Stem Cell Recruitment of Newly Formed Host Cells via a Successful Seduction? Filling the Gap between Neurogenic Niche and Injured Brain Site

Here, we report that a unique mechanism of action exerted by stem cells in the repair of the traumatically injured brain involves their ability to harness a biobridge between neurogenic niche and injured brain site. This biobridge, visualized immunohistochemically and laser captured, corresponded to an area between the neurogenic subventricular zone and the injured cortex. That the biobridge expressed high levels of extracellular matrix metalloproteinases characterized initially by a stream of transplanted stem cells, but subsequently contained only few to non-detectable grafts and overgrown by newly formed host cells, implicates a novel property of stem cells. The transplanted stem cells manifest themselves as pathways for trafficking the migration of host neurogenic cells, but once this biobridge is formed between the neurogenic site and the injured brain site, the grafted cells disappear and relinquish their task to the host neurogenic cells. Our findings reveal that long-distance migration of host cells from the neurogenic niche to the injured brain site can be achieved through transplanted stem cells serving as biobridges for initiation of endogenous repair mechanisms. This is the first report of a stem cell-paved “biobridge”. Indeed, to date the two major schools of discipline in stem cell repair mechanism primarily support the concept of “cell replacement” and bystander effects of “trophic factor secretion”. The present novel observations of a stem cell seducing a host cell to engage in brain repair advances basic science concepts on stem cell biology and extracellular matrix, as well as provokes translational research on propagating this stem cell-paved biobridge beyond cell replacement and trophic factor secretion for the treatment of traumatic brain injury and other neurological disorders.


Introduction
Initially employed for in-depth examination of cell development [1], stem cells have become a cornerstone for regenerative medicine in establishing cell-based therapies for neurological disorders [2,3]. A fundamental gap in our knowledge about the mechanism underlying stem cell therapy remains unresolved. Functional recovery has been observed in experimental models of neurological disorders despite few or even absent survival of transplanted stem cells within the injured brain site [4,5]. The original concept of direct cell replacement has been challenged by the view that stem cells afford indirect rescue of the injured tissue via secretion of therapeutic molecules [6,7].
The present study provides evidence of a novel therapeutic feature of stem cells involving their ability to harness a biobridge between neurogenic niche and injured brain site in a traumatic brain injury (TBI) model. This biobridge expressed high levels of extracellular matrix metalloproteinases (ECM) characterized initially by a stream of transplanted stem cells, but subsequently replaced by newly formed host cells. The transplanted stem cells serve as migratory cues for host neurogenic cells, guiding their exodus from the neurogenic site towards the injured brain site. Our findings reveal that longdistance migration of host cells from the neurogenic niche to the injured brain site can be achieved through transplanted stem cells serving as biobridges for initiation of endogenous repair mechanisms.

Summary
This study was designed to evaluate potential therapeutic value of intracerebral transplantation of cultured Notch-induced human bone marrow-derived mesenchymal stromal cells (MSCs) (referred to as SB623, supplied by SanBio Inc.) [26,28] in an animal model of TBI. Transplantation was carried out at 7 days after TBI with functional readouts of behavioral and histological deficits conducted during the subsequent three month period after TBI. We characterized locomotor and neurological performance at baseline (prior to TBI), then at 7 days after TBI (prior to transplantation), and monthly thereafter up to three months after TBI. Following completion of behavioral testing at one month or three months after TBI, animals were euthanized by transcardial perfusion and brains harvested to histologically characterize the extent of brain damage. The stem cell engraftment and host tissue endogenous repair mechanisms (e.g., increased host cell survival in peri-TBI lesion area) were examined by immunohistochemical analyses. A total of 40 animals identified at baseline (prior to TBI surgery) as exhibiting normal behaviors (EBST: 50-60% bias swing activity; Rotorod: 60 seconds staying time on rotating rod; Bederson: at most 0-0.5 mean neurologic score), received TBI surgery as described below. Only TBI animals reaching the criterion of behavioral impairment (EBST: at least 75% bias swing activity; Rotorod: 30 seconds or less staying time on rotating rod; Bederson: at least 2.5 mean neurologic score) were randomly assigned to either SB623 transplants (n=20) or vehicle infusion (n=20) on Day 7 post-TBI. All animals were monitored monthly postgrafting for behavioral outcomes. Randomly selected animals were euthanized at one month (n=10 per group) post-TBI, and the remaining animals euthanized at three months post-TBI by transcardial perfusion with 4% paraformaldehyde. For outcome measures, transplant outcome were evaluated using the following parameters: 1) locomotor behavior via elevated body swing test (EBST) and Rotorod; (2) neurological performance via a Bederson-modified neurological examination; 3) lesion volume via hematoxylin and eosin (H&E) histologic stains; 4) graft survival via immunohistochemistry using specific antibody shown to detect human cells, and; 5) mechanism-based immunohistochemial analyses of neuroprotection and/or regeneration using antibodies directed against the grafted human cells and host cells.

