Optic axons ignore foreign denervated sites in goldfish tectum even after removal of some of their normal termination sites.

Previous experiments have shown that optic axons ignore foreign denervated territory in the goldfish tectum and preferentially reinnervate their normal layers of termination. In the present experiments ultrastructural morphometry was used to test whether this was still the case if optic axons were deprived of some of their normal termination sites. The stratum fibrosum marginale (SFM) of the tectum was partially denervated to provide foreign synaptic sites and the caudal half of the tectum was ablated at the same time. This removed approximately half the normal sites of optic termination. In the majority of cases optic axons grew into the SFM but there was no evidence of any significant formation of synapses by them in the SFM. Instead they synapsed in their normal layers. Myelinated optic axons in the SFM were present in significantly higher numbers in fish operated on in spring than in fish operated on in autumn after similar survival times. Although removal of some of the normal sites of termination promoted growth of optic axons into foreign denervated territory, the axons did not synapse there. Instead they passed through the foreign region and reinnervated their normal layers even though this meant reorganizing the optic projection within the reduced tectal space.