Sage Journals: Discover world-class research

Abstract

Over the past 20 years, the conception of brain development has radically changed from a fixed and limited hierarchical process to a more plastic and continuous one. Most surprising, the field has learned that postnatal neurogenesis is not just a seasonal phenomenon in songbirds but a process that occurs across species and seasons. Astrocytes, whose primary role in the central nervous system was thought to be strictly supportive, have emerged as a heterogeneous population, a subset of which is the neural stem cell. Postnatal neurogenesis persists in specialized niches within the rostral subventricular zone and hippocampal dentate gyrus and, for a limited period, within the white matter tracts and external granular layer of the cerebellum. These specialized microenvironments are influenced by factors in the blood, cerebrospinal fluid, and local extracellular matrix. This article reviews the current understanding of adult neurogenesis, which is conserved across many vertebrate species, underscoring the value of animal models in past and present studies of human neurogenesis and neurogenic disease.

Keywords

adult neurogenesis animal models dentate gyrus neural precursor cells neural stem cells subventricular zone

In the 1960s, Altman was among the first to document neurogenesis within the postnatal rodent central nervous system (CNS).^2–4 Despite this body of work, postnatal neurogenesis was not widely accepted until 2 decades later when Nottebohm demonstrated neuronal replacement in the adult avian forebrain.^42,77 While evidence of postnatal neurogenesis accumulated in avian and mammalian species, another 15 years elapsed before adult neurogenesis was recognized in humans.³² Thirty years after Altman’s first descriptions of adult neurogenesis, neural stem cells (NSCs) were finally isolated from the adult mammalian brain by selective response to epidermal growth factor (EGF) and basic fibroblast growth factor (FGF2).⁹⁴ ^,95 Subsequently, multipotent NSCs have been isolated from the postnatal subventricular zone (SVZ) and its rostral and caudal extensions into the olfactory bulb and hippocampus,⁶⁷ ^,68,84 as well as within the white matter tracts of the postnatal rodent cerebellum.⁵⁹ ^,64,128,129 In addition, glial progenitors in regions not associated with neurogenesis demonstrate multipotency when exposed to appropriate growth factors in vitro.⁹ ^{,60,78,83,105}

The identification of neural stem and precursor cells in the adult brain broaches the possibility of regenerative medicine to improve brain function, learning, and memory. Apart from the ethical and moral implications associated with human embryonic research, postnatal NSCs are more readily available than embryonic stem cells for basic stem cell research and therapy and afford the opportunity for autologous stem cell transplantation.^{35, 46, 59, 64, 71, 79, 108, 119} In addition to replacement of dead or dying neural tissue, NSCs have the potential to elaborate trophic factors to rescue dysfunctional endogenous neurons, as well as inhibit inflammation in the transplanted areas.^31,81,89

Identification of postnatal NSCs and the environments in which they reside can provide insight into the pathogenetic mechanisms of various neurodegenerative and neoplastic diseases through in vitro and in vivo studies. To date, neural stem or precursor cells have been successfully isolated from rodents, primates, sheep, pigs, cats, and dogs.^{17,30,71,79,82,84,94,111,121–123,130} In vitro and in vivo studies of NSCs from multiple animal species have demonstrated remarkable species conservation of phenotype and neurogenic mechanisms.^{21,27,41,67,97,124} Accordingly, rodents, domesticated animal species, and nonhuman primates are valuable models for human neurogenesis and neurogenic disease and are used in investigative and therapeutic studies of human neurologic diseases.

Neural Precursor Cells

NSCs are rigorously defined as multipotent, capable of extended self-renewal, and able to generate a large number of progeny.⁹³ ^,101 Cells that demonstrate only multipotency, or are bipotent or unipotent, are more appropriately termed neural progenitor cells.^20,101 Neural stem and progenitor cells are both present in postnatal neurogenic regions. Thus, for the purposes of this review, unless a population is known to conform to the definition of a stem cell, the term neural precursor cell (NPC) is used to include both neural stem and progenitor cell populations.

Neurogenesis within the CNS occurs in the early postnatal period in the cerebellar external granule layer (EGL),^34,88 whereas routine adult neurogenesis is thought to occur only in the SVZ/olfactory bulb and subgranular zone (SGZ) of the hippocampal dentate gyrus.¹³¹ Extra-CNS neurogenesis, such as that which occurs in the nasal mucosa,^8,53 is beyond the scope of this review. In the CNS of mice, adult neurogenesis maintains and reorganizes the olfactory bulb interneuron network, modulates and refines existing neuronal circuits in the dentate gyrus, and is required for hippocampal-dependent learning and memory.^22,49

Only within the past decade has the in vivo identity of postnatal NSCs gradually been ascertained. The origin of adult SVZ NSCs is proposed to be the ventricular zone (VZ) neuroepithelial (NE) cell. In this view, NSCs persist from development through adulthood, undergoing a morphologic transformation from NE cell to radial glial cell to astrocyte (Fig. 1 ).²⁴ During fetal development, NE cells of the developing neural tube VZ are NSCs. Similar to radial glia, NE cells have a radial morphology characterized by a bipolar shape with a short apical process and single cilium contacting the ventricle, as well as a long basalar process extending toward the pia. The NE cells also express radial glial markers, such as vimentin, RC1, RC2, nestin, and, in primates, glial fibrillary acidic protein (GFAP).⁷⁶ ^,90,114 Radial glia arising from the NE cells differentiate into neurons, glia, and ependymal cells.³⁶ ^{,70,76,90,114} After birth, radial glia disappear by “differentiating” into astrocytes and ependymal cells. The ependyma replaces the VZ, and astrocytes localize beneath this layer in the SVZ. The adult SVZ lines the entire ventricular system; however, the majority of adult SVZ neurogenesis occurs rostrally, especially in the lateral ventricles of the frontal horn.^18,28

Figure 1.

Current conception of neurogenesis in the rostral periventricular region throughout development. The neural stem cell niche shifts from the ventricular zone (VZ) to subventricular zone (SVZ) after birth. NE, neuroepithelial.

