Regeneration of adult dorsal root axons into transplants of fetal spinal cord and brain: a comparison of growth and synapse formation in appropriate and inappropriate targets

Transplanting neural progenitor cells to restore connectivity after spinal cord injury

Spinal cord injury remains a scientific and therapeutic challenge with great cost to individuals and society. The goal of research in this field is to find a means of restoring lost function. Recently we have seen considerable progress in understanding the injury process and the capacity of CNS neurons to regenerate, as well as innovations in stem cell biology. This presents an opportunity to develop effective transplantation strategies to provide new neural cells to promote the formation of new neuronal networks and functional connectivity. Past and ongoing clinical studies have demonstrated the safety of cell therapy, and preclinical research has used models of spinal cord injury to better elucidate the underlying mechanisms through which donor cells interact with the host and thus increase long-term efficacy. While a variety of cell therapies have been explored, we focus here on the use of neural progenitor cells obtained or derived from different sources to promote connectivity in sensory, motor and autonomic systems.

Putative Inhibitory Extracellular Matrix Molecules Do Not Prevent Dorsal Root Regeneration into Fetal Spinal Cord Transplants

We examined the distribution of several extracellular matrix molecules (ECM) and their relationship to regenerating axons in embryonic day 14 spinal cord transplants 1 to 12 weeks after transplantation into adult rats. We used immunocytochemical tech niques to label chondroitin sulfate proteoglycans (CSPGs) and tenascin-C in adjacent sections. Synthesis of these molecules by astrocytes is thought to be one mechanism by which astrocytes inhibit regeneration in the central nervous system (CNS); glial fibrillary acidic protein antibody was used to label astrocytes and examine their rela tionship to both the ECM molecules and regenerating calcitonin gene-related pep tide (CORP)-contammg dorsal roots. We also compared the expression and distribu tion of these five markers in transplants with normal spinal cord development.

Neurotrophic effects on dorsal root regeneration into the spinal cord

Dorsal root ganglion neurons exhibit a robust and generally successful regenerative response following injury of their peripheral processes. Regeneration fails, however, after section of their central processes in the dorsal roots or dorsal columns. Experiments characterizing the attenuated response of these neurons to injury, and the inhibition of regeneration exerted by astrocytes and oligodendrocytes within the dorsal root entry zone and spinal cord, have contributed important insights into the failure of regeneration after injury to the central nervous system (CNS). Interventions that have enhanced the metabolic response of injured dorsal root ganglion neurons, and altered the inhospitable environment, have increased sensory afferent regeneration and recovery. There is reason to expect that these strategies will help to develop clinically applicable treatments of CNS injuries.

Strategies for repair of the deafferented spinal cord

Deafferentation of the spinal cord by interruption of the sensory fibers in the dorsal roots highlights the problem of regeneration failure in the central nervous system. The injured dorsal root axons regenerate steadily, albeit slowly, in the peripheral compartment of the dorsal root, but abruptly cease to elongate when confronted with the interface between the peripheral and central nervous system, the dorsal root transitional zone (DRTZ). The glial cells of the CNS and their products together form this regeneration barrier. Recent years have witnessed several successful approaches to, at least in part, overcome this barrier. Particularly promising results have been obtained by (1). the replacement of adult non-regenerating dorsal root ganglion neurons with corresponding cells from embryonic or fetal donors, (2). the implantation of olfactory ensheathing cells at the DRTZ, and (3). immediate intrathecal infusion of growth factors to which dorsal root ganglion cells respond. In all these instances, growth of sensory axons into the adult spinal cord, as well as return of spinal cord connectivity, have been demonstrated. These findings suggest routes towards treatment strategies for plexus avulsion, and contribute to our understanding of possibilities to overcome regeneration failure in the spinal cord.

