Identification of neural stem cells in the adult vertebrate brain.
Neurogenesis continues into adult life in restricted germinal layers. The identification of the neural stem cells that give rise to these new neurons has important clinical implications and provides fundamental information to understand the origins of the new neurons. Work in adult birds and rodents yielded a surprising result: the neural stem cells appear to have characteristics of glia. In adult birds, the primary neuronal precursors are radial glia. In adult mammals, the primary neuronal precursors have properties of astrocytes. Radial glial cells have previously been shown to transform into astrocytes; both cell types are classically considered part of a committed astroglial lineage. Instead, we propose that neural stem cells are contained within this astroglial lineage. These findings in adult vertebrate brain, together with recent work in the developing mammalian cerebral cortex, force us to reexamine traditional concepts about the origin of neurons and glia in the central nervous system. In particular, neural stem cells possess a surprisingly elaborate structure, suggesting that in addition to their progenitor role, they have important structural and metabolic support functions. The very same cells that give birth to new neurons also seem to nurture their maturation and support their function.