Fig. 9. Schematic indicating genetic pathways involved in the regulation of Vax genes and pax2.1 expression and patterning of the early eye primordium. Cyc/Nodal signals from axial tissues promote medial fates in the anterior neural plate (including optic stalks and preoptic area), in part through the regulation of Hh expression. Hh proteins promote both pax2.1 and Vax gene expression in parallel. Within the forebrain, Bel and Fgf activity is required for Hh to promote Vax expression. We currently do not know the epistatic relationship between Bel and Fgf signals. It is also probable that Fgf signals act more widely to pattern optic stalks as well as the preoptic area. Both pax2.1 and Vax genes are required for closure of the choroid fissure, whereas Vax genes have a more prominent role than pax2.1 in limiting retinal differentiation to the eye cup. See text for further details.

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