Crossed and uncrossed projections to the cat sacrocaudal spinal cord: III. Axons expressing calcitonin gene-related peptide immunoreactivity

We have investigated the projection patterns of peptidergic small-diameter primary afferent fibers to the cat sacrocaudal spinal cord, a region associated with midline structures of the lower urogenital system and of the tail. Calcitonin gene-related peptide (CGRP)-immunoreactive (CGRP-IR) primary afferent fibers were observed within the superficial laminae, rostrally as the typical inverted U-shaped band that capped the separate dorsal horns (S1 to rostral S2) and caudally as a broad band that spanned the entire mediolateral extent of the fused dorsal horns (caudal S2 and caudal). Within the dorsal gray commissure, labeling was seen as a periodic vertical, midline band. CGRP-IR labeling was prevalent in an extensive mediolateral distribution at the base of the dorsal horn, originating from both lateral and medial collateral bundles that extend from the superficial dorsal horn. Some bundles, in part traveling within the dorsal commissure, conspicuously crossed the midline. In addition to the robust projection to the superficial dorsal horn, there was a more extensive distribution of CGRP-IR fibers within the deeper portions of the cat sacrocaudal dorsal horn than has been reported for other regions of the cat spinal cord. Presumably, these deep projections convey visceral information to projection or segmental neurons at the neck of the dorsal horn and in the region of the central canal. This deep distribution overlaps the reported projections of the pelvic and pudendal nerves. In addition, the contralateral projections of CGRP-IR fibers may form an anatomical substrate of the bilateral receptive fields for selective dorsal horn neurons. The density and variety of CGRP-IR projection patterns is a reflection of the functional attributes of the innervated structures.

Neural transplantation in spinal cord injury

Although medical advancements have significantly increased the survival of spinal cord injury patients, restoration of function has not yet been achieved. Neural transplantation has been studied over the past decade in animal models as a repair strategy for spinal cord injury. Although spinal cord neural transplantation has yet to reach the point of clinical application and much work remains to be done, reconstructive strategies offer the greatest hope for the treatment of spinal cord injury in the future. This article presents the scientific basis of neural transplantation as a repair strategy and reviews the current status of neural transplantation in spinal cord injury.

Functional regeneration of sensory axons into the adult spinal cord

The arrest of dorsal root axonal regeneration at the transitional zone between the peripheral and central nervous system has been repeatedly described since the early twentieth century. Here we show that, with trophic support to damaged sensory axons, this regenerative barrier is surmountable. In adult rats with injured dorsal roots, treatment with nerve growth factor (NGF), neurotrophin-3 (NT3) and glial-cell-line-derived neurotrophic factor (GDNF), but not brain-derived neurotrophic factor (BDNF), resulted in selective regrowth of damaged axons across the dorsal root entry zone and into the spinal cord. Dorsal horn neurons were found to be synaptically driven by peripheral nerve stimulation in rats treated with NGF, NT3 and GDNF, demonstrating functional reconnection. In behavioural studies, rats treated with NGF and GDNF recovered sensitivity to noxious heat and pressure. The observed effects of neurotrophic factors corresponded to their known actions on distinct subpopulations of sensory neurons. Neurotrophic factor treatment may thus serve as a viable treatment in promoting recovery from root avulsion injuries. I

Embryonic central nervous system transplants mediate adult dorsal root regeneration into host spinal cord

OBJECTIVE

The aim of this study was to determine whether embryonic central nervous system transplants assisted cut dorsal root axons of adult rats to regenerate into the spinal cord.

METHODS

Rats received transplants of embryonic spinal cord, hippocampus, or neocortex into dorsal quadrant cavities aspirated in the lumbar enlargement. The transected L5 dorsal root stump was secured between the transplant and the spinal cord. Regenerated dorsal roots were subsequently labeled by using immunohistochemical methods to detect calcitonin gene-related peptide.

RESULTS

Calcitonin gene-related peptide-immunoreactive axons extended into all host spinal cords examined, but the patterns of regrowth differed in rats that had received embryonic spinal cord and brain transplants. In rats with embryonic spinal cord transplants, regenerated axons traversed the dorsal root/spinal cord interface, entered the spinal cord, and frequently formed plexuses with arborizations in motoneuron pools; some of these axons established synapses on spinal cord neurons. In rats with embryonic brain transplants, regenerated axons were diffusely distributed in the spinal cord but did not form plexuses. Few axons regenerated into the spinal cords of lesion-only animals. The results of quantitative analyses confirmed these findings.

CONCLUSION

These findings suggest that transplants of embryonic spinal cord and brain supply cues that enable cut dorsal roots to regenerate into the host spinal cord and that the cues provided by spinal cord transplants favor more extensive growth than do those provided by brain transplants. These cues are likely to depend in part on neurotrophic effects of embryonic central nervous system tissues. Therefore, embryonic central nervous system transplants, especially spinal cord grafts, may contribute to techniques for restoring interrupted spinal reflex arcs.

