Regeneration of the septohippocampal pathways in adult rats is promoted by utilizing embryonic hippocampal implants as bridges

Expression of glial antigens C1 and M1 in the peripheral nervous system during development and regeneration

The expression of C1 and M1 antigens was studied by indirect immunofluorescence methods in histological sections of peripheral nerves and ganglia of C57BL/6J mice during development and regeneration. In sciatic nerves of adult mice, C1 but not M1 antigen is found in vimentin- and glial fibrillary acidic protein (GFAP)-positive Schwann cells. A similar distribution is also seen in trigeminal nerve, dorsal root and superior cervical ganglia, and olfactory nerve. In all cases vimentin-positive structures outnumber GFAP- or C1 antigen-positive ones. At birth, C1 antigen and vimentin are expressed in sciatic nerves, but GFAP is not yet detectable. M1 antigen cannot be detected in Schwann cells. In monolayer cultures of neonatal mouse dorsal root ganglia, C1 antigen is expressed in a fibrillary staining pattern in some, but not all morphologically identified Schwann cells. In vitro, M1 antigen is not detectable in Schwann cells. After lesioning sciatic nerves of adult mice by cut or crush, detectable levels of C1 antigen rise after 4-6 days: The number of immunofluorescently labeled structures and their relative intensities are drastically augmented, first distally more so than proximally, over control values from non-lesioned, i.e. contralateral nerves. A similar augmentation is also observed for vimentin and GFAP. M1 antigen expression does not reach detectable levels in Schwann cells under these conditions. The increased detectability of C1 antigen persists up to 150 days after lesioning, the longest time period tested.

Ultrastructure of fetal spinal cord and cortex implants into adult rat spinal cord

The ultrastructure of cortex and spinal cord from 11-, 12-, and 15-day-old fetuses implanted into the spinal cord of adult rats was studied over 3 months. Under deep Chloropent anesthesia, a 0.5 X 1.0-mm square of fetal cortex or a 1.0-mm segment of fetal spinal cord was implanted subpially between the left dorsal column and the dorsal horn of 70 adult rats. Implants grew toward gray matter, usually interfacing with the host at the isthmus between the horns of the spinal cord. However, implants were observed that occupied the entire left dorsal and ventral horns of the left half of the host spinal cord. Implants had concentric zones: A central zone with basal lamina lined joined channels and subjacent neuroglia; a zone of differentiating implant nervous system; a zone with basal lamina lined implant with overlying pial cells on the dorsal and lateral surfaces of the implant; a zone that interfaced with the host with overlapping neuropil on the lateral and ventral surfaces of the implant. Neuron types were typical for cortical or spinal implants. Implants survived for 3 months and reached stages of neuronal and neuroglial maturation similar to controls. Both fetal spinal cord and brain were successful as implants, had delayed differentiation, and formed complex neuropils. The zone of overlapping interface of the donor and host is an anatomical indication of physiological and functional integration.

Fetal CNS transplants into adult spinal cord: techniques, initial effects, and caveats

Transplantation of 11 day gestation rat fetal cortex and spinal cord into adult rat thoracic spinal cord is feasible. However, the techniques used at present for the implantation of the fetal transplant result in host spinal gray matter necrosis. One day after implantation the transplant is in a fluid-filled cyst in the host. The transplanted fetal tissue forms spherical neuroepithelia and unorganized cellular arrays. At Day 3 after transplantation the implant has sedimented to the ventral aspects of the fluid-filled cyst. By 10 days, there is an active neuroepithium with differentiating neurons and neuroglia lining the basal portion of the cyst. The transplant then proceeds to fill the cavity formed by host phagocytosis of the debris in the fluid-filled cyst.

