Unique astrocyte ribbon in adult human brain contains neural stem cells but lacks chain migration

The relationship between nature of social change, age, and position of new neurons and their survival in adult zebra finch brain

Some kinds of neurons are spontaneously recruited in the intact, healthy adult brain, but the variables that affect their survival are not always clear. We show that in caudal nidopallium of adult male zebra finches, the rostrocaudal position of newly recruited neurons, their age (1 vs 3 months), and the nature of social change (complex vs simple) after the neurons were born affect their survival. Greater social complexity promoted the survival of younger new neurons, and the demise of older ones; a less marked social change promoted the survival of older new neurons. These effects were position dependent. We suggest that functional correlations between new neuron recruitment/survival and its inferred benefit to the animal might be better perceived when taking into account the position of cells, their age at the time of life style changes, and the nature and magnitude of the life style change.

Genesis of neuronal and glial progenitors in the cerebellar cortex of peripuberal and adult rabbits

Adult neurogenesis in mammals is restricted to some brain regions, in contrast with other vertebrates in which the genesis of new neurons is more widespread in different areas of the nervous system. In the mammalian cerebellum, neurogenesis is thought to be limited to the early postnatal period, coinciding with end of the granule cell genesis and disappearance of the external granule cell layer (EGL). We recently showed that in the rabbit cerebellum the EGL is replaced by a proliferative layer called ‘subpial layer’ (SPL) which persists beyond puberty on the cerebellar surface. Here we investigated what happens in the cerebellar cortex of peripuberal rabbits by using endogenous and exogenously-administered cell proliferation antigens in association with a cohort of typical markers for neurogenesis. We show that cortical cell progenitors extensively continue to be generated herein. Surprisingly, this neurogenic process continues to a lesser extent in the adult, even in the absence of a proliferative SPL. We describe two populations of newly generated cells, involving neuronal cells and multipolar, glia-like cells. The genesis of neuronal precursors is restricted to the molecular layer, giving rise to cells immunoreactive for GABA, and for the transcription factor Pax2, a marker for GABAergic cerebellar interneuronal precursors of neuroepithelial origin that ascend through the white matter during early postnatal development. The multipolar cells are Map5+, contain Olig2 and Sox2 transcription factors, and are detectable in all cerebellar layers. Some dividing Sox2+ cells are Bergmann glia cells. All the cortical newly generated cells are independent from the SPL and from granule cell genesis, the latter ending before puberty. This study reveals that adult cerebellar neurogenesis can exist in some mammals. Since rabbits have a longer lifespan than rodents, the protracted neurogenesis within its cerebellar parenchyma could be a suitable model for studying adult nervous tissue permissiveness in mammals.

Mechanisms of Disease: the role of stem cells in the biology and treatment of gliomas

The study of neural stem cell and progenitor cell biology has improved our understanding of the biology of brain tumors in a developmental context. Recent work has demonstrated that brain tumors may harbor small subpopulations of cells that share characteristics of neural stem cells. There is still an ongoing debate about the specific role of these stem-like cells in cancer initiation, development and progression. Nonetheless, the concept of cancer stem cells has offered a new paradigm to understand tumor biology and resistance to current treatment modalities. Molecular aberrations in these cancer stem cells might be crucial targets for therapeutic intervention, with the hope of achieving more durable clinical responses. Recent studies have demonstrated that endogenous and transplanted neural stem cells and progenitor cells show a marked tropism to brain tumors. Although the mechanisms that govern these processes are poorly understood, the use of neural stem cells and progenitor cells as delivery vehicles for molecules toxic to tumors offers a promising experimental treatment strategy. This Review summarizes recent advances in the basic understanding of neural stem cell and cancer stem cell biology and the progress towards translating these novel concepts into the clinic.

Hoxb1 controls cell fate specification and proliferative capacity of neural stem and progenitor cells

The directed differentiation of embryonic stem cells (ESCs) into neural stem cells (NSCs) of specific identities and the identification of endogenous pathways that may mediate expansion of NSCs are fundamental goals for the treatment of degenerative disorders and trauma of the nervous system. We report that timely induction of a Hoxb1 transgene in ESC-derived NSCs resulted in the specification of NSCs toward a hindbrain-specific identity through the activation of a rhombomere 4-specific genetic program and the repression of anterior neural identity. This change was accompanied by changes in signaling pathways that pattern the dorsoventral (DV) axis of the nervous system and concomitant changes in the expression of DV neural progenitor markers. Furthermore, Hoxb1 mediated the maintenance and expansion of posterior neural progenitor cells. Hoxb1(+) cells kept proliferating upon mitogen withdrawal and became transiently amplifying progenitors instead of terminally differentiating. This was partially attributed to Hoxb1-dependent activation of the Notch signaling pathway and Notch-dependent STAT3 phosphorylation at Ser 727, thus linking Hox gene function with maintenance of active Notch signaling and the JAK/STAT pathway. Thus, timely expression of specific Hox genes could be used to establish NSCs and neural progenitors of distinct posterior identities. ESC-derived NSCs have a mixed DV identity that is subject to regulation by Hox genes. Finally, these findings set the stage for the elucidation of molecular pathways involved in the expansion of posterior NSCs and neural progenitors. Disclosure of potential conflicts of interest is found at the end of this article.

Stem cell markers in gliomas

Gliomas are the most common tumours of the central nervous system (CNS) and a frequent cause of mental impairment and death. Treatment of malignant gliomas is often palliative because of their infiltrating nature and high recurrence. Genetic events that lead to brain tumours are mostly unknown. A growing body of evidence suggests that gliomas may rise from cancer stem cells (CSC) sharing with neural stem cells (NSC) the capacity of cell renewal and multipotency. Accordingly, a population of cells called "side population" (SP), which has been isolated from gliomas on the basis of their ability to extrude fluorescent dyes, behaves as stem cells and is resistant to chemotherapeutic treatments. This review will focus on the expression of the stem cell markers nestin and CD133 in glioma cancer stem cells. In addition, the possible role of Platelet Derived Growth Factor receptor type alpha (PDGFR-alpha) and Notch signalling in normal development and tumourigenesis of gliomas are also discussed. Future work elucidating the mechanisms that control normal development will help to identify new cancer stem cell-related genes. The identification of important markers and the elucidation of signalling pathways involved in survival, proliferation and differentiation of CSCs appear to be fundamental for developing an effective therapy of brain tumours.

