Neural stem cells in the adult nervous system

The concept of immutability of the nervous tissue has been lately replaced with the new idea that a continuous neurogenic turnover does occur in some limited areas of the Central Nervous System (CNS). At least two neurogenic regions of the adult mammalian CNS are involved in this process: the sub-ventricular zone of the forebrain and the dentate gyrus of the hippocampus, which are considered to be a reservoir of new neural cells. Neural Stem Cells (NSCs) are multipotential progenitors that have self-renewal capability. While in vivo endogenous NSCs seem able to produce almost exclusively neurons, a single NSC in vitro is competent to generate neurons, astrocytes, and oligodendrocytes. NSCs lack of a specific morphology and unambiguous surface markers that could allow their identification. For this reason, one of the major difficulties in considering stem cells is that they are defined in terms of their functional capabilities which can only be assessed by testing the abilities of the cells. This procedure might alter the nature of the stem cell itself, a situation similar to the uncertainty principle in physics. The purpose of this review is to describe the characteristic of the NSCs of the adult mammalian CNS, their potentiality, in term of proliferation and differentiation capabilities, their stability in long-term culture, all attributes that make them a good tool for tissue replacement therapies.