It was once thought that with the disappearance of radial glia, the ability for neurogenesis was lost. However, it is now recognized that the radial glia-derived astrocytes in the SVZ have the ability to self-renew as well as differentiate into neurons and glia. Moreover, like NE cells, they share some of the molecular markers and structural features of radial glia, such as the presence of GFAP, contact with the ventricle, and a single cilium extended into the ventricle.⁹⁰ ^,114 In the lateral wall of the lateral ventricle, the apical process of SVZ astrocytes, occasionally containing a nucleus, squeezes between ependymal cells to allow contact with the ventricle (Fig. 1). Thus, the adult lateral wall ventricular surface actually comprises a mixture of ependymal cells and astrocytic NSCs, mimicking the embryonic VZ in which primary progenitors line the ventricular space.^72,107

The presence of a ventricle-contacting cilium in SVZ astrocytes obfuscated the initial identification of the SVZ stem cell. Ciliated ventricle-contacting ependymal cells were originally proposed to be the adult CNS stem cell.⁵² Subsequently, it was shown that the ciliated “true” stem cells were in fact SVZ astrocytes and not ependymal cells.²⁶ Recent studies suggest that there is a subpopulation of ependymal cells with progenitor cell characteristics and properties in the adult SVZ that produces neurons and astrocytes in response to stroke.^16,19,87 However, this population is not self-renewing under conditions of stroke and therefore would not constitute a true stem cell population.¹⁶

The astrocytic stem cells in the hippocampal SGZ are also derived from radial glia, are GFAP+, retain a radial morphology, and are thus of the neuroepithelial cell–radial glial cell–astrocyte lineage.¹⁰⁴ The GFAP+ astrocytic cells from neurogenic and nonneurogenic regions appear to have distinct identities as NSCs and astrocytes, respectively.⁵¹ In rodent neurogenic regions, the GFAP+ cells with stem cell ability represent a morphologically distinct population of cells with bipolar or unipolar morphology.³⁸ Subsequent studies demonstrated that GFAP+ cells cultured with leukemia inhibitory factor have a bipolar/tripolar morphology and possess NSC markers and differentiation ability, whereas bone morphogenetic protein-generated GFAP+ cells are astrocytes.¹⁰ Thus, the NSC in developing and adult vertebrates is a cell with markers and structural characteristics of an astrocyte.

In Vitro Propagation

NPCs grow in vitro as free-floating cellular clusters termed neurospheres or as adherent cell cultures (Fig. 2 ).⁸⁴ ^,91,94,95 It was widely held that a neurosphere represented the clonal expansion of a single stem cell. However, there is evidence that neurospheres may also be generated by progenitor cells.²⁹ ^,93 Thus, the standard neurosphere assay that assesses the ability of single cells to form neurospheres overestimates the proportion of true stem cells in a population.⁹³

Figure 2. Tissue culture olfactory bulb–derived neural precursor cells; dog, postnatal day 3. The neural precursor cells are plated into 25-cm² tissue culture flasks coated with poly-D-lysine and grown as an adherent culture; a single neurosphere is present where the cells do not adhere to the culture surface (arrow). Bar = 50 μm.

Figure 3. A schematic of the neurogenic region in the adult mouse hippocampal dentate gyrus. The white box illustrates the location of the subgranular zone (SGZ) between the granular cell layer (GCL) and hilus. Inset depicts the organization of the SGZ. B, type B cell; D, type D cell; GC, granule cell.

NPCs in the developing and postnatal brain were first isolated by their in vitro response to EGF or FGF2 with heparin, factors known to stimulate precursor proliferation in the developing neuroepithelium.⁹¹ ^,93,94,95 Subsequently it was shown that NSCs in the embryonic mouse telencephalic germinal zone are responsive to FGF as early as embryonic day 8.5, but EGF responsiveness does not occur until embryonic day 14.5.¹¹⁵ This study also suggested that EGF-responsive NSCs were lineage descendants of the FGF-responsive NSCs. Indeed, later studies showed that FGF2 upregulates EGF receptor expression in NPCs.¹⁴ ^,66 In support of these findings, exhaustive in vivo evaluation of the EGF-responsive NPC type in the adult mouse SVZ showed that it corresponds to the rapidly cycling SVZ neuronal progenitor cell (Fig. 1; GFAP– type C cell), rather than the true NSC, which is quiescent (GFAP+ type B cell).²⁹

Protocols for in vitro NPC culture differ with respect to concentration and combination of growth factors and substrates.^{17,34,56,63,75,84,91,94,95} There is ample evidence that NPC response to growth factors varies not only with the developmental stage but also with factor composition and concentration.¹²⁵ ^,132 For example, although NPCs grown in either EGF or FGF2 with heparin were multipotential, 2 separate studies showed a preferential gliogenic effect from EGF-derived cultures and a neurogenic effect in cultures derived from FGF2 and heparin.¹²⁵ ^,132 In one study,¹³² this effect was noted in NPCs derived from embryonic and early postnatal rats but not in adults, whereas a second group reported these results for adult rat NPCs.¹²⁵ The combination of EGF and FGF2 with heparin, rather than either alone, was shown to have a synergistic effect with embryonic rat and human NPCs and with adult mouse subventricular NPCs.¹² ^,13,45,117 Addition of leukemia inhibitory factor or a laminin substrate with EGF and FGF2 had a preferential neurogenic effect in postnatal canine NPC cultures.¹²² In light of these findings, direct comparisons of NPCs grown under variable culture conditions must consider the conditions under which the cells are grown. Moreover, optimal growth conditions must be determined for NPCs according to developmental stage, species, and intended use.

Neurogenic Regions

While NPCs have been isolated from multiple areas of the postnatal brain, it is currently under debate whether neurogenesis occurs under normal conditions in areas outside the SVZ and SGZ. There is evidence of low-level adult neurogenesis in other regions, such as the neocortex, amygdala, and hypothalamus, but results are not always replicable, possibly because of the relatively small number of new neurons produced compared with the SVZ and SGZ.⁴⁴ There is limited postnatal neurogenesis in the rodent cerebellum, although multipotential cerebellar NPCs have been isolated from adult mice.⁶⁴

The SVZ and Rostral Migratory Stream

The astrocytic type B cells of the forebrain SVZ give rise to neuroblasts via a rapidly dividing population of neuronal progenitors (type C cells; Fig. 1).²⁷ Remarkably, neuroblasts then undergo chain migration over long distances from the lateral ventricular wall to the olfactory bulb, where they differentiate into granule and periglomerular cell interneurons throughout life. This chain migration, known as the rostral migratory stream (RMS), has been well characterized in rodents and nonhuman primates;^{21,27,61,68,86} new evidence suggests that this migration is guided by the flow of cerebrospinal fluid produced by the beating of ependymal cilia.¹⁰⁰ Until recently, the RMS was thought to be absent in humans⁹⁹ likely due to the fact that its orientation is slightly different from that of rodents.²¹ There is now evidence of the RMS and olfactory bulb neurogenesis in humans.^7,21

SGZ of the Dentate Gyrus

The hippocampal SGZ (Fig. 3 ), located between the granule cell layer and hilus, contains a population of quiescent astrocytic stem cells (GFAP+ type B cells), which give rise to a proliferating population of GFAP– type D cells (analogous to the SVZ type C cells) that differentiate into new granule neurons.^103,104 Type D cells differ from the type C cells of the SVZ because they divide less frequently and express early markers of neuronal differentiation.²⁵ The astrocytic NSCs in the SGZ have basal processes running under the dentate gyrus along blood vessels and apical processes extending into the granule layer. Unlike the SVZ, the new neurons migrate only a very short distance to the adjacent granular layer. Another difference between the 2 neurogenic regions is the presence of endothelial cell precursors intermingled with NPCs in the SGZ, the first evidence of a relationship between angiogenesis and neurogenesis in the adult brain.⁸⁵

Figure 4.