Neurotrophic agents in fibrin glue mediate adult dorsal root regeneration into spinal cord

OBJECTIVE

The aim of the present study was to determine whether neurotrophic factors (NTFs) exogenously administered in fibrin glue assisted cut dorsal root axons of adult rats to regenerate into the spinal cord.

METHODS

Rats received intraspinal implants of fibrin glue containing neurotrophin-3, brain-derived NTF, ciliary NTF, or Dulbecco's modified Eagle's medium (control) into left dorsal quadrant cavities aspirated in the lumbar enlargement. The transected L5 dorsal root stump was placed at the bottom of the lesion cavity and was secured between the fibrin glue and the spinal cord. Regenerated dorsal root axons were subsequently labeled with immunohistochemical methods to demonstrate those that contained calcitonin gene-related peptide.

RESULTS

Calcitonin gene-related peptide-immunoreactive dorsal root axons regenerated across the dorsal root-spinal cord interface of rats with fibrin glue containing neurotrophin-3, brain-derived NTF, or ciliary NTF, entered the spinal cord, and frequently arborized within clusters of motoneuronal cell bodies. Only a few axons regenerated into the spinal cord of animals with fibrin glue implants that lacked NTF, and their growth within the spinal cord was extremely limited. The results of quantitative studies confirmed these observations.

CONCLUSION

Our results indicate that neurotrophin-3, brain-derived NTF, and ciliary NTF enhance dorsal root regeneration into spinal cord and that fibrin glue is an effective medium for intraspinal delivery of NTF. This method of delivering NTF may therefore provide a strategy for restoring injured spinal reflex arcs.

Ensheathing glia transplants promote dorsal root regeneration and spinal reflex restitution after multiple lumbar rhizotomy

Previously, we have shown that transplants of olfactory bulb ensheathing cells promoted regeneration of transected dorsal roots into the spinal cord. In this study, we assessed the ability of regenerating axons to make functional connections in the cord. Dorsal roots L3 to L6 were sectioned close to their entrance into the spinal cord and reapposed after injecting a suspension of ensheathing cells into each dorsal root entry zone (Group G). Afferent regeneration into the cord and recovery of spinal reflexes were compared with animals that received no injection (Group S) or culture medium without cells (Group C). Electrophysiological tests, to measure nerve conduction and spinal reflexes (H response and withdrawal reflex) evoked by stimulation of afferents of the sciatic nerve, were performed. At 14 days after surgery, H response was found in only 1 of 7 rats of Group G, and withdrawal reflexes were absent from all animals. At 60 days, the H response reappeared in 7 of 10 rats of Group G, and 1 of 5 of each of Groups C and S. The withdrawal reflex recovered in 4 of 10 rats of Group G, but in none of Groups C and S. Immunohistochemical labeling for calcitonin gene-related peptide (CGRP) in rats of Group G showed immunoreactive fibers entering the dorsal horn from sectioned roots, although at lower density than in the contralateral side. In conclusion, transplanted ensheathing cells promoted central regeneration and functional reconnection of regenerating sensory afferents.

Fetal spinal cord transplants support growth of supraspinal and segmental projections after cervical spinal cord hemisection in the neonatal rat

Cervical spinal cord injury at birth permanently disrupts forelimb function in goal-directed reaching. Transplants of fetal spinal cord tissue permit the development of skilled forelimb use and associated postural adjustments (, companion article). The aim of this study was to determine whether transplants of fetal spinal cord tissue support the remodeling of supraspinal and segmental pathways that may underlie recovery of postural reflexes and forelimb movements. Although brainstem-spinal and segmental projections to the cervical spinal cord are present at birth, skilled forelimb reaching has not yet developed. Three-day-old rats received a cervical spinal cord overhemisection with or without transplantation of fetal spinal cord tissue (embryonic day 14); unoperated pups served as normal controls. Neuroanatomical tracing techniques were used to examine the organization of CNS pathways that may influence target-directed reaching. In animals with hemisections only, corticospinal, brainstem-spinal, and dorsal root projections within the spinal cord were decreased in number and extent. In contrast, animals receiving hemisections plus transplants exhibited growth of these projections throughout the transplant and over long distances within the host spinal cord caudal to the transplant. Raphespinal axons were apposed to numerous propriospinal neurons in control and transplant animals; these associations were greatly reduced in the lesion-only animals. These observations suggest that after neonatal cervical spinal cord injury, embryonic transplants support axonal growth of CNS pathways and specifically supraspinal input to propriospinal neurons. We suggest that after neonatal spinal injury in the rat, the transplant-mediated reestablishment of supraspinal input to spinal circuitry is the mechanism underlying the development of target-directed reaching and associated postural adjustments.