Control of neuroblast production and migration by converging GABA and glutamate signals in the postnatal forebrain

The production of adult-born neurons is an ongoing process accounting for > 10,000 immature neurons migrating to the olfactory bulb every day. This high turnover rate necessitates profound control mechanisms converging onto neural stem cells and neuroblasts to achieve adequate adult-born neuron production. Here, we elaborate on a novel epigenetic control of adult neurogenesis via highly coordinated, non-synaptic, intercellular signalling. This communication engages the neurotransmitters GABA and glutamate, whose extracellular concentrations depend on neuroblast number and high affinity uptake systems in stem cells. Previous studies show that neuroblasts release GABA providing a negative feedback control of stem cell proliferation. Recent findings show an unexpected mosaic expression of glutamate receptors leading to calcium elevations in migrating neuroblasts. We speculate that stem cells release glutamate that activates glutamate receptors on migrating neuroblasts providing them with migratory and survival cues. In addition, we propose that the timing of neurotransmitter release and their spatial diffusion will determine the convergent coactivation of neuroblasts and stem cells, and provide a steady-state level of neuroblast production. Upon external impact or injury this signalling may adjust to a new steady-state level, thus providing non-synaptic scaling of neuroblast production.

Relationship of glioblastoma multiforme to the lateral ventricles predicts survival following tumor resection

OBJECTIVE

There has been an increased focus on the region adjacent to the lateral ventricles (LV) as a potential source of malignant tumors and/or more aggressive disease. We set out to determine if glioblastoma multiforme (GBM) bordering the LV was associated with decreased survival as compared to non-LV GBM.

METHODS

We reviewed the clinical records of 69 consecutive patients undergoing craniotomy for GBM at a single academic institution. Twenty-six patients were identified with contrast-enhancing lesions (CEL) bordering the LV (LV CEL). These 26 patients were matched with 26 patients with CEL not bordering the LV (non-LV CEL). These cohorts were matched for factors consistently shown to be associated with survival, which were age, tumor size, Karnofsky performance score, extent of resection, Gliadel implantation, and Temodar chemotherapy. Overall survival was compared between the cohorts via Log-rank analysis.

RESULTS

Despite similarities in pre-operative clinical status, tumor size, peri-operative outcome, and treatment regimens, the median survival for patients with LV CEL was significantly decreased as compared to patients with non-LV CEL (8 months vs. 11 months), P = 0.02. Additionally, survival analysis in patients stratified by primary and secondary resection also demonstrated a strong trend towards decreased survival after resection of LV CEL. After primary and secondary resection, patients with LV CEL versus non-LV CEL had a median survival of 11 months vs. 14 months (P = 0.10) and 7 months vs. 10 months (P = 0.11), respectively.

CONCLUSION

While the causal factors underlying this observation are not provided with this observational study, GBM bordering the LV may carry a prognostic significance.

Radiation, retardation and the developing brain: time is the crucial variable

BACKGROUND

Widespread radiation is a threat unique to the modern world. A recent report reveals that sub-clinical damage to human foetuses between 8 and 25 weeks of gestation can result in cognitive deficits still manifest 16-18 years after birth. These previously unrecognised, long-term effects are apparently produced by a relatively short pulse of exposure to radioactive fallout at levels that were previously thought not to be deleterious. This idea is plausible given the nature of the developmental events occurring in the brain during this period of gestation.

CONCLUSION

This exposed population should be examined for other neurological and psychiatric syndromes. If these findings are corroborated, in the event of future radiation exposures, steps should be taken to shield pregnant women who are within this window of vulnerability.

Developing cell transplantation for temporal lobe epilepsy

Mesial temporal lobe epilepsy (MTLE) is presumed to develop progressively as a consequence of synaptic reorganization and neuronal loss, although the exact etiology of seizure development is unknown. Nearly 30% of patients with MTLE have disabling seizures despite pharmacological treatment, and the majority of these patients are recommended for resection. The authors review cell transplantation as an alternative approach to the treatment of epilepsy. Recent work in animal models shows that grafted neuronal precursors that differentiate into inhibitory interneurons can increase the level of local inhibition. Grafts of these inhibitory neurons could help restore equilibrium in MTLE. Developing a sound transplantation strategy involves careful consideration of the etiology of MTLE and the expected functional role of transplanted cells. These issues are reviewed, with a focus on those factors most likely to influence clinically applicable results.

Cellular genesis in the postnatal piglet

Because of the anatomical and developmental similarity of the piglet brain to the human brain we were interested in characterizing the areas of cellular genesis which occur postnatally to validate the model for subsequent neurobiological research. In this study, four piglets were injected with 5-bromodeoxyuridine (BrdU) at 6, 7 and 8 days of age. The animals were sacrificed at 13 days of age and the brains were analyzed to characterize areas of cellular genesis. BrdU was seen throughout the brain and found to be most abundant in the subventricular zone (SVZ); doublecortin (DCX) expressing cells were found throughout the white matter-with an extensive DCX network in the SVZ. Here we describe for the first time the use of immunohistochemistry for BrdU and DCX to study cellular genesis in the piglet brain.

Neural stem cells in the mammalian brain

New fundamental results on stem cell biology have been obtained in the past 15 years. These results allow us to reinterpret the functioning of the cerebral tissue in health and disease. Proliferating stem cells have been found in the adult brain, which can be involved in postinjury repair and can replace dead cells under specific conditions. Numerous genomic mechanisms controlling stem cell proliferation and differentiation have been identified. The involvement of stem cells in the genesis of malignant tumors has been demonstrated. Neural stem cell tropism toward tumors has been shown. These findings suggest new lines of research on brain functioning and development. Stem cells can be used to develop radically new treatments of neurodegenerative and cancer diseases of the brain.

Use of human neural tissue for the generation of progenitors

Accumulating evidence suggests that a better understanding of normal human brain stem cells and tumor stem cells (TSCs) will have profound implications for treating central nervous system disease during the next decade. Neurosurgeons routinely resect excess surgical tissue containing either normal brain stem cells or TSCs. These cells are immediately available for expansion and use in basic biological assays, animal implantation, and comparative analysis studies. Although normal stem cells have much slower kinetics of expansion than TSCs, they are easily expandable and can be frozen for future use in stem cell banks. This nearly limitless resource holds promise for understanding the basic biology of normal brain stem cells and TSCs, which will likely direct the next major shift in therapeutics for brain tumors, brain and spinal cord injury, and neurodegenerative disease. This report reviews the progress that has been made in harvesting and expanding both normal and tumor-derived stem cells and emphasizes the integral role neurosurgeons will play in moving the neural stem cell field forward.

