Adult spinal cord ependymal layer: a promising pool of quiescent stem cells to treat spinal cord injury

The activation of dormant ependymal cells following spinal cord injury

Ependymal cells, a dormant population of ciliated progenitors found within the central canal of the spinal cord, undergo significant alterations after spinal cord injury (SCI). Understanding the molecular events that induce ependymal cell activation after SCI represents the first step toward controlling the response of the endogenous regenerative machinery in damaged tissues. This response involves the activation of specific signaling pathways in the spinal cord that promotes self-renewal, proliferation, and differentiation. We review our current understanding of the signaling pathways and molecular events that mediate the SCI-induced activation of ependymal cells by focusing on the roles of some cell adhesion molecules, cellular membrane receptors, ion channels (and their crosstalk), and transcription factors. An orchestrated response regulating the expression of receptors and ion channels fine-tunes and coordinates the activation of ependymal cells after SCI or cell transplantation. Understanding the major players in the activation of ependymal cells may help us to understand whether these cells represent a critical source of cells contributing to cellular replacement and tissue regeneration after SCI. A more complete understanding of the role and function of individual signaling pathways in endogenous spinal cord progenitors may foster the development of novel targeted therapies to induce the regeneration of the injured spinal cord.

Ependymal cells and neurodegenerative disease: outcomes of compromised ependymal barrier function

Within the central nervous system, ependymal cells form critical components of the blood-cerebrospinal fluid barrier and the cerebrospinal fluid-brain barrier. These barriers provide biochemical, immunological and physical protection against the entry of molecules and foreign substances into the cerebrospinal fluid while also regulating cerebrospinal fluid dynamics, such as the composition, flow and removal of waste from the cerebrospinal fluid. Previous research has demonstrated that several neurodegenerative diseases, such as Alzheimer's disease and multiple sclerosis, display irregularities in ependymal cell function, morphology, gene expression and metabolism. Despite playing key roles in maintaining overall brain health, ependymal barriers are largely overlooked and understudied in the context of disease, thus limiting the development of novel diagnostic and treatment options. Therefore, this review explores the anatomical properties, functions and structures that define ependymal cells in the healthy brain, as well as the ways in which ependymal cell dysregulation manifests across several neurodegenerative diseases. Specifically, we will address potential mechanisms, causes and consequences of ependymal cell dysfunction and describe how compromising the integrity of ependymal barriers may initiate, contribute to, or drive widespread neurodegeneration in the brain.

Where the central canal begins: endoscopic in vivo description

OBJECTIVE

Although evidence and descriptions of the central canal (CC) along the medulla oblongata and the spinal cord have been provided by several anatomical and radiological studies, a clear picture and assessment of the opening of the CC, or apertura canalis centralis (ACC), into the fourth ventricle is lacking, due to its submillimetric size and hidden position in the calamus scriptorius.

METHODS

The authors reviewed all of their cases in which patients underwent ventricular transaqueductal flexible endoscopic procedures and selected 44 cases in which an inspection of the region of the calamus scriptorius had been performed and was suitable for study inclusion. Patients were divided into different groups, based on the presence or absence of a chronic pathological process involving the fourth ventricle. In each case, the visual appearance of the opening of the CC of the ACC was classified as no evidence (A0), indirect evidence (A1), or clear evidence (A2). Morphometric measurements were inferred from surrounding structures and the size of surgical tools visible in the field.

RESULTS

The opening of the CC could be clearly observed in all cases (A1 4.5%, A2 95.5%). In normal cases, a lanceolate shape along the median sulcus was most frequently found, with an average size of 600 × 250 µm that became rounded and smaller in size in cases of hydrocephalus. The distance between the caudal margin of the ACC and the obex was about 1.8 mm in normal cases, 2.1 mm in cases of obstructive hydrocephalus, and 1 mm in cases of normal pressure hydrocephalus. The two wings of the area postrema, variable in size and shape, were sited just caudal to the opening.

CONCLUSIONS

A flexible scope inserted through the cerebral aqueduct can approach the hidden calamus scriptorius like a pen fits into an inkpot. With this privileged viewpoint, the authors provide for the first time, to their knowledge, a clear and novel vision of the opening of the CC in the fourth ventricle, along with the precise location of this tiny structure compared to other anatomical landmarks in the inferior triangle.