Activated macrophage/microglial cells can promote the regeneration of sensory axons into the injured spinal cord

A prominent role for phagocytic cells in the regenerative response to CNS or PNS injury has been suggested by numerous studies. In the present work we tested whether increasing the presence of phagocytic cells at a spinal cord injury site could enhance the regeneration of sensory axons from cut dorsal roots. Nitrocellulose membranes treated with TGF-beta or coated with microglial cells were cotransplanted with fetal spinal cord tissue into an injured adult rat spinal cord. Cut dorsal roots were apposed to both sides of the nitrocellulose. Four weeks later, animals were sacrificed and spinal cord tissue sections were processed for immunocytochemical detection of calcitonin gene-related peptide (CGRP-ir) to identify regenerated sensory axons. Adjacent sections were processed with the antibody ED-1 or the lectin GSA-B4 for detection of macrophage/microglial cells in association with the regrowing axons. Qualitative and quantitative data indicate a correlation between the pattern and extent of axonal regeneration and the presence of phagocytic cells along the nitrocellulose implant. Axonal regeneration could be experimentally limited by implanting a nitrocellulose strip treated with macrophage inhibitory factor. These results indicate that increasing the presence of activated macrophage/microglial cells at a spinal cord injury site can provide an environment beneficial to the promotion of regeneration of sensory axons, possibly by the release of cytokines and interaction with other nonneuronal cells in the immediate vicinity.

Transplant therapy: recovery of function after spinal cord injury

Spinal cord injuries (SCI) result in devastating loss of function and altered sensation. Presently, victims of SCI have few remedies for the loss of motor function and the altered sensation often experienced subsequent to the injury. A goal in SCI research is to improve function in both acute and chronic injuries. Among the most successful interventions is the utilization of transplanted tissues toward improved recovery. The theory is that the transplanted tissue could (1) bridge the spinal lesion and provide chemical and/or mechanical guidance for host neurons to grow across the lesion, (2) bridge the spinal lesion and provide additional cellular elements to repair the damaged circuitry, (3) provide factors that would rescue neurons that would otherwise die and/or modulate neural circuits to improve function. A variety of tissues and cells have been added to the adult mammalian spinal cord to encourage restoration of function. These include Schwann cells, motor neurons, dorsal root ganglia, adrenal tissue, hybridomas, peripheral nerves, and fetal spinal cord (FSC) tissue en bloc or as disassociated cells. It is postulated that these tissues would rescue or replace injured adult neurons, which would then integrate or promote the regeneration of the spinal cord circuitry and restore function. In some instances, host-appropriate circuitry is supplied by the transplant and functional improvement is demonstrated. In this presentation, specific examples of recent work with transplanted tissue and cells that demonstrate improved behavioral outcome are presented. New recent work describing the in vitro propagation and characterization of human fetal spinal cord multipotential progenitor cells are also described in the context of a potential resource for transplantable cells. Additionally, data from transplantation experiments of human FSC cells into nonimmunosuppressed rat spinal cord are described, and the resultant improvements in behavioral outcome reported. Lastly, directions for future SCI research are proposed.

Fetal neural grafts and repair of the injured spinal cord

Solid or suspension grafts of fetal spinal cord (FSC), caudal brainstem (FBSt), neocortex (FNCx) or a combination of either FSC/FNCx or FSC/FBSt were placed into cavities produced by static loading (i.e., compression) of the spinal cord of adult cats two to 30 weeks after injury. Extensively vascularized, viable graft tissue was found in all animals with the exception of two cats which showed active rejection of their transplants. Surviving grafts showed many immature characteristics 6-9 weeks after transplantation. However, by 20-30 weeks, FSC and FBSt grafts were more mature. Grafts integrated with the host gray and white matter and neuritic processes from both host and graft were seen crossing the host-graft interface. Host calcitonin gene related peptide (CGRP)-like immunoreactive axons could be traced into FSC and FBSt grafts. A more restricted ingrowth of host serotonin (5-HT)-like immunoreactive fibers was seen in FSC grafts. Our results suggest that the capacity of homotypic transplants to promote recovery of function is greater than heterotypic transplants. Additionally, it appears that the functional capacity of the graft depends upon graft survival, the time interval between injury and transplantation, and whether or not the lesion cavity was debrided prior to grafting.

