Visual projections to the optic tecta in Xenopus after partial extirpation of the embryonic eye

The temporal, nasal or dorsal half (40–60%) of the eye primordium was excised in Xenopus frog embryos stage 25/26 (optic vesicle), 31/32 (optic cup) or 38 ± 1 (early larval eye). The residual nasal (NF) or temporal (TF) or ventral (VF) fragment gradually rounded‐up to form a grossly normal eye of normal or reduced size. Electrophysiologic analysis of the visuotectal projection after metamorphosis showed that (independent of stage of surgery and final eye size) most eyes of each type generated normal retinotectal maps (orderly and continuous retinotopic projection across the entire tectum). A minority of NF eyes generated double‐nasal compound maps, in which nasal and temporal halves of the adult retina projected as mirror images (of the nasal pattern) over the whole tectum. A minority of TF eyes generated double‐temporal compound maps, while a minority of VF eyes generated double‐ventral compound maps. Respectively, these compound maps simulated those generated by NN, TT or VV “compound‐eyes” prepared by fusing two nasal half eyes, or two temporal half eyes, or two ventral half eyes. The incidence of double‐maps was lowest in TF eyes at all stages, and after stage 25/26 surgery in all eye types. Lure tests and post‐synaptic recording confirmed that these eyes formed functional retinotectal synapses, and that the (presynaptic) map accurately mirrored the connectivity pattern. The results are considered in terms of (1) the patterning of locus specificities in the retinal ganglion cell population, (2) embryonic “regulation” and “reduplication” in non‐neural systems, and (3) alternative mechanisms by which partial extirpation might modify the deployment or expression of ganglion cell locus specificities to produce the retinotectal connectivity patterns we observed.