Neocortical transplants in the cerebellum of the rat: their afferents and efferents

Survival of iPSC-derived grafts within the striatum of immunodeficient mice: Importance of developmental stage of both transplant and host recipient

Degeneration of the striatum can occur in multiple disorders with devastating consequences for the patients. Infantile infections with streptococcus, measles, or herpes can cause striatal necrosis associated with dystonia or dyskinesia; and in patients with Huntington's disease the striatum undergoes massive degeneration, leading to behavioral, psychological and movement issues, ultimately resulting in death. Currently, only supportive therapies are available for striatal degeneration. Clinical trials have shown some efficacy using transplantation of fetal-derived primary striatal progenitors. Large banks of fetal progenitors that give rise to medium spiny neurons (MSNs), the primary neuron of the striatum, are needed to make transplantation therapy a reality. However, fetal tissue is of limited supply, has ethical concerns, and is at risk of graft immunorejection. An alternative potential source of MSNs is induced pluripotent stem cells (iPSCs), adult somatic tissues reprogrammed back to a stem cell fate. Multiple publications have demonstrated the ability to differentiate striatal MSNs from iPSCs. Previous publications have demonstrated that the efficacy of fetal progenitor transplants is critically dependent upon the age of the donor embryo/fetus as well as the age of the transplant recipient. With the advent of iPSC technology, a question that remains unanswered concerns the graft's "age," which is crucial since transplanting pluripotent cells has an inherent risk of over proliferation and teratoma formation. Therefore, in order to also determine the effect of transplant recipient age on the graft, iPSCs were differentiated to three stages along a striatal differentiation paradigm and transplanted into the striatum of both neonatal and adult immunodeficient mice. This study demonstrated that increased murine transplant-recipient age (adult vs neonate) resulted in decreased graft survival and volume/rostro-caudal spread after six weeks in vivo, regardless of "age" of the cells transplanted. Importantly, this study implicates that the in vivo setting may provide a better neurogenic niche for iPSC-based modeling as compared to the in vitro setting. Together, these results recapitulate findings from fetal striatal progenitor transplantation studies and further demonstrate the influence of the host environment on cellular survival and maturation.

The hippocampus and neurotransplantation

The present article is a review of our own results from histological and electron microscopic studies of hippocampal neurotransplants with different levels of integration with recipient brains. A model providing complete isolation from the brain was obtained using transplants developing in the anterior chamber of the eye. The growth, development, and cytological composition of transplanted tissue was found to depend on factors such as the age of the donor embryo tissue, the genetic compatibility between the donor and recipient, and the level of integration with the brain. Ultrastructural analysis of intraocular and intracortical transplants showed that overall, nerve and glial cells have the characteristics of highly differentiated, mature elements; the numerical density and structures of synaptic contacts were similar to those in normal conditions. However, transplanted tissues contained morphological features providing evidence of continuing growth of several nerve processes and increases in non-synaptic and transport-metabolic intercellular interactions. The ultrastructural deviations observed here are regarded as the manifestations of compensatory-adaptive changes during the development of tissues in conditions deficient in natural afferent synaptic influences. It is also demonstrated that the axons of transplanted neurons lacking adequate cellular targets can establish functional synaptic contacts with neuronal elements in the recipient brain which are not their normal targets.

Transplanted neurons alter the course of neurodegenerative disease in Lurcher mutant mice

Embryonic cerebellar, neocortical, and striatal tissues derived from NSE-LacZ transgenic mice were transplanted into the right cerebellar hemisphere of 8- to 10-day-old Lurcher or wild-type mice. Host mice survived for 30-90 days and the transplanted tissue was examined by light microscopy using Nissl staining, X-gal histochemistry, and immunohistochemistry for calcium binding protein and glutamic acid decarboxylase. Transplantation of cerebellar tissue, but not neocortical or striatal progenitors, resulted in robust infiltration of the lurcher mutant host cerebellar cortex by transgenic Purkinje neurons. Deep to the infiltrated molecular layer, the host granular layer was thicker and denser than the mutant granular layer, but transgenic cells did not contribute to the spared granular layer. The host inferior olivary complex consistently exhibited a noticeable bilateral asymmetry in Nissl-stained sections. A quantitative analysis of the olivary complex was performed in 10 90-day-old host mice. The results indicate that the left inferior olivary complex of 90-day-old host mice contained more neurons than the right inferior olive of the host mice and contained more neurons than was observed in 90-day-old Lurcher control mice. Analysis by olivary subdivision indicates that increased neuron numbers were present in all subdivisions of the host left inferior olive. These studies confirm the specific attractive effect of the mutant cerebellar cortex on transplanted Purkinje neuron progenitors and indicate that neural transplants may survive the neurodegenerative period to interact with developing host neural systems. The unilateral rescue of Lurcher inferior olivary neurons in cerebellar transplant hosts indicates that transplanted neurons may interact with diseased host neural circuits to reduce transneuronal degeneration in the course of a neurodegenerative disease.