Differential regenerative response of Purkinje cell and inferior olivary axons confronted with embryonic grafts: environmental cues versus intrinsic neuronal determinants

Regeneration of severed central axons is supposed to depend on two factors: a permissive local environment and the particular intrinsic properties of axotomized neurones. To assess the role of each of these factors in axonal regeneration, the capability of two particular axon populations of the adult mouse cerebellum to grow into target-specific (cerebellum) and target-unspecific (neocortex) embryonic grafts was determined. Purkinje cell and inferior olivary axons were transected by passing a microscalpel through the axial white matter of the cerebellar folia, particularly those of the anterior lobe. Immediately after the injury, solid transplants were placed in the lesion cavity. Purkinje cell axons were labelled by using anticalbindin immunocytochemistry, and olivocerebellar fibres were visualized by biotinylated dextran amine anterograde axonal tracing. Following axotomy, Purkinje cell axons appeared as thickened processes ending with large terminal clubs. Their morphology and number did not change up to the longest survival time considered (2 months), thereby confirming previous demonstrations that Purkinje cells survive axon injury (I. Dusart and C. Sotelo, 1994, J. Comp. Neurol. 347:211-232). Inferior olivary axons were thinner and bore smaller terminal bulbs. When embryonic cerebellar grafts, containing cortical and deep nuclear precursors, were placed close to the injured axons, olivocerebellar fibres vigorously regenerated into the transplants and ended in new climbing fibres along the dendrites of grafted Purkinje cells. By contrast, host Purkinje cell axons never showed any outgrowth towards the graft. Similarly, these axons failed to regenerate into grafts containing solely the rostromedial portion of the cerebellar anlage, mostly consisting of deep nuclear neurones, their main targets. Comparable results were obtained by transplanting embryonic neocortical tissue: inferior olivary axons also regenerated into the grafts, although with distinct terminal arbours without the climbing fibre phenotype, whereas Purkinje cell axons always failed to grow. These results provide the first direct demonstration that severed inferior olivary axons are able to regenerate. In addition, they show that the growth-permissive/-promoting conditions created by embryonic nervous tissue are not sufficient to induce the regeneration of every axonal type and allow us to hypothesise that successful regeneration depends on the interplay between environmental cues and intrinsic properties of the axotomized neurones.

Grafts of fetal central nervous system tissue rescue axotomized Clarke's nucleus neurons in adult and neonatal operates

Many conditions are thought to contribute to neuron death after axotomy, including immaturity of the cell at the time of injury, inability to reestablish or maintain target contact, and dependence on trophic factors produced by targets. Exogenous application of neurotrophic factors and transplants of peripheral nerve and embryonic central nervous system (CNS) tissue temporarily rescue axotomized CNS neurons, but permanent rescue may require transplants that are normal targets of the injured neurons. We examined the requirements for survival of axotomized Clarke's nucleus (CN) neurons. Two months after hemisection of the spinal cord at the T8 segment, there was an ipsilateral 30% loss of neurons at the L1 segment in adult operates and a 40% loss in neonates. Transplants of embryonic spinal cord, cerebellum, and neocortex inserted into the T8 segment at the time of hemisection prevented virtually all of the cell death in both adults and neonates, but transplants of embryonic striatum were ineffective. None of the grafts prevented the somal atrophy of CN neurons caused by axotomy. Retrograde transport of fluoro-gold from the cerebellum demonstrated that 33% of all CN neurons at L1 project to the cerebellum, 50% of these died following a T8 hemisection, but all these projection neurons were rescued by a transplant of embryonic spinal cord. These results suggest that the rescue of axotomized CN neurons is relatively specific for the normal target areas of these neurons, but this specificity is not absolute and may depend on the distribution and synthesis of particular neurotrophic agents.

