Fig. 7. Genetic hierarchy underlying early retina development. X, Y and Z represent factors that pattern the anterior neuroectoderm, leading to the overlapping expression of Six3 and Pax6 in the retina anlage, which results in the determination of retinal fate. The crossregulatory interaction and the feedback loops of Six3 and Pax6 (arrows) result in the maintenance of retinal fate. Rx3 expression is regulated by these genes, but may also receive input of upstream factors X, Y and Z. Rx3 activity is required for morphogenesis (optic vesicle evagination) and organ size regulation (proliferation in optic vesicle). The retina specific expression of Rx2 does not depend on Rx3 activity. Genes involved in subsequent differentiation steps require preceding Rx3 activity (Tbx2/3, Vsx1/2).