Effects of leptin on proliferation of astrocyte- and tanycyte-like neural stem cells in the adult mouse medulla oblongata

Astrocyte- and tanycyte-like neural stem cells (NSCs) were recently detected in the area postrema (AP) and central canal (CC) of the adult medulla oblongata, respectively. The present study aimed to examine dynamical behaviors of the astrocyte- and tanycyte-like NSCs of the mouse medulla oblongata to leptin. The neurosphere assay identified astrocytes in the AP and tanycytes in the CC as NSCs based on their self-renewing neurospherogenic potential. Both NSCs in neurosphere cultures were multipotent cells that generate astrocytes, oligodendrocytes, and neurons. Astrocyte-like NSCs actively proliferated and tanycyte-like NSCs were quiescent under physiologically-relevant in vivo conditions. Chronic leptin treatment promoted proliferation of astrocyte-like NSCs in the AP both in vitro and in vivo. Leptin receptors were expressed in astrocyte-like, but not tanycyte-like NSCs. Food deprivation significantly diminished proliferation of astrocyte-like NSCs. Therefore, the present study indicates that proliferation of astrocyte-like, but not tanycyte-like NSCs is regulated by nutritional conditions.

High mobility group box 1 promotes the differentiation of spinal ependymal cells into astrocytes rather than neurons

Spinal ependymal cells are involved in proliferation, differentiation and migration after spinal cord injury (SCI) and represent an endogenous source of repair cells for treating SCI. However, 95% of activated ependymal cells eventually differentiate into astrocytes after SCI and ultimately contribute more than half of the new astrocytes that form glial scars in vivo. The factors that regulate the fate of ependymal cells after SCI remain unclear. High mobility group box 1 (HMGB1) is regarded as an important proinflammatory factor in nerve injury, and recent studies have shown that HMGB1 can regulate the fate of stem cells after injury. In this study, we investigated whether HMGB1 released from reactive astrocytes after SCI regulates the proliferation and differentiation of ependymal cells in vitro. Ependymal cells extracted and cultured from the spinal cord of mice were separately treated with astrocyte culture medium (ACM), IL-1β, ACM (IL-1β) and the HMGB1 protein, and the proliferation and differentiation of ependymal cells were detected. Additionally, an HMGB1-neutralizing antibody (anti-HMGB1) was added to further verify the regulatory effect of HMGB1 on ependymal cells. The results showed that HMGB1 released from reactive astrocytes promoted ependymal cell differentiation into astrocytes and inhibited ependymal cell differentiation into neurons in vitro; however, the effect disappeared after the addition of anti-HMGB1. HMGB1 had no significant effect on ependymal cell proliferation. Our findings demonstrate that HMGB1 can regulate the differentiation of ependymal cells after SCI. These results provide a new strategy for the treatment of SCI.

A Guide to Extract Spinal Cord for Translational Stem Cell Biology Research: Comparative Analysis of Adult Human, Porcine, and Rodent Spinal Cord Stem Cells

Improving the clinical translation of animal-based neural stem/progenitor cell (NSPC) therapies to humans requires an understanding of intrinsic human and animal cell characteristics. We report a novel in vitro method to assess spinal cord NSPCs from a small (rodent) and large (porcine) animal model in comparison to human NSPCs. To extract live adult human, porcine, and rodent spinal cord tissue, we illustrate a strategy using an anterior or posterior approach that was simulated in a porcine model. The initial expansion of primary NSPCs is carried out using the neurosphere assay followed by a pharmacological treatment phase during which NSPCs derived from humans, porcines, and rodents are assessed in parallel using the same defined parameters. Using this model, NSPCs from all species demonstrated multi-lineage differentiation and self-renewal. Importantly, these methods provide conditions to enable the direct comparison of species-dependent cell behavior in response to specific exogenous signals.

Crosstalk between stem cell and spinal cord injury: pathophysiology and treatment strategies

The injured spinal cord is difficult to repair and regenerate. Traditional treatments are not effective. Stem cells are a type of cells that have the potential to differentiate into various cells, including neurons. They exert a therapeutic effect by safely and effectively differentiating into neurons or replacing damaged cells, secreting neurotrophic factors, and inhibiting the inflammatory response. Many types of stem cells have been used for transplantation, and each has its own advantages and disadvantages. This review discusses the possible mechanisms of stem cell therapy for spinal cord injury, and the types of stem cells commonly used in experiments, to provide a reference for basic and clinical research on stem cell therapy for spinal cord injury.