Schematic representation of fractones. Vascular and perivascular basal laminae and fractones are represented in dark black. The schema is based on the confocal distribution of laminin immunoreactivity, which defines the vascular/perivascular basal laminae and fractones. The depicted perivascular macrophages are part of a fibroblast/macrophage network originating from the meninges. Art, artery; Cap, capillary; Ep, ependyma; SVZ, subventricular zone; LV, lateral ventricle. Modified with permission from Mercier et al.⁶⁹

Cerebellar White Matter Tracts and the EGL

The cerebellum is the third location of postnatal neurogenesis. However, depending on species and location, neurogenesis appears to be limited to weeks to a year postnatally. Neuronal progenitors are present in the postnatal rodent cerebellar EGL up to 25 days and in the canine EGL until around 70 days; in humans, the EGL remains proliferative up to a year.^34,88 Neuronal progenitors of the EGL are restricted to generating granule cell neurons.¹²⁹ Cortical interneurons, as well as astrocytes and oligodendrocytes, are generated from multipotential NPCs that migrate from the VZ into white matter tracts after birth. The neurogenic potential of these NPCs appears limited to the first 2 postnatal weeks in rats.¹²⁸ It is of interest to note that cerebellar Bergmann glia, the only cells that retain radial glia morphology in the adult brain, originate from NPCs migrating from the VZ through the white matter within the first postnatal week. Moreover, adult Bergmann glia express the NSC markers Sox1, Sox2, and Sox9, as well as CD15 and GFAP.^1,5,6,37 Although overt neurogenesis has not been observed in the adult cerebellum, NSC potential is demonstrated in a subpopulation of cells isolated from the adult cerebellum in general⁵⁹ and Sox1-positive Bergmann glial cells in particular.¹

The NSC Niche

The concept of a stem cell niche arose from the observation that stem cells were evident in limited areas within adult tissues. The importance of the microenvironment’s role in cell fate is illustrated by in vitro experiments in which glial progenitors and NPCs from nonneurogenic areas demonstrate stem cell potential when exposed to the appropriate mixture of growth factors.^29,60,83 These studies suggest that neurogenesis is restricted to specific areas of the brain as a result of differentials in regulatory signals rather than an absence of NPCs in nonneurogenic regions. The SVZ and SGZ stem cell niches are microenvironments uniquely suited to the maintenance and regulation of stem cells. Fundamental niche properties identified to date include a specialized extracellular matrix, basal laminae, and NSC localized near blood vessels.

Vascular Niche

The unique relationship of NSCs to the vasculature was first identified in the SGZ of the hippocampal dentate gyrus.⁸⁵ Immunophenotyping of the proliferating cells in the SGZ revealed the presence of dividing cells in aggregates closely associated with blood vessels. Unexpectedly, more than a third of these proliferating cells were immunopositive for endothelial markers, and the surrounding area stained positively for vascular endothelial growth factor. The intimate association of NPCs and endothelial precursors around microvessels suggested a relationship between angiogenesis and neurogenesis. Further evidence of the importance of the vasculature in the NSC niche is provided by in vitro studies. In endothelial/NSC cocultures, diffusible factors released from endothelial cells inhibited NSC differentiation, stimulated self-renewal, and enhanced neurogenesis. Stem cell renewal appears to be promoted via Notch and Hes1 activation subsequent to enhanced neuroepithelial cell contact induced by soluble endothelial factors. Thus, while FGF2 is necessary for NSC proliferation, maintenance of self-renewal requires endothelial factors acting in concert with FGF2.¹⁰⁶

In the SVZ, the majority of proliferating type C cells and astrocytic stem cells are positioned within 10 μm of blood vessels, whereas most neuroblasts are found more distant to the microvasculature.¹¹³ SVZ astrocytic NSCs (type B cells) are present in a superficial layer just under and sometimes intercalating with the ependyma; these astrocytes have apical membranes snaking between ependymal cells and a basal process terminating on blood vessels.^72,107 Type B cells possessing long processes running parallel to the ventricle are also found subjacent to the apical type B cell layer.¹⁰⁷ The blood-brain barrier, typified by tight junctions between endothelial cells and astrocyte endfeet, appears to be modified in the SVZ. Type B and C cells have direct contact with blood vessels in areas that do not have pericyte coverage or astrocyte endfeet; accordingly, small molecules circulating in the blood have access to the SVZ. This unique relationship of NPCs to SVZ blood vessels is not detected in other nonneurogenic regions.¹¹³ The connection of type B cells to both the ventricle and the basal lamina of blood vessels provides 2 direct routes by which signal molecules can reach NSCs. However, unlike the SGZ, no evidence of proliferating endothelial precursors is detected in the adult SVZ.^107,113

Extracellular Matrix and Basal Laminae

Stem cells outside the brain are closely associated with basal laminae, which compartmentalize and store factors such as FGF2, bone morphogenetic proteins, vascular endothelial growth factor, and EGF.^40,69 Basal laminae are defined as aggregates of extracellular matrix rich in heparan sulfate glycosaminoglycans, fibronectin, laminin, and collagen IV.⁶⁹ In the SVZ, basal laminae are highly organized extravascular structures with a fractal arrangement, dubbed fractones.⁶⁹ Fractones have apical stems ending in bulbs immediately beneath the ependyma and extensively folded basal contacts with blood vessels. The fractal basal laminae enfold astrocytes, ependymal cells, NPCs, microglia, and the perivascular fibroblast/macrophage network (Fig. 4 ). Fractal basal laminae in the SVZ of mice, rats, and humans comprise laminin (β1 and γ1), collagen IV, the heparan sulfate proteoglycan (HSPG) perlecan, and glycoprotein nidogen.⁵⁵ An N-sulfated HSPG, seemingly distinct from perlecan, is present in a subpopulation of fractones and smaller subpopulation of blood vessels. Mitotic NPCs are highly associated with N-sulfated HSPG fractones compared with non-N-sulfated HSPG fractones, and mitotically active NPCs are located significantly closer to N-sulfated HSPG-containing fractones than blood vessels.⁵⁵ Of most interest is the finding that FGF2 binds mainly to N-sulfated HSPG fractones (and blood vessels) in situ and in vivo and that treatment with a heparitinase–heparanase mixture abolishes FGF2 binding in the SVZ.⁵⁵

Figure 5. Brain, septal subventricular zone; dog, postnatal day 21. Antinestin polyclonal and anti–glial fibrillary acidic protein (anti-GFAP) monoclonal antibodies. Confocal z-stack with immunostaining for nestin (green) and GFAP (red); step size is 0.2 μm. Note that there is colocalization of nestin and GFAP staining (yellow) in the processes that extend to the ventricle. Nuclei stain blue with 4,6 diamidino-2-phenylindole.

