Drebrin Regulates Neuroblast Migration in the Postnatal Mammalian Brain

After birth, stem cells in the subventricular zone (SVZ) generate neuroblasts that migrate along the rostral migratory stream (RMS) to become interneurons in the olfactory bulb (OB). This migration is crucial for the proper integration of newborn neurons in a pre-existing synaptic network and is believed to play a key role in infant human brain development. Many regulators of neuroblast migration have been identified; however, still very little is known about the intracellular molecular mechanisms controlling this process. Here, we have investigated the function of drebrin, an actin-binding protein highly expressed in the RMS of the postnatal mammalian brain. Neuroblast migration was monitored both in culture and in brain slices obtained from electroporated mice by time-lapse spinning disk confocal microscopy. Depletion of drebrin using distinct RNAi approaches in early postnatal mice affects neuroblast morphology and impairs neuroblast migration and orientation in vitro and in vivo. Overexpression of drebrin also impairs migration along the RMS and affects the distribution of neuroblasts at their final destination, the OB. Drebrin phosphorylation on Ser142 by Cyclin-dependent kinase 5 (Cdk5) has been recently shown to regulate F-actin-microtubule coupling in neuronal growth cones. We also investigated the functional significance of this phosphorylation in RMS neuroblasts using in vivo postnatal electroporation of phosphomimetic (S142D) or non-phosphorylatable (S142A) drebrin in the SVZ of mouse pups. Preventing or mimicking phosphorylation of S142 in vivo caused similar effects on neuroblast dynamics, leading to aberrant neuroblast branching. We conclude that drebrin is necessary for efficient migration of SVZ-derived neuroblasts and propose that regulated phosphorylation of drebrin on S142 maintains leading process stability for polarized migration along the RMS, thus ensuring proper neurogenesis.     

Early specification of GAD67 subventricular derived olfactory interneurons

Olfactory bulb interneurons are continuously generated in the subventricular zone (SVZ) and migrate along the rostral migratory stream (RMS) into the olfactory bulb (OB) where the majority becomes local GABAergic interneurons. We previously showed that SVZ-derived progenitor cells expressed glutamic acid decarboxylase 65 kDa (GAD65) very early in the migratory pathway. However, only approximately half of OB GABAergic interneurons use GAD65, an equal number express the 67 kDa GAD enzyme. To investigate the differentiation of these GABAergic interneurons we examined their migration in a transgenic mouse expressing green fluorescent protein (GFP) under the control of the GAD67 promoter. In adult, GFP was expressed by a subpopulation of migratory cells in the SVZ and along the RMS. Using Doublecortin (DCX) as a marker of migrating neuroblasts and bromodeoxyuridine (BrdU) incorporation, we show that these GAD67-GFP neurons co-express DCX and incorporate BrdU indicating they are newly born migratory neuroblasts. This is similar to GAD65 transgene expression, and in contrast to dopaminergic interneuron transgene expression which occurs only after cells reach the olfactory bulb. Although the GAD65/67 transgenes are expressed early in migration, there is minimal protein production in the cells prior to reaching the OB. These results suggest that migrating SVZ-derived neuroblasts acquire GABAergic identity prior to reaching their final location in the olfactory bulb.     

Unique neuronal tracers show migration and differentiation of SVZ progenitors in organotypic slices

Continual neurogenesis in the subventricular zone (SVZ) of postnatal and adult mammalian forebrain has been well documented, but the mechanisms underlying cell migration and differentiation in this region are poorly understood. We have developed novel in vivo and in vitro methods to investigate these processes. Using stereotaxic injections of a variety of tracers/tracker [Cholera Toxin β subunit (CTb‐), Fluorogold (FG), and Cell Tracker Green (CTG)], we could efficiently label SVZ cells. Over several days, labeled cells migrate along the rostral migratory stream (RMS) to their final differentiation site in the olfactory bulb (OB). The compatibility of these tracers/trackers with immunohistochemistry allows for cell labeling with multiple dyes (e.g., CTb and CTG) and/or specific cell antigens. To investigate the dynamics of migration we labeled SVZ progenitor cells with small injections of CTG and monitored the movements of individual cells in fresh parasagittal brain slices over several hours using time‐lapse confocal microscopy. Our observations suggest that tangential cell migration along the RMS occurs more rapidly than radial cell migration into the OB granule cell layer. To investigate migration over longer time periods, we developed an in vitro organotypic slice in which labeled SVZ progenitors migrate along the RMS and differentiate within the OB. The phenotypic characteristics of these cells in vitro were equivalent to those observed in vivo. Taken together, these methods provide useful tools investigating cell migration and differentiation in a preparation that maintains the anatomical organization of the RMS. © 2001 John Wiley & Sons, Inc. J Neurobiol 49: 326–338, 2001     

Unique neuronal tracers show migration and differentiation of SVZ progenitors in organotypic slices.

