The segmental organization of the developing shark brain based on neurochemical markers, with special attention to the prosencephalon.

Brain regionalization has been extensively studied in tetrapods, teleosts and cyclostomes. In contrast, it has not been investigated in elasmobranchs, despite their key phylogenetic position to understand brain evolution in jawed vertebrates. In this study we provide a schematic view of the segmental pattern of the developing shark brain based on mapping of the expression of Pax6 and neurochemical markers such as calretinin, tyrosine hydroxylase, serotonin, and glutamic acid decarboxylase. By correlating the cytoarchitectonic limits with the specific location of these markers, we identify transverse and longitudinal boundaries and domains, which suggest a segmental pattern, reminiscent of the one described in other vertebrates. Taken together, these data provide an initial scheme, which will be further tested and refined using a broader range of genetic markers involved in patterning and differentiation.