Figure 6. Brain, septal subventricular zone; dog, postnatal day 21. Higher magnification of the same region as in Figure 5. The arrow points to one of the double-labeled processes extending to the ventricular surface.

Collagen I, usually associated with basal laminae and growth factors during developmental morphogenesis, is detected in the SVZ in a broad, diffuse band, as well as within some fractone stems and bulbs.⁶⁹ This parenchymal band of collagen I is not detected in the parenchyma of surrounding brain structures. Other extracellular matrix molecules that are strongly expressed in the adult SVZ but are rare to absent in surrounding regions include glial-derived tenascin C and chondroitin sulfate glycosaminoglycans, especially dermatan sulfate–dependent (DSD-1) proteoglycan.⁴⁰

Tenascin C, is produced by type B NSCs and forms a layer between the SVZ and adjacent striatum.⁵⁴ The glycoprotein functions as a developmental regulator, enhancing and diminishing sensitivity to FGF2 and bone morphogenetic protein 4, respectively, which accelerates the developmental expression of EGF receptors in NPCs.³⁹ In contrast, chondroitin sulfate glycosaminoglycans are necessary for the proliferation, self-renewal, and maintenance of FGF2-responsive, but not EGF-responsive, NPCs.^109,110 In particular, the CS proteoglycan DSD-1, expressed on radial glia and retained on adult SVZ NPCs, is shown to have these properties. Targeted interference with the DSD-1 proteoglycan epitope recognized by the antibody 473HD results in significant diminution of neurosphere forming ability.¹¹⁸

Laminin is abundant around vascular cells in the SVZ and comprises part of the fractal basal laminae. The importance of laminin in the neurogenic niche is demonstrated by the expression of the laminin receptor α6ß1 on NPCs.^14,107 The integrin α6 is thus far the only gene identified in common between embryonic, hematopoietic, and NSCs.³³ In the adult mouse, α6-integrin is expressed by NPCs in acutely dissociated SVZ cultures and within neurospheres.¹⁰⁷ When the α6-integrin is blocked in vivo by intraventricular anti-α6-antibody infusion, there is roughly a 50% increase in NPCs located > 10 μm from blood vessels and nearly a 35% increase in proliferative NPCs.¹⁰⁷ These findings suggest that α6-integrin expression is integral in maintaining NPCs within the vascular niche and regulating NPC proliferation and differentiation.

Immunomodulation of Neurogenesis

The NSC niche, like the rest of the brain, is not immunologically privileged as once thought but is an immunologically specialized region in which microglia and lymphocytes appear to interact with NPCs in both health and disease.^{11,73,120,126,133} Recent studies have shown that in disease, inflammation may either enhance or suppress neurogenesis depending on how microglia, newly recruited blood monocytes, and/or astrocytes are activated. Activation of microglia with helper T-cell cytokines interferon γ and interleukin-4 (IL-4) is shown to promote neurogenesis and oligodendrogenesis, whereas tumor necrosis factor α, expressed by endotoxin-activated microglia, inhibits neurogenesis.^11,73 Subsequent investigations into the nondiseased adult neurogenic niche in rats revealed that increased hippocampal neurogenesis induced by environmental enrichment was associated with microglial activation and expression of MHC-II proteins, as well as insulin-like growth factor.¹³³ Microglial MHC-II expression suggests activation by Th2 cytokines (eg, IL-4); MHC-II is not expressed after innate immune activation of microglia (eg, endotoxin).¹²⁷ In contrast, exercise-induced increases in hippocampal neurogenesis in mice showed no evidence of microglial activation, microglial MHC II expression, or increase in T cells in the hippocampus.⁸⁰ The discrepancies between these 2 studies may be attributable to the differential effects of environmental enrichment versus physical activity on neurogenesis and/or to the species, rat versus mouse.

The importance of T cells in adult physiologic neurogenesis is supported by studies showing that neurogenesis is impaired in the hippocampus and SVZ of genetically immune-deficient mice and can be restored by systemic repopulation with T cells, specifically CD4+ T cells.^126,133 Moreover, immune-depleted and genetically immune-deficient mice show impaired spatial learning and memory compared with wild-type controls.^58,126,133 Interestingly, peripheral T-cell activation results in a transient increase in hippocampal neurogenesis and NPC proliferation.¹²⁷ In health, T cells are rare to absent in the CNS parenchyma but present in the meningeal spaces and choroid plexus/ventricular walls. Under physiologic conditions, the neurogenic effects of the adaptive immune response may originate largely from T cells within the meningeal spaces rather than the hippocampus; these effects appear to be mediated via IL-4.²³

While data indicate that adult neurogenesis is dependent on systemic CD4+ T-cell activity, controversy remains with respect to T-cell antigen specificity. Ziv et al showed that neurogenesis was enhanced in transgenic mice with an excess of monospecific T cells directed against a CNS-specific antigen (myelin basic protein) and reduced in transgenic mice with an excess of T cells directed against nonself antigen (ovalbumin) relative to matched wild-type controls.¹³³ Wolf et al found that repopulation of immune-deficient RAG2^–/– mice with a heterogeneous population of CD4+ T cells was sufficient to increase neurogenesis. However, repopulation with CNS-specific T cells was not evaluated.¹²⁶

NPC Phenotype

Phenotypic characterization of NPCs aids in the identification of true stem cells, allows their prospective isolation and enrichment for therapeutic purposes, and helps in elucidating the mechanisms by which the nervous system develops and regenerates. Among the first NSC markers identified were nestin and vimentin, which are intracellular intermediate filament proteins.⁴⁷ ^,65 Vimentin and nestin are not ideal NPC markers, because their expression is not restricted to NPCs. Vimentin is present in many other cell types in the CNS besides NPCs, and nestin is expressed in fibroblastic cell lines, reactive astrocytes, and skeletal muscle.⁹⁸ ^,102,112 Moreover, neural progenitor/stem cells have been identified that do not express nestin.⁶²

Work in the past decade has shown that NSCs in the developing fetus and in neurogenic regions of the postnatal brain express GFAP.²⁶ ^{,50,70,74,76,104} The quiescent astrocytic type B cell identified as the NSC in the adult mouse SVZ is both GFAP+ and nestin+.²⁷ Like nestin and vimentin, GFAP is an intracellular filament protein and is expressed in nonprecursor cells as well as NSCs. Although none of these proteins alone is a definitive NSC marker, labeling for combinations of markers in conjunction with cellular morphology and location may serve to accurately identify NSCs (Figs. 5, 6).