Morphometric Study of Fetal Brain Transplants in the Insular Cortex and NGF Effects on Neuronal and Glial Development

Homotopic grafts supplemented with nerve growth factor (NGF) speed the recovery from learning deficits observed following electrolytic lesions of the insular cortex in rats. NGF also reduces the time in which the activity of choline acetyltransferase (ChAT) is first detected inside the graft by histochemical techniques. It is not known whether this behavioral and biochemical recovery correlates with an advanced maturation of the cellular elements within the graft, presumably induced by NGF. To investigate the degree of maturation of neurons, glial cells and blood vessels in NGF-supplemented grafts, adult rats were lesioned electrolytically in the insular cortex, and homotopic embryonic grafts (E16) with or without NGF supplementation were transplanted into the lesion. Fifteen days post grafting, the rats were perfused and the brains stained using silver impregnation techniques. Our results showed that neuronal maturation, as evaluated through several morphometric parameters, was advanced in NGF-supplemented grafts when compared with other experimental groups. Furthermore, grafts supplemented with NGF also showed significant increases in the number of neurons, oligodendrocytes, astrocytes and blood vessels. These observations indicated that the addition of NGF to insular cortex grafts promoted the maturation of neuronal and glial elements within the graft. They also support the possibility that the advanced morphological maturation of insular cortex grafts supplemented with NGF underlies the accelerated functional and biochemical recovery of animals with lesions of the insular cortex.

Integration of hippocampal suspension grafts with host neocortex

The possibility of histological and functional integration of nervous tissue heterotopically grafted into the adult host brain was investigated. Suspensions of embryonic (E17-18) rat hippocampus with dentate fascia were placed into acute cavities in the barrel field of young adult rats (n = 25). Golgi-Cox silver impregnation and Cresyl Violet stain were used for histological analysis 3-4 months postgrafting. The surviving grafts were present in 80% of the grafted animals. Only three out of 20 surviving grafts were completely isolated from the surrounding host brain; other grafts had areas of direct confluence with the host neuropil. Extracellular recording of neuronal activity revealed normal spontaneous activity typical of the hippocampus in the majority of the grafts. Electrical stimulation of the posterior nucleus of the thalamus, homolateral motor neocortex, contralateral barrel field, and sensory stimulation of the host evoked responses in 50-60% of the grafted neurons. This did not differ significantly from the responsiveness of the similarly tested neurons of homotopic neocortical suspension grafts. The latencies of the responses in the hippocampal grafts were consistently longer (by about 10 ms) than in the neocortical ones. Comparison of the hippocampal suspension grafts with other types of hippocampal and neocortical grafts suggests that under certain conditions heterotopic tissue can be successfully integrated into the host brain. Development of the host-graft interconnections depends on topical proximity, the presence of denervated synaptic loci in both tissues, elimination of the intragraft neuronal targets and disruption of the intrinsic connections between them.

Transplants of embryonic cortical tissue placed in the previously damaged frontal cortex of adult rats: local cerebral glucose utilization following execution of forelimb movements

Transplantation of fetal cortical tissue into the motor cortex of adult rats was used as an experimental model to examine the functional integration of homotopic fetal neocortical grafts into the motor pathways of adult host brain. We have employed the [14C]2-deoxy-D-glucose method to analyse the metabolic activity of the transplant and host sensorimotor cortex: (i) in animals solicited to perform specific lever-pressing movements with the limb contralateral to the transplant (experimental group); and (ii) in non-solicited animals or in animals using the limb ipsilateral to the transplant (control group). Grafts in the control group displayed homogeneous uptake of 2-deoxy-D-glucose throughout the rostrocaudal extent of the transplant. The local cerebral glucose utilization levels were low as compared to those of the surrounding cortex but were at least two-times higher than in the corpus callosum. Increase in 2-deoxy-D-glucose uptake by the transplant cells was found only in the experimental group. In this group, 2-deoxy-D-glucose uptake was higher in the caudal (AP: +3.0 to +1.7 mm, relative to Bregma) than in the rostral sectors of the transplants suggesting the existence of a topographic organization within the transplant. In addition, except in the rostral part, glucose utilization was higher in the transplant of the experimental group than in the sensorimotor areas of the non-activated cortex in the control group. Moreover, glucose utilization of the transplant cells was systematically higher in the experimental than in the control group. The transplants appear to display a certain level of metabolic integration with the host sensorimotor cortex since, in the experimental group, there was no significant differences in local cerebral glucose utilization values in the caudal sector of the transplant and in the surrounding sensorimotor cortical areas of the host. The 2-deoxy-D-glucose uptake was even higher in the caudal sector of the transplant than in some of the subfields of the contralateral sensorimotor cortex. The present findings indicate for the first time that motor activation of the contralateral forelimb produces an increase in metabolic activity in distinct transplant sectors, the topographic distribution of which matches the normal topographic organization of the forelimb somatomotor map. This suggests that transplants of embryonic frontal neocortex placed in the frontal cortex of adult hosts become functionally integrated with the host motor system.(ABSTRACT TRUNCATED AT 400 WORDS)

Morphological assessment of grafted rat and mouse cortical neurons: a light and electron microscopic study

