Demonstration of a polarizing signal that reverses future retinotectal patterns across Nuclepore filter barriers, in Xenopus embryonic eye.

We have studied the interactions which occur in surgically disarranged eye rudiments by recombining a left anterior half-eye graft from a donor Xenopus embryo with a right host posterior half eye, across a variety of physical barriers, at embryonic stages 31 or 32. The anterior half-eye graft and barrier were removed 18 h later at stage 38, and the host posterior half-eye was allowed to reconstitute a whole eye whose visuotectal projection could be mapped electrophysiologically after metamorphosis. Anteroposteriorly reversed maps and double-anterior twinned maps that are characteristic of anterior half-eyes, were found in 50-65% of cases in each of the experimental series using no barrier (N = 16), or using Nuclepore filter barriers (N = 47), including 5 of 8 cases when a filter of 0.015 micrometer pore diameter was used. The latter cases are especially interesting, because the filter pores were much smaller than the minimum size known to permit cell-cell contact through the pores. No animals showed AP-reversed retinotectal maps or double-anterior twinned maps when the graft and host half-eyes were separated by a tantalum or plastic barrier (N = 21). Only a single case of AP-reversed mapping was found in 115 control animals including simple posterior half-eye preparations at stage 32 or 38 (N = 13), sham fusions (30 min) across Nuclepore filters (N = 35), or chronic application of a filter (or plastic or tantalum) barrier from stages 32-38 (N = 55) without a left anterior half-eye graft. We conclude that signals from an anterior half-eye can act to repolarize a posterior half-eye in the absence of cell transfer and under conditions which permit little or no direct cell-cell contact.