Mechanisms of function of neural grafts in the adult mammalian brain.

Evidence for the survival, growth and function of grafted neural tissues in the adult mammalian brain is reviewed. In addition to considering the viability of grafts in the different model systems that have been investigated, consideration is given to alternative mechanisms by which the grafts might exert a functional influence over the host brain and the host animal's behaviour: (a) acute influence over spontaneous recovery of function, (b) chronic but diffuse secretion of neurochemicals into the host neuropile, (c) tonic reinnervation of the host brain, (d) bridging grafts, and (e) reciprocal reinnervation and full incorporation of graft tissue into host circuitry. It is concluded that no one mechanism is primary, but that different levels of reorganization can take place in different graft paradigms and neural systems.