Long-term expansion of adult human brain subventricular zone precursors

OBJECTIVE

Many common neurosurgical procedures, including anterior temporal lobectomy and endoscopic ventricular puncture, allow neurosurgeons to retrieve portions of the germinal subventricular zone (SVZ). Isolation and maintenance of precursor cells from this zone can be used for hypothesis-driven experiments with a goal of improving our understanding of the basic mechanisms of central nervous system injury or disease and the potential of cell-based therapies to treat them. This article details our ability to reliably harvest, isolate, characterize, and maintain normal adult human brain SVZ precursor cells.

METHODS

Normal SVZ specimens were retrieved as part of anterior temporal lobe resections during planned epilepsy surgery. Dissociated SVZ specimens were plated and incubated in epidermal growth factor and basic fibroblast growth factor for more than 1 year to select for and expand normal neural precursor cells.

RESULTS

Self-renewal and immunocytochemical experiments proved the feasibility of long-term expansion of a slowly dividing nestin+, vimentin+, and glial fibrillary acidic protein-positive astrocyte capable of generating new neurons and glia. These mitotically active bipotent human precursors generated a large number of progeny and possessed significant self-renewal capacity, demonstrated by their ability to generate neurospheres. Cryopreservation was reliable with no loss of the precursor phenotype.

CONCLUSION

We have adapted techniques to allow for the isolation and long-term propagation of human adult neural precursors that are capable of generating both neurons and astrocytes in vitro. We have exploited the cell's self-renewal capacity to significantly and consistently expand human neural precursor cells for as long as 20 months. These findings suggest that cells derived from the SVZ during routine surgery may provide a renewable source of human neural precursor cells to study the biological mechanism of central nervous system disease or for application in cell-based human transplantation paradigms.

Glial progenitor-like phenotype in low-grade glioma and enhanced CD133-expression and neuronal lineage differentiation potential in high-grade glioma

BACKGROUND

While neurosphere- as well as xenograft tumor-initiating cells have been identified in gliomas, the resemblance between glioma cells and neural stem/progenitor cells as well as the prognostic value of stem/progenitor cell marker expression in glioma are poorly clarified.

METHODOLOGY/PRINCIPAL FINDINGS

Viable glioma cells were characterized for surface marker expression along the glial genesis hierarchy. Six low-grade and 17 high-grade glioma specimens were flow-cytometrically analyzed for markers characteristics of stem cells (CD133); glial progenitors (PDGFRalpha, A2B5, O4, and CD44); and late oligodendrocyte progenitors (O1). In parallel, the expression of glial fibrillary acidic protein (GFAP), synaptophysin and neuron-specific enolase (NSE) was immunohistochemically analyzed in fixed tissue specimens. Irrespective of the grade and morphological diagnosis of gliomas, glioma cells concomitantly expressed PDGFRalpha, A2B5, O4, CD44 and GFAP. In contrast, O1 was weakly expressed in all low-grade and the majority of high-grade glioma specimens analyzed. Co-expression of neuronal markers was observed in all high-grade, but not low-grade, glioma specimens analyzed. The rare CD133 expressing cells in low-grade glioma specimens typically co-expressed vessel endothelial marker CD31. In contrast, distinct CD133 expression profiles in up to 90% of CD45-negative glioma cells were observed in 12 of the 17 high-grade glioma specimens and the majority of these CD133 expressing cells were CD31 negative. The CD133 expression correlates inversely with length of patient survival. Surprisingly, cytogenetic analysis showed that gliomas contained normal and abnormal cell karyotypes with hitherto indistinguishable phenotype.

CONCLUSIONS/SIGNIFICANCE

This study constitutes an important step towards clarification of lineage commitment and differentiation blockage of glioma cells. Our data suggest that glioma cells may resemble expansion of glial lineage progenitor cells with compromised differentiation capacity downstream of A2B5 and O4 expression. The concurrent expression of neuronal markers demonstrates that high-grade glioma cells are endowed with multi-lineage differentiation potential in vivo. Importantly, enhanced CD133 expression marks a poor prognosis in gliomas.

Neuronal maturation in an experimental model of brain tissue heterotopia in the lung

Neural maturation involves diverse interaction and signaling mechanisms that are essential to the development of the nervous system. However, little is known about the development of neurons in heterotopic brain tissue in the lung, a rare abnormality observed in malformed babies and fetuses. The aim of this study was to identify the neurons and to investigate their maturation in experimental brain tissue heterotopia during fetal and neonatal periods. The fetuses from 24 pregnant female Swiss mice were used to induce brain tissue heterotopia on the 15th gestational day. Briefly, the brain of one fetus of each dam was extracted, disaggregated, and injected into the right hemithorax of siblings. Six of these fetuses with pulmonary brain tissue implantation were collected on the 18th gestational day (group E18), and six others were collected on the 8th postnatal day (group P8). The brain of each fetus from dams not submitted to any experimental procedure was collected on the 18th gestational day (group CE18) and on the 8th postnatal day (group CP8) to serve as a control for neuronal quantitation and maturation. Immunohistochemical staining of NeuN was used to assess neuron quantity and maturation. The NeuN labeling index was greater in the postnatal period than in the fetal period for the experimental and control groups (P8 > E18 and CP8 > CE18), although there were fewer neurons in experimental than in control groups (P8 < CP8 and E18 < CE18) (P < 0.005). These results indicate that fetal neuroblasts/neurons not only survive a dramatic event such as mechanical disaggregation, in the same way as it happens in human cases, but also they retain their development in heterotopia, irrespective of local tissue influences.

Brain tumour stem cells: the undercurrents of human brain cancer and their relationship to neural stem cells

Conceptual and technical advances in neural stem cell biology are being applied to the study of human brain tumours. These studies suggest that human brain tumours are organized as a hierarchy and are maintained by a small number of tumour cells that have stem cell properties. Most of the bulk population of human brain tumours comprise cells that have lost the ability to initiate and maintain tumour growth. Although the cell of origin for human brain tumours is uncertain, recent evidence points towards the brain's known proliferative zones. The identification of brain tumour stem cells has important implications for understanding brain tumour biology and these cells may be critical cellular targets for curative therapy.