Subjects
The University of South Florida Institutional Animal Care and Use Committee approved all procedures used in this study. Animals had free access to food and water, and all were housed under normal conditions (20°C, 50% relative humidity, and a 12 hour light/dark cycle).

TBI surgery
All surgical procedures were conducted under aseptic conditions. Adult male Sprague-Dawley (SD) rats (8-weeks old) were anesthetized with 1.5% isofluorane and checked for pain reflexes. Under deep anesthesia, animals underwent the moderate TBI model. Each animal was placed in a stereotaxic frame (anesthesia maintained via gas mask) with 1-2% isoflurane. After exposing the skull, a 4.0 mm craniectomy was performed over the left frontoparietal cortex (center at −2.0 mm anteroposterior (AP) and +2.0 mm mediolateral (ML) to bregma) [29]. A pneumatically operated metal impactor (diameter = 3.0 mm) impacted the brain at a velocity of 6.0 m/s reaching a depth of 1.0 mm below the dura mater layer and remained in the brain for 150 milliseconds. The impactor rod was angled 15° to the vertical to be perpendicular to the tangential plane of the brain curvature at the impact surface. A linear variable displacement transducer (Macrosensors, Pennsauken, NJ) connected to the impactor measured velocity and duration to verify consistency. After controlled cortical impact injury, the incision was sutured after bleeding ceased. An integrated heating pad and rectal thermometer unit with feedback control allowed maintenance of body temperature at normal limits. All animals were monitored until recovery from anesthesia. In addition, animals were weighed and observed daily for the next three consecutive days following TBI surgery, weighed twice a week thereafter, and monitored daily for health status and any signs that indicate problems or complications throughout the study.

Grafting procedures
All surgical procedures were conducted under aseptic conditions. Animals were anesthetized with 1.5% isofluorane and checked for pain reflexes. Once deep anesthesia was achieved (by checking for pain reflexes), hair was shaved around the area of surgical incision (skull area) with enough border to prevent contaminating the operative site, followed by two surgical germicidal scrubs of site, and draping with sterile drapes. The animal was then fixed to a stereotaxic apparatus (Kopf Instruments). A 26-gauge Hamilton syringe was then lowered into a small burred skull opening (transplant coordinates were adjusted to correspond with the cortical area adjacent to the core injury site: 0.5 mm anterior and 1.0 mm lateral to bregma and 2.0 mm below the dural surface [29]). Within this single needle pass, 3 deposits of the test article (100,000 cells in 3 µl per deposit or a total of 300,000 cells in 9 µl of Plasmalyte A for 3 deposits) were made. The target area was the medial cortex which corresponded to the peri-injured cortical area, based on previously established target sites for similar stereotaxic implants. Each deposit consisted of 100,000 viable cells in 3 µl volume infused over a period of 3 minutes. Following an additional 2-minute absorption time, the needle was retracted and the wound closed stainless steel wound clip. A heating pad and a rectal thermometer allowed maintenance of body temperature at about 37°C throughout surgery and following recovery from anesthesia.

