Ontogeny of galanin-immunoreactive neurons in the central nervous system of the chicken.

1. The ontogenic development and differentiation of the galanin-immunoreactive (GA-IR) neuron systems in the avian brain was studied by the aid of immunocytochemistry. 2. The first traces of GA-immunoreactivity can be detected already on day 2 of embryonic life within the neural tube, and the system appears to be fully developed around day 16. 3. GA-IR neuron system develops parallel in the hypothalamus and in extrahypothalamic sites. 4. The first well-defined groups of GA-IR perikarya are visible within the diencephalon and myelencephalon as early as day 6. 5. In the embryonic hypothalamus, the periventricular and the tuberal area are especially rich in GA-IR neurons, while in extrahypothalamic regions, the mesencephalon and lower brain stem is prominent in this respect. 6. Since similarities have been observed in developmental patterns of the GA-IR system and the releasing-hormonal systems, it might be assumed that the galaninergic system might have functional relationship with these latter neuronal structures during development.