Reciprocal transplantations between the optic tectum and the cerebellum in adult goldfish.

1. The topographic pattern of re-established visual projections after a reciprocal transplantation between tectal and cerebellar tissues was studied in adult goldfish. 2. A rectangular tissue was dissected from the left tectum, and a similar piece from the cerebellum in the same fish. The cerebellar piece was rotated by either 180 or 0 degrees around the dorsoventral axis, and transplanted into the tectum in place of the tectal piece. This tectal tissue was likewise grafted into the cerebellum after either 180 or 0 degrees rotation in the same fish. 3. The tectal graft disappeared from the cerebellum within 2 months after surgery. The operated cerebellum showed a remarkable capability for healing its excised part. No visual responses were recorded from the cerebellum. 4. The cerebellar grafts remained in place within the operated tectum in twenty fish. In fifteen of these fish, tested 3 or 4 months after surgery, the cerebellar grafts did not give any visual responses, unlike the surrounding responsive area of the tectum. These fish showed a partial scotoma in the central area of the visual field, which corresponded to the unresponsive transplanted area of the tectum. Autoradiographic examination after intraocular injection of L-[3H]proline showed that these cerebellar grafts did not contain any noticeable label, in contrast to the extensively labelled surrounding tectal tissues. 5. Sporadic visual responses were recorded from deep layers in the transplanted area of the tectum in five of the twenty fish at early post-operative periods. The receptive fields of these responses were distributed in a correct retinotopic order, regardless of whether the cerebellar tissue had been rotated by either 180 or 0 degrees. Autoradiographic examination, however, revealed that these cerebellar grafts were not invaded by regenerating optic fibres. Instead, they bypassed the interposed cerebellar tissue by making detours beneath the graft on the way towards their appropriate target zones within the tectal tissue. 6. This selective avoidance of a foreign (cerebellar) tissue and the orderly reinnervation of the proper tectal tissue by regenerating optic fibres provide us with further evidence for neuronal specificity.