Behavioral and neurological tests
All investigators testing the animals were blinded to the treatment condition. Animals were subjected to elevated body swing test (EBST), neurological exam, and Rotorod. EBST involved handling the animal by its tail and recording the direction of the swings. The test apparatus consisted of a clear Plexiglas box (40 x 40 x 35.5 cm). The animal was gently picked up at the base of the tail, and elevated by the tail until the animal's nose was at a height of 2 inches (5 cm) above the surface. The direction of the swing, either left or right, was counted once the animals head moved sideways approximately 10 degrees from the midline position of the body. After a single swing, the animal was placed back in the Plexiglas box and allowed to move freely for 30 seconds prior to retesting. These steps were repeated 20 times for each animal. Intact rats display a 50% swing bias, that is, the same number of swings to the left and to the right. A 75% swing bias indicated 15 swings in one direction and 5 in the other during 20 trials. We have previously utilized the EBST, and noted that unilaterally lesioned animals display >75% biased swing activity at one month after a nigrostriatal lesion or unilateral hemispheric injury; asymmetry is stable for up to six months [3,26]. About one hour after the EBST, a modified Bederson-Neurological exam was conducted following the procedures previously described [3,26] with minor modifications. Neurologic score for each rat was obtained using 3 tests which include (1) forelimb retraction, which measured the ability of the animal to replace the forelimb after it was displaced laterally by 2 to 3 cm, graded from 0 (immediate replacement) to 3 (replacement after several seconds or no replacement); (2) beam walking ability, graded 0 for a rat that readily traversed a 2.4-cm-wide, 80-cm-long beam to 3 for a rat unable to stay on the beam for 10 seconds; and (3) bilateral forepaw grasp, which measured the ability to hold onto a 2-mm-diameter steel rod, graded 0 for a rat with normal forepaw grasping behavior to 3 for a rat unable to grasp with the forepaws. The scores from all 3 tests, which were done over a period of about 15 minutes on each assessment day, were added to give a mean neurologic deficit score (maximum possible score, 9 points divided by 3 tests = 3). After an hour of completion of the neurological exam, the animals were then subjected to the Rotorod test. The Rotorod test involved placement of the animal on an accelerating Rotorod (Accuscan, Inc.) that used a rotating treadmill that accelerates from 4 rpm to 40 rpm over a 60-second period. The total number of seconds maintained on the Rotorod was recorded and used as index of motor coordination. We have previously shown that TBI model animals exhibited significantly shorter time staying on the Rotorod compared to sham operated or normal controls. Animals were subjected to this battery of tests at baseline (prior to TBI), then at 7 days after TBI (prior to transplantation) and monthly thereafter up to three months post-TBI.

Histology
Perfusion. Brain section preparation was designed to identify extent of brain damage and host cell survival. At scheduled intervals (1 month or 3 months) after TBI, randomly selected rats were euthanized (n=10 per group), and perfused by transcardial perfusion with 4% paraformaldehyde. The brains were dissected, post-fixed for overnight in 4% paraformaldehyde, then subsequently immersed in 30% sucrose. Each forebrain was cut into 40 µm thick coronal tissue sections with anterior-posterior coordinates corresponding from bregma 5.2 mm to bregma -8.8 mm per animal and subsequently processed for measurements of brain damage and analyses of cell survival in the peri-TBI lesion area (see below).
Measurements of brain damage. At least 4 coronal tissue sections per brain were processed for H&E staining. Every sixth coronal tissue sections per brain were collected beginning at AP -2.20 and ending at AP +0.20 anterior to the bregma, and randomly selected for measurement of cortical core and periinjury area [29]. The indirect lesion area, in which the intact area of the ipsilateral hemisphere was subtracted from the area of the contralateral hemisphere, was calculated to reveal cerebral damage. The lesion volume was presented as a volume percentage of the lesion compared to the contralateral hemisphere.
Analyses of cell survival in peri-TBI lesion area. Randomly selected high powerfield corresponding to the peri-injured cortical area was used to quantitatively count host cells surviving in this region.

Immunohistochemistry
Free floating sections were processed for immunofluorescent microscopy. Briefly, 40 µm cryostat sectioned tissues were examined at 4X magnification and digitized using a PC-based Image Tools computer program. Brain sections were blindcoded and Abercrombie's formula was used to calculate the total number of immunopositive cells [3,26]. Cell engraftment index for SB623 was assessed using monoclonal human specific antibody (HuNu) that did not cross-react with rodent proteins. Additional brain sections were processed for mechanism-based immunohistochemical analyses of brain tissue samples focusing on cell proliferation (Ki67), migration (doublecortin or DCX) and immature neural marker (nestin).

