Mapping retinal projections from double nasal and double temporal compound eyes to dually innervated tectum in...

Right double nasal (NN) and left double temporal (TT) compound eyes were formed in the same animal by the fusion of two similar halves at early embryonic stages in Xenopus. After metamorphosis either the right or left optic nerve was sectioned to induce optic fiber regeneration from one of the two eyes to both tecta. Retinal projections from each eye to the monocularly and dually innervated tecta were later investigated by visuotectal mapping and by [3H]proline autoradiography. Visuotectal and autoradiographic maps from NN and TT eyes showed the restoration of typical compound eye projections where each of the hemiretinal projections extended across the entire contralateral (monocularly innervated) tectum. In contrast, the tectum ipsilateral to the optic nerve section was dually innervated by fibers from the TT and NN eyes. The rostral and caudal poles of the tectum were innervated by fibers from the TT and NN eyes respectively, whereas the middle third received partially overlapping projections from both eyes. Thus the full expansion of the hemiretinal projection across the dually innervated tectum was prevented by fiber-fiber interactions between optic fibers from the TT and NN eyes. It is concluded that tectal polarity cues determine the proper orientation, and fiber-fiber interaction the extent and orderliness of the retinotectal map. The formation of the latter two features of the retinotectal map does not require the existence of independent tectal positional markers.