Employing Endogenous NSCs to Promote Recovery of Spinal Cord Injury

Endogenous neural stem cells (NSCs) exist in the central canal of mammalian spinal cords. Under normal conditions, these NSCs remain quiescent and express FoxJ1. After spinal cord injury (SCI), the endogenous NSCs of a heterogeneous nature are activated and proliferate and migrate towards the lesion site and mainly differentiate into astrocytes to repair the injured tissue. In vitro, spinal cord NSCs are multipotent and can differentiate into neurons, astrocytes, and oligodendrocytes. The altered microenvironments after SCI play key roles on the fate determination of activated NSCs, especially on the neuronal specification potential. Studies show that the activated spinal cord NSCs can generate interneurons when transplanted into the adult hippocampus. In addition, the spinal cord NSCs exhibit low immunogenicity in a transplantation context, thus implicating a promising therapeutic potential on SCI recovery. Here, we summarize the characteristics of spinal cord NSCs, especially their properties after injury. With a better understanding of endogenous NSCs under normal and SCI conditions, we may be able to employ endogenous NSCs for SCI repair in the future.

Directed glial differentiation and transdifferentiation for neural tissue regeneration

Glial cells which are indispensable for the central nervous system development and functioning, are proven to be vulnerable to a harmful influence of pathological cues and tissue misbalance. However, they are also highly sensitive to both in vitro and in vivo modulation of their commitment, differentiation, activity and even the fate-switch by different types of bioactive molecules. Since glial cells (comprising macroglia and microglia) are an abundant and heterogeneous population of neural cells, which are almost uniformly distributed in the brain and the spinal cord parenchyma, they all create a natural endogenous reservoir of cells for potential neurogenerative processes required to be initiated in response to pathophysiological cues present in the local tissue microenvironment. The past decade of intensive investigation on a spontaneous and enforced conversion of glial fate into either alternative glial (for instance from oligodendrocytes to astrocytes) or neuronal phenotypes, has considerably extended our appreciation of glial involvement in restoring the nervous tissue cytoarchitecture and its proper functions. The most effective modulators of reprogramming processes have been identified and tested in a series of pre-clinical experiments. A list of bioactive compounds which are potent in guiding in vivo cell fate conversion and driving cell differentiation includes a selection of transcription factors, microRNAs, small molecules, exosomes, morphogens and trophic factors, which are helpful in boosting the enforced neuro-or gliogenesis and promoting the subsequent cell maturation into desired phenotypes. Herein, an issue of their utility for a directed glial differentiation and transdifferentiation is discussed in the context of elaborating future therapeutic options aimed at restoring the diseased nervous tissue.

Methylprednisolone inhibits the proliferation of endogenous neural stem cells in nonhuman primates with spinal cord injury

OBJECTIVE

The aim of this work was to investigate the effects of methylprednisolone on the proliferation of endogenous neural stem cells (ENSCs) in nonhuman primates with spinal cord injury (SCI).

METHODS

A total of 14 healthy cynomolgus monkeys (Macaca fascicularis) (4–5 years of age) were randomly divided into 3 groups: the control group (n = 6), SCI group (n = 6), and methylprednisolone therapy group (n = 2). Only laminectomy was performed in the control animals at T-10. SCI was induced in monkeys using Allen’s weight-drop method (50 mm × 50 g) to injure the posterior portion of the spinal cord at T-10. In the methylprednisolone therapy group, monkeys were intravenously infused with methylprednisolone (30 mg/kg) immediately after SCI. All animals were intravenously infused with 5-bromo-2-deoxyuridine (BrdU) (50 mg/kg/day) for 3 days prior to study end point. The small intestine was dissected for immunohistochemical examination. After 3, 7, and 14 days, the spinal cord segments of the control and SCI groups were dissected to prepare frozen and paraffin sections. The proliferation of ENSCs was evaluated using BrdU and nestin immunofluorescence staining.

RESULTS

Histological examination showed that a larger number of mucosa epithelial cells in the small intestine of all groups were BrdU positive. Nestin-positive ependymal cells are increased around the central canal after SCI. After 3, 7, and 14 days of SCI, BrdU-positive ependymal cells in the SCI group were significantly increased compared with the control group, and the percentage of BrdU-positive cells in the left/right ventral horns and dorsal horn was significantly higher than that of the control group. Seven days after SCI, the percentages of both BrdU-positive ependymal cells around the central canal and BrdU– and nestin–double positive cells in the left/right ventral horns and dorsal horn were significantly lower in the methylprednisolone therapy group than in the SCI group.

CONCLUSIONS

While ENSCs proliferate significantly after SCI in nonhuman primates, methylprednisolone can inhibit the proliferation of ependymal cells after SCI.