Continual neurogenesis in the subventricular zone (SVZ) of postnatal and adult mammalian forebrain has been well documented, but the mechanisms underlying cell migration and differentiation in this region are poorly understood. We have developed novel in vivo and in vitro methods to investigate these processes. Using stereotaxic injections of a variety of tracers/tracker [Cholera Toxin beta subunit (CTb-), Fluorogold (FG), and Cell Tracker Green (CTG)], we could efficiently label SVZ cells. Over several days, labeled cells migrate along the rostral migratory stream (RMS) to their final differentiation site in the olfactory bulb (OB). The compatibility of these tracers/trackers with immunohistochemistry allows for cell labeling with multiple dyes (e.g., CTb and CTG) and/or specific cell antigens. To investigate the dynamics of migration we labeled SVZ progenitor cells with small injections of CTG and monitored the movements of individual cells in fresh parasagittal brain slices over several hours using time-lapse confocal microscopy. Our observations suggest that tangential cell migration along the RMS occurs more rapidly than radial cell migration into the OB granule cell layer. To investigate migration over longer time periods, we developed an in vitro organotypic slice in which labeled SVZ progenitors migrate along the RMS and differentiate within the OB. The phenotypic characteristics of these cells in vitro were equivalent to those observed in vivo. Taken together, these methods provide useful tools investigating cell migration and differentiation in a preparation that maintains the anatomical organization of the RMS.     

Adult Mouse Subventricular Zone Stem and Progenitor Cells Are Sessile and Epidermal Growth Factor Receptor Negatively Regulates Neuroblast Migration

Background

The adult subventricular zone (SVZ) contains stem and progenitor cells that generate neuroblasts throughout life. Although it is well accepted that SVZ neuroblasts are migratory, recent evidence suggests their progenitor cells may also exhibit motility. Since stem and progenitor cells are proliferative and multipotential, if they were also able to move would have important implications for SVZ neurogenesis and its potential for repair.

Methodology/Principal Findings

We studied whether SVZ stem and/or progenitor cells are motile in transgenic GFP+ slices with two photon time lapse microscopy and post hoc immunohistochemistry. We found that stem and progenitor cells; mGFAP-GFP+ cells, bright nestin-GFP+ cells and Mash1+ cells were stationary in the SVZ and rostral migratory stream (RMS). In our search for motile progenitor cells, we uncovered a population of motile βIII-tubulin+ neuroblasts that expressed low levels of epidermal growth factor receptor (EGFr). This was intriguing since EGFr drives proliferation in the SVZ and affects migration in other systems. Thus we examined the potential role of EGFr in modulating SVZ migration. Interestingly, EGFr^low neuroblasts moved slower and in more tortuous patterns than EGFr-negative neuroblasts. We next questioned whether EGFr stimulation affects SVZ cell migration by imaging Gad65-GFP+ neuroblasts in the presence of transforming growth factor alpha (TGF-α), an EGFr-selective agonist. Indeed, acute exposure to TGF-α decreased the percentage of motile cells by approximately 40%.

Conclusions/Significance

In summary, the present study directly shows that SVZ stem and progenitor cells are static, that EGFr is retained on some neuroblasts, and that EGFr stimulation negatively regulates migration. This result suggests an additional role for EGFr signaling in the SVZ.     