While the use of intracellular NPC markers is a valuable tool for in situ studies of neural development and regeneration, prospective isolation and enrichment of NPCs for in vitro and transplantation studies require a cell surface marker. Candidate cell surface markers used to enrich for NPCs include the hematopoietic stem cell marker CD133 and differential binding of peanut agglutinin and heat-stable antigen.^64,96,116 Human and murine hematopoietic stem cells can be fractionated using the cell surface antigen CD133 or the differential binding of peanut agglutinin and heat-stable antigen. When applied to single cell suspensions from the ependyma/subependyma of adult mice, approximately 80% of the cells that had poor peanut agglutinin binding and expressed very low levels of heat-stable antigen were capable of forming neurospheres; this represents nearly a 250-fold enrichment of NPC activity compared to unsorted cells.⁹⁶ The cell surface marker CD133 selected for a nearly 24-fold increase of NPC activity from human fetal brain tissue and prospectively identified a multipotent NPC in the postnatal mouse cerebellum.⁶⁴ ^,116 Interestingly, CD133 expression in the murine postnatal SVZ is confined to a subpopulation of ependymal cells capable of forming neurospheres with limited self-renewal ability in vitro.^19,87

Three other cell surface markers associated with NPCs in developing and adult brains are Lewis X antigen (CD15), an epitope in phosphacan (DSD-1 proteoglycan) recognized by the monoclonal antibody 473HD, and the integrin β1—specifically, α6β1-integrin.^{14,15,107,118} Enriching for CD15+ cells from the adult mouse subependyma resulted in a 25-fold increase in NPC activity,¹⁵ whereas 473HD+ cells showed a 6-fold increase in neurosphere formation compared with unsorted cells.¹¹⁸ Integrin β1-expressing cells from postnatal mice and rats generated 3 to 12 times as many neurospheres as β1-negative cells.¹⁴

A method of hematopoietic stem cell purification, side population analysis, has been successfully used to enrich for NPCs. Hematopoietic stem cells exclude the nuclear dye Hoechst 33342, resulting in the presence of a side population of cells with low fluorescence emission.⁴³ Differential dye efflux was shown to be a property of NPCs, as it is with hematopoietic stem cells.⁴⁸ ^,57 In the side population derived from adult neurospheres, there was a 7.5-fold enrichment in NPC activity.⁵⁷ Moreover, combining side population analysis with CD15 labeling further enriched NSC activity beyond that achieved by side population analysis alone. This enrichment was much more pronounced with embryonic NPCs (38% enrichment) than with adult NPCs (10% enrichment).⁵⁷

Conclusion

The current body of knowledge concerning developmental and adult human neurogenesis is largely attributable to research animals, including avian species. Postnatal neurogenesis was discovered in rats in the 1960s but was not widely accepted until 2 decades later when research studies in adult songbirds elegantly demonstrated seasonal neurogenesis in specialized regions. As more animal species are investigated, fundamental mechanisms of neurogenesis and NSC characteristics appear to be well conserved across diverse species. Interspecies conservation of neurogenesis (along with the existence of large and small animal models of neurogenic diseases) affords unique investigative and therapeutic research opportunities relevant to both humans and animals.

Footnotes

Acknowledgements

I thank Dr John H. Wolfe for mentorship. Figures 2, 5, and were obtained from research funded by National Institutes of Health grants DK42707, DK46637, and DK63973 to J. H. Wolfe and by a National Center for Research Resources fellowship to R. M. Walton (RR07063).

The author declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

The author received no financial support for the research, authorship, and/or publication of this article.

References

Alcock

Sottile

. Dynamic distribution and stem cell characteristics of Sox1-expressing cells in the cerebellar cortex. Cell Res. 2009;19:1324–1333.

Altman

. Are new neurons formed in the brains of adult mammals? Science. 1962;135:1127–1128.

Altman

Das

. Autoradiographic and histological evidence of postnatal hippocampal neurogenesis in rats. J Comp Neurol. 1965;124:319–335.

Altman

Das

. Post-natal origin of microneurones in the rat brain. Nature. 1965;207:953–956.

Baboval

Crandall

Kinnally

Chou

Smith

. Restriction of high CD15 expression to a subset of rat cerebellar astroglial cells can be overcome by transduction with adenoviral vectors expressing the rat alpha 1,3-fucosyltransferase IV gene. Glia. 2000;31:144–154.

Bartsch

Mai

. Distribution of the 3-fucosyl-N-acetyl-lactosamine (FAL) epitope in the adult mouse brain. Cell Tissue Res. 1991;263:353–366.

Bedard

Parent

. Evidence of newly generated neurons in the human olfactory bulb. Brain Res Dev Brain Res. 2004;151:159–168.

Beites

Kawauchi

Crocker

Calof

. Identification and molecular regulation of neural stem cells in the olfactory epithelium. Exp Cell Res. 2005;306:309–316.

Belachew

Chittajallu

Aguirre

Postnatal NG2 proteoglycan-expressing progenitor cells are intrinsically multipotent and generate functional neurons. J Cell Biol. 2003;161:169–186.

10.

Bonaguidi

McGuire

LIF and BMP signaling generate separate and discrete types of GFAP-expressing cells. Development. 2005;132:5503–5514.

11.

Butovsky

Ziv

Schwartz

Microglia activated by IL-4 or IFN-gamma differentially induce neurogenesis and oligodendrogenesis from adult stem/progenitor cells. Mol Cell Neurosci. 2006;31:149–160.

12.

Caldwell

Garcion

terBorg

Heparin stabilizes FGF-2 and modulates striatal precursor cell behavior in response to EGF. Exp Neurol. 2004;188:408–420.

13.

Caldwell

Svendsen

. Heparin, but not other proteoglycans potentiates the mitogenic effects of FGF-2 on mesencephalic precursor cells. Exp Neurol. 1998;152:1–10.

14.

Campos

Leone

Relvas

Beta1 integrins activate a MAPK signalling pathway in neural stem cells that contributes to their maintenance. Development. 2004;131:3433–3444.

15.

Capela

Temple

. LeX/ssea-1 is expressed by adult mouse CNS stem cells, identifying them as nonependymal. Neuron. 2002;35:865–875.

16.

Carlen

Meletis

Goritz

Forebrain ependymal cells are Notch-dependent and generate neuroblasts and astrocytes after stroke. Nat Neurosci. 2009;12:259–267.

17.

Carpenter

Cui

. In vitro expansion of a multipotent population of human neural progenitor cells. Exp Neurol. 1999;158:265–278.

18.

Chiasson

Tropepe

Morshead

Adult mammalian forebrain ependymal and subependymal cells demonstrate proliferative potential, but only subependymal cells have neural stem cell characteristics. J Neurosci. 1999;19:4462–4471.

19.

Coskun

Blanchi

CD133+ neural stem cells in the ependyma of mammalian postnatal forebrain. Proc Natl Acad Sci U S A. 2008;105:1026–1031.

20.

Craig

Tropepe

Morshead

. In vivo growth factor expansion of endogenous subependymal neural precursor cell populations in the adult mouse brain. J Neurosci. 1996;16:2649–2658.

21.

Curtis

Kam

Nannmark

Human neuroblasts migrate to the olfactory bulb via a lateral ventricular extension. Science. 2007;315:1243–1249.

22.

