INTRODUCTION

Although adult-to-adult organ transplantation has developed in the past 50 years in the surgical arenas, neurosurgeons have had no options to take out damaged brain areas and to implant new tissue from adult donors. Adult neurons do not survive isolation and transplantation. The neurosurgeon, moreover, cannot take out malfunctioning brain tissue or cells without severely damaging the nervous system in its still intact parts. However, the possibility of neural tissue repair by implantation rather than by transplantation became an option, with observations in animal research that has shown that immature nerve cells not only survive and mature following implantation in the adult nervous system, but also integrate and become functionally active in existing networks. Implantation of neurons to supplement lost neurons in cases of neurodegenerative diseases and neurotrauma thus became a challenging perspective for the neurosurgeon.

Parkinson's disease (PD) was the test bed disease for this approach, as it is primarily characterized by a defined loss of neurons in the substantia nigra serving a dopaminergic input in the striatum of the central nervous system (CNS). Grafting fetal substantia nigra dopaminergic cells into the striatum of substantia nigra-lesioned rats reversed the motor disturbances, and similar studies in non-human primate models were successful as well. These results prompted clinical trials with human fetal dopaminergic neurons implanted into the striatum of PD patients. The grafted neurons indeed survived and became active cells as shown by functional brain scans.