Transplanted Schwann cells, not olfactory ensheathing cells, myelinate optic nerve fibres

In a previous study we found that olfactory ensheathing cells transplanted into complete retrobulbar transections of the rat optic nerve mediated regeneration of severed retinal ganglion cell axons through the graft region. Although the regenerating axons were ensheathed by the transplanted cells, none of the regenerating axons became myelinated by either central or peripheral type myelin. In the present study we used the same operative procedure but transplanted Schwann cells instead of olfactory ensheathing cells. As with the olfactory ensheathing cell transplants the Schwann cells transplants also induced regeneration of the severed retinal ganglion cell axons into the graft region. In contrast to the situation with the olfactory ensheathing cell transplants, however, a considerable number of the regenerating axons became myelinated by peripheral type myelin produced by the transplanted Schwann cells. This observation identifies a further distinction between these two cell types which are phenotypically similar in many ways, but which have been shown to have major functional differences with regard to regeneration in spinal cord lesions.     

GDNF‐treated acellular nerve graft promotes motoneuron axon regeneration after implantation into cervical root avulsed spinal cord

T‐H. Chu, L. Wang, A. Guo, V. W‐K. Chan, C. W‐M. Wong and W. Wu (2012) Neuropathology and Applied Neurobiology 38, 681–695 GDNF‐treated acellular nerve graft promotes motoneuron axon regeneration after implantation into cervical root avulsed spinal cord It is well known that glial cell line‐derived neurotrophic factor (GDNF) is a potent neurotrophic factor for motoneurons. We have previously shown that it greatly enhanced motoneuron survival and axon regeneration after implantation of peripheral nerve graft following spinal root avulsion. Aims: In the current study, we explore whether injection of GDNF promotes axon regeneration in decellularized nerve induced by repeated freeze‐thaw cycles. Methods: We injected saline or GDNF into the decellularized nerve after root avulsion in adult Sprague–Dawley rats and assessed motoneuron axon regeneration and Schwann cell migration by retrograde labelling and immunohistochemistry. Results: We found that no axons were present in saline‐treated acellular nerve whereas Schwann cells migrated into GDNF‐treated acellular nerve grafts. We also found that Schwann cells migrated into the nerve grafts as early as 4 days after implantation, coinciding with the first appearance of regenerating axons in the grafts. Application of GDNF outside the graft did not induce Schwann cell infiltration nor axon regeneration into the graft. Application of pleiotrophin, a trophic factor which promotes axon regeneration but not Schwann cell migration, did not promote axon infiltration into acellular nerve graft. Conclusions: We conclude that GDNF induced Schwann cell migration and axon regeneration into the acellular nerve graft. Our findings can be of potential clinical value to develop acellular nerve grafting for use in spinal root avulsion injuries.     

Nanoparticles carrying neurotrophin-3-modified Schwann cells promote repair of sciatic nerve defects

Schwann cells and neurotrophin-3 play an important role in neural regeneration,but the secretion of neurotrophin-3 from Schwann cells is limited,and exogenous neurotrophin-3 is inactived easily in vivo.In this study,we have transfected neurotrophin-3 into Schwann cells cultured in vitro using nanoparticle liposomes.Results showed that neurotrophin-3 was successfully transfected into Schwann cells,where it was expressed effectively and steadily.A composite of Schwann cells transfected with neurotrophin-3 and poly(lactic-co-glycolic acid) biodegradable conduits was transplanted into rats to repair 10-mm sciatic nerve defects.Transplantation of the composite scaffold could restore the myoelectricity and wave amplitude of the sciatic nerve by electrophysiological examination,promote nerve axonal and myelin regeneration,and delay apoptosis of spinal motor neurons.Experimental findings indicate that neurotrophin-3 transfected Schwann cells combined with bridge grafting can promote neural regeneration and functional recovery after nerve injury.     

