Histological studies on the development of retinotectal projections from nasoventral quarter-eyes in Xenopus laevis

Abstract

In Xenopus larvae, the size and location of the retinotectal projection of nasoventral quarter-eyes was analyzed in early stages (43–47), midlarval stages (50 and 53) and metamorphic stages (56 and 60), by labelling the optic nerve with the cobalt-lysine complex or with horseradish-peroxidase (HRP). For direct comparison, both fragment and normal eye projections were determined simultaneously in the same specimen in brain whole mounts. During early stages (up to stage 47), the projection fields of normal eyes and quarter-eyes are confined to the rostral part of the tectum. The extension of the projection in rostrocaudal direction of eye fragments does not differ from that of normal eyes. During later development up to metamorphosis, normal eyes expand their projection over the newly formed tectal surface in a caudal direction, whereas the fiber terminations of nasoventral quarter-eyes still remain in the rostral part of the tectum. Quantitative studies revealed that there is no difference in the size of both halves of the tectum. At least for quarter-eyes, however, a strict correlation between eye size and extension of the contralateral projection field could be established. According to our results, it is unlikely that during development local tectal markers are involved in determining the location of the projection field and the retinotopic ordering of the optic fibers. Instead we suggest that the optic fibers separate in accordance with their retinal specificity.