Development of embryonic spinal cord transplants in the rat

Although fetal brain tissue, grafted into the CNS of neonatal and adult animals, has been shown to survive and differentiate, relatively little information has been obtained regarding the development of embryonic spinal cord transplants, especially in the injured host CNS. The survival and differentiation of fetal spinal cord transplants in either intracerebral cavities or the lateral ventricles of the adult rat brain were thus examined with light and electron microscopy. Approximately 90% of the spinal cord implants taken from 12-15-day fetuses persisted in either transplantation site with some surviving for as long as 8 months (latest interval studied). The survival rate was considerably lower (22%), however, with tissues obtained from older fetuses. Within 3 weeks, the transplants obtained from 12-15-day donors had become extensively myelinated and contained many neurons of different sizes, including some clusters of large neurons resembling ventral horn cells of the intact spinal cord. In addition, all of the mature grafts were characterized by multiple myelin-free regions of neuropil, containing many small neurons (20 micron in diameter). [3H]Thymidine labelling of the transplants and intact cords of the surviving littermates of the donor fetuses suggested that these myelin-free areas corresponded to the substantia gelatinosa of the adult spinal cord. In many cases, the transplants were confluent with the host CNS parenchyma without an intervening glial scar. Furthermore, multiple spinal cord transplants, placed into the same lesion site, were often fused, and injection of one of the transplants with horseradish peroxidase demonstrated many retrogradely labelled neurons in the adjacent implant. The results of this study suggest that some topographical features of the normal spinal cord may be represented in mature spinal cord transplants. In addition, these findings establish a basis for future investigations aimed at repair of the injured host spinal cord with homologous fetal tissue.

Recovery of choline acetyltransferase and acetylcholinesterase activities in the ipsilateral hippocampus following unilateral, partial transection of the fimbria in rats

An initial decrement followed by a significant recovery of choline acetyltransferase and acetylcholinesterase activities in the ipsilateral hippocampus was observed in rats with unilateral, partial transection of the fimbria. Regeneration of hippocampal cholinergic terminals could account for the parallel return towards normal of both the enzyme activities. We suggest that the still intact fimbrial fibres, homologous to those severed, contribute to the observed recovery.

Development of fetal retina, tectum, and cortex transplanted to the superior colliculus of adult rats

Earlier studies showed that embryonic retina, cortex, or tectum transplanted adjacent to the superior colliculus of newborn host rats differentiated many of the histological features appropriate for the donor region and developed interconnections with the host nervous system. In the study presented here, the same regions were transplanted to the brain of adult host rats and the development of these transplants was compared to those into newborn hosts. Retina, rostral tectum, or occipital cortex was dissected from donor rat embryos on gestational day 14 or 15. A portion of cortex was aspirated in 2-month-old host rats to expose the right superior colliculus, and one of the donor tissues was placed adjacent to the colliculus in each host. Two to 4 months after transplantation, transplant histology and neuronal interconnections between the transplant and host nervous system were studied by using Nissl and neurofibrillar stains and 3H-proline and HRP tract tracing techniques. Four main points can be drawn from these results. First, 80% of the transplants survived in adult hosts - a percentage comparable to that found in newborn hosts. Second, each of the types of tissues transplanted differentiated histological characteristics appropriate for its site or origin, although the degree of differentiation was always much less than in transplants to newborns. Third, the transplants developed only relatively local projections into the host cortex and superior colliculus. This contrasts with the extensive projections found from the transplants into the brain of newborn hosts. Fourth, no definitive projections from the host retina or brain were identified to any of the transplants into adults, whereas both cortical and tectal transplants into newborns received projections from the host.

Postnatally induced formation of the corpus callosum in acallosal mice on glia-coated cellulose bridges

Developing axons of the corpus callosum of mice are guided across the cerebral midline by a slinglike glial structure that forms transiently between the hemispheres. If the "sling" is cut at precallosal stages, the would-be callosal fibers whirl into paired neuromas adjacent to the longitudinal cerebral fissure. In experiments on such surgically acallosal animals, the aberrant commissural axons maintained a potential to regrow across the hemispheres at prenatal and early postnatal stages if they were presented with a properly aligned, glia-covered scaffold spanning the hemispheres.