The expression of nerve growth factor receptor on Schwann cells and the effect of these cells on the regeneration of axons in traumatically injured human spinal cord

To investigate the effects of Schwann cells and nerve growth factor receptor (NGFR) on the regeneration of axons, autopsy specimens of spinal cord from 21 patients with a survival time of 2 h to 54 years after spinal cord trauma were studied using immunohistochemistry and electron microscopy. Regenerating sprouts of axons could be observed as early as 4 days after trauma. At 4.5 months after trauma, many regenerating nests of axons appeared in the injured spinal cord. The regeneration nests contained directionally arranged axons and Schwann cells. Some axons were myelinated. In injured levels of the spinal cord, the Schwann cells exhibited an increased expression of NGFR within spinal roots. These results show that an active regeneration process occurs in traumatically injured human spinal cord. The NGFR expressed on Schwann cells could mediate NGF to support and induce the axon regeneration in the central nervous system. Received: 20 June 1995 / Revised, accepted: 18 September 1995     

骨髓基质细胞移植治疗脊髓全横断损伤超微结构观察

目的观察骨髓基质细胞（MSCs）移植治疗脊髓全横断损伤（SCI）超微结构,探讨内源性细胞与再生轴突关系。方法通过全骨髓法培养、纯化MSCs,SCI9d后移植MSCs,通过免疫荧光组化观察细胞移植后损伤区轴突再生情况,免疫荧光双标、免疫电镜观察再生轴突与内源性细胞关系。结果移植8W后实验组脊髓损伤区可见大量神经微丝蛋白200（NF200）阳性纤维,对照组脊髓损伤区未见明显的NF200阳性纤维。免疫荧光双标结果显示损伤区NF200阳性纤维和2,3＇-环核苷酸磷酸而酯酶（CNP）阳性细胞之间存在密切的空间关系,免疫电镜显示CNP阳性细胞通过伸长丝状伪足形成再生轴突支架,内源性施万细胞参与再生轴突髓鞘形成。结论MSCs移植可促进损伤区轴突再生,宿主自身CNP阳性细胞和施万细胞参与损伤轴突的再生和髓鞘形成。     

Adaptive plasticity of Xenopus glial cells in vitro and after CNS fiber tract lesions in vivo

Xenopus oligodendrocytes and aspects of their differentiation were analyzed in vitro and in vivo using cell- and stage-specific antibodies. Undifferentiated oligodendrocytes were derived from optic nerves or spinal cords. They divided in vitro, were of elongated shape, were glial fibrillary acidic protein and O4 positive, transiently exhibited several antigens including HNK-1 and L1, and promoted axon growth as do Schwann cells. With forskolin they differentiated and, much like myelin-forming oligodendrocytes in the intact optic nerve and spinal cord, they expressed sets of advanced myelin markers. These advanced myelin markers disappeared from the regenerating optic nerve 4 weeks after lesion. The optic nerve instead was populated by cells with radial processes and somata in the center of the nerve; among them were cells and processes that were O4 positive and that are suspected to represent undifferentiated oligodendrocytes. Where processes of these cells reached to the retinal axons in the nerve's periphery, advanced myelin markers typical of differentiated oligodendrocytes reappeared 8 weeks after lesion. These glial changes did not occur in the absence of retinal axons. Thus, the apparent capability of Xenopus oligodendrocytes to adapt to the transient absence, reappearance, and regenerative state of the axons enables them to contribute to central nervous system fiber tract repair. This occurs in the lesioned optic nerve but not in the spinal cord, where no such glial changes were observed and where axons fail to regenerate. © 1996 Wiley-Liss, Inc.     

A combination of insulin-like growth factor-I and platelet-derived growth factor enhances myelination but diminishes axonal regeneration into Schwann cell grafts in the adult rat spinal cord

Insulin-like growth factor-I (IGF-I) promotes axonal regeneration in the peripheral nervous system and this effect is enhanced by platelet-derived growth factor (PDGF). We decided, therefore, to study the effects of these factors on axonal regeneration in the adult rat spinal cord. Semipermeable polymer tubes, closed at the distal end, containing Matrigel mixed with cultured rat Schwann cells and IGF-I/PDGF, were placed at the proximal stump of the spinal cord after removal of the thoracic T9-11 segments. Control animals received implants of only Matrigel and Schwann cells or only Matrigel and IGF-I/PDGF. Four weeks after implantation, electron microscopic analysis showed that the addition of IGF-I/PDGF resulted in an increase in the myelinated:unmyelinated fiber ratio from 1:7 to 1:3 at 3 mm in the Schwann cell graft, and that myelin sheath thickness was increased 2-fold. The reduced number of unmyelinated axons was striking in electron micrographs. These results suggested that IGF-I/PDGF enhanced myelin formation of regenerated axons in Schwann cell implants, but there was a 36% decrease in the total number of myelinated axons at the 3 mm level of the graft. This finding and the altered myelinated:unmyelinated fiber ratio revealed that the overall fiber regeneration into Schwann cell implants was diminished up to 63% by IGF-I/PDGF. Histological evaluation revealed that there were more larger cavities in tissue at the proximal spinal cord-graft interface in animals receiving a Schwann cell implant with IGF-I/PDGF. Such cavitation might have contributed to the reduction in axonal ingrowth. In sum, the results indicate that whereas the combination of IGF-I and PDGF enhances myelination of regenerating spinal cord axons entering implants of Matrigel and Schwann cells after midthoracic transection, the overall regeneration of axons into such Schwann cell grafts is diminished. GLIA 19:247–258, 1997. © 1997 Wiley-Liss, Inc.     

