Mapping the developing retinotectal projection in frog tadpoles by a double label autoradiographic techinque

Tadpoles of Xenopus at developmental stages 45-59 were given an intraocular injection of [3H]proline and an intraperitoneal injection of [3H]thymidine, killed either after a short (1-2 day) or long (16-37 day) survival period, and processed for autoradiography. In all cases, there was a regular pattern of labelling. In the retina, the proline-labelled cells were always central to the thymidine-labelled cell nuclei, and in the long survivors there was a sharp transition between the cells occupying the central labelled retina and the surrounding unlabelled cells at the retinal periphery. In the tectum, unlabelled optic axons were confined to a rostrolateral zone which was bordered caudomedially by a zone of labelled tectal cell nuclei, and in the long survivors there was a completely unlabelled zone at the caudomedial margin of the tectum. The size of the tectal zone of labelled axons was not significantly different in the long survivors from that in short survivors injected at the same stage of development. Labelled optic axons remained close to the tectal cells that were present before the time of injection and did not move later into the zone of cells added to the tectum in the survival period after the injections.