Regeneration of rat hippocampal fimbria fibers after fimbria transection and peripheral nerve or fetal hippocampal implantation

After a unilateral hippocampal fornix-fimbria transection in adult rats, either autologous peripheral nerve or fetal hippocampus was implanted into the transection site. After 2 to 4 weeks, 2 to 3 months, and 6 to 8 months fimbria fiber regeneration was analyzed by acetylcholinesterase (AChE) histochemistry and retrograde transport of horseradish peroxidase after injection into the denervated host hippocampus. Prominent innervation of both types of implant by central AChE-staining axons occurred by 2 to 3 weeks postimplantation and was sustained to at least 8 months. Reinnervation of the adjacent host hippocampal terminal zone was also apparent, but was sparse compared with innervation of implants.

Viability, growth, and maturation of fetal brain and spinal cord in the sciatic nerve of adult rat

The feasibility for growth, maturation, and differentiation of fetal nervous system implanted into adult mammalian peripheral nervous system was studied. Thirty-five adult rats had the epineurium of the sciatic nerve crushed, perineurium minced, and fetal rat cortex or spinal cord implanted. Rats were utilized 7, 14, and 21 days, and 1, 2, 3, and 4 months later. A 1-mm cube of cortex or a 1-mm segment of spinal cord of 11-, 12-, or 15-day gestation fetuses was placed into the epineurium. Age-matched controls (7 DPI (days postimplantation) control for E15 implant was a 1-day pup, 21 days' gestation) were utilized for comparison (two per time group). Five animals had sciatic crush and perineurial mince only, and the gait and toe-spreading response were observed over 4 months. All implanted animals walked normally at 30 days. All implants were successful and survived the duration of the experiment. Cortical implants produced prominent bulges in the epineurium (21 days-4 months). Maximal neuronal and neurolgial cell division was observed at 7-21 days, decreased at 30 days, and stopped by 60 days. Most implanted cortical neurons had mature nuclei and immature dendritic patterns (apolar), and rarely had mature dendritic patterns. Neuroglia were abundant. The younger the cortical implant, the larger the cell mass produced (E11 greater than E15). The spinal cord implants survived, were viable, contained mainly neuroglia, and grew minimally. Host Schwann cells and nerve fibers were found in and around the implants. These data show that adult peripheral nervous system can act as an environment for growth and viability of fetal CNS implants.

Growth, differentiation, and viability of fetal rat cortical and spinal cord implants into adult rat spinal cord

Successful transplantation of the fetal brain into adult host brain has been accomplished. These studies explore the growth, differentiation, and viability of E11, E12, and E15 rat fetal cortex and fetal spinal cord implantation into the spinal cord of adult rats (donor and host, Sprague-Dawley). Under deep Chloropent anesthesia, 70 rats had 1-mm cubes of fetal cortex inserted with pressure or by stylus injection subpially between the dorsal horn and dorsal column (left side), or implantation of whole segments of fetal spinal cord. Animals were prepared for light microscopy 14 and 21 days and 1, 2, and 3 months later. Implants by both fetal tissues had a 69% survival rate. The younger the fetal implant the higher the success of the implant (E11 greater than E15). The diameter of fetal spinal cord implants reached the diameter of control postnatal animals after 30 days. The implants not only increased in mass (up to 7-fold in some cases) but differentiated and matured (apolar, unipolar, bipolar, and multipolar) neurons were observed one to three months postimplantation. By 30 days postimplantation, fetal neurons had large, often crenated nuclei, with a large single prominent nucleolus. The most successful implants were the young E11 fetal spinal cord into the adult host spinal cord. These implants represent an initial successful transplantation of fetal spinal cord into adult spinal cord.