Deng

Aimone

Gage

. New neurons and new memories: how does adult hippocampal neurogenesis affect learning and memory? Nat Rev Neurosci. 2010;11:339–350.

23.

Derecki

Cardani

Yang

Regulation of learning and memory by meningeal immunity: a key role for IL-4. J Exp Med. 2010;207:1067–1080.

24.

Doetsch

. The glial identity of neural stem cells. Nat Neurosci. 2003;6:1127–1134.

25.

Doetsch

. A niche for adult neural stem cells. Curr Opin Genet Dev. 2003;13:543–550.

26.

Doetsch

Caille

Lim

Subventricular zone astrocytes are neural stem cells in the adult mammalian brain. Cell. 1999;97:703–716.

27.

Doetsch

Garcia-Verdugo

Alvarez-Buylla

. Cellular composition and three-dimensional organization of the subventricular germinal zone in the adult mammalian brain. J Neurosci. 1997;17:5046–5061.

28.

Doetsch

Garcia-Verdugo

Alvarez-Buylla

Regeneration of a germinal layer in the adult mammalian brain. Proc Natl Acad Sci U S A. 1999;96:11619–11624.

29.

Doetsch

Petreanu

Caille

EGF converts transit-amplifying neurogenic precursors in the adult brain into multipotent stem cells. Neuron. 2002;36:1021–1034.

30.

Duittoz

Hevor

. Primary culture of neural precursors from the ovine central nervous system (CNS). J Neurosci Methods. 2001;107:131–140.

31.

Einstein

Karussis

Grigoriadis

Intraventricular transplantation of neural precursor cell spheres attenuates acute experimental allergic encephalomyelitis. Mol Cell Neurosci. 2003;24:1074–1082.

32.

Eriksson

Perfilieva

Bjork-Eriksson

Neurogenesis in the adult human hippocampus. Nat Med. 1998;4:1313–1317.

33.

Fortunel

Otu

. Comment on “‘Stemness’: transcriptional profiling of embryonic and adult stem cells” and “a stem cell molecular signature.” Science. 2003;302:393.

34.

Gage

Ray

Fisher

. Isolation, characterization, and use of stem cells from the CNS. Annu Rev Neurosci. 1995;18:159–192.

35.

Gage

Coates

Palmer

. Survival and differentiation of adult neuronal progenitor cells transplanted to the adult brain. Proc Natl Acad Sci U S A. 1995;92:11879–11883.

36.

Gaiano

Nye

Fishell

. Radial glial identity is promoted by Notch1 signaling in the murine forebrain. Neuron. 2000;26:395–404.

37.

Ganat

Silbereis

Cave

Early postnatal astroglial cells produce multilineage precursors and neural stem cells in vivo. J Neurosci. 2006;26:8609–8621.

38.

Garcia

Doan

Imura

GFAP-expressing progenitors are the principal source of constitutive neurogenesis in adult mouse forebrain. Nat Neurosci. 2004;7:1233–1241.

39.

Garcion

Halilagic

Faissner

Generation of an environmental niche for neural stem cell development by the extracellular matrix molecule tenascin C. Development. 2004;131:3423–3432.

40.

Gates

Thomas

Howard

Cell and molecular analysis of the developing and adult mouse subventricular zone of the cerebral hemispheres. J Comp Neurol. 1995;361:249–266.

41.

Gil-Perotin

Duran-Moreno

Belzunegui

Ultrastructure of the subventricular zone in Macaca fascicularis and evidence of a mouse-like migratory stream. J Comp Neurol. 2009;514:533–554.

42.

Goldman

Nottebohm

. Neuronal production, migration, and differentiation in a vocal control nucleus of the adult female canary brain. Proc Natl Acad Sci U S A. 1983;80:2390–2394.

43.

Goodell

Brose

Paradis

Isolation and functional properties of murine hematopoietic stem cells that are replicating in vivo. J Exp Med. 1996;183:1797–1806.

44.

Gould

. How widespread is adult neurogenesis in mammals? Nat Rev Neurosci. 2007;8:481–488.

45.

Gritti

Frolichsthal-Schoeller

Galli

Epidermal and fibroblast growth factors behave as mitogenic regulators for a single multipotent stem cell-like population from the subventricular region of the adult mouse forebrain. J Neurosci. 1999;19:3287–3297.

46.

Herrera

Garcia-Verdugo

Alvarez-Buylla

. Adult-derived neural precursors transplanted into multiple regions in the adult brain. Ann Neurol. 1999;46:867–877.

47.

Hockfield

McKay

. Identification of major cell classes in the developing mammalian nervous system. J Neurosci. 1985;5:3310–3328.

48.

Hulspas

Quesenberry

. Characterization of neurosphere cell phenotypes by flow cytometry. Cytometry. 2000;40:245–250.

49.

Imayoshi

Sakamoto

Ohtsuka

Roles of continuous neurogenesis in the structural and functional integrity of the adult forebrain. Nat Neurosci. 2008;11:1153–1161.

50.

Imura

Kornblum

Sofroniew

. The predominant neural stem cell isolated from postnatal and adult forebrain but not early embryonic forebrain expresses GFAP. J Neurosci. 2003;23:2824–2832.

51.

Imura

Nakano

Kornblum

Phenotypic and functional heterogeneity of GFAP-expressing cells in vitro: differential expression of LeX/CD15 by GFAP-expressing multipotent neural stem cells and non-neurogenic astrocytes. Glia. 2006;53:277–293.

52.

Johansson

Momma

Clarke

Identification of a neural stem cell in the adult mammalian central nervous system. Cell. 1999;96:25–34.

53.

Kawauchi

Beites

Crocker

Molecular signals regulating proliferation of stem and progenitor cells in mouse olfactory epithelium. Dev Neurosci. 2004;26:166–180.

54.

Kazanis

Belhadi

Faissner

The adult mouse subependymal zone regenerates efficiently in the absence of tenascin-C. J Neurosci. 2007;27:13991–13996.

55.

Kerever

Schnack

Vellinga

Novel extracellular matrix structures in the neural stem cell niche capture the neurogenic factor fibroblast growth factor 2 from the extracellular milieu. Stem Cells. 2007;25:2146–2157.

56.

Kilpatrick

Bartlett

. Cloning and growth of multipotential neural precursors: requirements for proliferation and differentiation. Neuron. 1993;10:255–265.

57.

Kim

Morshead

. Distinct populations of forebrain neural stem and progenitor cells can be isolated using side-population analysis. J Neurosci. 2003;23:10703–10709.

58.

Kipnis

Cohen

Cardon

T cell deficiency leads to cognitive dysfunction: implications for therapeutic vaccination for schizophrenia and other psychiatric conditions. Proc Natl Acad Sci U S A. 2004;101:8180–8185.

59.

Klein

Butt

Machold

Cerebellum- and forebrain-derived stem cells possess intrinsic regional character. Development. 2005;132:4497–4508.

60.

