Retinal ganglion cells: specification of central connections in larval Xenopus laevis.

In a series of Xenopus larvae (stages 28 to 35), the left eye cup was dorsoventrally and anteroposteriorly inverted. After metamorphosis, the retinotectal projection was mapped by recording action potentials evoked in the tectum by a small spot of light projected on the retina. Normal retinotectal projection was found following rotation of the eye cup at stage 28 to 29. Rotation of the eye cup at stage 30 resulted in anteroposterior inversion of the retinotectal projection; rotation at stages 31, 32, and 35 resulted in inversion of the projection in anteroposterior and dorsoventral axis of the retina. Therefore the retinal ganglion cells were unspecified at stage 28 to 29; spatial specification of ganglion cells occurred in the anteroposterior axis of the eye cup at stage 30 and in the dorsoventral axis between stages 30 and 31.