Intracerebral neural implants: neuronal replacement and reconstruction of damaged circuitries.

Neural grafting has emerged over the last decade as a viable approach to studying the development and regeneration of neuronal connections in the CNS of mammals. In this review we focus on one particular aspect of this technique: the use of intracerebral neural implants for the reestablishment of severed connections, the substitution of lost pathways, and the replacement of tissue defects in the adult mammalian CNS. A survey of the literature shows that grafting of neuronal tissue to the mammalian CNS has been frequently attempted since the end of the last century, but that the results of these earlier studies were generally very poor. Thompson (1890) and Saltykow (1905) were possibly the first to report results from grafts of adult CNS tissue, and Del Conte (1907) the first to try grafts of embryonic tissues to the brains of mammals. Their results were generally unsuccessful and no clear-cut evidence of good long-term survival was obtained. Del Conte, in particular, concluded that the brain was an unfavorable transplantation site. Similar negative results were subsequently published, e.g. by Altobelli (1914), Willis (1935), Glees (1955), Wenzel & B~lehner (1969), and Frotscher et al (1970). Some of the early investigators were, however, more successful. Ranson (1914) and Tidd (1932) obtained partial survival of grafted sensory ganglia in the cerebral cortex of developing rats, and Dunn (1917) reported survival of four out of 44 grafts of neonatal cerebral cortex, implanted into the cortex of nine to ten day-old