Kondo

Raff

. Oligodendrocyte precursor cells reprogrammed to become multipotential CNS stem cells. Science. 2000;289:1754–1757.

61.

Kornack

Rakic

. The generation, migration, and differentiation of olfactory neurons in the adult primate brain. Proc Natl Acad Sci U S A. 2001;98:4752–4757.

62.

Kukekov

Laywell

Thomas

A nestin-negative precursor cell from the adult mouse brain gives rise to neurons and glia. Glia. 1997;21:399–407.

63.

Laywell

Rakic

Kukekov

Identification of a multipotent astrocytic stem cell in the immature and adult mouse brain. Proc Natl Acad Sci U S A. 2000;97:13883–13888.

64.

Lee

Kessler

Read

Isolation of neural stem cells from the postnatal cerebellum. Nat Neurosci. 2005;8:723–729.

65.

Lendahl

Zimmerman

McKay

. CNS stem cells express a new class of intermediate filament protein. Cell. 1990;60:585–595.

66.

Lillien

Raphael

. BMP and FGF regulate the development of EGF-responsive neural progenitor cells. Development. 2000;127:4993–5005.

67.

Liu

Martin

. Olfactory bulb core is a rich source of neural progenitor and stem cells in adult rodent and human. J Comp Neurol. 2003;459:368–391.

68.

Lois

Alvarez-Buylla

. Long-distance neuronal migration in the adult mammalian brain. Science. 1994;264:1145–1148.

69.

Mercier

Kitasako

Hatton

. Anatomy of the brain neurogenic zones revisited: fractones and the fibroblast/macrophage network. J Comp Neurol. 2002;451:170–188.

70.

Merkle

Tramontin

Garcia-Verdugo

Radial glia give rise to adult neural stem cells in the subventricular zone. Proc Natl Acad Sci U S A. 2004;101:17528–17532.

71.

Milward

Lundberg

Isolation and transplantation of multipotential populations of epidermal growth factor-responsive, neural progenitor cells from the canine brain. J Neurosci Res. 1997;50:862–871.

72.

Mirzadeh

Merkle

Soriano-Navarro

Neural stem cells confer unique pinwheel architecture to the ventricular surface in neurogenic regions of the adult brain. Cell Stem Cell. 2008;3:265–278.

73.

Monje

Toda

Palmer

. Inflammatory blockade restores adult hippocampal neurogenesis. Science. 2003;302:1760–1765.

74.

Morshead

Garcia

Sofroniew

The ablation of glial fibrillary acidic protein-positive cells from the adult central nervous system results in the loss of forebrain neural stem cells but not retinal stem cells. Eur J Neurosci. 2003;18:76–84.

75.

Moyer

Johnson

Zompa

Culture, expansion, and transplantation of human fetal neural progenitor cells. Transplant Proc. 1997;29:2040–2041.

76.

Noctor

Flint

Weissman

Dividing precursor cells of the embryonic cortical ventricular zone have morphological and molecular characteristics of radial glia. J Neurosci. 2002;22:3161–3173.

77.

Nottebohm

. Neuronal replacement in adulthood. Ann N Y Acad Sci. 1985;457:143–161.

78.

Nunes

Roy

Keyoung

Identification and isolation of multipotential neural progenitor cells from the subcortical white matter of the adult human brain. Nat Med. 2003;9:439–447.

79.

Oka

Honmou

Akiyama

Autologous transplantation of expanded neural precursor cells into the demyelinated monkey spinal cord. Brain Res. 2004;1030:94–102.

80.

Olah

Ping

De Haas

Enhanced hippocampal neurogenesis in the absence of microglia T cell interaction and microglia activation in the murine running wheel model. Glia. 2009;57:1046–1061.

81.

Ourednik

Lynch

Neural stem cells display an inherent mechanism for rescuing dysfunctional neurons. Nat Biotechnol. 2002;20:1103–1110.

82.

Pagano

Impagnatiello

Girelli

Isolation and characterization of neural stem cells from the adult human olfactory bulb. Stem Cells. 2000;18:295–300.

83.

Palmer

Ray

Gage

. FGF-2-responsive neuronal progenitors reside in proliferative and quiescent regions of the adult rodent brain. Mol Cell Neurosci. 1995;6:474–486.

84.

Palmer

Takahashi

Gage

. The adult rat hippocampus contains primordial neural stem cells. Mol Cell Neurosci. 1997;8:389–404.

85.

Palmer

Willhoite

Gage

. Vascular niche for adult hippocampal neurogenesis. J Comp Neurol. 2000;425:479–494.

86.

Pencea

Bingaman

Freedman

Neurogenesis in the subventricular zone and rostral migratory stream of the neonatal and adult primate forebrain. Exp Neurol. 2001;172:1–16.

87.

Pfenninger

Roschupkina

Hertwig

CD133 is not present on neurogenic astrocytes in the adult subventricular zone, but on embryonic neural stem cells, ependymal cells, and glioblastoma cells. Cancer Res. 2007;67:5727–5736.

88.

Phemister

Young

. The postnatal development of the canine cerebellar cortex. J Comp Neurol. 1968;134:243–254.

89.

Pluchino

Quattrini

Brambilla

Injection of adult neurospheres induces recovery in a chronic model of multiple sclerosis. Nature. 2003;422:688–694.

90.

Rakic

. Elusive radial glial cells: historical and evolutionary perspective. Glia. 2003;43:19–32.

91.

Ray

Peterson

Schinstine

Proliferation, differentiation, and long-term culture of primary hippocampal neurons. Proc Natl Acad Sci U S A. 1993;90:3602–3606.

92.

Reynolds

Rietze

. Neural stem cells and neurospheres: re-evaluating the relationship. Nat Methods. 2005;2:333–336.

93.

Reynolds

Weiss

. Clonal and population analyses demonstrate that an EGF-responsive mammalian embryonic CNS precursor is a stem cell. Dev Biol. 1996;175:1–13.

94.

Reynolds

Weiss

. Generation of neurons and astrocytes from isolated cells of the adult mammalian central nervous system. Science. 1992;255:1707–1710.

95.

Richards

Kilpatrick

Bartlett

. De novo generation of neuronal cells from the adult mouse brain. Proc Natl Acad Sci U S A. 1992;89:8591–8595.

96.

Rietze

Valcanis

Brooker

Purification of a pluripotent neural stem cell from the adult mouse brain. Nature. 2001;412:736–739.

97.

Rodriguez-Perez

Perez-Martin

Jimenez

Immunocytochemical characterisation of the wall of the bovine lateral ventricle. Cell Tissue Res. 2003;314:325–335.

98.

Sahin Kaya

Mahmood

Expression of nestin after traumatic brain injury in rat brain. Brain Res. 1999;840:153–157.

99.

Sanai

Tramontin

Quinones-Hinojosa

Unique astrocyte ribbon in adult human brain contains neural stem cells but lacks chain migration. Nature. 2004;427:740–744.

100.