The morphology of cortical neurons grafted into (or near) the rat striatum was studied by means of intracellular Lucifer yellow injections in fixed slices. Rat donor syngeneic cortical tissue (from postnatal day 1 old rats; AO strain) as well as mouse donor xenogeneic cortical tissue (prenatal day 19; C3H/HE strain) were grafted as solid pieces into 8-12 week-old rats (AO strain). Recipients of mouse xenografts were immunosuppressed with a monoclonal antibody against the interleukin-2 receptor. After perfusion and sectioning of the graft-containing areas, individual slices were incubated in the DNA stain 4.6-diamidino-2-phenylindole (DAPI) to visualize the cell nuclei. Grafts could be easily identified by a surrounding rim of astrocytes which outline the border between grafted and host tissue. Grafted cortical neurons were intracellularly filled with Lucifer yellow, DAB-photoconverted, and further processed for light and electron microscopy. In general, no cortical lamination could be observed in the grafted rat and mouse cortical tissue, but neurons were loosely packed throughout the graft. Two major cell types could be identified in all grafts investigated so far. The majority resembled those described as spiny neurons (85%), which could be further classified into pyramid-like, spiny stellate-like or fusiform spiny neurons, with somata ranging between 15 and 25 microns in diameter. The remaining 15% resembled non-spiny neurons with either a multipolar basket-like or fusiform morphology. Dendrites of spiny and non-spiny neurons, which could extend to distances up to 400 microns, were never seen to cross the astrocytic border, but some main axon and axonal collaterals of spiny neurons were found to leave the graft. On the basis of light microscopic observations no difference was found between mouse and rat grafted cortical neurons. The results of this study show that grafted cortical neurons retain some of the characteristic features of neurons in the intact adult cerebral cortex, although there appears to be a greater preponderance of spiny neurons in grafted tissue. This may reflect an immaturity of the grafted tissue or a response to the striatal environment.

Intrastriatal cerebellar grafts: differentiation of cerebellar anlage and sprouting of Purkinje cell axons

Pieces of cerebellar primordia were obtained from G16 (day 16 of gestation) rat fetuses and stereotaxically injected into the striatum of adult Wistar rats. The transplants were allowed to integrate with the host brain for 2 h up to 6 months after implantation. Ninety four out of 105 transplants perfectly integrated with the host brain (90%) and established the typical trilaminar histoarchitecture of cerebellar cortex. The transplants were sufficiently vascularized. Vessels seen within the grafts provided all ultrastructural elements of a blood-brain barrier. Light microscopic evaluation of graft development showed no considerable retardation of cerebellar histogenesis. Electron microscopic examination disclosed normal ultrastructure of cerebellar neurons, as well as elements of regular synaptic organization. The topic of efferent graft-to-host projections was investigated 2.5 months after transplantation using the monoclonal Purkinje cell marker anti-Leu-4 (CD3). This method allowed us to detect immunoreactive, morphologically intact axons of grafted Purkinje cells running over long distances (at least 500 microns) within the host striatum. Whilst afferent but in no case efferent connections of heterotopic cerebellar transplants had been demonstrated elsewhere, we could now prove the reciprocal modus of graft-host interaction with heterotopic cerebellar grafts.

New aspects of neurotransplantation

We have examined the possibility of promoting axonal regeneration within lesioned neural tissue using grafted artificial gel matrices. Polymeric matrices which feature a three-dimensional crosslinked macromolecular network were implanted into preformed lesions of the central nervous system (CNS). The host response consisted of matrix invasion by glial elements and the deposition of newly synthesized extracellular molecules. This rearrangement of the brain scarring process into an organized cellular coating promoted axonal regeneration into the gels. Entrapment of embryonic neurons and embryonal carcinoma (EC)-derived neurons, within the gels, was performed to explore the possibility of using polymer brain implants as neural graft microcarriers. Our results suggest that this approach will be useful for the delivery of cells and the promotion of axonal elongation required for successful neurotransplantation.

Olfactory bulb transplantation into the olfactory bulb of neonatal rats: a WGA-HRP study

After unilateral bulbectomy in neonatal (P1-P5) rats, autoradiographically prelabeled presumptive olfactory bulbs from E15 and E17 embryos were transplanted in place of the removed tissue. After 2-7 months, the animals received injections of wheat germ agglutinin-horseradish peroxidase (WGA-HRP) into the piriform cortex. Nine of the twenty animals revealed WGA-HRP-positive neurons among neurons autoradiographically labeled, providing thus evidence that the axons of the output neurons from the homotopically transplanted olfactory bulb reconnect with the host piriform cortex.

