Anuran olfactory bulb organization: embryology, neurochemistry and hodology.

In Xenopus laevis, we analyzed the origin of the projection neurons and interneurons in the developing olfactory bulbs by studying the expression patterns of the genes x-Eomes, x-Lhx5, x-Dll3 and x-Pax6. Olfactory bulb interneurons were characterized by using four conserved molecular markers for distinct subpopulations: gamma-aminobutyric acid, calretinin, calbindin, and tyrosine hydroxylase. Immunohistochemistry was combined with tract-tracing experiments to demonstrate the projection neurons and the interneurons of the olfactory bulbs simultaneously. Taken together, the results showed: (1) the pallial nature of the olfactory bulb and its projection neurons in Xenopus, like in mammals with comparable central projection areas, (2) the subpallial origin of the interneurons that, most likely, follow migratory pathways comparable to those described for mammals, (3) the different interneuron types possess neurochemical characteristics similar to mammals. Therefore, the present results show that the origin, chemoarchitecture and central connections of the olfactory bulbs are highly conserved in evolution.