Differentiation of cerebellar anlage heterotopically transplanted to adult rat brain: a light and electron microscopic study

Pieces of cerebellar primordia from (days 14 or 15 of gestation) E14 or E15 rat embryos were dissected out and transplanted into a cavity of the occipital cortex and underlying hippocampus, over the superior colliculus of 2-month-old rats. The host animals were allowed to survive for 2 to 3 months. The cytoarchitectonic and the synaptic organizations were analyzed in 16 of such transplants. Only 4 of the implants established connections with the host brain through several thin peduncles composed of myelinated fibers. The remaining 12 implants survived in an extraparenchymal situation. Independently of its partial linking to the host brain, the graft grew and developed a cerebellar structure composed of nuclear and cortical regions. The latter exhibited normal lamination and foliation, and contained the five categories of neurons which characterize normal cerebellar cortex. Electron microscopic examination disclosed that the synaptic connections normally present in the cerebellar cortex were also formed in the implants with the exception of climbing fibers, which were absent. The cerebellar interneurons kept their normal topographic distribution and gave origin to numerous synapses which maintained their own specificity. Some mossy fibers were present in the granule cell layer at the center of typical glomeruli. However, abnormal synaptic arrangements were also observed within the neuropil of this granule cell layer. They consisted of pseudoglomerular formations composed of clusters of tightly packed small axon terminals covered by granule cell dendrites. The origin of these boutons was not established. Since they did not correspond to the classes of presynaptic elements normally synapsing on these dendrites, they constitute a new example of cerebellar heterologous synapses. Their presence could be related to changes in the cellular environment due to the rarity of mossy afferents. HRP tracing experiments, carried out in extraparenchymal transplants, have allowed us to determine that the corticonucleocortical loop of normal cerebellum is also developed in the implants. Nuclear neurons are at the origin of the mossy fibers involved in glomerular formations, whereas Purkinje cells project to the nuclear region. The establishment of these reciprocal connections could determine the functional stabilization of both kinds of cerebellar neurons and thus the long survival of extraparenchymal grafts. These results allow the conclusion that the presence of extracerebellar afferents is not necessary for the organotypic and synaptotypic differentiation of cerebellar anlage.

Septal transplants restore maze learning in rats with fornix-fimbria lesions

In a study of the capacity of neural grafts to promote functional recovery in rats with fimbria-fornix lesions, 5 groups of rats were studied behaviourally and with acetylcholinesterase (AChE) histochemistry: (1) sham-operated controls; (2) bilateral fimbria-fornix lesions; (3) bilateral lesions plus bilateral solid embryonic septal grafts to the lesion cavity; (4) bilateral lesions plus bilateral embryonic septal suspension injections into the hippocampus; and (5) bilateral lesions plus bilateral solid embryonic locus coeruleus grafts to the lesion cavity. Seven months were allowed for growth of the grafts and reinnervation of the host hippocampus prior to behavioural testing. The control rats were able to rapidly learn a rewarded alternation task, while the performance of animals with bilateral fimbria-fornix lesions alone remained at a chance level. Both types of septal grafts (rich in cholinergic neurones) but not the locus coeruleus grafts (rich in noradrenergic neurones) reversed the impairment. Behavioural recovery correlated significantly with AChE-positive fibre ingrowth from the grafts into the denervated host hippocampus. However, the septal grafts did not ameliorate the lesion-induced disturbances in spontaneous activity or spontaneous alternation. Thus, the observed behavioural recovery appears specific to the conditioned alternation task and dependent upon cholinergic reinnervation of the hippocampus.

Spontaneous regeneration of cut axons in adult rat brain

Lesions with precisely defined boundaries were made, in adult rat brains, with a new type of cutting device that remained implanted during various survival times. The subsequent events were followed histologically in silver-stained (including Fink-Heimer) horizontal sections, with myelin and nuclear counterstains. The devices, inserted vertically through the dorsal surface of the brain until they met the floor of the skull, consisted of a horizontal cutting wire (0.9--2.6 mm long) between two vertical support wires, all 90 micrometers diameter. Following fixation of the brain, the device was removed from its ventral surface; thus, the channels left by the support wires appeared as two holes in horizontal sections, clearly marking the limits of the cut. The extents of the tissue cut by, and that passively deflected around, such devices were characterized from mearsurements of brains examined immediately after an incision was made and from degeneration studies after 3-day implantations. After implantations of 18--230 days, the severed axons no longer abutted the line of such lesions, as seen initially; now axons were often observed to extend alongside the lesion, then to bend and course around the support wire. Such "detours' ' curved back into the injured tracts, seeming to reconnect them appropriately. Detours had begun by 3--8 days, and end-swellings were frequently seen on their reoriented axons. Neither collateral sprouting of spared axons nor passive deformation of brain tissue was consistent with the findings. It is concluded that a massive and reconstructive regeneration of cut axons occurred around the ends of lesions made by the cutting devices.

