both sexes, most of the development, in terms of distribution and morphology of the neurones, appears to be completed by the middle of gestation (term being on day 145). On day 85 GnRH-immunoreactive neuronal systems of male and female fetuses have also been reported to be very similar to GnRH neuronal systems of adult females. Attention should now be focused on the earliest developmental steps.