Migration and differentiation of PC12 cells transplanted into the rat spinal cord

To test the hypothesis that transplanted neuronal or neuronal-like cell lines, grown in vitro, might survive and differentiate in the mammalian spinal grey matter, adult male Sprague-Dawley rats (N = 5) were injected with a suspension of between 3 x 10(5) and 1.0 x 10(6) DiI labeled, undifferentiated rat pheochromocytoma (PC12) cells in sterile phosphate buffered saline. The PC12 cell line was chosen since, in certain in vitro conditions, this cell line serves as a model of neuronal differentiation, which includes the ability to conduct action potentials and form functional synapses. After a survival time of 7 or 8 days, the spinal cords were removed, cryosectioned longitudinally and examined for detection of DiI labeled PC12 cells using fluorescent microscopy. The number of DiI labeled profiles and the proportions of the DiI cells which were differentiated were counted per section in at least five non-contiguous sections per animal. Differentiation was defined as cells with neurite-like extension which exceeded twice the soma diameter. Results demonstrated the following: (1) from 2 to 15% of the transplanted PC12 cells survived; (2) migration within the spinal grey matter occurred since PC12 cells were found as much as 510 microns away from the injection site; (3) of the surviving PC12 cell population, a proportion of between 60 and 80% were differentiated, many with two or more neurite-like processes, in all of the rats; (4) no mitotic profiles were observed in DiI labelled cells; (5) undifferentiated PC12 cells were juxtaposed to the lumens of small blood vessels or within the lesion cavity. Although the specific factors remain to be elucidated, the observed PC12 migration and differentiation within the host spinal grey matter appears to be controlled by factors in the microenvironment. These data support the use of a homogeneous in vitro population of neuronal or neuronal-like cells, which are readily accessible to transfection with the appropriate genes, as transplant sources for the injured spinal cord.

Grafting in acute spinal cord injury: morphological and immunological aspects of transplanted adult rat enteric ganglia

We have studied allogeneic transplants of adult rat enteric ganglia in order to evaluate their use as donor tissue for eventual autografts in rodent spinal cord injury models. Female Sprague-Dawley rats of similar weights served either as transplant donors or as recipients. A glass micropipette of 0.8 mm diameter was used to create a local penetrating injury of the lower thoracic spinal cord and the transplant material was pressure injected through the pipette within the neural parenchyma. Ganglia of the myenteric plexus adhering to the stratum longitudinal muscularis were dissected from portions of the jejunum and ileum. Following partial enzymatic digestion and mechanical disruption of the myenteric plexus and muscle tissue (labeled with adherent rhodamine conjugated microbeads), reaggregates of myenteric plexus and muscle were suspended in growth medium and cultured in vitro for one to two days prior to transplantation. Transplants were examined at three, four, six, and eight weeks after surgery. Some of the donor tissue was grown in vitro, in order to determine its cellular composition. These cultured explants were fixed after 10 days, and like myenteric plexus and muscle grafts, were stained histochemically for acetylcholinesterase and observed by fluorescence and light microscopy. At the earlier post-transplantation periods, grafts contained several clusters of enteric ganglion cells that were positive for acetylcholinesterase and exhibited ultrastructural features characteristic of the enteric nervous system. They had well-defined boundaries. Reactive astrocytes and their processes remained located within the host spinal cord adjacent to the boundary region of the grafts. Likewise, macrophages were located in areas abutting the graft. Newly formed vasculature penetrated the graft interior and appeared to be continuous with the host vessels. Grafts grown for at least eight weeks were characterized by interdigitating boundaries. Finger-like protrusions of graft tissue containing fibroblasts and collagen intermixed with adjacent gray and white matter of the host cord. Such transplants also had reactive astrocytes and ED1-positive macrophages. At this later stage, several groups of ganglion cells were identified that were intensely acetylcholinesterase-positive; however, only two of four grafts were recovered, whereas two of the transplants degenerated. We postulate that degeneration of allogeneic grafts may occur as a result of ongoing immune responses of the host which could be prevented by use of autogeneic enteric ganglia. Our studies show that fully differentiated enteric ganglia can survive transplantation to acutely injured spinal cord of adult rats.

Regeneration of adult dorsal root axons into transplants of dorsal or ventral half of foetal spinal cord

Several dorsal root axons regenerate into the transplants of foetal spinal cord (FSC) and form synapses there. It is unknown whether the growth is specific to transplants of dorsal half FSC, a normal target of most dorsal root axons, or whether it is due to properties shared by transplants of ventral half FSC. We used calcitonin gene-related peptide immunohistochemistry to label subsets of regenerated host dorsal root axons, and morphometric analysis to compared neuronal populations within both transplants. Adult Sprague-Dawley rats received intraspinal grafts of dorsal or ventral half FSC (E14), and the L4 or L5 dorsal root was cut and juxtaposed to the grafts. Three months later sagittal sections were prepared for immunohistochemistry and Nissl-Myelin stain. Histograms of the perikaryal area showed that the transplants of dorsal half FSC consisted of small neurons predominantly, whereas transplants of ventral half FSC consisted of neurons of variable sizes. Dorsal root axons regenerated into both transplants, but growth into dorsal half FSC was more robust. These results indicate that both transplants provide an environment that supports dorsal root regeneration, but that the environment provided by dorsal half FSC is more favorable. Transplants of dorsal half FSC may offer advantages for the long-term goal of repairing of damaged spinal cord circuits.