Zymography
A separate cohort of animals consisting of TBI plus SB623 cells, TBI plus vehicle, and control-sham operated agematched adult SD rats (n=3 per group) was subjected to the same experimental paradigm as above, but tissues were processed for zymography, a process involving electrophoretic separation of proteins for assessment of proteolytic activity [30,31]. The tissue corresponding to the biobridge formed by the migrating cells from the SVZ to the impacted cortex was laser captured. After extraction, the tissue was placed in cryotubes and flash frozen in liquid nitrogen. The tubes were stored in a -80°C freezer until homogenization. The samples were homogenized in 450 µL of cold working buffer containing 50 mM Tris-HCl (pH 7.5), 75 mM NaCl, and 1 mM PMSF. The tissue was processed with a homogenizer for 10 minutes and centrifuged at 4°C for 20 minutes at 13000 rpm. The supernatants were separated, frozen and kept at -80°C until use. The total protein concentration was assessed by the Bradford method. On the day of the zymography, the volume equivalent to 50 µg of total protein was loaded into fresh made gelatin zymography gels. The gels were then electrophoretically separated under non-reducing conditions and 100 V. After electrophoresis the zymogram gels were washed in 125 ml 2.5% Triton twice for 20 minutes. The gels were then incubated in activation buffer (Zymogram Development Buffer, Bio-Rad, Hercules, CA) for 20 hours at 37°C. The next day, the gels were stained with Coomassie Blue R-250 Staining Solution (Bio-Rad) for 3 hours and destained for 25 minutes with Destain Solution (Bio-Rad). The gelatinolytic activity of the samples was assessed by densitometric analysis (Gel-Pro v 3.1, Media Cybernetics, Carlsbad, CA) of the bands as a relative comparison to a standard band of recombinant enzyme. To minimize inter-gel variability, all gels had a control lane loaded with 0.5 ng recombinant enzyme, which was used as a standard optical density and enzyme amount (in ng). The lytic bands identified in the zymogram gels were subjected to molecular weight identification with the use of pre-stained standard protein marker (Bio-Rad). Membranes were blocked with blotting grade blocker non-fat dry milk (Bio-Rad). After washing with 0.1% tween 20-tris-buffered saline (TTBS), the membranes were incubated with anti-matrix metalloproteinase (MMP)-9 monoclonal mouse antibody overnight at 4°C. Membranes were washed again in TTBS, incubated with secondary antibody (goat anti-mouse IgG, horseradish peroxidase conjugated antibody, Calbiochem) for one hour and finally developed with horseradish peroxidase development solution (ECL advance detection kit, Amersham). The membranes were exposed to autoradiography films (Hyblot CL, Denville Scientific Inc.). The density of the sample bands for the zymograms was expressed as maximal optical density relative to the standard band.

Cell migration assay
Using a transwell assay, primary rat neuronal cells, PRNCs, (embryos at Day 18; BrainBits) (1×10 5 cells/well) seeded onto the upper chamber of a Boyden chamber (Costar Transwell assay, Corning, NY, USA) supplemented with NbActive4 (BrainBits) in the absence of antibiotics. The chamber was placed in a 24-well plate containing confluent SB623 cells (1×10 5 cells/well) and starved with serum-free DMEM/F-12 medium in the presence or absence of Cyclosporine-A (a known MMP-9 inhibitor; 10 4 ng/mL in dimethyl sulfoxide; Sigma-Aldrich Inc., St Louis, MO, USA) for 24 h in the cell incubator. Next, the upper chamber was removed and wiped clean, then the lower side of the filter was washed and fixed in 4% paraformaldehyde. For quantification, migratory cells that reached the lower chamber and attached to the lower side of the filter were counted from five randomly captured microscopic fields (X400) and averaged for each treatment condition. This migratory assay was performed in triplicates.

