Specification of positional information in retinal ganglion cells of Xenopus: stability of the specified state.

In the normal development of retinotectal connections, the site in the tectum at which an optic fiber synapses is related to the position of its ganglion-cell body in the retina. How and when the ganglion cells acquire information about their positions is unknown, but the positional information that each ganglion cell will ultimately act upon is determined or specified at embryonic stages 28-32 in the clawed frog, Xenopus laevis. Here we report that once positional information has been so specified, it remains stable when the eye is "back-grafted" into the orbit of a stage-28 host, or cultured in vitro for up to 10 days before grafting into the orbit of a stage-38 host. Thus, the ganglion cells of these eyes form tectal connections appropriate to their original positions in the donor orbits and independent of their final positions in the host orbits. We conclude that specification of positional information involves stable changes in the phenotypic properties of the differentiating retinal cells that (i) render the cells refractory to information about changes in their positions after stage 32 and (ii) commit each ganglion cell to the development of a unique property (locus specificity) that predisposes its axon to synapse at a particular locus in the retinotectal map.