Axonal guidance during development of the great cerebral commissures: descriptive and experimental studies, in vivo, on the role of preformed glial pathways

Do structures exist within the embryonic central nervous system that guide axons across the midline during development of the great cerebral commissures (corpus callosum, anterior commissure)? With the use of serial section and reconstructive computer graphic techniques we have found that during normal ontogeny of the mouse forebrain and before the arrival of the pioneer fibers of the corpus callosum at the midline, a population of primitive glial cells migrates medially (through the fused walls of the dorsal septum) from the ependymal zones of each hemisphere. At the midline, and well rostral to the lamina terminalis, these cells unite to form a bridgelike structure or "sling" suspended below the longitudinal cerebral fissure. The first callosal axons grow along the surface of this cellular bridge as they travel toward the contralateral side of the brain. The "sling" disappears neonatally. The fibers of the anterior commissure grow within the lamina terminalis along a different type of preformed glial structure. Movement of these axons occurs through an aligned system of glial processes separated by wide extracellular spaces. Do these transient glial tissues actually provide guidance cues to the commissural axons? Analyses of three situations in which the glial "sling" is genetically or surgically impaired or nonexistent indicate that this structure does, indeed, play an essential role in the development of the corpus callosum. We have analyzed (1) the embryonic stages of a congenitally acallosal mouse mutant (strain BALB/cCF), (2) several pouch stages of a primitive acallosal marsupial, Didelphys virginiana (opossum), and (3) animals in which the "sling" had been lesioned surgically through the uterine wall in the normal embryo (strain C57BL/6J). In the acallosal mouse mutant fusion of the septal midline is delayed by about 72 hours and the "sling" does not form. Although the would-be callosal axons approach the midline on schedule, they do not cross. Instead, the callosal fibers whirl into a pair of large neuromas adjacent to the longitudinal fissure. Similarly, in the opossum, fusion of the medial septal walls and formation of the glial "sling" are also lacking. However, in this species, instead of traveling dorsally, the "callosal" axons turn ventrally and pass contralaterally by way of the anterior commissure pathway. Surgical disunion of the glial "sling" also resulted in acallosal individuals. The callosal pathology in these affected animals mimicked exactly that of the genetically lesioned mutant. Our observations suggest that many different types of oriented glial tissues exist within the embryonic neural anlage. We propose that such tissues have the ability to influence the directionality of axonal movements and, thereby, play a crucial role in establishing orderly fiber projections within the developing central nervous system.

Brain injury causes a time-dependent increase in neuronotrophic activity at the lesion site

A cavity was made in the brain (entorhinal cortex) of developing or adult rats, and a small piece of Gelfoam was emplaced to collect fluid secreted into the wound. The neuronotrophic activity of the fluid was assayed with sympathetic and parasympathetic neurons in culture. The results show that wounds in the brain of developing or adult rats stimulate the accumulation of neuronotrophic factors and that the activity of these factors increases over the first few days after infliction of the damage.