Musashi and Plasticity of Xenopus and Axolotl Spinal Cord Ependymal Cells

The differentiated state of spinal cord ependymal cells in regeneration-competent amphibians varies between a constitutively active state in what is essentially a developing organism, the tadpole of the frog Xenopus laevis, and a quiescent, activatable state in a slowly growing adult salamander Ambystoma mexicanum, the Axolotl. Ependymal cells are epithelial in intact spinal cord of all vertebrates. After transection, body region ependymal epithelium in both Xenopus and the Axolotl disorganizes for regenerative outgrowth (gap replacement). Injury-reactive ependymal cells serve as a stem/progenitor cell population in regeneration and reconstruct the central canal. Expression patterns of mRNA and protein for the stem/progenitor cell-maintenance Notch signaling pathway mRNA-binding protein Musashi (msi) change with life stage and regeneration competence. Msi-1 is missing (immunohistochemistry), or at very low levels (polymerase chain reaction, PCR), in both intact regeneration-competent adult Axolotl cord and intact non-regeneration-competent Xenopus tadpole (Nieuwkoop and Faber stage 62+, NF 62+). The critical correlation for successful regeneration is msi-1 expression/upregulation after injury in the ependymal outgrowth and stump-region ependymal cells. msi-1 and msi-2 isoforms were cloned for the Axolotl as well as previously unknown isoforms of Xenopus msi-2. Intact Xenopus spinal cord ependymal cells show a loss of msi-1 expression between regeneration-competent (NF 50-53) and non-regenerating stages (NF 62+) and in post-metamorphosis froglets, while msi-2 displays a lower molecular weight isoform in non-regenerating cord. In the Axolotl, embryos and juveniles maintain Msi-1 expression in the intact cord. In the adult Axolotl, Msi-1 is absent, but upregulates after injury. Msi-2 levels are more variable among Axolotl life stages: rising between late tailbud embryos and juveniles and decreasing in adult cord. Cultures of regeneration-competent Xenopus tadpole cord and injury-responsive adult Axolotl cord ependymal cells showed an identical growth factor response. Epidermal growth factor (EGF) maintains mesenchymal outgrowth in vitro, the cells are proliferative and maintain msi-1 expression. Non-regeneration competent Xenopus ependymal cells, NF 62+, failed to attach or grow well in EGF+ medium. Ependymal Msi-1 expression in vivo and in vitro is a strong indicator of regeneration competence in the amphibian spinal cord.

The BAF45D Protein Is Preferentially Expressed in Adult Neurogenic Zones and in Neurons and May Be Required for Retinoid Acid Induced PAX6 Expression

Adult neurogenesis is important for the development of regenerative therapies for human diseases of the central nervous system (CNS) through the recruitment of adult neural stem cells (NSCs). NSCs are characterized by the capacity to generate neurons, astrocytes, and oligodendrocytes. To identify key factors involved in manipulating the adult NSC neurogenic fate thus has crucial implications for the clinical application. Here, we report that BAF45D is expressed in the subgranular zone (SGZ) of the dentate gyrus, the subventricular zone (SVZ) of the lateral ventricle, and the central canal (CC) of the adult spinal cord. Coexpression of BAF45D with glial fibrillary acidic protein (GFAP), a radial glial like cell marker protein, was identified in the SGZ, the SVZ and the adult spinal cord CC. Quantitative analysis data indicate that BAF45D is preferentially expressed in the neurogenic zone of the LV and the neurons of the adult CNS. Furthermore, during the neuroectoderm differentiation of H9 cells, BAF45D is required for the expression of PAX6, a neuroectoderm determinant that is also known to regulate the self-renewal and neuronal fate specification of adult neural stem/progenitor cells. Together, our results may shed new light on the expression of BAF45D in the adult neurogenic zones and the contribution of BAF45D to early neural development.

Spinal Cord Injury and Related Clinical Trials

Spinal cord injury (SCI) has been considered an incurable condition and it often causes devastating sequelae. In terms of the pathophysiology of SCI, reducing secondary damage is the key to its treatment. Various researches and clinical trials have been performed, and some of them showed promising results; however, there is still no gold standard treatment with sufficient evidence. Two therapeutic concepts for SCI are neuroprotective and neuroregenerative strategies. The neuroprotective strategy modulates the pathomechanism of SCI. The purpose of neuroprotective treatment is to minimize secondary damage following direct injury. The aim of neuroregenerative treatment is to enhance the endogenous regeneration process and to alter the intrinsic barrier. With advancement in biotechnology, cell therapy using cell transplantation is currently under investigation. This review discusses the pathophysiology of SCI and introduces the therapeutic candidates that have been developed so far.