Brain micro-ecologies: neural stem cell niches in the adult mammalian brain

Neurogenesis persists in two germinal regions in the adult mammalian brain, the subventricular zone of the lateral ventricles and the subgranular zone in the hippocampal formation. Within these two neurogenic niches, specialized astrocytes are neural stem cells, capable of self-renewing and generating neurons and glia. Cues within the niche, from cell-cell interactions to diffusible factors, are spatially and temporally coordinated to regulate proliferation and neurogenesis, ultimately affecting stem cell fate choices. Here, we review the components of adult neural stem cell niches and how they act to regulate neurogenesis in these regions.

Mechanisms and functional implications of adult neurogenesis

The generation of new neurons is sustained throughout adulthood in the mammalian brain due to the proliferation and differentiation of adult neural stem cells. In this review, we discuss the factors that regulate proliferation and fate determination of adult neural stem cells and describe recent studies concerning the integration of newborn neurons into the existing neural circuitry. We further address the potential significance of adult neurogenesis in memory, depression, and neurodegenerative disorders such as Alzheimer's and Parkinson's disease.

Adult mammalian neurogenesis and the New Zealand white rabbit

Although the central nervous system is unable to undergo spontaneous repair and is hostile to the integration of exogenously delivered cells, various examples of adult structural plasticity have been shown to occur. It is now widely accepted that endogenous proliferative activity leading to the production of new neurons exists, at least within two restricted brain sites: the hippocampal dentate gyrus and the forebrain subventricular zone. A substantial insight into spontaneous neurogenesis within these allocortical regions in rodents has been obtained, but less is known regarding its occurrence in other mammalian brain regions. In this review, differences in the structural and temporal characteristics of protracted neurogenesis in mammals will be considered. Attention will be focused on the rabbit cerebrum and cerebellum, where unexpected features of structural plasticity have been found to occur despite the relative closeness of the Orders Lagomorpha and Rodentia.

Neurogenesis in the aging brain

Neurogenesis, or the birth of new neural cells, was thought to occur only in the developing nervous system and a fixed neuronal population in the adult brain was believed to be necessary to maintain the functional stability of adult brain circuitry. However, recent studies have demonstrated that neurogenesis does indeed continue into and throughout adult life in discrete regions of the central nervous systems (CNS) of all mammals, including humans. Although neurogenesis may contribute to the ability of the adult brain to function normally and be induced in response to cerebral diseases for self-repair, this nevertheless declines with advancing age. Understanding the basic biology of neural stem cells and the molecular and cellular regulation mechanisms of neurogenesis in young and aged brain will allow us to modulate cell replacement processes in the adult brain for the maintenance of healthy brain tissues and for repair of disease states in the elderly.

Epidermal growth factor plays a crucial role in mitogenic regulation of human brain tumor stem cells

A cancer stem cell population in malignant brain tumors takes an essential part in brain tumor initiation, growth, and recurrence. Growth factors, such as epidermal growth factor, fibroblast growth factor-2, vascular endothelial growth factor, platelet-derived growth factor, and hepatocyte growth factor, are shown to support the proliferation of neural stem cells and also may play key roles in gliomagenesis. However, the responsible growth factor(s), which controls maintenance of brain tumor stem cells, is not yet uncovered. We have established three cancer stem cell lines from human gliomas. These cells were immunoreactive with the neuronal progenitor markers, nestin and CD133, and established tumors that closely resembled the features of original tumor upon transplantation into mouse brain. Three cell lines retained their self-renewal ability and proliferation only in the presence of epidermal growth factor (>2.5 ng/ml). In sharp contrast, other growth factors, including fibroblast growth factor-2, failed to support maintenance of these cells. The tyrosine kinase inhibitors of epidermal growth factor signaling (AG1478 and gefitinib) suppressed the proliferation and self-renewal of these cells. Gefitinib inhibited phosphorylation of epidermal growth factor receptor as well as Akt kinase and extracellular signal-regulated kinase 1/2. Flow cytometric analysis revealed that epidermal growth factor concentration-dependently increased the population of CD133-positive cells. Gefitinib significantly reduced CD133-positive fractions and also induced their apoptosis. These results indicate that maintenance of human brain tumor stem cells absolutely requires epidermal growth factor and that tyrosine kinase inhibitors of epidermal growth factor signaling potentially inhibit proliferation and induce apoptosis of these cells.

Immature and mature neurons coexist among glial scars after rat traumatic brain injury

OBJECTIVES

Glial scars around a damaged area after brain injury inhibit neurite elongation from surviving neurons and axonal plasticity, and thus prevent neural network regeneration. However, the generation, differentiation and maturation of neural stem cells (NSCs) among glial scars after brain injury have not yet been reported.

METHODS

In the present study, we investigated the chronological relationship between gliosis and maturation of new neurons around a damaged area using a rat traumatic brain injury (TBI) model.

RESULTS

Between 1 and 7 days after injury, many nestin-positive cells were observed around the damaged area. Three days after injury, many small nestin-positive cells showed an astrocytic morphology. Between 1 and 30 days after injury, doublecortin (DCX)-positive cells were present around the damaged area. Three and 7 days after injury, a small number of nestin-positive cells were immunopositive for glial fibrillary acidic protein (GFAP). Seven days after injury, there were DCX-positive cells in the gliosis occurring in the lesion. Thirty days after injury, DCX-positive cells were observed near and among the glial scars and a small number of these cells were immunopositive for NeuN.

DISCUSSION

These results suggest that DCX-positive cells were present near and among the glial scars after brain injury, and that these cells changed from immature to mature neurons. It is considered that promotion of the maturation and differentiation of newly formed immature neurons near and among glial scars after injury may improve the brain dysfunction induced by glial scars after brain injury.

Mature astrocytes in the adult human neocortex express the early neuronal marker doublecortin

Doublecortin (DCX) is a microtubule-associated protein expressed by migrating neuroblasts and is considered to be a reliable marker of neurogenesis. DCX has been used to study the relation between neurogenesis in adult human brain and neurological and neurodegenerative disease processes in the search for putative therapeutic strategies. Using autopsy and surgically resected tissue from a total of 60 patients, we present evidence that DCX is present in several cellular compartments of differentiated astrocytes in the adult human neocortex. One of these compartments consisted of peripheral processes forming punctate envelopes around mature neuronal cell bodies. Markers of glial activation, such as GFAP and HLA, were not associated with DCX immunoreactivity, however, the presence of cytoarchitectural alterations tended to correlate with reduced DCX staining of astrocytic somata. Interestingly, local Alzheimer pathology that showed no relation with cytoarchitectural abnormalities appeared to correlate negatively with the expression of DCX in the astrocytic somata. In combination with the literature our data support the view that DCX in the adult human neocortex may have a function in glia-to-neuron communication. Furthermore, our results indicate that in the adult human neocortex DCX is neither a reliable nor a selective marker of neurogenesis.