Contactin1a expression is associated with oligodendrocyte differentiation and axonal regeneration in the central nervous system of zebrafish

Contactin1a (Cntn1a) is a zebrafish homolog of contactin1 (F3/F11/contactin) in mammals, an immunoglobulin superfamily recognition molecule of neurons and oligodendrocytes. We describe conspicuous Cntn1a mRNA expression in oligodendrocytes in the developing optic pathway of zebrafish. In adults, this expression is only retained in glial cells in the intraretinal optic fiber layer, which contains 'loose' myelin. After optic nerve lesion, oligodendrocytes re-express Cntn1a mRNA independently of the presence of regenerating axons and retinal ganglion cells upregulate Cntn1a expression to levels that are significantly higher than those during development. After spinal cord lesion, expression of Cntn1a mRNA is similarly increased in axotomized brainstem neurons and white matter glial cells in the spinal cord. In addition, reduced mRNA expression in the trigeminal/anterior lateral line ganglion in erbb3-deficient mutant larvae implies Cntn1a in Schwann cell differentiation. These complex regulation patterns suggest roles for Cntn1a in myelinating cells and neurons particularly in successful CNS regeneration.     

Studies on the control of myelinogenesis. 3. Signalling of oligodendrocyte myelination by regenerating peripheral axons

Continuing from earlier work which demonstrated the peripheral axonal regulation of Schwann cell myelination, this study has investigated the possibility that a peripheral axon can stimulate oligodendrocyte myelination. To test this hypothesis, regenerating PNS axons were allowed to interact with uncommitted oligodendrocytes by transecting a rat peroneal nerve and inserting a segment of the autologous optic nerve between the cut ends. Grafts were maintained for 4–28 weeks and then examined by light and electron microscopy. A few regenerating peripheral myelinated nerve fibers penetrated the optic nerve graft. Some axons penetrated the outer margin of the graft, were myelinated by Schwann cells, and surrounded by astrocyte processes bordered by basal lamina. More centrally in the optic nerve graft, regenerating peripheral axons displayed myelin of CNS type. The outer myelin lamella abutted directly on the plasmalemma surface of surrounding astrocytic processes and was expanded focally to form a glial tongue. These observations demonstrate the experimental induction of central myelination by regenerating peripheral axons and suggest the existence of a common neuronal mechanism to stimulate myelin formation by both the Schwann cell and the oligodendrocyte.     

Aberrant remyelination of axons after heat injury in the dorsal funiculus of rat spinal cord

Summary We studied the course of demyelination and subsequent remyelination of nerve fibers after heat injury in the dorsal funiculus of the rat spinal cord. Four weeks after heat treatment, we observed, in addition to normally remyelinated axons, a few aberrantly remyelinated axons which had both CNS-and PNS-type myelin sheaths: the CNS-type myelin sheaths were always situated inside the PNS-type sheaths. This finding indicates that in some conditions Schwann cells can form myelin sheaths around those formed by oligodendrocytes.     

The use of cultured autologous Schwann cells to remyelinate areas of persistent demyelination in the central nervous system

Areas of persistent demyelination were created in the dorsal columns of the cat spinal cord by injecting ethidium bromide into white matter which had previously been exposed to 40 Grays of X-irradiation. In the centre of such lesions demyelinated axons occurred in a glial-free area while axons next to normal tissue were separated by astrocyte processes. No remyelination occurs in such lesions (Blakemore 1984). Autologous Schwann cells and fibroblasts cultured from a peripheral nerve biopsy were injected into such lesions and the extent of Schwann cell remyelination examined. Only lesions injected with viable cells showed remyelination by Schwann cells; in no lesion were all the demyelinated axons remyelinated. Three forms of association of Schwann cell with axons were detected. In the centre of the lesions Schwann cells either remyelinated axons around or near to blood vessels, or lay next to demyelinated axons and did not form myelin. Schwann cell remyelination was also detected in the astrocyte-containing areas around the edges of some lesions. It was concluded that the extent of Schwann cell remyelination was influenced by the mode of entry of the cells into the lesion and by the architecture of the lesion. The presence or absence of stable extracellular matrix is believed to be the prime factor which influenced Schwann cell remyelination. The relevance of these observations to artificial repair of the lesions of multiple sclerosis is discussed.     