Results
Adult male SD rats were initially evaluated in motor and neurological tests (all performed by two investigators blinded to the treatment condition throughout the study) to confirm that all animals included here were displaying normal behaviors at baseline (i.e., prior to brain insult). Animals were exposed to experimental TBI, then seven days later subjected to the same behavioral tests to confirm the typical TBI-induced motor and neurological impairments, and thereafter (also at 7 days post-TBI) assigned in a random fashion to receive either stereotaxic transplants of either SB623 cells [26,32] or vehicle infusion into the cortex (see Methods). At one month and three months post-TBI, transplanted animals displayed significantly improved motor and neurological functions coupled with significantly reduced damage to the cortical core and peri-injured cortical areas compared to traumatically injured animals that received vehicle only (Figure 1). These behavioral and histological improvements were achieved with modest graft survival of 0.60% and 0.16% at one month and three months post-TBI, respectively. Based on the robust functional recovery despite lack of graft persistence, we next examined the status of the host tissue. At one month post-TBI, immunofluorescent and confocal microscopy revealed a surge of endogenous cell proliferation (Ki67) and immature neural differentiation (nestin) in the peri-injured cortical areas and SVZ, with a stream of migrating cells (DCX) along the corpus callosum (CC) of the transplanted animals, while those that received vehicle alone displayed limited cell proliferation, neural differentiation, and scattered migration in the peri-injured cortical areas and almost absent expression of newly formed cells in the SVZ (Figure 2). At three months post-TBI, the brains from transplanted animals exhibited a much more massive cell proliferation and neural differentiation encasing the peri-injured cortical areas (CTX) accompanied by a solid stream of neuronally labeled cells (nestin as well as DCX) migrating not just along but across the CC from the SVZ to the impacted CTX (Figure 3). In contrast, the brains from vehicle-infused animals while producing a much more elevated cell proliferation, showed that newly formed cells were "trapped" within the SVZ and the CC and only sporadic cells were able to reach the impacted CTX ( Figure 3). Quantitative analyses of Ki67, nestin and DCX immunolabeled cells in SVZ, DG, CC and CTX revealed statistically significant differences between transplanted and vehicle-infused animals ( Figure S1). We next focused on the biobridge formed by the migrating cells from the SVZ to the impacted cortex and laser captured this corresponding tissue in a separate cohort of animals consisting of TBI plus SB623 cells, TBI plus vehicle, and control-sham operated agematched adult SD rats (n=3 per group) using the same experimental paradigm as above. Zymographic assays revealed two-fold and nine-fold upregulation of the MMP-9 expression/activity in TBI transplanted animals at one month and three months post-transplantation compared to vehicleinfused TBI animals or control-sham operated animals ( Figure  4A). Parallel in vitro studies provided further evidence that SB623 cells promoted cell migration via an ECM-mediated mechanism ( Figure 4B).

Discussion
The present results revealed that SB623 transplants remodeled the traumatically injured brain by harnessing a biobridgebetween SVZ and the peri-injured cortex ( Figure 5). This new mechanism of stem cell therapy opens the possibility of creating similar biobridges between neurogenic and nonneurogenic sites to facilitate injury-specific migration of cells across tissues that otherwise are non-conducive barriers against cell motility.
A Phase I/IIa transplantation study of SB623 cells in chronic stroke patients is underway. The clinical product entails allogeneic SB623 cells. In cell culture and animal models of brain disorders, SB623 cells have been shown to attenuate behavioral and histological deficits associated with stroke, spinal cord injury, and Parkinson's disease [33][34][35]. The present study is designed to extend the utility of SB623 in TBI. The US FDA recently approved a limited clinical trial of transplanting SB623 cells in TBI, in part based on the data being reported here. Thus, a major human implication impetus in this study was to provide the preclinical basis for initiating a clinical trial of SB623 in TBI.
The novel finding of SB623-facilitated migration of endogenous cells via a biobridge implicates the active role of MMPs and ECMs in stroke pathology [36,37] and their increasingly recognized role as therapeutic targets for stroke [38,39]. A variety of stem cells, including those derived from umbilical cord blood, peripheral blood, and adult brain, have been demonstrated to alter levels and functions of MMPs and ECMs [40][41][42], which would suggest their potential to similarly serve as biobridges as seen with the present Notch-induced SB623 MSCs.
Although neurogenic niches in the adult brain, such as the SVZ, have now been documented to exist and demonstrated to be critical in the repair of the stroke brain [43][44][45][46][47], a key limiting factor for endogenous repair is the successful migration of these newly formed host cells to reach the ischemic brain area. Our present results suggest that SB623 cell transplantation boosted endogenous repair mechanisms by guiding the migration of new cells from the neurogenic SVZ, across a nonneurogenic brain area, and eventually reaching the site of injury. The fundamental mechanism of action of SB623 cells involves their capacity to form biobridges consisting of MMPs and ECMs which serve as a gateway to ferry the newly formed cells from the neurogenic niche into the ischemic tissue. Although the grafted SB623 pioneered the formation of these biobridges, they subsequently relinquished these biobridges to the endogenous stem cells, altogether facilitating the host brain remodeling process. Our findings directly advance the concept of a biobridge mechanism as a robust stem cell-mediated brain repair strategy in TBI, and provide pivotal guidance on the translational applications of cell therapy in TBI patients. Future studies require closely monitoring the long-term efficacy and safety of SB623 cell therapy in chronic TBI animals in order to further optimize the conduct of the clinical trial of these cells in TBI patients.
A basic knowledge gap in functional restoration after stem cell transplantation is the elusive demonstration of integration of grafted cells into the recipient brain tissue and their subsequent interaction with host cells. The cellular interaction between the transplanted cell and host cell becomes extremely essential when graft survival is mediocre, indicating that for robust and stable therapeutic benefits an endogenous repair mechanism must be set in motion by the graft, in particular finding a way for the host cells to reach their destination even across non-neurogenic and injured tissues. MMPs have been implicated in recovery in chronic brain injury [32], with MMP inhibition abrogating neurogenic migration from SVZ into damaged tissues and retarding neurovascular remodeling [48]. Stem cells may serve as biobridges expressing MMP profiles that recapitulate the neurovascular unit abetting the transplantmediated host cell migration towards injured brain areas in affording functional recovery in TBI.