The neurosphere assay, a method under scrutiny

OBJECTIVES

The aim of this review is to provide an overview of the fundamental features of the neurosphere assay (NSA), which was initially described in 1992, and has since been used not only to detect the presence of stem cells in embryonic and adult mammalian neural tissues, but also to study their characteristics in vitro. Implicit in this review is a detailed examination of the limitations of the NSA, and how this assay is most accurately and appropriately used. Finally we will point out criteria that should be challenged to design alternative ways to overcome the limits of this assay.

METHODS

NSA is used to isolate putative neural stem cells (NSCs) from the central nervous system (CNS) and to demonstrate the critical stem cell attributes of proliferation, extensive self-renewal and the ability to give rise to a large number of differentiated and functional progeny. Nevertheless, the capability of neural progenitor cells to form neurospheres precludes its utilisation to accurately quantify bona fide stem cell frequency based simply on neurosphere numbers. New culture conditions are needed to be able to distinguish the activity of progenitor cells from stem cells.

CONCLUSION

A commonly used, and arguably misused, methodology, the NSA has provided a wealth of information on precursor activity of cells derived from the embryonic through to the aged CNS. Importantly, the NSA has contributed to the demise of the 'no new neurogenesis' dogma, and the beginning of a new era of CNS regenerative medicine. Nevertheless, the interpretations arising from the utilisation of the NSA need to take into consideration its limits, so as not to be used beyond its specificity and sensitivity.

The therapeutic plasticity of neural stem/precursor cells in multiple sclerosis

Adult multipotent neural stem/precursor cells (NPCs) have the capacity to self-renew and generate functional differentiated cells (e.g. neurons, astrocytes or oligodendrocytes) within discrete tissue-specific germinal niches, such as the subventricular zone of the lateral ventricles and the subgranular zone of the dentate gyrus of the hippocampus. Due to their intrinsic plasticity NPCs can be considered an essential part of the cellular mechanism(s) by which the central nervous system (CNS) tries to repair itself after an injury and, as a consequence, they also represents an attractive therapeutic tool for the treatment of neurological disorders. Here we will discuss not only the role of NPC-based transplantation therapies in multiple sclerosis (MS) but also recent data suggesting that endogenous NPCs, while contributing to CNS repair in MS, may also become the target of the disease itself.

Characterization of adult neural stem cells and their relation to brain tumors

The adult mammalian brain contains neural stem cells that are capable of generating new neurons and glia over the course of a lifetime. Neural stem cells reside in 2 germinal niches, the subventricular zone (SVZ) and the dentate gyrus subgranular zone. These primary progenitors have been identified in their niche in vivo; these cells have characteristics of astrocytes. Recent studies have shown that adult SVZ stem cells are derived from radial glia, the stem cells in the developing brain, which in turn are derived from the neuroepithelum, the earliest brain progenitors. Thus, SVZ stem cells are a continuum from neuroepithelium to radial glia to astrocytes, and are contained within what has been considered the lineage for astrocytes. However, it seems that only a small subset of the astrocytes present in the adult brain have stem cell properties. Recent findings have shown that SVZ stem cell astrocytes express a receptor for platelet-derived growth factor (PDGF), suggesting that the ability to respond to specific growth factor stimuli, such as PDGF, epidermal growth factor and others, may be unique to these stem cell astrocytes. Intriguingly, activation of these same signaling pathways is widely implicated in brain tumor formation. Since the adult brain has very few proliferating cells capable of accumulating the numerous mutations required for transformation, the adult neural stem and/or progenitor cells may be likely candidates for the brain tumor cell of origin. Indeed, activation of the PDGF or epidermal growth factor pathways in adult neural stem or progenitor cells confers tumor-like properties on these cells, lending support to this hypothesis.

Adult neurogenesis and systemic adaptation: animal experiments and clinical perspectives for PTSD

The life-long persistence of neuron production in the adult mammalian central nervous system was established at the end of the 20th century and since then, intensive studies have been carried out to determine the biological role of neuronal turnover in the mature brain. To date, evidence has been found of involvement in learning/memory function and stress-related mental disorders. With a discussion of speculative link between impaired amygdala-relevant neurogenesis and PTSD in an animal model, we here review across species the functional significance of adult neurogenesis from the point of view of systemic adaptation.

The effect of CXCL1 on human fetal oligodendrocyte progenitor cells

Chemokine CXCL1 is abundantly present in proliferative zones during brain development and in regions of remyelination, suggesting that it influences development of oligodendrocyte progenitors (OPC) in these regions. We studied in vitro the effects and possible mechanisms by which CXCL1 acts on human fetal OPC. In organotypic slice cultures of human fetal cortical ventricular/subventricular (VZ/SVZ) zones, blocking of CXCL1 signaling reduced significantly the proliferation of OPC. Moreover, exogenously added CXCL1 induced increase of OPC proliferation. Treatments of purified OPC cultures and cell depletion experiments demonstrated that this effect of CXCL1 was mainly indirect, mediated through astrocytes. We identified that CXCL1 acted through the extracellular signal regulated kinase (ERK1/2) pathway, activated primarily in astrocytes. In vitro, astrocytes stimulated with CXCL1 released several cytokines, but only the release of interleukin-6 (IL-6) was completely blocked by inhibition of ERK1/2 pathway. When released IL-6 was neutralized in slices, a decrease in OPC proliferation was demonstrated, while addition of IL-6 was able to return OPC proliferation in astrocyte-depleted slices to the control level. These results suggest that in the human fetal brain CXCL1 promotes proliferation of early OPC, acting in part through an ERK1/2-dependent pathway and release of IL-6 from astrocytes.

Myelin repair: the role of stem and precursor cells in multiple sclerosis

Multiple sclerosis is the most common potential cause of neurological disability in young adults. The disease has two distinct clinical phases, each reflecting a dominant role for separate pathological processes: inflammation drives activity during the relapsing-remitting stage and axon degeneration represents the principal substrate of progressive disability. Recent advances in disease-modifying treatments target only the inflammatory process. They are ineffective in the progressive stage, leaving the science of disease progression unsolved. Here, the requirement is for strategies that promote remyelination and prevent axonal loss. Pathological and experimental studies suggest that these processes are tightly linked, and that remyelination or myelin repair will both restore structure and protect axons. This review considers the basic and clinical biology of remyelination and the potential contribution of stem and precursor cells to enhance and supplement spontaneous remyelination.

