Readjustment of retinotectal projection following reimplantation of a rotated or inverted tectal tissue in adult goldfish.

1. The pattern of visual projection from the retina on to the optic tectum following reimplantation of a piece of the tectal tissue was studied with neurophysiological mapping methods in adult goldfish. 2. When a rectangular piece of the tectum was dissected, lifted free, and then reimplanted to the same tectum after rotation by 180 degrees around the dorsoventral axis, the re-established visual projection later showed a complete reversal of retinotopic order within the reimplanted area with reference to the normal projection on to the intact surrounding area of the same tectum. The localized reversal was observed as early as 65 days, and also as late as 721 days after the 180 degree rotated reimplantation. 3. If a square piece of the tectal tissue was reimplanted after rotation by 90 degrees anticlockwise around the dorsoventral axis, the restored visual projection later showed a corresponding localized 90 degrees rotation within the reimplanted ares. 4. When the entire laminar structure of a dissected tectal tissue was inverted, and the reimplanted upside-down along the same rostrocaudal axis of the tectum, the restored visual projection on to the inverted tectal reimplant was found to be organized in a reverse retinotopic order along only the mediolateral axis within the reimplanted area. The restored visual projection retained a correct retinotopic order along the rostrocaudal axis. The same trends were also observed after regeneration of the optic fibres following section of the contralateral optic nerve. 5. If the inverted tectal tissue was reimplanted along the same mediolateral axis of the tectum, the re-established visual projection showed a localized reversal of retinotopic order along only the rostrocaudal axis within the reimplanted area. Sectioning the contralateral optic nerve made no difference to the result. 6. These results suggest that a piece of adult tectal tissue retains its original topographic polarity regardless of the orientation of reimplantation after either a rotation or an inversion. Furthermore the retention is not a short-lived transitory phenomenon. It persisted as long as the reimplanted tissue survived. 7. Histological examination of the operated tecta revealed that the reimplanted tectal tissues underwent a severe derangement in their laminar structures. It was impossible to identify the main target zone of retinotectal projection (the stratum fibrosum et griseum superficiale) or the central cellular layer (the stratum griseum centrale) in the reimplants. The prominent feature of the deranged tectal tissue was irregular vortices of tangled fibre bundles. Sparse tectal neurones of bipolar and granular types were irregularly scattered in the deranged structure of the reimplant. 8. Thus, the retention of original topographic polarity did not require an integrity of the cytoarchitectonic structure of the reimplanted tectal tissue.