The data obtained highlight a crucial role for the Otx genes in specification, regionalization and terminal differentiation of rostral central nervous system and lead to hypothesize that modification of their regulatory control may have influenced the morphogenesis and evolution of the brain. In addition, in most cases the entire RPE territory is affected in Otx -deficient mice, whereas only patches of the dorsal RPE is affected when the entire Mitf gene is lost ( Mitf vga-9/vga. In order to verify this hypothesis, a series of mouse models have been generated in which the functions of the murine Otx genes were: (i) fully inactivated, (ii) replaced with each other, and (iii) replaced with the Drosophila otd gene. The data obtained highlight a crucial role for the Otx genes in specification, regionalization and terminal differentiation of rostral central nervous system. However, we believe that Otx genes have a prevalent role in this process, because the expression of Otx precedes that of Mitf (Nguyen and Arnheiter, 2000). In mouse, Drosophila and intermediate species otd/Otx genes have shown a remarkable similarity in expression pattern suggesting that they could be part of a conserved control system operating in the brain and different from that coded by the HOX complexes controlling the hindbrain and spinal cord. Among these, the orthodenticle group, including the Drosophila orthodenticle (otd) and the vertebrate Otx1 and Otx2 genes, is mostly involved in fundamental processes of anterior neural patterning. An Otx related gene is present already in Cnidarians, primitive metazoans with a defined body plan and radial symmetry. We have already shown that the orthodenticle-related pro- tein (HpOtx) gene derived from sea urchin Hemicentrotus pulcherrimus encodes two distinct isoforms, HpOt圎 and HpOtxL, which are dierentially expressed and play important roles during the regulation of early development 5. Findings from several groups now confirm the importance of Otx2 in the early specification of neuroectoderm destined to become foremidbrain, the existence of an Otx gene dosage-dependent mechanism in patterning the developing brain, and the involvement of Otx1 in corticogenesis. Most of the gene candidates for the control of developmental programmes that underlie brain morphogenesis in vertebrates are the orthologues of Drosophila genes coding for signalling molecules or transcription factors.
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