Neural stem cells reside in the subventricular zone (SVZ) of the adult mammalian brain. This germinal region, which continually generates new neurons destined for the olfactory bulb, is composed of four cell types: migrating neuroblasts, immature precursors, astrocytes, and ependymal cells. Here we show that SVZ astrocytes, and not ependymal cells, remain labeled with proliferation markers after long survivals in adult mice. After elimination of immature precursors and neuroblasts by an antimitotic treatment, SVZ astrocytes divide to generate immature precursors and neuroblasts. Furthermore, in untreated mice, SVZ astrocytes specifically infected with a retrovirus give rise to new neurons in the olfactory bulb. Finally, we show that SVZ astrocytes give rise to cells that grow into multipotent neurospheres in vitro. We conclude that SVZ astrocytes act as neural stem cells in both the normal and regenerating brain.

Cerebral dopamine depletion is the hallmark of Parkinson disease. Because dopamine modulates ontogenetic neurogenesis, depletion of dopamine might affect neural precursors in the subependymal zone and subgranular zone of the adult brain. Here we provide ultrastructural evidence showing that highly proliferative precursors in the adult subependymal zone express dopamine receptors and receive dopaminergic afferents. Experimental depletion of dopamine in rodents decreases precursor cell proliferation in both the subependymal zone and the subgranular zone. Proliferation is restored completely by a selective agonist of D2-like (D2L) receptors. Experiments with neural precursors from the adult subependymal zone grown as neurosphere cultures confirm that activation of D2L receptors directly increases the proliferation of these precursors. Consistently, the numbers of proliferating cells in the subependymal zone and neural precursor cells in the subgranular zone and olfactory bulb are reduced in postmortem brains of individuals with Parkinson disease. These observations suggest that the generation of neural precursor cells is impaired in Parkinson disease as a consequence of dopaminergic denervation.

Neural stem cells in the subventricular zone (SVZ) continue to generate new neurons in the adult brain. SVZ cells exposed to EGF in culture grow to form neurospheres that are multipotent and self-renewing. We show here that the majority of these EGF-responsive cells are not derived from relatively quiescent stem cells in vivo, but from the highly mitotic, Dlx2(+), transit-amplifying C cells. When exposed to EGF, C cells downregulate Dlx2, arrest neuronal production, and become highly proliferative and invasive. Killing Dlx2(+) cells dramatically reduces the in vivo response to EGF and neurosphere formation in vitro. Furthermore, purified C cells are 53-fold enriched for neurosphere generation. We conclude that transit-amplifying cells retain stem cell competence under the influence of growth factors.

The lateral walls of the forebrain lateral ventricles are the richest source of stem cells in the adult mammalian brain. These stem cells give rise to new olfactory neurons that are renewed throughout life. The neurons originate in the subventricular zone (SVZ), migrate within the rostral extension (RE) of the SVZ along the rostral migratory stream (RMS) within tube-like structures formed of glial cells, to eventually reach the olfactory bulb (OB). We demonstrate that, contrary to the current view, multipotential (neuronal-astroglial-oligodendroglial) precursors with stem cell features can be isolated not only from the SVZ but also from the entire RE, including the distal portion within the OB. Specifically, these stem cells do not derive from the migratory neuroblasts coming from the SVZ. Interestingly, stem cells isolated from the proximal RE generate significantly more oligodendrocytes, and those from the distal RE proliferate significantly more slowly than stem cells derived from the SVZ and other RE regions. These findings demonstrate that stem cells are not confined to the forebrain periventricular region and indicate that stem cells endowed with different functional characteristics occur at different levels of the SVZ-RE pathway. Key words: adult neural stem cells; multipotent precursors; forebrain subventricular region; rostral extension; olfactory bulb; neurogenesisMultipotential neural stem cells can be isolated from the forebrain periventricular region of the adult mammalian brain (Reynolds and Weiss, 1992;Richards et al., 1992;Morshead et al., 1994;Gritti et al., 1996;Temple and Alvarez-Buylla, 1999;Doetsch et al., 1999a, Johansson et al., 1999. The subventricular zone (SVZ) lines most of the lateral wall of the lateral ventricles in adult rodents. Although in vitro forebrain periventricular stem cells display multipotency (for review, see Weiss et al., 1996;McKay, 1997;Temple and Alvarez Buylla, 1999;Gage, 2000), in vivo SVZ precursors generate primarily committed neuronal precursors that migrate tangentially along the rostral extension (RE) of the SVZ toward the olfactory bulb (OB), constituting the rostral migratory stream (RMS). Within the RMS these neuroblasts (type A cells) form elongated aggregates called chains and continue to divide while migrating (Menezes et al., 1995;Wichterle et al., 1997) through glial tunnels formed by the processes of astrocytes (type B cells; Lois et al., 1996). After reaching the core of the OB they move radially into the granular and periglomerular layers, where they differentiate into mature neurons (Luskin, 1993;Lois and Alvarez-Buylla, 1994). These findings gave rise to the current view that the forebrain periventricular region is a stem cell reservoir, and the RE is a conduit for the neuronal progeny of these cells that are targeted to more rostral brain regions. This view renders the idea that the RE itself could be a primary source of multipotential stem cells rather counterintuitive. However, it has been observed that the ratio of proliferating to nonprolife...