Mats made from fibronectin support oriented growth of axons in the damaged spinal cord of the adult rat

A variety of biological as well as synthetic implants have been used to attempt to promote regeneration into the damaged spinal cord. We have implanted mats made from fibronectin (FN) into the damaged spinal cord to determine their effectiveness as a substrate for regeneration of axons. These mats contain oriented pores and can take up and release growth factors. Lesion cavities 1 mm in width and depth and 2 mm in length were created on one side of the spinal cord of adult rats. FN mats containing neurotrophins or saline were placed into the lesion. Mats were well integrated into surrounding tissue and showed robust well-oriented growth of calcitonin gene-related peptide, substance P, GABAergic, cholinergic, glutamatergic, and noradrenergic axons into FN mats. Transganglionic tracing using cholera toxin B indicated large-diameter primary afferents had grown into FN implants. Schwann cells had also infiltrated FN mats. Electron microscopy confirmed the presence of axons within implants sites, with most axons either ensheathed or myelinated by Schwann cells. Mats incubated in brain-derived neurotrophic factor and neurotrophin-3 showed significantly more neurofilament-positive and glutamatergic fibers compared to saline- and nerve growth factor-incubated mats, while mats incubated with nerve growth factor showed more calcitonin gene-related peptide-positive axons. In contrast, neurotrophin treatment had no effect on PGP 9.5-positive axons. In addition, in some animals with neurotrophin-3-incubated mats, cholera toxin B-labelled fibers had grown from the mat into adjoining intact areas of spinal cord. The results indicate that FN mats provide a substrate that is permissive for robust oriented axonal growth in the damaged spinal cord, and that this growth is supported by Schwann cells.     

Determinants of directional specificity in the regeneration of lamprey spinal axons

The projection patterns of regenerating spinal axons in the larval sea lamprey (Petromyzon marinus) were determined by intracellular injection of HRP. Four hundred and eighty-six of 562 stained axons and axon-like neurites (87%) arising from Muller and Mauthner axons, giant interneurons, and dorsal cells terminated in an orientation similar to that of their counterpart control cells. Therefore, lamprey spinal axons regenerate selectively along their normal projection paths. During the first 4 weeks of recovery, i.e., before any had regenerated beyond the transection site, 91 of 114 axons and long neurites (80%) projected in the proper direction. Thus, the correctness of the final projection patterns did not result from selective retraction of randomly directed long neurites. When the cords were doubly transected 1 cm apart, orientation of regenerating neurites remained normal both within the 1 cm island and in the adjacent spinal cord. This suggests that the directional specificity of axonal regeneration was determined neither by the location of the scar nor by the availability of channels formed by the degenerating fibers. Finally, removing 1 cm of spinal cord eliminated potential synaptic targets for regenerating axons on either side of the lesion, but did not affect the direction of axonal growth. These findings are consistent with the hypothesis that the regeneration of lamprey spinal axons is guided by local chemical cues that persist long after the pathways are formed early in development.     

Remyelination by cells introduced into a stable demyelinating lesion in the central nervous system

Schwann cells from an autogeneic peripheral nerve source were injected into an established demyelinating lesion produced by the direct micro-injection of diphtheria toxin into the cat spinal cord. In control diphtheria toxin lesions, which were not injected with Schwann cells, demyelination and some oligodendrocyte remyelination was seen but Schwann cell remyelination was not observed. In diphtheria toxin lesions which were wholly confined to the posterior columns, Schwann cell myelin was not seen before 3 weeks after cell injection. The Schwann cell myelinated fibres occurred singly or in small groups within the posterior columns and were considered to have been myelinated by injected Schwann cells. By one month Schwann cell myelinated fibres had thick myelin sheaths but many demyelinated axons remained. By contrast, in more extensive diphtheria toxin lesions there was widespread Schwann cell remyelination of central axons at all stages examined after cell injection. The Schwann cell myelinated fibres were grouped in large numbers around the damaged dorsal root entry zones, the likely source of Schwann cells in these lesions. It is concluded that CNS remyelination may be improved by the injection of peripheral Schwann cells although the extent of remyelination is limited. One facet limiting remyelination may be the concentration of Schwann cells that it is possible to inject with present techniques. Functional recovery remains to be investigated.     

