Schematic view of a generalized vertebrate CNS. (A) Topological location of the ZLI and IsO organizers and the key genes determining their position in a schematic view of a generalized developing vertebrate central nervous system. The most anterior of the internal brain secondary organizers, the ZLI, develops within the diencephalon, at the interface of the expression of Fezf (blue) and Irx (yellow). The IsO organizer will be located at the midbrain hindbrain boundary (MHB), delimited by the expression of members of the Otx (green) and Gbx (orange) gene families. (B) Scheme of comparative gene expression domains in schematic representations of (left) prototypical vertebrate and (right) amphioxus larva brains (based on ). Whereas some patterning genes involved in ZLI location have a conserved relative expression pattern, others like Lfng/Fng (white) or Wnt8 (black) do not. CV = cerebral vesicle; D = diencephalon; H = hindbrain; M = midbrain; T = telencephalon. Note that adult/larval brain structures from vertebrates and amphioxus are not fully comparable.