Introduction

The first attempts to implant central nervous system (CNS) tissue into the adult mammalian brain were performed a century ago (Thompson, 1890), seemingly with very poor graft survival. Although developing CNS tissue had been shown by Dunn (1917) and Le Gros Clark (1940) to survive transplantation, the ability of such grafts to establish extensive afferent and efferent connections with the recipient's brain and to influence the behavior of the host was not demonstrated until the late 1970s (for a historical overview see e.g., Bjorklund & Stenevi, 1985). Since then, grafting of fetal neural tissue into the mammalian CNS has emerged as a widely used experimental tool with which to study a diversity of neurobiological problems, for example mechanisms of neural development, plasticity and regeneration. Much basic research has been devoted to the morphological and functional analysis of neural grafts in animal models of human neurological disorders, such as Parkinson's disease, Huntington's disease, and dementia. This has led to the first clinical trials with intrastriatal implantation of fetal dopamine-rich mesencephalic tissue in patients with Parkinson's disease (see e.g., Lindvall et al., 1990).