Transplant connectivity in the rat cerebral cortex. A carbocyanine study.

Previous reports suggest that the specificity of connectivity between cortical areas and their related thalamic nuclei is established during development. We have used heterotopic E15, E17 and E19 transplants to determine at which embryonic age the cortex is mature enough to establish such specific connections. Carbocyanine DiI or Fluoro-Gold were used for labelling transplants. Efferent cortico-cortical, callosal and collicular connections were observed when DiI was applied in fixed tissue. Thalamic nuclei were labelled when DiI was used 'in vivo' or the transplants were labelled with Fluoro-Gold. The connections of the transplants were those corresponding to the area in which the transplants were placed. This suggests that up to E19 the embryonic cortex is too immature to define the specific thalamo-cortical connections.