Effects of neural transplantation on seizures in the immature genetically epilepsy-prone rat

To study the hypothesis that neural transplantations can alter seizure susceptibility in a genetic animal model of epilepsy, 93 pubescent genetically epilepsy-prone rats with stage 9 seizures received either bilateral inferior colliculi (N = 21) or lateral ventricle (N = 42) transplants or sham transplants (N = 30). The grafts consisted of embryonic locus ceruleus, neocortical, or cerebellar tissue. Starting 2 days after the transplantation the rats were subjected to audiogenic stimulations every other day for 61 days. Latency to the running and tonic phase, seizure severity score, and duration of the tonic and clonic phase were compared in the neural transplant and sham-operated controls. Rats that received transplants had a longer latency to the tonic phase and a shorter duration of the clonic phase than the controls. At age 110 days the rats had electrodes implanted bilaterally into the angular bundle and were kindled. No difference in kindling rate was found between the rats that received neural grafts and the sham-operated controls. Cerebrospinal fluid concentration of norepinephrine was not altered by the transplants. This study demonstrates that the anticonvulsant effects of neural transplants, using the genetically epilepsy-prone model of epilepsy, are mild.

Effect of neural transplants on seizure frequency and kindling in immature rats following kainic acid

To study the hypothesis that neural transplantations can alter seizure susceptibility in a chronic animal model of epilepsy 260 immature rats (30- to 32-days-old) were administered a convulsant dosage of kainic acid (KA). Ten days later rats that had severe seizures following KA received either bilateral intracerebroventricular transplants of hippocampal (n = 27), neocortical (n = 29), cerebellar (n = 30), or locus ceruleus (n = 32) tissue, or underwent sham transplantation (n = 66). Spontaneous seizure frequency was assessed for 230 days following which the rats underwent entorhinal kindling. The percentage of rats developing spontaneous recurrent seizures was similar in the 4 transplant groups and the sham-operated controls. Rats receiving hippocampal and locus ceruleus transplants had fewer spontaneous seizures than the sham-operated controls or other transplant groups. However, there were no differences in afterdischarge thresholds or kindling rates in the 5 groups. This study demonstrates that the anticonvulsant effects of neural transplants, using this animal model are mild. Tissue type of the graft appears to be an important variable in the alteration of seizure frequency.

A novel assay for the in vivo study of Schwann cells

An in vivo assay was developed for long-term analyses of Schwann cells at the single cell level. Schwann cells were isolated from sciatic nerves and labeled with the fluorescent gold label Fluoro-Gold. Cells were then transplanted into severed sciatic nerves of young adult rats. Gold-labeled Schwann cells, identified by double-labeling with S100 antibodies, were observed up to 10 mm away from the transplant site and after 90 and 120 days of survival in vivo.

Organization of visual cortical projections to fetal tectal transplants in rats: a study using multiple retrograde tracers

Retrograde tracing techniques have been used to study the host visual cortical projection to fetal tectal tissue grafted to the midbrain of newborn host rats. To determine whether there is any topographic order in these cortical afferents, different parts of the grafts were injected with 3 different tracers: Fast blue (FB), Diamidino yellow dihydrochloride (DY), and either horseradish peroxidase (HRP) or rhodamine-labelled microspheres (Rh). The comparative visual cortical distribution of cells retrogradely labelled with the different dyes was then examined. Tectal tissue from 15-day-old pigmented rat embryos was injected via a glass micropipette onto the dorsal midbrain of anaesthetised newborn rats of the same strain. In adulthood, host rats were examined for the presence of grafts; 21 grafts were injected with retrograde tracers and the cortices of 12 of these animals were mapped to show the relative location of FB-, DY-, HRP- or Rh-labelled cells. Qualitative inspection of area 17 did not reveal consistent evidence of point-to-point visuotopic mapping in the cortico-transplant projection. However, within area 17 statistical analysis (chi 2 tests) revealed significant differences in most brains in the relative distribution of FB-, DY-, HRP- or Rh-labelled neurons. Areas 18 and 18a contained greater numbers of retrogradely labelled cells. In these extrastriate regions, statistical analysis also indicated significant differences in the relative distribution of neurons labelled with different tracers. These data thus provide evidence for a non-random pattern of cortical innervation of tectal grafts. Possible reasons for the absence of coherent, topographically organized cortico-transplant maps typical of the normal corticotectal projection are considered.

The response of the cerebral hemisphere of the rat to injury. II. The neonatal rat

The response to injury of the cerebrum of the neonatal rat was studied in knife wounds by using both light and electron microscopical, and immunohistochemical, techniques. The rats were injured at 2, 4, 8, 12, 16 and 20 dayspost natumand the tissues examined 8 days later. A mature scar, that is, a layer of fibrous tissue separated from the injured neuropile by a glia limitans, is not formed in the brains of rats lesioned before 8 dayspost natum. Before this time, the neuropile of the severed hemisphere grows together and both the glia limitans externa and ventricular lining are repaired. The only evidence of the wound, 20 days after injury, is a subpial and periventricular accumulation of astrocytes and occasional groups of blood vessels; elsewhere glial and neuronal processes traverse the wound obliterating all signs of the original lesion. After 8 dayspost natum, scar tissue is deposited. The scar first appears in the superficial cortex as fibroblasts and macrophages invade from the meninges. With increasing age at injury, these cells penetrate more deeply and, after 16 dayspost natumat injury, the entire lesion contains these cells. Concomitantly, a glia limitans is formed over the walls of the lesion, firstly in the superficial cortex continuous with the glia limitans externa, and successively in the deeper cortex, white matter and corpus striatum as the meningeal fibroblasts and macrophages invade these regions. In the developing cerebrum, injured before 8 dayspost natum, the failure to form a scar is unrelated to the maturity of the astrocytes and fibroblasts, because both interact to regenerate the glia limitans externa. The development of a scar, in animals injured after 8 dayspost natum, is correlated with the failure of both axonal and dendritic regeneration. Because there are few oligodendrocytes, and no myelin, it appears that inhibition of axonal and dendritic growth is linked to scar formation, and not to putative inhibitory substrates such as those on the surface of oligodendrocytes, CNS scarring may be initiated by the invasion of fibroblasts and macrophages from the meninges into the injured neuropile. The possible reasons why these mesenchymal cells fail to penetrate before 8 dayspost natumare discussed.