An ultrastructural analysis of the development of foetal rat retina transplanted to the occipital cortex, a site lacking appropriate target neurons for optic fibres

Foetal retina was removed from donor rats at 15 days of gestation and transplanted to the occipital cortex of neonatal host rats. The purpose of this procedure was to examine the development of retinal neurons and photoreceptors, and document synaptic patterns during maturation of the transplanted retina in an environment lacking a normal target for optic axons. Host animals were sacrificed at 5, 10, 15, 20 and 30 days and samples of cortex containing the transplant were subjected to a light and electron microscopic analysis. During early stages of development, (5 days) the retina assumes a radial orientation with the scleral (outer) surface located centrally and the vitreal (inner) surface occupying the periphery. Numerous mitotic figures are found at the centre of the transplant and columns of primitive neuroblasts appear to radiate out from this zone. By 10 to 15 days after transplantation the retinal tissue contains numerous small rosettes each of which displays a histotypic organization with recognizable layers of sensory cells and their centrally-projecting processes, an outer limiting membrane, made up of a network of zonulae adherentes, and a rudimentary outer and inner plexiform layer which delineate the cells of the inner nuclear layer. Ultrastructural analysis of such rosettes confirmed the presence of typical bipolar, amacrine, horizontal and ganglion cells, but revealed that while the plexiform layers were occupied by numerous processes from these neurons, few if any, of these exhibited synaptic vesicles. By 20 to 30 days following transplantation sensory cells have completely differentiated, giving rise to prominent inner and outer segments which display typical cilia, centrioles and basal bodies, together with numerous stacked lamellae of photoreceptors which were contorted, presumably due to growth in an abnormal site. It should be further emphasized that these structures developed in the absence of pigment cells. Synaptic development ensues during this period to form characteristic dyads within the outer and inner plexiform layers. Additionally, clusters of amacrine to amacrine contacts occurred in the inner plexiform layer and were found to be increased relative to other types of junctions. In general, synaptogenesis takes place in the outer and inner plexiform layers and all categories of retinal synapses are established, but the process was found to be significantly delayed in comparison to normal retina at the same stage of development. Quantitative analysis revealed a reduced number of presumptive ganglion cells in proportion to the other categories of neurons. Optic fibres remained small and failed to myelinate. It is suggested that lack of an appropriate target for optic axons induced this alteration and may be indirectly related to the delay in the onset of synaptic development.

Cholinergic axons from delayed septal implants and sympathetic fibers co-exist in the denervated dentate gyrus

Adult female rats received a unilateral fornix/fimbria lesion 6 weeks prior to obtaining an implant of embryonic septal tissue. The ingrowth of cholinergic axons from the delayed implants into the denervated dentate gyrus was visualized by acetylcholinesterase (AChE) histochemistry. Since sprouting of anomalous sympathetic fibers also occurs following fornix/fimbria transection, the organization of these fibers was identified with the monoamine histofluorescence technique. Lesion control specimens with fornix/fimbria lesions demonstrate that sympathetic fibers are present along the septo-temporal axis of the dentate gyrus by 6 weeks postlesion. In specimens with delayed septal implants there is ingrowth of AChE fibers along the septo-temporal axis of the dentate gyrus with the densest distribution of fibers located at the septal pole of the dentate. The sympathetic fibers which are present in the dentate prior to the implantation of the septal tissue still persist in regions which contain a moderate density of AChE fibers but appear absent or diminished in regions with a dense cholinergic ingrowth. The data suggest that the postsynaptic signals necessary for the selective reinnervation of the dentate gyrus by the septal cholinergic axons are not abolished by synaptic reorganization in the neuropil which occurs prior to the implantation of the septal tissue. Moreover, there may be some competition between the cholinergic and sympathetic fibers for this postsynaptic signal and for space within the dentate neuropil.

The development of cerebellar primordia transplanted to the neocortex of the rat

Pieces of cerebellar primordia were dissected from the developing fetus at day 18 of gestation in Sprague-Dawley rats and transplanted to the neocortex of a 10- to 12-day-old rat. The histological development of 53 such transplants was analyzed at a series of survival times ranging from 5 min to 426 days. The cerebellar cortex developed much as it does in situ. However, only migratory sequences were strictly followed, while Purkinje cell differentiation and folia formation were initially retarded. In deep parts of the transplant, and throughout transplants confined to deep layers of the neocortex, the external granule layer was associated with penetrating blood vessels. An inverted cytoarchitectural pattern developed as concentric cylindrical layers around these vessels. In contrast, normal lamination and foliation were found only in transplants growing on the neocortical surface. Axons coursing between host and transplant were seen frequently and were especially pronounced at those sites beneath the internal granule layer which appeared to contain deep cerebellar nuclei. At the longest survival time there was preliminary evidence that suggested morphological deterioration of the transplant. Surface transplants, in addition to developing a normal orientation of cells and layers, have the added benefit of being accessible for further experimental manipulations after the transplant has become established.

