Two healing patterns correlate with different adult neural connectivity patterns in regenerating embryonic Xenopus retina.

Nasal and temporal one-third-sized eye fragments, formed by ablation at stage 32-33 of Xenopus laevis embryos, heal and, in about 50% of the cases, survive to make eyes in the postmetamorphic animal which have mappable visuotectal projections. The majority of nasal one-third eyes have duplicate projections whereas the majority of temporal one-third eyes have unduplicated projections. Most nasal one-third eye fragments and a minority of temporal eye fragments heal by the extrusion of cells from the center of the cut edge into the region of the ablation, forming a tongue of cells between the distal cut edges. This healing pattern is correlated with duplicated visuotectal projections. Most temporal one-third fragments and a minority of nasal one-third fragments heal by rounding up; that is, the distal cut edges collapse to meet in the region of the ablation. This healing pattern is correlated with the formation of unduplicated visuotectal projections. During tongue formation, neurons and undifferentiated cells are transferred from the original fragment into the tongue in a disorderly array, but quickly re-form normal retinal order. We propose that the tongue cells retain their original determination to connect to the same tectal positions as the fragment from which they originated, despite their new positions, and that this mosaicism, coupled with cell movement into the tongue, established duplicate visuotectal projections.