EGF-Induced Expansion of Migratory Cells in the Rostral Migratory Stream

The presence of neural stem cells in the adult brain is currently widely accepted and efforts are made to harness the regenerative potential of these cells. The dentate gyrus of the hippocampal formation, and the subventricular zone (SVZ) of the anterior lateral ventricles, are considered the main loci of adult neurogenesis. The rostral migratory stream (RMS) is the structure funneling SVZ progenitor cells through the forebrain to their final destination in the olfactory bulb. Moreover, extensive proliferation occurs in the RMS. Some evidence suggest the presence of stem cells in the RMS, but these cells are few and possibly of limited differentiation potential. We have recently demonstrated the specific expression of the cytoskeleton linker protein radixin in neuroblasts in the RMS and in oligodendrocyte progenitors throughout the brain. These cell populations are greatly altered after intracerebroventricular infusion of epidermal growth factor (EGF). In the current study we investigate the effect of EGF infusion on the rat RMS. We describe a specific increase of radixin⁺/Olig2⁺ cells in the RMS. Negative for NG2 and CNPase, these radixin⁺/Olig2⁺ cells are distinct from typical oligodendrocyte progenitors. The expanded Olig2⁺ population responds rapidly to EGF and proliferates after only 24 hours along the entire RMS, suggesting local activation by EGF throughout the RMS rather than migration from the SVZ. In addition, the radixin⁺/Olig2⁺ progenitors assemble in chains in vivo and migrate in chains in explant cultures, suggesting that they possess migratory properties within the RMS. In summary, these results provide insight into the adaptive capacity of the RMS and point to an additional stem cell source for future brain repair strategies.     

Longterm quiescent cells in the aged human subventricular neurogenic system specifically express GFAP‐δ

A main neurogenic niche in the adult human brain is the subventricular zone (SVZ). Recent data suggest that the progenitors that are born in the human SVZ migrate via the rostral migratory stream (RMS) towards the olfactory bulb (OB), similar to what has been observed in other mammals. A subpopulation of astrocytes in the SVZ specifically expresses an assembly‐compromised isoform of the intermediate filament protein glial fibrillary acidic protein (GFAP‐δ). To further define the phenotype of these GFAP‐δ expressing cells and to determine whether these cells are present throughout the human subventricular neurogenic system, we analysed SVZ, RMS and OB sections of 14 aged brain donors (ages 74‐93). GFAP‐δ was expressed in the SVZ along the ventricle, in the RMS and in the OB. The GFAP‐δ cells in the SVZ co‐expressed the neural stem cell (NSC) marker nestin and the cell proliferation markers proliferating cell nuclear antigen (PCNA) and Mcm2. Furthermore, BrdU retention was found in GFAP‐δ positive cells in the SVZ. In the RMS, GFAP‐δ was expressed in the glial net surrounding the neuroblasts. In the OB, GFAP‐δ positive cells co‐expressed PCNA. We also showed that GFAP‐δ cells are present in neurosphere cultures that were derived from SVZ precursors, isolated postmortem from four brain donors (ages 63‐91). Taken together, our findings show that GFAP‐δ is expressed in an astrocytic subpopulation in the SVZ, the RMS and the OB. Importantly, we provide the first evidence that GFAP‐δ is specifically expressed in longterm quiescent cells in the human SVZ, which are reminiscent of NSCs.     

Reduced proliferation in the adult mouse subventricular zone increases survival of olfactory bulb interneurons

Neurogenesis in the adult brain is largely restricted to the subependymal zone (SVZ) of the lateral ventricle, olfactory bulb (OB) and the dentate subgranular zone, and survival of adult-born cells in the OB is influenced by factors including sensory experience. We examined, in mice, whether survival of adult-born cells is also regulated by the rate of precursor proliferation in the SVZ. Precursor proliferation was decreased by depleting the SVZ of dopamine after lesioning dopamine neurons in the substantia nigra compacta with 6-hydroxydopamine. Subsequently, we examined the effect of reduced SVZ proliferation on the generation, migration and survival of neuroblasts and mature adult-born cells in the SVZ, rostral migratory stream (RMS) and OB. Proliferating cells in the SVZ, measured by 5-bromo-2-deoxyuridine (BrdU) injected 2 hours prior to death or by immunoreactivity against Ki67, were reduced by 47% or 36%, respectively, 7 days after dopamine depletion, and by 29% or 31% 42 days after dopamine depletion, compared to sham-treated animals. Neuroblast generation in the SVZ and their migration along the RMS were not affected, neither 7 nor 42 days after the 6-hydroxydopamine injection, since the number of doublecortin-immunoreactive neuroblasts in the SVZ and RMS, as well as the number of neuronal nuclei-immunoreactive cells in the OB, were stable compared to control. However, survival analysis 15 days after 6-hydroxydopamine and 6 days after BrdU injections showed that the number of BrdU+ cells in the SVZ was 70% higher. Also, 42 days after 6-hydroxydopamine and 30 days after BrdU injections, we found an 82% increase in co-labeled BrdU+/γ-aminobutyric acid-immunoreactive cell bodies in the granular cell layer, while double-labeled BrdU+/tyrosine hydroxylase-immunoreactive cell bodies in the glomerular layer increased by 148%. We conclude that the number of OB interneurons following reduced SVZ proliferation is maintained through an increased survival of adult-born precursor cells, neuroblasts and interneurons.     