Proliferation, neurogenesis and regeneration in the non-mammalian vertebrate brain

Post-embryonic neurogenesis is a fundamental feature of the vertebrate brain. However, the level of adult neurogenesis decreases significantly with phylogeny. In the first part of this review, a comparative analysis of adult neurogenesis and its putative roles in vertebrates are discussed. Adult neurogenesis in mammals is restricted to two telencephalic constitutively active zones. On the contrary, non-mammalian vertebrates display a considerable amount of adult neurogenesis in many brain regions. The phylogenetic differences in adult neurogenesis are poorly understood. However, a common feature of vertebrates (fish, amphibians and reptiles) that display a widespread adult neurogenesis is the substantial post-embryonic brain growth in contrast to birds and mammals. It is probable that the adult neurogenesis in fish, frogs and reptiles is related to the coordinated growth of sensory systems and corresponding sensory brain regions. Likewise, neurons are substantially added to the olfactory bulb in smell-oriented mammals in contrast to more visually oriented primates and songbirds, where much fewer neurons are added to the olfactory bulb. The second part of this review focuses on the differences in brain plasticity and regeneration in vertebrates. Interestingly, several recent studies show that neurogenesis is suppressed in the adult mammalian brain. In mammals, neurogenesis can be induced in the constitutively neurogenic brain regions as well as ectopically in response to injury, disease or experimental manipulations. Furthermore, multipotent progenitor cells can be isolated and differentiated in vitro from several otherwise silent regions of the mammalian brain. This indicates that the potential to recruit or generate neurons in non-neurogenic brain areas is not completely lost in mammals. The level of adult neurogenesis in vertebrates correlates with the capacity to regenerate injury, for example fish and amphibians exhibit the most widespread adult neurogenesis and also the greatest capacity to regenerate central nervous system injuries. Studying these phenomena in non-mammalian vertebrates may greatly increase our understanding of the mechanisms underlying regeneration and adult neurogenesis. Understanding mechanisms that regulate endogenous proliferation and neurogenic permissiveness in the adult brain is of great significance in therapeutical approaches for brain injury and disease.

Molecular pathogenesis of adult brain tumors and the role of stem cells

Primary brain tumors consist of neoplasms with varied molecular defects, morphologic phenotypes, and clinical outcomes. The genetic and signaling abnormalities involved in tumor initiation and progression of the most prevalent adult primary brain tumors, including gliomas, meningiomas, and medulloblastomas, are described in this article. The current understanding of the cell-of-origin of these neoplasms is reviewed, which suggests that the malignant phenotype is propelled by cells with stem-like qualities. A comprehensive understanding of the molecular basis of transformation and the cell-of-origin of these neoplasms will enable the formulation of more targeted treatment alternatives that could improve survival and quality of life.

Comparison of neuron-like cells derived from bone marrow stem cells to those differentiated from adult brain neural stem cells

Bone marrow-derived stem/progenitor cells have been shown by independent investigators to give rise to neural-like cells (neurons and glia) both in vitro and in vivo. The objective of the present study was to determine whether nestin-enriched cells derived from bone marrow can differentiate into cells with the same morphological and functional characteristics as neurons derived from adult brain neurogenic zones. Cell culture methods were used for generation of adult bone marrow and brain stem/progenitor cells and for studying their differentiation into neural-like cells. The proportion of cells expressing neuronal markers was greater in cultures derived from adult hippocampal neural stem cells than in the bone marrow-derived cells, but the electrophysiological and functional characteristics of the cells were similar. Action potentials with electrical characteristics corresponding to those exhibited by adult neural stem cell-derived neurons were recorded from approximately 2.5% of patched neuron-like cells differentiated from bone marrow cells. The active uptake of tritium-labeled neurotransmitters gamma-aminobutyric acid ([(3)H]GABA) and dopamine ([(3)H]DA) was measured in both sets of cultures. [(3)H]GABA uptake, but not [(3)H]DA, was significantly increased in differentiated neurons in both neural stem cell cultures and bone marrow-derived cultures. [(3)H]GABA uptake was greater in differentiated neurons derived from brain neural stem cells. In summary, both the nestin-expressing bone marrow and the adult brain neural stem/progenitors developed into cells with morphological, immunocytochemical, and functional characteristics of neurons. Even though a smaller proportion of neuron-like cells was generated from bone marrow-derived progenitors than from brain-derived neural stem cells, these cells may be useful in the cellular therapy of neurodegenerative diseases and traumatic brain and spinal cord injury.

Neural stem cells and the future treatment of neurological diseases: raising the standard

Neural stem and progenitor cells offer great potential for treatment of neurological disorders.AQ[16]Both neurologic and neurological were used in text; the latter was more common. Note changes have been made to match in title and text body. The current strategies of isolation, expansion, and characterization of these cells require in vitro manipulations that can change their intrinsic properties, specifically with the acquisition of chromosomal abnormalities. We have analyzed the rationale of using neural stem cells in neurological disorders, the caveats of current isolation and in vitro culture protocols of neural precursors. Addressing these challenges is crucial for translation of neural stem cell therapy to the clinic.

Brain plasticity and tumors

Brain plasticity is the potential of the nervous system to reshape itself during ontogeny, learning or following injuries. The first part of this article reviews the pathophysiological mechanisms underlying plasticity at different functional levels. Such plastic potential means that the anatomo-functional organization of the brain in humans, both physiological and pathological, has flexibility. Patterns of reorganization may differ according to the time-course of cerebral damage, with better functional compensation in more slowly growing lesions. The second part of this review analyzes the interactions between tumor growth and brain reshaping, using non-invasive (neuroimaging) and invasive (electrophysiological) methods of functional mapping. Finally, the therapeutic implications provided by a greater understanding of these mechanisms of cerebral redistribution are explored from a surgical point of view. Enhanced preoperative prediction of an individual's potential for reorganization might be integrated into surgical planning and preserving quality of life through tailored rehabilitation programmes to optimize functional recovery following resection of a brain tumor.