Wnts Are Expressed in the Ependymal Region of the Adult Spinal Cord

The Wnt family of proteins plays key roles during central nervous system development and in several physiological processes during adulthood. Recently, experimental evidence has linked Wnt-related genes to regulation and maintenance of stem cells in the adult neurogenic niches. In the spinal cord, the ependymal cells surrounding the central canal form one of those niches, but little is known about their Wnt expression patterns. Using microdissection followed by TaqMan® low-density arrays, we show here that the ependymal regions of young, mature rats and adult humans express several Wnt-related genes, including ligands, conventional and non-conventional receptors, co-receptors, and soluble inhibitors. We found 13 genes shared between rats and humans, 4 exclusively expressed in rats and 9 expressed only in humans. Also, we observed a reduction with age on spontaneous proliferation of ependymal cells in rats paralleled by a decrease in the expression of Fzd1, Fzd8, and Fzd9. Our results suggest a role for Wnts in the regulation of the adult spinal cord neurogenic niche and provide new data on the specific differences in this region between humans and rodents.

Connexin 50 modulates Sox2 expression in spinal-cord-derived ependymal stem/progenitor cells

Ion channels included in the family of Connexins (Cx) have been reported to influence the secondary expansion of traumatic spinal cord injury (SCI) and neuropathic pain following SCI. However, Cxs also contribute to spinal cord neurogenesis during the remyelinating process and functional recovery after SCI. Certain Cxs have been recently related to the control of cell proliferation and the differentiation of neuronal progenitors. Adult spinal-cord-derived ependymal stem progenitor cells (epSPC) show high expression levels of Cx50 in non-pathological conditions and lower expression when they actively proliferate after injury (epSPCi). We explore the role of Cx50 in the ependymal population in the modulation of Sox2, a crucial factor of neural progenitor self-renewal and a promising target for promoting neuronal-cell-fate induction for neuronal tissue repair. Short-interfering-RNA ablation or over-expression of Cx50 regulates the expression of Sox2 in both epSPC and epSPCi. Interestingly, Cx50 and Sox2 co-localize at the nucleus indicating a potential role for this ion channel beyond cell-to-cell communication in the spinal cord. In vivo and in vitro experiments with Clotrimazole, a specific pharmacological modulator of Cx50, show the convergent higher expression of Cx50 and Sox2 in the isolated epSPC/epSPCi and in spinal cord tissue. Therefore, the pharmacological modulation of Cx50 might constitute an interesting mechanism for Sox2 induction to modulate the endogenous regenerative potential of neuronal tissue with a potential application in regenerative therapies.

The Spinal Ependymal Layer in Health and Disease

Ependymal cells are epithelial support cells that line the central canal and ventricular cavities of the central nervous system, providing the interface between the cerebrospinal fluid and the parenchyma of the brain and spinal cord. The spinal ependymal layer (SEL) is composed of 3 main cell types: tanycytes, ependymocytes, and cerebrospinal fluid-contacting neurons. A fourth cell type, termed the supraependymal cell, is also occasionally described. Cells of the SEL show restricted proliferative capacity in health but display neural stem cell properties both in vitro and in vivo in various disease states. A growing body of literature is devoted to the regenerative roles of the SEL, particularly in the context of spinal cord injury, where mechanical damage to the spinal cord leads to a significant increase in SEL proliferation. SEL-derived cell progeny migrate to sites of injury within the injured spinal cord parenchyma and contribute primarily to glial scar formation. In additional to their role as endogenous neural stem cells, cells of the SEL may be an important source of cytokines and other cell signaling molecules, such as tumor necrosis factor, heat shock proteins, and various growth factors. The SEL has become of recent interest to neuroscience researchers because of its potential to participate in and respond to diseases affecting the spinal cord (eg, traumatic spinal cord injury) and neurodegenerative disease. The intimate association of the SEL with the cerebrospinal fluid makes intrathecal therapies a viable option, and recent studies highlight the potential promise of treatments that augment SEL responses to disease.