Identification and characterization of neuroblasts in the subventricular zone and rostral migratory stream of the adult human brain

It is of great interest to identify new neurons in the adult human brain, but the persistence of neurogenesis in the subventricular zone (SVZ) and the existence of the rostral migratory stream (RMS)-like pathway in the adult human forebrain remain highly controversial. In the present study, we have described the general configuration of the RMS in adult monkey, fetal human and adult human brains. We provide evidence that neuroblasts exist continuously in the anterior ventral SVZ and RMS of the adult human brain. The neuroblasts appear singly or in pairs without forming chains; they exhibit migratory morphologies and co-express the immature neuronal markers doublecortin, polysialylated neural cell adhesion molecule and βIII-tubulin. Few of these neuroblasts appear to be actively proliferating in the anterior ventral SVZ but none in the RMS, indicating that neuroblasts distributed along the RMS are most likely derived from the ventral SVZ. Interestingly, no neuroblasts are found in the adult human olfactory bulb. Taken together, our data suggest that the SVZ maintains the ability to produce neuroblasts in the adult human brain.     

Rostral migratory stream neuroblasts turn and change directions in stereotypic patterns

Neuroblasts generated in the adult subventricular zone (SVZ) migrate through the rostral migratory stream (RMS) to the olfactory bulb (OB). Previous work uncovered motility ranging from straight to complex, but it was unclear if directional changes were stochastic or exhibited stereotypical patterns. Here, we provide the first in-depth two-photon time-lapse microscopy study of morphological and dynamic features that accompany turning and direction reversals in the RMS. We identified three specific kinds of turning (30–90 degrees): bending of the leading process proximal to the cell body (P-bending 47% of cases), bending of the distal leading process (D-bending 30%) or branching of the leading process or lamellipodium (23%). Bending and branching angles were remarkably constrained and were significantly different from one another. Cells reversed direction (>90 degrees) through D-bendings (54%), branching (11%) or de novo growth of processes from the soma (23%), but not P-bending. Direction reversal was often composed of several iterations of D-bending or branching as opposed to novel modalities. Individual neuroblasts could turn or change direction in multiple patterns suggesting that the patterns are not specific for different lineages. These findings show that neuroblasts in the RMS use a limited number of distinct and constrained modalities to turn or reverse direction.Key words: neurogenesis, subventricular zone, migration, direction, motility     

Gap junctions are involved in cell migration in the early postnatal subventricular zone

The massive migration of neuroblasts and young neurons through the anterior extension of the postnatal subventricular zone (SVZ), known as the rostral migratory stream (RMS) is still poorly understood on its molecular basis. In this work, we investigated the involvement of gap junctional communication (GJC) in the robust centrifugal migration from SVZ/RMS explants obtained from early postnatal (P4) rats. Cells were dye‐coupled in homocellular and heterocellular pairings and expressed at least two connexins, Cx 43 and 45. Treatment with the uncoupler agent carbenoxolone (CBX, 10–100 μM) reversibly reduced outgrowth from SVZ explants, while its inactive analog, glycyrhizinic acid (GZA), had no effect. Consistent with a direct effect on cell migration, time‐lapse video microscopy show that different pharmacological uncouplers cause an abrupt and reversible arrest of cell movement in explants. Our results indicate that GJC is positively involved in the migration of neuroblasts within the SVZ/RMS. © 2009 Wiley Periodicals, Inc. Develop Neurobiol, 2009     

Histone deacetylase inhibitors de-repress tyrosine hydroxylase expression in the olfactory bulb and rostral migratory stream