Neuronal and glial elements of fetal neostriatal grafts in the adult neostriatum

The cellular components of striatal grafts into the host striatum of rats were studied using [3H]thymidine autoradiography, histochemistry, immunocytochemistry and Golgi-staining. Autoradiography revealed that a layer of glial cells, somas smaller than 8 microns in diameter, stained positive with glial fibrillary acidic protein, and demarcating transplant from host, is derived mainly from the donor. Golgi studies revealed that many neuronal fibers fail to cross the glial layer to reach the host striatum. Migration of transplanted striatal cells into the host milieu was evident. The density of migrated cells decreased linearly as a function of distance from the transplant. Most of the far-migrated cells were glial cells. Neuronal migration was limited. In the transplant, donor cells marked by [3H]thymidine constituted at least 70% of the population. Neurons which stained positively for GABA, substance P, and acetylcholinesterase were identified in the transplant. Fibers of two of these three neuronal types, substance P and acetylcholinesterase, formed patchy patterns in the transplant. Detailed morphology on GABAergic fiber is not available to date, because of the limited antibodies or the method used. GABA is the highest population in the striatal transplant. Two types of GABA-positive cells were clearly distinguishable according to cell size. A majority resembled the medium-sized cell commonly found in striatum, while those of the other type resembled the larger GABA cells usually found in the globus pallidus.

Basal forebrain lesions facilitate adult host fiber ingrowth into neocortical transplants

The ability of mature host thalamic neurons to innervate embryonic (E19) cortex when implanted into the cortex of adult hosts was compared in normal and basal forebrain lesioned mice. The ingrowth of mature horseradish peroxidase-labeled thalamic axons into the transplants is facilitated by prior basal forebrain lesions. We discuss the possible reasons for the lesion-induced enhancement of axonal ingrowth, including the possibility that the enhanced ingrowth of thalamic fiber systems may be related to the loss of cortical innervation by extrathalamic brainstem inputs, especially cholinergic afferent fibers. The results support the interpretation that extrathalamic inputs to cortex play a modulatory role in regulating the growth and connections of specific sensory fiber systems during brain responses to injury.

Morphological study of cerebellar transplant cocultivated with cerebral cortical graft in the anterior eye chamber. I. Granular layer

Fetal cerebral cortex and cerebellar anlage from rat fetuses of 15-16 gestational day were grafted simultaneously to the anterior eye chamber of adult female albino rat recipients. Two months after transplantation the cerebellar portion of the double graft consisted of foliated cerebellar cortex surrounding a well-defined cerebellar nucleus. In the absence of pia mater or glial scar the cerebral and cerebellar grafts were observed to establish direct contact with each other. Although much thinner than in the normal cerebellum, the overall morphological organization of the granular layer in the transplant was similar to that described for "in situ" normal cerebellum, with some remarkable differences, though. In normal cerebellum all mossy terminals contain spheroid synaptic vesicles, a characteristic morphological feature of excitatory endings. In the transplant, however, although the majority of mossy terminals contained (small or large) spheroid synaptic vesicles, numerous mossy terminals were filled with ovoid, or pleomorphic synaptic vesicles, a morphological marker of inhibitory terminals. GABA-immunogold reaction, revealed, indeed, the presence of this inhibitory transmitter in mossy terminals containing ovoid synaptic vesicles. Both GABA (-) and GABA (+) mossy terminals formed asymmetric (Gray I-type) synaptic junctions with the surrounding dendritic digits of granule cells. It is suggested that GABA-ergic fibers as well as most non-GABA-ergic axons (originating either from the cerebral cortical graft, or from the cerebellar nucleus) may develop to mossy terminal-like structures as a consequence of the hugh deficit in "natural" mossy fibers in this model.

Transplantation of cultured human spinal cord cells into the rat motor cortex: use of Phaseolus vulgaris leucoagglutinin as a cell marker

Successful transplantations have been made of cultured explants of human fetal spinal cord into surgically created cavities in the motor cortical area of non-immunosuppressed young adult rats. The cultured cells were marked by brief incubation with Phaseolus vulgaris leucoagglutinin (PHA) just prior to transplantation. Following sacrifice of the rats 1.5 months later, PHA immunohistochemistry clearly outlined the demarcation zone of the explants. The transplanted neurons possessed long, somewhat tortuous fibers with occasional varicosities, as well as some thick processes. These findings extend our previous studies in which it was shown that cultured human fetal adrenal medulla and sympathetic ganglia cells could be successfully transplanted to non-immunosuppressed rat brain. They also suggest that PHA may be a valuable marker for transplanted cells at least for 1.5 months post-transplantation.