Clonal analyses and cryopreservation of neural stem cell cultures

The discovery of stem cell populations in the adult central nervous system (CNS) that continually produce neurons and glial cells, and the hypothesis that they could contribute to neural plasticity/repair, has opened new and exciting areas of research in basic cell biology and regenerative medicine. The success of these studies relies on understanding the functional features and the normal fate of neural stem cells (NSCs) in vivo as well on the development of in vitro culture conditions enabling isolation, extensive propagation, and rigorous characterization of the "putative" NSCs. The neurosphere assay (NSA) has emerged as a valuable tool for isolating embryonic and adult CNS stem cells and for studying their biology. However, because this assay may select and expand a heterogeneous stem/progenitor cell population, rigorous clonal and serial subcloning analyses are required to detect and document stem cell activity and to unequivocally identify bona fide stem cells. We illustrate and discuss methods for the isolation, propagation, cryopreservation, and functional characterization of NSCs, focusing on the essential issue of their clonogenic capacity.

Gliomagenesis and neural stem cells: Key role of hypoxia and concept of tumor “neo-niche”

Gliomas represent the most common primary brain tumors and the most devastating pathology of the central nervous system. Despite progress in conventional treatments, the prognosis remains dismal. Recent studies have suggested that a glioma brain tumor may arise from a "cancer stem cell". To understand this theory we summarize studies of the concepts of neural stem cell, and its specialized microenvironment, namely the niche which can regulate balanced self-renewal, differentiation and stem cell quiescence. We summarize the molecular mechanism known or postulated to be involved in the disregulation of normal stem cells features allowing them to undergo neoplasic transformation. We seek data pointing out the key role of hypoxia in normal homeostasis of stem cells and in the initiation, development and aggressiveness of gliomas. We develop the concept of tumor special microenvironment and we propose the new concept of neo-niche, surrounding the glioma, in which hypoxia could be a key factor to recruit and deregulate different stem cells for gliogenesis process. Substantial advances in treatment would come from obtaining better knowledge of molecular impairs of this disease.

Cancer stem cells as mediators of treatment resistance in brain tumors: status and controversies

Malignant primary brain tumors are characterized by a short median survival and an almost 100% tumor-related mortality. Despite the addition of new chemotherapy regimes, the overall survival has improved marginally, and radiotherapy is only transiently effective, illustrating the profound impact of treatment resistance on prognosis. Recent studies suggest that a small subpopulation of cancer stem cells (CSCs) has the capacity to repopulate tumors and drive malignant progression and mediate radio- and chemoresistance. This implies that future therapies should turn from the elimination of the rapidly dividing, but differentiated tumor cells, to specifically targeting the minority of tumor cells that repopulate the tumor. Although there exists some support for the CSC hypothesis, there remain many uncertainties regarding theoretical, technical, and interpretational aspects of the data supporting it. If correct, the CSC hypothesis could have profound implications for the way tumors are classified and treated. In this review of the literature, we provide original data and hypotheses supporting alternative explanations and outline some of the therapeutic implications that can be derived.

The biology of cancer stem cells

Cancers originally develop from normal cells that gain the ability to proliferate aberrantly and eventually turn malignant. These cancerous cells then grow clonally into tumors and eventually have the potential to metastasize. A central question in cancer biology is, which cells can be transformed to form tumors? Recent studies elucidated the presence of cancer stem cells that have the exclusive ability to regenerate tumors. These cancer stem cells share many characteristics with normal stem cells, including self-renewal and differentiation. With the growing evidence that cancer stem cells exist in a wide array of tumors, it is becoming increasingly important to understand the molecular mechanisms that regulate self-renewal and differentiation because corruption of genes involved in these pathways likely participates in tumor growth. This new paradigm of oncogenesis has been validated in a growing list of tumors. Studies of normal and cancer stem cells from the same tissue have shed light on the ontogeny of tumors. That signaling pathways such as Bmi1 and Wnt have similar effects in normal and cancer stem cell self-renewal suggests that common molecular pathways regulate both populations. Understanding the biology of cancer stem cells will contribute to the identification of molecular targets important for future therapies.

From bench to bed: the potential of stem cells for the treatment of Parkinson's disease

Parkinson's disease (PD) is the most common movement disorder. The neuropathology is characterized by the loss of dopamine neurons in the substantia nigra pars compacta. Transplants of fetal/embryonic midbrain tissue have exhibited some beneficial clinical effects in open-label trials. Neural grafting has, however, not become a standard treatment for several reasons. First, the supply of donor cells is limited, and therefore, surgery is accompanied by difficult logistics. Second, the extent of beneficial effects has varied in a partly unpredictable manner. Third, some patients have exhibited graft-related side effects in the form of involuntary movements. Fourth, in two major double-blind placebo-controlled trials, there was no effect of the transplants on the primary endpoints. Nevertheless, neural transplantation continues to receive a great deal of interest, and now, attention is shifting to the idea of using stem cells as starting donor material. In the context of stem cell therapy for PD, stem cells can be divided into three categories: neural stem cells, embryonic stem cells, and other tissue-specific types of stem cells, e.g., bone marrow stem cells. Each type of stem cell is associated with advantages and disadvantages. In this article, we review recent advances of stem cell research of direct relevance to clinical application in PD and highlight the pros and cons of the different sources of cells. We draw special attention to some key problems that face the translation of stem cell technology into the clinical arena.

Comment on "Human neuroblasts migrate to the olfactory bulb via a lateral ventricular extension"

Curtis et al. (Research Articles, 2 March 2007, p. 1243) claimed discovery of a human neuronal migratory stream to the olfactory bulb along a putative lateral ventricular extension. However, high levels of proliferation reported with proliferating cell nuclear antigen were not confirmed using different markers, neuronal chain migration was not demonstrated, and no serial reconstruction shows a true ventricular extension.

