Fetal cortical transplants in the cerebral hemisphere of newborn rats: a retrograde fluorescent analysis of connections.

Fetal cerebral neocortex (E15-17) was grafted into the cortex of newborn (0-1 day old) rats. In some animals the tissue was grafted just caudal to cortical aspiration lesions made immediately prior to grafting. At maturity, transplant efferents were examined by use of the retrogradely transported fluorescent dyes Fast Blue and Diamidino Yellow. Dyes were principally injected in various combinations into the host pyramidal tract decussation and into cervical and lumbar spinal cord levels. In addition some of these animals received injections into the host cortex opposite the transplant, into the ipsilateral thalamus or into the transplant. Transplants were recovered in 26 of 30 animals, and they were typically larger in recipients sustaining aspiration lesions. Histologically, transplants commonly demonstrated laminar patterns that resembled normal cortical supra- and infragranular laminae. Numerous retrogradely-labeled neurons were counted within transplants after injections of fluorescent tracers into the host pyramidal tract decussation or cervical spinal cord. These cells were often located appropriately in areas resembling infragranular layers. Retrogradely labeled neurons were also found within transplants after injections into the host cortex or thalamus. Additionally, numerous host neurons were labeled after Diamidino Yellow injections into the transplant. These findings demonstrate an exchange of connections between host and transplant and suggest the establishment of normal connection patterns.