Transplantation of embryonic occipital cortex to the brain of newborn rats. An autoradiographic study of transplant histogenesis.

The neurogenesis of immature cerebral cortex transplants was investigated using tritiated thymidine (3HT) autoradiography. Cortical tissue taken from rat fetuses during their last week of gestation (E15-E21) was transplanted to the tectum or cerebral cortex of newborn rat hosts. At different times after transplantation, a single injection of 3HT was given to the host. Most of the experimental animals were killed after the transplants had grown to maturity (5-12 weeks), and some were studied shortly after the tracer had been given. In other experiments, donor tissue was used that was labeled in utero up to 1 day before being transplanted on E16, E17, E18, or E19. It was found that neurons labeled before transplantation survived and differentiated in the graft. Removal of the graft from its natural context did not prevent 3HT incorporation into surviving precursor neurons, indicating continuation of neurogenesis in the cortical transplants. In transplants from E16 donors neurons continued to be generated for 5-6 days after transplantation. Termination of neurogenesis occurred at successively earlier times in transplants taken from correspondingly older embryos. Independent of size and position of the transplant, application of 3HT after "projected" transplant ages of E23 and older labeled only nonneuronal cells. This suggests a time schedule of neuron generation in the cortical transplants similar to that observed during normal development of the cerebral cortex, which is not disturbed by the transplanting procedure.