Basic Behavior of Migratory Schwann Cells in Peripheral Nerve Regeneration

In axonal regeneration after a peripheral nerve injury, Schwann cells migrate from the two nerve ends and at last form a continuous tissue cable across the gap which guides the axons toward the bands of Büngner. However, the behavior of migratory Schwann cells and their possible role are obscure. Using a film model in which the proximal stump of a transected nerve in mice was sandwiched between two thin plastic films, we analyzed neural regeneration in the early phase up to the 6th day after axotomy. Regenerating neurites emerged from the nodes of Ranvier adjacent to the axotomized nerve stump within 3 h after axotomy and extended along the parent nerve onto the film. All of the regenerating neurites on the surface of the film consisted of naked axons for at least 2 days after axotomy. Thereafter, Schwann cells from the proximal nerve migrated along a network of the regenerating axons and then closely attached to the axons, ensheathing them. Some of the Schwann cells advanced ahead of the axonal growth cones and were distributed over regions in which axonal extension was not yet present. As calculated from the time course of regenerating neurites, the velocity of axonal regeneration showed two phases: an initial slow phase (77 μm/day) up to the 2nd post-operative day followed by a faster phase (283 μm/day). The first observation of Schwann cells coincided with the onset of the second phase. In addition, the length of regenerating axons on the surface of the film containing many Schwann cells was significantly greater than that on the surface where Schwann cells were not yet present. It meant that migratory Schwann cells stimulated axons to elongate for a longer distance. Furthermore, Schwann cells from a distal stump showed a stronger ability to accelerate the axonal outgrowth than these from a proximal stump.     

E6020, a synthetic TLR4 agonist,accelerates myelin debris clearance,Schwann cell infiltration,and remyelination in the rat spinal cord

Oligodendrocyte progenitor cells (OPCs) are present throughout the adult brain and spinal cord and can replace oligodendrocytes lost to injury, aging, or disease. Their differentiation, however, is inhibited by myelin debris, making clearance of this debris an important step for cellular repair following demyelination. In models of peripheral nerve injury, TLR4 activation by lipopolysaccharide (LPS) promotes macrophage phagocytosis of debris. Here we tested whether the novel synthetic TLR4 agonist E6020, a Lipid A mimetic, promotes myelin debris clearance and remyelination in spinal cord white matter following lysolecithin‐induced demyelination. In vitro, E6020 induced TLR4‐dependent cytokine expression (TNFα, IL1β, IL‐6) and NF‐κB signaling, albeit at ～10‐fold reduced potency compared to LPS. Microinjection of E6020 into the intact rat spinal cord gray/white matter border induced macrophage activation, OPC proliferation, and robust oligodendrogenesis, similar to what we described previously using an intraspinal LPS microinjection model. Finally, a single co‐injection of E6020 with lysolecithin into spinal cord white matter increased axon sparing, accelerated myelin debris clearance, enhanced Schwann cell infiltration into demyelinated lesions, and increased the number of remyelinated axons. In vitro assays confirmed that direct stimulation of macrophages by E6020 stimulates myelin phagocytosis. These data implicate TLR4 signaling in promoting repair after CNS demyelination, likely by stimulating phagocytic activity of macrophages, sparing axons, recruiting myelinating cells, and promoting remyelination. This work furthers our understanding of immune–myelin interactions and identifies a novel synthetic TLR4 agonist as a potential therapeutic avenue for white matter demyelinating conditions such as spinal cord injury and multiple sclerosis.     