Transplantation of reaggregates of embryonic neural retinae to neonatal rat brain: differentiation and formation of connections

Embryonic day 14 neural retinae were dissociated into single cells and reaggregated prior to transplantation over the superior colliculi of newborn rats. One month after transplantation, the brains of the host rats were examined for transplant differentiation and the formation of projections from the transplant to the host brain. All reaggregated retinal transplants differentiated in the host brain, showing normal lamination and cellular morphology. Electron microscopic examination demonstrated normal synaptology with the transplanted retinal neuropil. Horseradish peroxidase injections into the host superior colliculus retrogradely filled cells within the transplant. These cells were found in the lamina corresponding to the ganglion cell layer and displayed a morphology characteristic of normal ganglion cells. Lesions of the transplants confirmed the projection of the reaggregated retinal transplants to superior colliculus. Degeneration was also traced into a number of ther nuclei that are normally retinorecipient. The presence of degenerating synaptic terminals resembling those seen after eye removal in control rats was confirmed by electron microscopic examination. It appears that despite disruption of initial cell-cell associations early in retinal development and prior to transplantation, the reaggregated transplants retain the ability to differentiate and form appropriate connection with the host brain.

Reconstruction of the contused cat spinal cord by the delayed nerve graft technique and cultured peripheral non-neuronal cells

Previously, surgical reconstruction of the transected dog spinal cord by the delayed nerve graft technique has been shown to result in reinnervation of the nerve graft by axons. In the present study, we compared the results of surgical reconstruction of the severely contused cat spinal cord by the delayed nerve graft technique alone to those after reconstruction with a similar nerve graft plus cultured peripheral non-neuronal cells implanted between the grafted nerve and the spinal cord stumps. The spinal cord-nerve graft junction was examined by light and electron microscopy. The cultured cells were prelabelled with tritiated thymidine and their location after implantation determined by autoradiography. By 3 days after spinal cord reconstruction, the prelabelled cells were present at the junction and had migrated into the nerve graft and also into the spinal cord stumps where they were observed near axons. By 7 days, physical connections were observed bridging the junction between the spinal cord and nerve graft and axons ensheathed by Schwann cells had already penetrated at least 1 mm into the nerve graft. Wound healing took at least a week longer in animals repaired with a nerve graft alone. At one year or later after reconstructive surgery, in both groups of animals, the grafted nerve was reinnervated with myelinated and unmyelinated axons. Thus, the severely contused cat spinal cord could be reconstructed with the delayed nerve graft technique alone but the use of the cultured cells appeared to enhance wound healing and decrease the time required for axon elongation into the nerve graft.

In vivo evidence for a hippocampal adrenergic neuronotrophic factor specifically released on septal deafferentation