Most olfactory bulb (OB) interneurons are derived from neural stem cells in the subventricular zone (SVZ) and migrate to the OB via the rostral migratory stream (RMS). Mature dopaminergic interneurons in the OB glomerular layer are readily identified by their synaptic activity-dependent expression of tyrosine hydroxylase (TH). Paradoxically, TH is not expressed in neural progenitors migrating in the RMS, even though ambient GABA and glutamate depolarize these progenitors. In forebrain slice cultures prepared from transgenic mice containing a GFP reporter gene under the control of the Th 9 kb upstream regulatory region, treatment with histone deacetylase (HDAC) inhibitors (either sodium butyrate, Trichostatin A or Scriptaid) induced Th-GFP expression specifically in the RMS independently of depolarizing conditions in the culture media. Th-GFP expression in the glomerular layer was also increased in slices treated with Trichostatin A, but this increased expression was dependent on depolarizing concentrations of KCl in the culture media. Th-GFP expression was also induced in the RMS in vivo by intra-peritoneal injections with either sodium butyrate or valproic acid. Quantitative RT-PCR analysis of neurosphere cultures confirmed that HDAC inhibitors de-repressed Th expression in SVZ-derived neural progenitors. Together, these findings suggest that HDAC function is critical for regulating Th expression levels in both neural progenitors and mature OB dopaminergic neurons. However, the differential responses to the combinatorial exposure of HDAC inhibitors and depolarizing culture conditions indicate that Th expression in mature OB neurons and neural progenitors in the RMS are regulated by distinct HDAC-mediated mechanisms.     

Targeted Deletion of the ERK5 MAP Kinase Impairs Neuronal Differentiation,Migration, and Survival during Adult Neurogenesis in the Olfactory Bulb

Recent studies have led to the exciting idea that adult-born neurons in the olfactory bulb (OB) may be critical for complex forms of olfactory behavior in mice. However, signaling mechanisms regulating adult OB neurogenesis are not well defined. We recently reported that extracellular signal-regulated kinase (ERK) 5, a MAP kinase, is specifically expressed in neurogenic regions within the adult brain. This pattern of expression suggests a role for ERK5 in the regulation of adult OB neurogenesis. Indeed, we previously reported that conditional deletion of erk5 in adult neurogenic regions impairs several forms of olfactory behavior in mice. Thus, it is important to understand how ERK5 regulates adult neurogenesis in the OB. Here we present evidence that shRNA suppression of ERK5 in adult neural stem/progenitor cells isolated from the subventricular zone (SVZ) reduces neurogenesis in culture. By contrast, ectopic activation of endogenous ERK5 signaling via expression of constitutive active MEK5, an upstream activating kinase for ERK5, stimulates neurogenesis. Furthermore, inducible and conditional deletion of erk5 specifically in the neurogenic regions of the adult mouse brain interferes with cell cycle exit of neuroblasts, impairs chain migration along the rostral migratory stream and radial migration into the OB. It also inhibits neuronal differentiation and survival. These data suggest that ERK5 regulates multiple aspects of adult OB neurogenesis and provide new insights concerning signaling mechanisms governing adult neurogenesis in the SVZ-OB axis.     

Deficits of olfactory interneurons in polysialyltransferase‐ and NCAM‐deficient mice

The neurogenic niche of the anterior subventricular zone (SVZ) persistently generates neuroblasts, which migrate along the rostral migratory stream (RMS) into the olfactory bulb (OB), where they differentiate into granule and periglomerular cells. Loss of the neural cell adhesion molecule NCAM or its post‐translational modification polysialic acid (polySia) impairs migration causing accumulations of cells in the proximal RMS and decreased OB volume. Polysialylation of NCAM is implemented by two polysialyltransferases, ST8SIA2 and ST8SIA4, with overlapping functions. Here, we used mice with Ncam1 and polysialyltransferase deletions to analyze how partial or complete loss of polySia synthesis or a combined loss of polySia and NCAM affects the RMS and the interneuron composition in the OB. Numerous calretinin (CR)‐positive cells were detected dispersed around the RMS in Ncam1 knockout, St8sia2, St8sia4 double‐knockout, and St8sia2, St8sia4, Ncam1 triple‐knockout mice, as well as in St8sia2 ^?/? but not in St8sia4 ^?/? mice. These changes were not reflected by reductions of CR‐positive cells in the granule or glomerular layer of the OB. Instead, calbindin‐positive periglomerular interneurons were strongly reduced in all polySia‐NCAM negative mice and slightly attenuated in St8sia2 ^?/? as well as in the St8sia4 ^?/? mice, which were devoid of ectopic CR‐positive cells along the RMS. Consistent with the early developmental generation of calbindin‐ as compared with CR‐positive OB interneurons, this phenotype was fully developed at postnatal day 5. Together, these results demonstrate that the early development of calbindin‐positive periglomerular interneurons depends on the presentation of polySia on NCAM and requires the activity of both polysialyltransferases. © 2015 Wiley Periodicals, Inc. Develop Neurobiol 76: 421–433, 2016     