Failure of neocortical transplants to alter seizure susceptibility in previously kindled rats

Transplantation of embryonic tissue into the brains of host animals has been demonstrated to totally or partially correct functional lesions and neurohormonal deficits in a variety of animals. This study evaluated the hypothesis that "naive" embryonic neural tissue implanted into previously kindled animals will alter subsequent seizure susceptibility. At age 16 days, male rats were electrically kindled in the right amygdala (AM). Following kindling (age 19 days), rats underwent either transplants of embryonic neocortical tissue into the left dorsal hippocampus or a sham procedure. At age 84 days, both groups underwent transfer kindling in the left AM. In addition, two other groups of 19-day-old pups that had had electrode implantation in the right AM without subsequent kindling received either embryonic neocortical implants in the left dorsal hippocampus or a sham procedure and were kindled in the left AM for the first time at age 84 days. There were no differences in rate of kindling between the animals that received successfully grafted transplants and those that underwent sham procedures. Using the kindling model, transplantation of embryonic neocortical tissue into the hippocampus does not significantly alter seizure susceptibility in the host animal.

Xenografts of brain cells labeled in cell suspensions show growth and differentiation in septo-hippocampal transplants

Embryonic mouse brain cells from the basal forebrain region were labeled in cell suspensions and transplanted into the denervated hippocampal formation of adult rats. Many labeled cells had the appearance of typical pyramidal neurons with dendrites that had both growth cones and neurites. Labeled neurons and glia were seen at several sites in the hippocampal formation. The neurons were located predominantly along the dentate granule cell layer and the pyramidal neurons had a preferred orientation of their apical dendrites toward the molecular layer. Since it was rare to see a surviving labeled neuron within the injection site, migration away from the injection site seemed important for survival of the cells. The methods used in these experiments should become an important adjunct to the methods for studying the migration, differentiation and growth of neurons and glia.

Three morphologically distinct types of interface develop between adult host and fetal brain transplants: implications for scar formation in the adult central nervous system

The development of the host/graft interface of cerebellar and cerebral transplants was studied 1-60 days after operation. Grafts from fetal Wistar rats were transplanted to a cavity over the superior colliculus of adult rats by removing parts of the overlying cortex and hippocampus according to the Björklund/Stenevi technique. In sham-operated control rats, in which a cavity was made in the brain but no graft was implanted, the parenchyma bordering the entire cavity developed a complete glial-meningeal scar within 2 weeks after operation consisting of multilayered glial processes, a basal lamina, and fibroblasts (meningeal cells). A similar interface also developed between graft and host in the most superficial parts of the transplantation cavity. In the basal parts of the transplantation cavity, the host/graft interface consisted either of an incomplete sheet of astrocyte processes aligned in parallel to each other but without a covering basal lamina or of completely fused neuropil without any morphological signs of separation between host and transplant. It is concluded that these three zones of host/graft interface are established by differential interaction between the growing transplant and the host cicatrix. At the basal host/graft parenchymatous interface the fetal transplant interferes with the normal adult cicatrization process of the host, possibly by either releasing inhibitory factors or by preventing contact between the astroglia of the host and fibroblasts (meningeal cells). In white matter regions of the transplantation cavity, voluminous cysts developed, both in sham-operated controls and in graft recipients, which were invaded by transplanted neurons.

Neural transplantation: autoradiographic analysis of histogenesis in neocortical transplants

Neurohistogenesis in neocortical transplants obtained from 15-, 16-, 17-, 18-, 19-, 20- and 21-day-old embryos, was studied employing [3H]thymidine autoradiography. The neural tissues were transplanted in the midvermis of cerebellum of the host animals. Following transplantation the host animals in different groups were injected with the radiochemical at 6 hr, 1, 2, 3, 4, 5, 6, 7, 8, 9 and 10 day intervals, to label the neurons forming on different days in the developing transplants. Analysis of autoradiograms showed that all the neocortical transplants did undergo histogenesis in the host cerebellum, and that it was similar to that seen in a normally developing neocortex. Transplants from the 15-day embryos showed histogenesis lasting for 9 days, and at the other extreme transplants from the 21-day embryos showed histogenesis lasting only for 1 day. Histogenesis in other transplants fell between these two extremes in a graded fashion in relation to the age of the donor embryos. The magnitude of histogenesis in transplants from different donor embryos was closely related to the final size of the transplants. Transplants from 15-day embryos were the largest in size, and they were followed by those from 16-, 17-, 18-, 19-, 20- and 21-day donor embryos in a graded fashion. All transplants were intraparenchymal, and histologically appeared normal. They contained fully differentiated neurons, and were anatomically integrated with the host cerebellum without any glial scar tissue or necrotic tissue intervening between them.

