During the development of the neural retina, neighbouring neurons in the retina send projections to adjacent regions in higher brain centres. Retinal gradients of axonal guidance molecules underlie the formation of this topographic map, but what establishes these gradients? On p. , Sen et al. report that in the chick retina, retinoic acid (RA) regulates the expression of the dorsoventral (DV) topographic guidance molecules EphB2, EphB3 and ephrin B2. They show that expression of a dominant-negative RA receptor reduces expression of EphB2 and EphB3 in the ventral retina and of ephrin B2 in the dorsal retina, and that the transcription factor Vax, which regulates the expression of these guidance molecules, functions upstream or parallel to RA. The researchers conclude that RA signalling is important for the establishment of the topographic map in the retina through its regulation of axon guidance.