Sawamoto

Wichterle

Gonzalez-Perez

New neurons follow the flow of cerebrospinal fluid in the adult brain. Science. 2006;311:629–632.

101.

Seaberg

van der Kooy

. Stem and progenitor cells: the premature desertion of rigorous definitions. Trends Neurosci. 2003;26:125–131.

102.

Sejersen

Lendahl

. Transient expression of the intermediate filament nestin during skeletal muscle development. J Cell Sci. 1993;106(pt 4):1291–1300.

103.

Seri

Garcia-Verdugo

Collado-Morente

Cell types, lineage, and architecture of the germinal zone in the adult dentate gyrus. J Comp Neurol. 2004;478:359–378.

104.

Seri

Garcia-Verdugo

McEwen

Astrocytes give rise to new neurons in the adult mammalian hippocampus. J Neurosci. 2001;21:7153–7160.

105.

Seri

Herrera

Gritti

Composition and organization of the SCZ: a large germinal layer containing neural stem cells in the adult mammalian brain. Cereb Cortex. 2006;16(suppl 1):i103–111.

106.

Shen

Goderie

Jin

Endothelial cells stimulate self-renewal and expand neurogenesis of neural stem cells. Science. 2004;304:1338–1340.

107.

Shen

Wang

Kokovay

Adult SVZ stem cells lie in a vascular niche: a quantitative analysis of niche cell-cell interactions. Cell Stem Cell. 2008;3:289–300.

108.

Shihabuddin

Numan

Huff

Intracerebral transplantation of adult mouse neural progenitor cells into the Niemann-Pick-A mouse leads to a marked decrease in lysosomal storage pathology. J Neurosci. 2004;24:10642–10651.

109.

Sirko

von Holst

Weber

Chondroitin sulfates are required for fibroblast growth factor-2-dependent proliferation and maintenance in neural stem cells and for epidermal growth factor-dependent migration of their progeny. Stem Cells. 2010;28:775–787.

110.

Sirko

von Holst

Wizenmann

Chondroitin sulfate glycosaminoglycans control proliferation, radial glia cell differentiation and neurogenesis in neural stem/progenitor cells. Development. 2007;134:2727–2738.

111.

Smith

Blakemore

. Porcine neural progenitors require commitment to the oligodendrocyte lineage prior to transplantation in order to achieve significant remyelination of demyelinated lesions in the adult CNS. Eur J Neurosci. 2000;12:2414–2424.

112.

Steinert

Chou

Prahlad

A high molecular weight intermediate filament-associated protein in BHK-21 cells is nestin, a type VI intermediate filament protein: limited co-assembly in vitro to form heteropolymers with type III vimentin and type IV alpha-internexin. J Biol Chem. 1999;274:9881–9890.

113.

Tavazoie

Van der Veken

Silva-Vargas

A specialized vascular niche for adult neural stem cells. Cell Stem Cell. 2008;3:279–288.

114.

Tramontin

Garcia-Verdugo

Lim

Postnatal development of radial glia and the ventricular zone (VZ): a continuum of the neural stem cell compartment. Cereb Cortex. 2003;13:580–587.

115.

Tropepe

Sibilia

Ciruna

Distinct neural stem cells proliferate in response to EGF and FGF in the developing mouse telencephalon. Dev Biol. 1999;208:166–188.

116.

Uchida

Buck

Direct isolation of human central nervous system stem cells. Proc Natl Acad Sci U S A. 2000;97:14720–14725.

117.

Vescovi

Parati

Gritti

Isolation and cloning of multipotential stem cells from the embryonic human CNS and establishment of transplantable human neural stem cell lines by epigenetic stimulation. Exp Neurol. 1999;156:71–83.

118.

von Holst

Sirko

Faissner

. The unique 473HD-Chondroitinsulfate epitope is expressed by radial glia and involved in neural precursor cell proliferation. J Neurosci. 2006;26:4082–4094.

119.

Walton

Magnitsky

Seiler

Transplantation and magnetic resonance imaging of canine neural progenitor cell grafts in the postnatal dog brain. J Neuropathol Exp Neurol. 2008;67:954–962.

120.

Walton

Sutter

Laywell

Microglia instruct subventricular zone neurogenesis. Glia. 2006;54:815–825.

121.

Walton

Wolfe

. Abnormalities in neural progenitor cells in a dog model of lysosomal storage disease. J Neuropathol Exp Neurol. 2007;66:760–769.

122.

Walton

Wolfe

. In vitro growth and differentiation of canine olfactory bulb-derived neural progenitor cells under variable culture conditions. J Neurosci Methods. 2008;169:158–167.

123.

Wang

Martin

Baker

Neural progenitor cell transplantation and imaging in a large animal model. Neurosci Res. 2007;59:327–340.

124.

Weissman

Noctor

Clinton

Neurogenic radial glial cells in reptile, rodent and human: from mitosis to migration. Cereb Cortex. 2003;13:550–559.

125.

Whittemore

Morassutti

Walters

Mitogen and substrate differentially affect the lineage restriction of adult rat subventricular zone neural precursor cell populations. Exp Cell Res. 1999;252:75–95.

126.

Wolf

Steiner

Akpinarli

CD4-positive T lymphocytes provide a neuroimmunological link in the control of adult hippocampal neurogenesis. J Immunol. 2009;182:3979–3984.

127.

Wolf

Steiner

Wengner

. Adaptive peripheral immune response increases proliferation of neural precursor cells in the adult hippocampus. FASEB J. 2009;23:3121–3128.

128.

Zhang

Goldman

. Developmental fates and migratory pathways of dividing progenitors in the postnatal rat cerebellum. J Comp Neurol. 1996;370:536–550.

129.

Zhang

Goldman

. Generation of cerebellar interneurons from dividing progenitors in white matter. Neuron. 1996;16:47–54.

130.

Zhang

Lipsitz

Duncan

. Self-renewing canine oligodendroglial progenitor expanded as oligospheres. J Neurosci Res. 1998;54:181–190.

131.

Zhao

Deng

Gage

. Mechanisms and functional implications of adult neurogenesis. Cell. 2008;132:645–660.

132.

Zhu

Mehler

Mabie

Developmental changes in progenitor cell responsiveness to cytokines. J Neurosci Res. 1999;56:131–145.

133.

Ziv

Ron

Butovsky

Immune cells contribute to the maintenance of neurogenesis and spatial learning abilities in adulthood. Nat Neurosci. 2006;9:268–275.

Postnatal Neurogenesis

Abstract

Keywords

Neural Precursor Cells

In Vitro Propagation

Neurogenic Regions

The SVZ and Rostral Migratory Stream

SGZ of the Dentate Gyrus

Cerebellar White Matter Tracts and the EGL

The NSC Niche

Vascular Niche

Extracellular Matrix and Basal Laminae

Immunomodulation of Neurogenesis

NPC Phenotype

Conclusion

Footnotes

Acknowledgements

References