Genomic and functional profiling of human Down syndrome neural progenitors implicates S100B and aquaporin 4 in cell injury

Down syndrome (DS) is caused by trisomy of chromosome 21 and is characterized by mental retardation, seizures and premature Alzheimer's disease. To examine neuropathological mechanisms giving rise to this disorder, we generated multiple human DS neural progenitor cell (NPC) lines from the 19-21 week frontal cortex and characterized their genomic and functional properties. Microarray profiling of DS progenitors indicated that increased levels of gene expression were not limited to chromosome 21, suggesting that increased expression of genes on chromosome 21 altered transcriptional regulation of a subset of genes throughout the entire genome. Moreover, many transcriptionally dysregulated genes were involved in cell death and oxidative stress. Network analyses suggested that upregulated expression of chromosome 21 genes such as S100B and amyloid precursor protein activated the stress response kinase pathways, and furthermore, could be linked to upregulation of the water channel aquaporin 4 (AQP4). We further demonstrate in DS NPCs that S100B is constitutively overexpressed, that overexpression leads to increased reactive oxygen species (ROS) formation and activation of stress response kinases, and that activation of this pathway results in compensatory AQP4 expression. In addition, AQP4 expression could be induced by direct exposure to ROS, and siRNA inhibition of AQP4 resulted in elevated levels of ROS following S100B exposure. Finally, elevated levels of S100B-induced ROS and loss of AQP4 expression led to increased programmed cell death. These findings suggest that dysregulation of chromosome 21 genes in DS neural progenitors leads to increased ROS and thereby alters transcriptional regulation of cytoprotective, non-chromosome 21 genes in response to ongoing cellular insults.

The role of SVZ-derived neural precursors in demyelinating diseases: from animal models to multiple sclerosis

We will review the role of endogenous neural stem cells in myelin repair both in animal models of demyelination and multiple sclerosis. The mammalian sub-ventricular zone (SVZ) is the largest germinative zone of the adult brain, which contains a well characterized stem cell niche. While most studies highlight the neurogenic potential of SVZ progenitors, recent data indicate that SVZ cells become reactived in response to different pathological cues, like trauma, ischemia, neurodegeneration, inflammation and demyelination. Experimental models of demyelination in rodent demonstrate enhanced proliferation and recruitment of SVZ progenitors into myelin lesions, in response to demyelination. Moreover, cell lineage tracing experiments showed that SVZ progenitor cells can give rise to oligodendrocytes in demyelinated lesions, that could potentially contribute to remyelination. To examine the relevance of these studies in myelin diseases, we recently examined the human SVZ in post-mortem MS brains. The human SVZ is characterized by a ribbon of SVZ astrocytes lining the ependymal border of the lateral ventricles and which behave as multipotential progenitors in vitro. We showed that cellular density and proliferation were enhanced in MS SVZ compared to non-neurological controls. This high cellular density was correlated with the increased number of progenitor cells in MS SVZ, as well as in sub-ventricular lesions. Interestingly, some of these progenitors expressed transcription factors involved in oligodendrogenesis, such as Sox9, Olig2 and Sox10. These data indicate that gliogenesis occurs also in MS SVZ and suggest the recruitment of SVZ-derived oligodendrocyte precursors to peri-ventricular demyelinated lesions. Further investigation of adult neural stem cells and their progenitors in the brain of rodents and non-human primates should help to gain insights in their process of activation in response to demyelination and their role in myelin repair.

Methamphetamine self-administration and voluntary exercise have opposing effects on medial prefrontal cortex gliogenesis

Psychostimulant abuse produces deficits in prefrontal cortex (PFC) function, whereas physical activity improves PFC-dependent cognition and memory. The present study explored the vulnerability of medial PFC (mPFC) precursor proliferation and survival to methamphetamine self-administration and voluntary exercise, factors that may have opposing effects on mPFC plasticity to facilitate functional consequences. Intermittent 1 h access to methamphetamine (I-ShA) increased, but daily 1 and 6 h access decreased, proliferation and survival, with dose-dependent effects on mature cell phenotypes. All groups showed increased cell death. Voluntary exercise enhanced proliferation and survival but, in contrast to methamphetamine exposure, did not alter cell death or mature phenotypes. Furthermore, enhanced cell survival by I-ShA and voluntary exercise had profound effects on gliogenesis with differential regulation of oligodendrocytes versus astrocytes. In addition, new cells in the adult mPFC stain for the neuronal marker neuronal nuclear protein, although enhanced cell survival by I-ShA and voluntary exercise did not result in increased neurogenesis. Our findings demonstrate that mPFC gliogenesis is vulnerable to psychostimulant abuse and physical activity with distinct underlying mechanisms. The susceptibility of mPFC gliogenesis to even modest doses of methamphetamine could account for the pronounced pathology linked to psychostimulant abuse.

Neurogenesis in the postnatal human epileptic brain

OBJECT

The normal adult human telencephalon does not reveal evidence of spontaneous neuronal migration and differentiation despite the robust germinal capacity of the subventricular zone (SVZ) astrocyte ribbon that contains neural stem cells. This might be because it is averse to accepting new neurons into an established neuronal network, probably representing an evolutionary acquisition to prevent the formation of anomalous neuronal circuits. Some forms of epilepsy, such as malformations of cortical development, are thought to be due to abnormal corticogenesis during the embryonic and early postnatal periods. The role of postnatal architectural reorganization and possibly postnatal neurogenesis in some forms of epilepsy in humans remains unknown. In this study the authors used resected specimens of epileptic brain to determine whether neurogenesis could occur in the diseased tissue.

METHODS

The authors studied freshly resected brain tissue obtained in 47 patients who underwent neurosurgical procedures and four autopsies. Forty-four samples were harvested in patients who underwent resection for the treatment of pharmacoresistant epilepsy.

RESULTS

Using organotypic brain slice preparations cultured with 5-bromodeoxyuridine (a marker for cell proliferation), immunohistochemistry, and cell trackers, the authors demonstrate the presence of spontaneous cell proliferation, migration, and neuronal differentiation in the adult human telencephalon that starts in the SVZ and progresses to the adjacent white matter and neocortex in human neocortical pathological structures associated with epilepsy. No cell migration or neuronal differentiation was found in the control group.

CONCLUSIONS

The presence of spontaneous neurogenesis associated with some forms of human neocortical epilepsy may represent an erroneous and maladaptive mechanism for neuronal circuitry repair, or it may be an intrinsic part of the pathogenic process.

Prospects for neural stem cell-based therapies for neurological diseases

Neural stem and progenitor cells have great potential for the treatment of neurological disorders. However, many obstacles remain to translate this field to the patient's bedside, including rationales for using neural stem cells in individual neurological disorders; the challenges of neural stem cell biology; and the caveats of current strategies of isolation and culturing neural precursors. Addressing these challenges is critical for the translation of neural stem cell biology to the clinic. Recent work using neural stem cells has yielded novel biologic concepts such as the importance of the reciprocal interaction between neural stem cells and the neurodegenerative environment. The prospect of using transplants of neural stem cells and progenitors to treat neurological diseases requires a better understanding of the molecular mechanisms of both neural stem cell behavior in experimental models and the intrinsic repair capacity of the injured brain.