Adult neurogenic process in the subventricular zone‐olfactory bulb system is regulated by Tau protein under prolonged stress

ObjectivesThe area of the subventricular zone (SVZ) in the adult brain exhibits the highest number of proliferative cells, which, together with the olfactory bulb (OB), maintains constant brain plasticity through the generation, migration and integration of newly born neurons. Despite Tau and its malfunction is increasingly related to deficits of adult hippocampal neurogenesis and brain plasticity under pathological conditions [e.g. in Alzheimer''s disease (AD)], it remains unknown whether Tau plays a role in the neurogenic process of the SVZ and OB system under conditions of chronic stress, a well‐known sculptor of brain and risk factor for AD.Materials and methodsDifferent types of newly born cells in SVZ and OB were analysed in animals that lack Tau gene (Tau‐KO) and their wild‐type littermates (WT) under control or chronic stress conditions.ResultsWe demonstrate that chronic stress reduced the number of proliferating cells and neuroblasts in the SVZ leading to decreased number of newborn neurons in the OB of adult WT, but not Tau‐KO, mice. Interestingly, while stress‐evoked changes were not detected in OB granular cell layer, Tau‐KO exhibited increased number of mature neurons in this layer indicating altered neuronal migration due to Tau loss.ConclusionsOur findings suggest the critical involvement of Tau in the neurogenesis suppression of SVZ and OB neurogenic niche under stressful conditions highlighting the role of Tau protein as an essential regulator of stress‐driven plasticity deficits.     

Rostral migratory stream neuroblasts turn and change directions in stereotypic patterns

Neuroblasts generated in the adult subventricular zone (SVZ) migrate through the rostral migratory stream (RMS) to the olfactory bulb (OB). Previous work uncovered motility ranging from straight to complex, but it was unclear if directional changes were stochastic or exhibited stereotypical patterns. Here, we provide the first in-depth two-photon time-lapse microscopy study of morphological and dynamic features that accompany turning and direction reversals in the RMS. We identified three specific kinds of turning (30-90 degrees): bending of the leading process proximal to the cell body (P-bending 47% of cases), bending of the distal leading process (D-bending 30%) or branching of the leading process or lamellipodium (23%). Bending and branching angles were remarkably constrained and were significantly different from one another. Cells reversed direction (>90 degrees) through D-bendings (54%), branching (11%) or de novo growth of processes from the soma (23%), but not P-bending. Direction reversal was often composed of several iterations of D-bending or branching as opposed to novel modalities. Individual neuroblasts could turn or change direction in multiple patterns suggesting that the patterns are not specific for different lineages. These findings show that neuroblasts in the RMS use a limited number of distinct and constrained modalities to turn or reverse direction.     

Initial stages of radial glia astrocytic transformation in the early postnatal anterior subventricular zone

In the early postnatal subventricular zone (SVZ), two seemingly unrelated events occur simultaneously: a massive tangential migration of neuroblasts towards the olfactory bulb, known as the rostral migratory stream (RMS), and the outward movement of radial glia (RG) undergoing astrocytic transformation. Because of the orthogonal arrangement between these two sets of cells, little, if any, relevance has been ascribed for their possible interactions. By depositing DiI at the pial surface we have studied RG transformation within the SVZ/RMS, from birth up to the end of the first postnatal week. While still within the SVZ/RMS, RG morphology changed from simple bipolar to highly complex branched profiles, attaining their highest degree of complexity at the interface of the SVZ with the overlying white matter. At this interface cell bodies of radial glia accumulate and their processes run tangentially, surrounding the SVZ/RMS. Processes of RG surrounding the SVZ/RMS could also be observed by immunostaining for vimentin, GFAP, and nestin. In contrast, in the white matter all DiI-labeled RG presented a simple bipolar profile. These results indicate that the outward radial migration of the transforming RG does not occur uniformly. Instead, the different morphologies and cell densities that RG assume when they cross the SVZ/RMS and overlying white matter imply different migratory behaviors. Finally, our data suggest that RG provide a cellular scaffold to the early postnatal SVZ/RMS, much in the same way as astrocytes in the adult RMS.