Neuromyelitis optica with intraspinal expansion of Schwann cell remyelination

We report a case of neuromyelitis optica (NMO) with an unusual pattern of remyelination in the spinal cord. A Japanese woman complained of pain and numbness in the left thumb at the age of 36 years. She mainly presented with optic and spinal symptoms and was initially diagnosed as multiple sclerosis (MS). Her bilateral eyesight decreased, which led to light perception only in the right eye. She became unable to walk without a wheelchair. In spite of steroid pulse therapy, plasma exchange therapy and immunosuppressive therapy, her symptoms gradually worsened. After 33 years of a relapsing–remitting course, she died of septic urinary tract infection at the age of 69 years. Autopsy revealed prominent demyelination in the optic tract and the spinal cord. The optic nerve showed extensive demyelination accompanied by axon depletion. The spinal cord lesions were found in C8 to L2 level (contiguous 15 segments), especially Th5 to Th11 level. The thoracic spinal cord showed extensive remyelination spreading from the entry zone of peripheral nerves to the central portion. Regenerative myelin showed immunopositivity for Schwann/2E, a marker of Schwann cells and myelin of the peripheral nervous system. Expressions of glial fibrillary acidic protein and aquaporin 4 (AQP4) were weakened in the area of Schwann cell remyelination, suggesting that the essential pathogenesis of this case was disturbance of astrocytes. Inhibition of gliosis probably led to cystic cavities, and destruction of basal lamina may have permitted Schwann cells of peripheral nerves to enter the spinal cord and proliferate within empty spaces. Compared with the optic tract and the spinal cord lesions, a large part of the brain plaques was vague and inactive. We pathologically diagnosed this case as NMO for optic neuritis, myelitis, a contiguous spinal cord lesion and loss or decrease of AQP4 expression.     

PERIPHERAL NERVE REGENERATION THROUGH GRAFTS OF LIVING AND FREEZE-DRIED CNS TISSUE

The ability of peripheral nerve fibres to regenerate through the central nervous system (CNS) extracellular matrix in the presence of CNS myelin debris was examined using living and freeze-dried optic nerve grafts. The grafts were placed end-to-end with the proximal stumps of severed common peroneal nerves of inbred mice. Within a 4 week period, regenerating peripheral nervous system fibres were found in only two of 14 living grafts. However axons always grew into freeze-dried grafts within one week, despite the presence of CNS myelin debris. The regenerating axons in freeze-dried grafts were accompanied by Schwann cells and were initially found associated with the inner aspect of the glial basal lamina. Although the extracellular matrix of the freeze-dried CNS tissue was subsequently reorganized by invading cells, it seems likely that neither the nature of the CNS extracellular matrix nor the presence of CNS myelin debris had a major inhibitory influence on peripheral nerve regeneration. It is suggested that the presence of living astrocytes covered by a basal lamina at the proximal end of the living optic nerve grafts may inhibit their penetration by regenerating axons.     

The growth of motoneurones and their neurites in relation to Schwann cells harvested from sciatic nerve

A study has been made of the effects of Schwann cells isolated from neonatal sciatic nerve on motoneurones in culture. Motoneurones were identified in dissociated spinal cord cultures from 6-day avian embryos by prior retrograde labelling with rhodamine-latex microspheres. Schwann cells trebled the expression of long neurites in these motoneurones over that in control media as well as maintaining them viable for 24 h. A transformed Schwann cell line, RN22, produced similar results. These effects were mediated by a soluble factor(s) released from the Schwann cells which was distinct from nerve growth factor. Schwann cells exerted this initiation of neurites on homogeneous cultures of motoneurones, indicating that other spinal cord cells are not necessary to mediate this effect. The observations are discussed in terms of the hypothesis that Schwann cells guide motor axons during regeneration and formation of limb and muscle nerves.     

Cell transplantation for the treatment of spinal cord injury–bone marrow stromal cells and choroid plexus epithelial cells

Transplantation of bone marrow stromal cells(BMSCs) enhanced the outgrowth of regenerating axons and promoted locomotor improvements of rats with spinal cord injury(SCI).BMSCs did not survive long-term,disappearing from the spinal cord within 2–3 weeks after transplantation.Astrocyte-devoid areas,in which no astrocytes or oligodendrocytes were found,formed at the epicenter of the lesion.It was remarkable that numerous regenerating axons extended through such astrocyte-devoid areas.Regenerating axons were associated with Schwann cells embedded in extracellular matrices.Transplantation of choroid plexus epithelial cells(CPECs) also enhanced axonal regeneration and locomotor improvements in rats with SCI.Although CPECs disappeared from the spinal cord shortly after transplantation,an extensive outgrowth of regenerating axons occurred through astrocyte-devoid areas,as in the case of BMSC transplantation.These findings suggest that BMSCs and CPECs secret neurotrophic factors that promote tissue repair of the spinal cord,including axonal regeneration and reduced cavity formation.This means that transplantation of BMSCs and CPECs promotes "intrinsic" ability of the spinal cord to regenerate.The treatment to stimulate the intrinsic regeneration ability of the spinal cord is the safest method of clinical application for SCI.It should be emphasized that the generally anticipated long-term survival,proliferation and differentiation of transplanted cells are not necessarily desirable from the clinical point of view of safety.