Survival of adrenal gland implants in the neocortex of the rat: a morphological study

The survival of adrenal gland implantation in the cerebral cortex of the rat was studied. In the present study, mature adrenal gland had survived after six months of implantation. No scar tissue was observed between the adrenal gland cortex and the host cerebral cortex. The implanted tissue showed some reorganization in its cortex and medulla. In the adrenal cortex there was an observable increase in connective tissue fibers and some degeneration of cells. In the medulla, again, both surviving and degenerating cells were observed. This study shows that mature adrenal gland has the capacity to survive after implantation in the cerebral cortex of the rats. Further studies are being carried out on fetal and mature tissue implantation and the ability of the adrenal medulla to secrete catecholamines.

Transplantation of fetal postmitotic neurons to rat cortex: survival, early pathway choices and long-term projections of outgrowing axons

A system for studying growth and development of transplanted subpopulations of postmitotic cerebral cortical neurons is described. The cytotoxic drug methylazoxymethanol (MAM) was given to pregnant rats on the fourteenth day of gestation to destroy precursor cells of layers II-IV of the fetal cerebral cortex. Layer V and VI precursor cells which had completed their final division before MAM treatment and were unaffected by it, were labeled by a prior injection of [3H]thymidine. This strategy provides a donor cerebral cortex containing mainly neurons destined to form layers V and VI of the adult cerebral cortex; these cells are postmitotic. Pieces of donor cerebral cortex were transplanted to the cerebral hemispheres of normal newborn hosts at one day, two days, or 6 days after MAM treatment; survival was assessed 1-12 weeks after transplantation by autoradiography of histological sections. Radiolabeled graft cells survived in 89% of recipients and many of these grew axons into the host, as indicated by retrograde labeling with horseradish peroxidase. Significant numbers of graft cells could also be stained immunocytochemically for glutamic acid decarboxylase or for the peptides, somatostatin, vasoactive intestinal polypeptide or cholecystokinin. A second group of experiments examined the routes of early axon outgrowth from normal and postmitotic fetal grafts. When the donor cortex had been incubated in a mixture of [3H]proline and [3H]leucine for 20 min prior to transplantation, the earliest axons growing out of the graft into normal newborn hosts could be assessed by autoradiography of axoplasmic transport after survivals in the host of 7 days. Normal and postmitotic grafts taken at E15 or E20 were capable of outgrowth, though the axons of E20 postmitotic cells did not grow far. The location of the transplant was the major determinant of where graft cells' axons grew and growth was mainly into existing axonal pathways of the host. In a third group of experiments, long term axonal projections from normal and postmitotic fetal transplants to 4 regions of the host brain--thalamus, contralateral cortex, striatum, and hippocampus--were examined with retrograde tracers 2-4 months after transplantation. Projections from grafts to the 4 host sites were highly dependent on the presence of nearby host axons connecting with those sites. Neurons in all types of graft projected to one or other of the 4 sites, but generally in small numbers. Higher proportions of cells in grafts from E15 MAM-treated donors projected to the host thalamus.(ABSTRACT TRUNCATED AT 400 WORDS)

Removal and reimplantation of the parietal cortex of the neonatal mouse: consequences for the barrelfield

The barrelfield, i.e., the cortical representation of the contralateral whiskerpad, develops in the mouse between postnatal days 4 and 6 (P4 and P6). The pattern of barrels in the barrelfield is dependent on that of the whiskers on the whiskerpad. We removed (unilaterally) and reimplanted, with normal orientation (n = 23) or after 180 degrees rotation (n = 23), that part of the pallium where the barrelfield is to develop. These operations were made at birth (PO), at P1 or at P3. Our aim was to know whether a homeomorphic representation of the periphery would be established, by regeneration or by retarded growth of surviving thalamic fibers into the cortex, and if so, whether it could be modified in its orientation. Of group (i) 16 animals survived of which 14 killed between P26 and P29 could be analyzed. We operated 11 at PO and 4 of these had a normal barrelfield, two a rotated one (by about 30 degrees) with some barrels in odd positions, one a wildly disorganized one and 4 no barrels at all. Three mice operated at P3 presented a normal barrelfield. In group (ii) operated at PO, the 17 survivors, killed between P13 and P68, had no barrels. The results indicate that a homeomorphic representation of the periphery may be established in the somatosensory cortex after an early, complete and massive interruption not only of all its afferent and efferent fibers, but also of its vascularization. With respect to animals of group (ii) we cannot rule out the ingrowth of thalamic afferents into the implant.(ABSTRACT TRUNCATED AT 250 WORDS)

Neural tissue grafts and repair of the injured spinal cord

Neural tissue grafting presently stands as one of the more intriguing experimental strategies being applied to the problem of spinal cord regeneration. The following annotation presents an overview of recent investigations which have shown: that peripheral nerve grafts can stimulate axonal outgrowth in many descending and ascending fibre populations of the injured spinal cord and that central nervous system (CNS) implants, derived from segmental and supraspinal levels of the embryonic neuraxis, may likewise have the potential for promoting repair of damaged intraspinal neural circuitries in adult and neonatal recipients.

