Healing and growth of half-eye "compound eyes" in Xenopus: application of an interspecific cell marker

Surgically constructed compound eyes have been widely used to examine the development of retinotectal projections in amphibians. Such studies have been limited, however, by the lack of an adequate cellular marker with which to assess the contributions of grafted and host tissues to the later larval and adult retina. We have followed the growth of graft- and host-derived ocular tissues in interspecific compound eyes prepared by orthotopic and heterotopic exchanges of half-eye fragments between pigmented Xenopus borealis and albino Xenopus laevis embryos. This genotypic combination allowed the growth of graft-derived choroid and pigment epithelium to be studied in the living animal, and permitted cell-by-cell resolution of graft- and host-derived neurons in quinacrine-stained paraffin sections. At mid- and late-larval stages graft-derived neurons occupied large and usually coherent territories of retina in all classes of orthotopic and heterotopic compound eyes. In successfully healed cases, sample means of the percentage of the total retina occupied by graft-derived neurons ranged from 43 to 51%. Graft-derived territories originated near the optic nerve head and extended into the germinal neuroepithelium at the retinal periphery. As compared to orthotopic compound eyes, graft-derived territories in all classes of heterotopic compound eyes were slightly smaller and significantly more variable. Despite this variability, the correlation between graft-derived pigment epithelial and retinal territories was quite good in heterotopic compound eyes. While graft-host borders were generally sharp, there was also evidence of local cell mixing over distances of several cell diameters in the inner nuclear and ganglion cell layers. Single displaced cells, isolated from other members of their genetic cohort by 5 or more cell diameters, were also present in the inner nuclear and ganglion cell layers.