Misexpression of the Emx-related homeobox genes cVax and mVax2 ventralizesthe retina and perturbs the retinotectal map

The mechanisms that establish the dorsal-ventral (D-V) axis of the eye are poorly understood. We isolated two homeobox genes from mouse and chicken, m Vax2 and cVax, whose expression during early eye development is restricted to the ventral retina. In chick, ectopic expression of either Var leads to ventralization of the early retina, as assayed by expression of the transcr iption factors Pax2 and Tbx5, and the Eph family members EphB2, EphB3, ephr inB1, and ephrinB2, all of which are normally dorsally or ventrally restric ted. Moreover, the projections of dorsal but not ventral ganglion cell axon s onto the optic tectum showed profound targeting errors following cVax mis -expression, mVax2/cVax thus specify positional identity along the D-V axis of the retina and influence retinotectal mapping.