Neuroanatomical organization of GnRH neuronal systems in the lizard (Podarcis s. sicula) brain during development

The ontogenesis of the GnRH neuronal systems was studied in the brain of the lizard, Podarcis s. sicula, by immunohistochemistry. The first GnRH neurons were seen in the mesencephalon on the 45th day of incubation. One week later GnRH-ir neurons appeared in the infundibulum as well. These neurons never appeared to be contiguous with midbrain GnRH neurons. Thus, the adult pattern of distribution of GnRH neurons was reached before hatching, which occurred on the 66th day of incubation at a temperature of 28 +/- 2 degrees C. Although mesencephalic and infundibular GnRH neurons and their fiber projections appeared to be distributed in anatomically distinct brain areas, both systems showed a positive reaction to chicken-I GnRH (cGnRH-I), chicken-II GnRH (cGnRH-II) and salmon GnRH (sGnRH). From the time of hatching, GnRH-ir fibers in the mesencephalon appeared to be reaching the optic tectum, tegmentum, cerebellum and rostral dorsal rhombencephalon, whereas GnRH fibers in the infundibulum were projecting to the caudal basal telencephalon, median eminence and rostral basal rhombencephalon. In 60-day-old juvenile lizards, the central area of telencephalon contained neurons reacting only with anti-cGnRH-I and anti-sGnRH. Such neurons were absent in the adult. Neither GnRH cells nor fibers were observed in the nasal area, terminal nerve and olfactory bulbs at any stage of development and in the adult. We hypothesize that the two GnRH neuronal systems have separate embryonic origins.