Denervation of the hippocampal formation in adult rats through lesions of the septohippocampal pathway was found to induce a trophic growth response in intracortical grafts of sympathetic superior cervical ganglia, and to stimulate regeneration of the intrinsic locus coeruleus adrenergic neurons following chemically induced axotomy. The grafted sympathetic adrenergic neurons grew very poorly into the adjacent hippocampus in animals with the septohippocampal pathways intact. A lesion of the ipsilateral fimbria-fornix or of the medial septum-diagonal band area caused a massive stimulation of axonal growth from the transplanted ganglionic neurons into the denervated hippocampus. This increase was more than 100-fold by 1 month after lesion and about 10-fold by 3 months after lesion. Fluorescence histochemistry revealed that the lesion-induced ingrowth occurred primarily into those areas of the dentate gyrus and hippocampus which were denervated of their septal cholinergic afferents. In addition, the septal and fimbria-fornix lesions induced a marked increase in size and noradrenaline fluorescence of the grafted sympathetic neurons, without any clear-cut effects on the numbers of surviving neurons in the graft. This lesion-induced trophic growth response (increases in axonal outgrowth, cell body size and noradrenaline content) was specific for lesions of the septal (probably primarily cholinergic) innervation of the hippocampus. Thus, extensive denervations induced by lesions of the commissural or perforant path afferents, as well as selective lesions of the intrinsic adrenergic afferents from the locus coeruleus, had no clear-cut effects. The intrinsic central adrenergic neurons were also found to be responsive to the lesion-induced growth-stimulating mechanism. Thus, lesions of the fimbria-fornix or the medial septum-diagonal band area had a marked stimulatory effect on the regeneration of the locus coeruleus noradrenergic neurons after selective chemical axotomy (induced by 5,7-dihydroxytryptamine; 5,7-DHT). Thus, the adrenergic reinnervation of the initially denervated hippocampus was significantly accelerated by 3 weeks after the fimbria-fornix or septal lesions, and the increase persisted for at least 8-10 months after transplantation. These results provide evidence for an adrenergic neuronotrophic factor whose production in the hippocampus normally is under the control of non-adrenergic (probably cholinergic) afferents originating in the septal-diagonal band area. The actions of this putative factor on sympathetic adrenergic neurons resemble those induced by nerve growth factor (NGF). Interestingly, however, the results obtained after 5,7-DHT-induced axotomy indicate that central and peripheral adrenergic neurons are equally responsive, and thus that the putative central adrenergic neuronotrophic factor may play a normal physiological role in the regulation of axonal growth and regeneration within the central nervous system.

Reformation in adult rats of functional septo-hippocampal connections by septal neurons regenerating across an embryonic hippocampal tissue bridge

Implants of embryonic hippocampus, placed into a cavity transecting the septo-hippocampal pathway in adult rats, have previously been shown to promote the regeneration of the lesional septo-hippocampal axons across the cavity. The present study provides electrophysiological evidence that these regenerating acetylcholinesterase-containing axons are able to re-establish atropine-sensitive excitatory synaptic connections in the grafted hippocampus as well as in the dorsal part of the initially denervated host hippocampus, thus demonstrating that intracerebral tissue implants can promote true, functional regeneration across a major tissue defect in the adult mammalian CNS.

Innervation of embryonic hippocampal implants by regene-rating axons of cholinergic septal neurons in the adult rat

The regeneration of the septal cholinergic system in adult rats has been studied in animals bearing transplants of hippocampus taken from 20-40 mm rat fetuses (approximately 17-21 days of gestation). The septal axons located within the fimbria and the dorsal fornix were lesioned and a cavity was prepared at the rostral end of the hippocampus. The embryonic tissue was placed adjacent to the severed end of the fornix-fimbria. The time-course of ingrowth of cholinergic fibers into the transplant was monitored by acetylcholine esterase (AChE) histochemistry and the determination of the levels of choline acetyltransferase (ChAT). Both methods indicate that there is a progressive ingrowth into the transplant of cholinergic fibers up to 3 months after transplantation. The newly-formed AChE-positive fibers in the transplant remain beyond one year after transplantation and are thus presumably permanent. Both horseradish peroxidase (HRP) injections into the implant and radiofrequency lesions of the septal-diagonal band area indicate that the principal source of these fibers is the AChE-positive neurons of the medial septum and the nucleus of the diagonal band which normally form the septohippocampal cholinergic projection. The results suggest: (1) that implants of a normal embryonic target tissue can promote axonal regeneration in mature neurons of the mammalian central nervous system; (2) that some neurons in the adult mammalian CNS retain at least part of their embryonic capacity to generate axons and recognize specific postsynaptic targets in developing CNS tissue; and (3) that this host-implant interaction can result in the formation of quite specific innervation patterns in the implanted target tissue.