Selection of appropriate medial branch of the optic tract by fibres of ventral retinal origin during development and in regeneration: an autoradiographic study in Xenopus.

The formation of the branches of the optic tract has been studied with the use of [3H]proline autoradiography, during development and during regeneration of the optic nerve in Xenopus with one compound ventral (VV) eye made by the embryonic fusion of two ventral eye fragments. The formation of the optic pathway was abnormal in that the lateral branch failed to develop, suggesting that fibres from a VV retina selectively entered the tectum via the medial branch during development. Three months after section of the optic nerve of a VV eye, regenerated fibres were present both in the contralateral and ipsilateral tecta. On the ipsilateral side regenerated fibres entered the tectum via the medial branch only. Retinal fibres entered the contralateral tectum through both branches in some animals and through the medial branch only in others. It is concluded that mechanical factors alone are insufficient to explain the phenomenon of selection of the appropriate medial branch by fibres of ventral retinal origin either during development or in regeneration. Some form of fibre-substrate interaction seems to be necessary; and this ability of fibres from a VV eye to take the path appropriate for ventral retina argues strongly that the VV eye is not a regulated system in terms of cell specificities.