Valproic Acid Arrests Proliferation but Promotes Neuronal Differentiation of Adult Spinal NSPCs from SCI Rats

Although the adult spinal cord contains a population of multipotent neural stem/precursor cells (NSPCs) exhibiting the potential to replace neurons, endogenous neurogenesis is very limited after spinal cord injury (SCI) because the activated NSPCs primarily differentiate into astrocytes rather than neurons. Valproic acid (VPA), a histone deacetylase inhibitor, exerts multiple pharmacological effects including fate regulation of stem cells. In this study, we cultured adult spinal NSPCs from chronic compressive SCI rats and treated with VPA. In spite of inhibiting the proliferation and arresting in the G0/G1 phase of NSPCs, VPA markedly promoted neuronal differentiation (β-tubulin III(+) cells) as well as decreased astrocytic differentiation (GFAP(+) cells). Cell cycle regulator p21(Cip/WAF1) and proneural genes Ngn2 and NeuroD1 were increased in the two processes respectively. In vivo, to minimize the possible inhibitory effects of VPA to the proliferation of NSPCs as well as avoid other neuroprotections of VPA in acute phase of SCI, we carried out a delayed intraperitoneal injection of VPA (150 mg/kg/12 h) to SCI rats from day 15 to day 22 after injury. Both of the newborn neuron marker doublecortin and the mature neuron marker neuron-specific nuclear protein were significantly enhanced after VPA treatment in the epicenter and adjacent segments of the injured spinal cord. Although the impaired corticospinal tracks had not significantly improved, Basso-Beattie-Bresnahan scores in VPA treatment group were better than control. Our study provide the first evidence that administration of VPA enhances the neurogenic potential of NSPCs after SCI and reveal the therapeutic value of delayed treatment of VPA to SCI.

Delayed Diagnosis of Enhancing Posterior Fossa Tumors Mimicking the Tela Choroidea of the Fourth Ventricle

BACKGROUND

Posterior fossa brain tumors are common in children. Symptoms typically develop when the tumors have reached sufficient size to cause compression of adjacent neural structures or cause obstructive hydrocephalus. Many tumors in this region originate from the tela choroidea and choroid plexus of the fourth ventricle. Enhancement of the fourth ventricular tela choroidea and choroid plexus is uncommon in children, and when such enhancement is present, it may represent early tumor growth.

METHODS

A 5-year-old girl with a history of congenital nystagmus, for whom initial work-up was reported as negative, presented again several years later with headache, nausea, and vomiting. She was found to have a large posterior fossa lesion on repeat neuroimaging that was retrospectively seen on the first neuroimaging scan as prominent enhancement in the region of the fourth ventricular choroid plexus. The second patient presented with congenital nystagmus and a lingual tremor and was found to have a slowly growing lesion situated in the fourth ventricle. Initial imaging was read as nodularly enhancing tela choroidea, but subsequent scans revealed enlargement of the lesion.

RESULTS

The first patient underwent gross total resection, and neuropathology was consistent with an atypical teratoid rhabdoid tumor. The patient has done well with postoperative adjuvant therapies. In the second patient, resection of the lesion revealed ependymoma; the patient has done well after adjuvant radiation therapy.

CONCLUSIONS

Pediatric patients who have enhancing tela choroidea or choroid plexus without an obvious mass lesion of the fourth ventricle may harbor early tumors. Surveillance imaging in these patients may be warranted given the aggressive nature of certain posterior fossa tumors in children. Failure to recognize abnormal enhancement patterns in this region may lead to delayed diagnosis.

Cell replacement therapy: lessons from teleost fish

Many disorders of the CNS are characterized by a massive loss of neurons. A promising therapeutic strategy to cure such conditions is based on the activation of endogenous stem cells. Implementation of this strategy will benefit from a better understanding of stem cell dynamics and the local CNS microenvironment in regeneration-competent vertebrate model systems. Using a spinal cord injury paradigm in zebrafish larvae, Briona and Dorsky (2014) have provided evidence for the existence of two distinct neural stem cell populations. One population has the characteristics of radial glia and expresses the homeobox transcription factor Dbx. The other lacks Dbx but expresses Olig2. These results are placed in the context of other studies that also support the notion of heterogeneity of adult stem cells in the CNS. The implication that differences among stem cell populations, in combination with specific factors from the local cellular microenvironment, might have a decisive impact on the fate choices of the new cells, is discussed. Reviewed evidence suggests that rather few modifications in the signaling pathways involved in the control of stem cell behavior have led, in the course of evolution, to the pronounced differences between mammals and regeneration-competent organisms. As a consequence, rather minor pharmacological manipulations may be sufficient to reactivate the hidden neurogenic potential of the mammalian CNS, and thus make it available for therapeutic applications.