Enhancing the self-repairing potential of the CNS after injury.

Embryonic CNS tissues can now be successfully transplanted into most regions of the CNS in neonate, adult, or aged host animals. The transplants generally project to their correct target areas in the host brain. However, transplants appear to receive only partial innervation by host fibers. Replacement of lost host neurons by transplants has proven to be functionally most successful in the cases of neurosecretory, monoaminergic, or cholinergic neurons. Their success probably depends on their ability to deliver the proper neurotransmitter (and perhaps trophic factors) close to their targets rather than on their capacity to reestablish all lost connections. Efforts to achieve complete integration of the transplants will probably require interventive strategies to guide the natural repair response of CNS tissue. Survival and outgrowth of the transplants can be promoted by taking advantage of the natural increase of neurotrophic factors in response to injury. Neurotrophic factors and substrate guidance factors, once purified and completely characterized, promise to be powerful new therapeutic agents for enhancing the survival and regrowth of the injured CNS.