Figure 1. Behavioral tests (performed by two investigators blinded to the treatment condition throughout the study)
were initially conducted at baseline (i.e., prior to brain insult) and revealed that all adult SD rats included in this study displayed normal behaviors (A, B, and C). At 7 days after TBI, the same behavioral tests showed that TBI produced significant impairments in motor and neurological tasks. At one month, two months, and three months post-TBI, transplanted animals displayed significantly improved motor and neurological functions compared to traumatically injured animals that received vehicle only. These behavioral improvements were accompanied by reduction in TBI core and peri-injury cell death (D and E) as revealed by H&E staining (a and b correspond to vehicle and transplant respectively at one month post-treatment, while c and d represent vehicle and transplant respectively at three months post-treatment. a-d are at 10X while a'-d' at 20X magnification). Asterisks (*) indicate significant improvements in behavioral and histological deficits in TBI transplanted cells compared to TBI animals that received vehicle only (p's < 0.05).   . Laser-captured biobridge, corresponding to the brain tissue between SVZ and impacted cortex, expressed high levels of MMP-9 gelatinolytic activities at one month and three months post-TBI in animals transplanted with SB623 which were significantly higher than those TBI animals that received vehicle only or sham-operated animals (*p's< 0.05 vs. vehicle or sham; Panel A). Although vehicle-infused TBI animals also showed a significantly upregulated MMP-9 gelatinolytic activity at one month post-TBI (**p< 0.05 vs. sham), the level of this neurovascular proteinase activity reverted back to control-sham levels at three months post-TBI. Each bar represents the mean ± standard deviation from n=3 per treatment group for each time point. Next, to further reveal that SB623 cells promoted cell migration via an ECM-mediated mechanism, primary rat cortical cells were either grown alone or co-cultured with SB623 in the presence or absence of the MMP-9 inhibitor Cyclosporine-A (Panel B). Migratory cell assay (see inset) revealed significantly enhanced migration of primary rat cortical cells into the chamber that contained SB623, which was significantly suppressed by treatment with the inhibitor (*p< 0.05 vs. all other treatment conditions). The absence of SB623 and inhibitor in the cell culture condition, the treatment of the inhibitor alone, and the combined treatment of SB623 and inhibitor did not significantly differ in the resulting cell migratory potential.
doi: 10.1371/journal.pone.0074857.g004 Figure 5. After TBI, endogenous repair mechanisms commenced, but are limited to the neurogenic SVZ and to a few quiescent resident neurogenic cells around the impacted cortex (A). This endogenous repair process is not sufficient to mount a robust and stable defense against the TBI-induced cell death cascade unless exogenous stem cells are introduced. A physical gap between the neurogenic SVZ and the non-neurogenic, impacted cortex prevents migration of neurogenic cells to the injured cortex. Transplantation of stem cells into the peri-injured cortical areas creates a neurovascular matrix of biobridge to bootleg newly formed endogenous cells from the SVZ to the peri-injured cortex (B). Once the biobridge is established, the endogenous repair mechanism is maintained by newly formed host cells even in the absence of stem cells (C). Such transplant-paved biobridge between neurogenic and non-neurogenic sites allows endogenous neurogenic cells to reach injury-specific brain sites. Figure S1. Quantifications of Ki67, nestin and DCX labeled cells are shown in panels A, B, and C, respectively. Asterisks (*) indicate significant increase in the number of phenotypically labeled cells counted per high-power field view (28,800 µm 2 ) selected at random in the region of interest in TBI animals transplanted with SB623 cells compared to TBI animals that received vehicle only (p's < 0.05).