Laboratory of Biochemical Genetics, National Heart, Lung and Blood Institute, National Institutes of Health, Bethesda, MD 20892.

The topographic map of cell position in the avian retina is conserved and inverted when retinal ganglion neurons synapse with neurons in the optic tectum. Developmental mechanisms based on molecular gradients that specify positional information and pattern formation have been postulated in the establishment of these topographic maps of cells in retina and optic tectum. Two cell surface proteins in retina, TOP(DV) and TOP(AP), are distributed in dorsoventral and anteroposterior topographic gradients, respectively. Corresponding gradients of TOP molecules present in the tectum are inverted with respect to the retinal gradients. These orthogonal gradients of TOP(DV) and TOP(AP) molecules provide a possible Cartesian coordinate system for designation of cell position at all points in the retinotectal map.

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