Introduction

Over most of the past century of modern neuroscience, it was thought that the adult brain was completely incapable of generating new neurons or having neurons added to its complex circuitry. However, in the last decade, the development of new techniques has resulted in an explosion of new research showing that neurogenesis, the birth of new neurons, normally occurs in only two regions of the adult mammalian brain, and that there are significant numbers of multipotent neural precursors, or “stem cells”, in many parts of the adult mammalian brain (Reynolds et al., 1992; Lois and Alvarez-Buylla, 1993; Palmer et al., 1995).

The idea of “making new neurons” is appealing for neurodegenerative disease or selective neuronal loss – whenever neurons are lost causing chronic neurological or psychiatric disorders. As is so often the case in science, however, the path from the intuitive idea and first reports of general feasibility to clinical realization has turned out to be a long one. However, contrary to previously widely held beliefs, we now know that the brain itself demonstrates how new neurons can be generated from neural precursors (“neurogenesis”) (Altman and Das, 1965; Altman, 1969; Cameron et al., 1993; Eriksson et al., 1998). One goal of neural precursor biology is to learn from this regionally limited, constitutive neurogenesis how to manipulate neural precursors toward therapeutic neuronal repopulation. Elucidation of the relevant molecular controls might allow both control over transplanted precursor cells and the development of neuronal replacement therapies based on the recruitment of endogenous cells.