Neural development and regeneration in the visual system of teleosts

Abstract

The structure of the developing optic tectum of the trout has been investigated using histological preparations and fresh material. The neuroblasts reached the pial surface and started to differentiate before separating from the ependymal processes. A new finding was the presence of inverted pear- shaped neurones in the plexiform external layer in Golgi sections of adult tectum. The organisation of the tectum was found to be similar to that described by Leghissa (1955).The first optokinetic responses occurred in trout embryos at a stage of development at which the thickness of the outer white zone was equal to that of the inner grey zone. The topographical representation of the visual field on the optic tectum of ten normal goldfish was mapped electro-physiologically using metal-filled micropipette electrodes.The topographical representation of the visual field on the optic tectum of ten normal goldfish was mapped electro- physiologically using metal -filled micropipette electrodes. Responses were evoked by visual stimulation with spots of light and with black discs of various sizes.Restoration of the visual projection over the right tectum following excision and reimplantation of the tectum in its normal orientation was studied in a series of goldfish at different times after the operation. There was a wide variation in the time taken for the restoration of the retinotectal projection map. From the third postoperative month some normal projections in the graft area were seen. It was concluded that optic nerve fibres can regenerate into the graft, and that the graft may retain its original specificities, viz. its specialization to represent a particular part of the retinaeCases where recovery occurred later than three months were those with larger areas of grafted tissue.When the graft was reimplanted 90° rotated it was predicted that the recovered projection would show a corresponding 90° rotation over the grafted area. Results tended to show the predicted distortion in the projection though not at the edges of the grafts.It was difficult to draw any useful conclusions from the retinotectal projection of 180° rotated tectum.In another series when the posterior half of the tectum was removed and the entire optic nerve allowed to regenerate, the resulting projection was compressed into the remaining half tectum.There was a gross abnormality in the histological structure of the grafted tectum even in those cases where the restoration of the visual map was more or less normal.An attempt has been made using various microscopic techniques to compare regeneration within tectal grafts with normal tectal development.