Time course of dorsal root axon regeneration into transplants of fetal spinal cord: I. A light microscopic study

Cut dorsal root axons regenerate into intraspinal transplants of fetal spinal cord and establish synaptic connections there. The aims of the present study were to describe the progression of dorsal root growth within the transplants and the maturation of transplant morphology and to determine whether the regenerated dorsal root axons persist within the transplants or eventually withdraw. Embryonic (E) day 14 spinal cord was grafted into the lumbar enlargement of adult Sprague-Dawley rats, and the L4 or L5 dorsal root was cut and juxtaposed to the transplants. The morphology of the transplants was examined from 1 day to over 1 year after surgery, and the regenerated dorsal roots were labeled with immunohistochemical methods to study the subset that contains calcitonin gene-related peptide (CGRP). Embryonic spinal cord transplants survived and grew within the host spinal cord in over 90% of the animals. Transplant volume increased and the morphology of the transplants matured over the first 12 weeks and then did not change for 48-60 weeks. During the first week the transplants were composed of dissociated neurons, glia, and hematogenous cells with considerable extracellular space between them. Subsequently, the grafted neurons became densely aggregated, and non-neuronal elements such as inflammatory cells and myelin debris disappeared. CGRP-immunoreactive dorsal roots began to regenerate into the transplants within 24 hours, formed dense bundles by 4 days, and were still present at 60 weeks, the longest survival period examined. Myelination of axons within transplants began at 2 weeks. Quantitative analysis showed that the area of the transplants occupied by CGRP-labeled axons and the distribution area of the labeled axons within the transplants increased until 12 weeks and persisted unchanged for over 48 weeks. These results indicate that regenerated dorsal root axons are permanently maintained within transplants of embryonic spinal cord and suggest that the transplants can contribute to the permanent restoration of damaged intraspinal neural circuits.

Fetal cell grafts into resection and contusion/compression injuries of the rat and cat spinal cord

This article reviews recent findings concerning the feasibility, basic neurobiology, and potential functional benefits of fetal CNS tissue grafts into acute and chronic lesions of the adult spinal cord. In the rat, neuro-anatomical observations suggest that transplants into resection cavities establish neuritic projections that could functionally reunite separated rostral and caudal segments of the host spinal cord. Furthermore, some complementary electrophysiological evidence has been obtained for synaptic connectivity between host and graft neurons. In these studies, extracellular single-unit activity was evoked in fetal spinal cord (FSC) transplants by stimulating host dorsal roots that had been juxtaposed to donor tissue at the time of transplantation. In other investigations, we examined whether grafts could also establish axonal projections to appropriate areas of gray matter in the chronically injured spinal cord. For this purpose, fetal serotoninergic (5-HT) neurons were injected caudal to complete spinal cord transections that had been made 1-3 months earlier. Immunocytochemistry revealed that these cells projected their axons into gray matter regions normally innervated by bulbospinal 5-HT neurons. To investigate transplantation in a more clinically relevant lesion model, a third group of experiments involved injection of dissociated cell suspensions into acute [less than 24 h postinjury (p.i.)]), subchronic (7-10 days p.i), and chronic (greater than or equal to one month, p.i.) contusion lesions. Such grafts routinely filled areas that otherwise would have been regions of cavitation extending rostral-caudal distances of approximately 7 mm. FSC transplants in such injuries also appeared to influence some aspects of motoneuron excitability and hindlimb locomotion. More recent studies of the cat spinal cord have extended these findings in the rat by showing long-term survival (greater than 2 years) of fetal CNS allografts in recipients with either subtotal transection or compression lesions. Preliminary studies of connectivity have also shown host-graft projection patterns similar to those seen in the rat. Behavioral analyses are currently underway to examine the effects of fetal grafts in cats with chronic postcompression lesions. These observations in the rat and cat are discussed in the general context of basic biological and clinical issues relevant to the long-term objective of promoting functional improvement in the damaged spinal cord.