Postnatal development of the serotonin innervation of the hippocampus and dentate gyrus following raphe implants

Serotoninergic (5-HT) neurons derived from the embryonic raphe nuclear area (brainstem, embryonic days (E 16-18) were implanted into the entorhinal cortex of 6-day-old (P6) neonatal rat recipients which had received a fimbria lesion and entorhinal cortex ablation on P3. The hippocampus, dentate gyrus, and the raphe implant area were examined with 5-HT immunohistochemistry 7, 14, 21, 30, and 60 days after implantation. The pattern of 5-HT reinnervation was compared to that of normal and lesioned animals, and to previous studies in which rats received septal or striatal implants. In the hippocampus adjacent to the implant 5-HT-immunoreactive fibers were first observed by 7 days postimplantation and increased in density and in their septotemporal and dorsoventral extent with increasing time postimplantation. Moderately dense fiber networks were diffusely distributed in the hippocampus and dentate gyrus at 30 and 60 days postimplant. Little, if any, indication of lamination was present. Retrogradely labeled neurons (the majority of which contained 5-HT immunoreactivity) were observed in the raphe implant following injections of Fast Blue into the hippocampal formation. A few retrogradely labeled cells did not contain 5-HT, methionine-enkephalin (ME), or substance P (SP) immunoreactivity, although ME- and SP- immunoreactive neurons were observed in the implants. The lamination patterns and the increased density of 5-HT-immunoreactive fibers following a raphe implant into the entorhinal cortex clearly differ from the normal 5-HT pattern and from the patterns of lamination following a striatal or septal implant.

Brain grafts can restore irradiation-damaged neuronal connections in newborn rats

Immature rat brain tissue grafted to the brain of other immature and adult rats can survive and establish nerve connections with the host brains. In addition to facilitating the study of factors involved in the formation of central neural connections, brain grafts may also be used to substitute damaged or maldeveloped neurones. With exceptions in the visual system, the restoration of specific central neural connections has to date involved grafts of cholinergic and monoaminergic neurones, which have good regenerative capacity. In the present study, rat hippocampal neurones were damaged by neonatal X-ray irradiation and replaced by transplantation of normal, developing neurones of the same type. The grafted neurones (dentate granule cells) are not cholinergic or monoaminergic, but when appropriately located in the host hippocampal region they established specific and highly ordered afferent and efferent connections with the damaged host brain. Moreover, simultaneous demonstration of afferent and efferent transplant pathways showed that serial host-transplant-host connections had formed, restoring the normal neuronal circuitry initially disrupted by the irradiation.

Neocortical transplants in the rat brain: an ultrastructural study

Electron microscopic analysis of neocortical transplants in the cerebellum of the host animals showed that the nerve cells, glial cells, and neuropil of the transplants were normal. These transplants showed anatomical integration with the host brain through various regions of interface. Neuropil interfaces were found to have a high density of synaptic profiles, and medullary interfaces had a very small number of synaptic profiles.

Neural transplantation in the spinal cord of adult rats. Conditions, survival, cytology and connectivity of the transplants

Embryonic neural tissues of various types were transplanted into the intact, completely transected, and partially transected spinal cords of adult rats. The host animals were killed 4-6 months after the surgery, and the spinal cords and transplants examined. The best results were obtained when embryonic neocortical tissues obtained from 16-day rat embryos were used for transplantation into host animals that had been subjected to partial sectioning of the spinal cord. Use of other types of neural tissue, or transplantation of tissues into the intact or completely severed spinal cords was not successful. The successful neocortical transplants had survived, grown, differentiated, and established anatomical integration with the host spinal cords. The anatomical integration was established through an interface with the host spinal cord along the basal aspect. Along the lateral aspect glial scar tissue was present separating the transplants from the spinal cord parenchyma. The transplants contained well-differentiated and normal-looking neurons. They received afferents from the spinal cord only through the interface and not through the glial scar formations. The findings indicated that it is possible to transplant embryonic neocortical tissues into the spinal cords of the adult animals that become integrated with the spinal cord parenchyma. The axonal fibers in the adult spinal cord appear capable of regeneration and growing into the transplants only when an appropriate neural milieu, in the form of a healthy and viable interface, is available. In its absence the severed axons of the adult spinal cord do not grow into the neural transplants.

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.

Astrocytic development in fetal parietal cortex grafted to cerebral and cerebellar cortex of immature rats

Pieces of cortex cerebri anlage were dissected out from 16- to 17-day-old fetuses and transplanted to the cortical and cerebellar regions of 5- to 6-day-old rat pups. Twelve animals with grafts in the cortical region and 5 animals with grafts in the cerebellar region were studied 1.5-4 months later. Cresyl violet stained sections revealed no gross difference in either cell morphology, cell density or cell distribution between grafts in the two locations. A molecular layer-like zone was present on all free surfaces of the grafts, whether facing a ventricle or the meninges. The astrocytic development was studied using immunohistochemistry with antibodies against glial fibrillary acidic protein, (GFA), and the S-100 protein. Both antibodies visualized starshaped astrocytes and perivascular membranes surrounding blood vessels. Semi-quantitative measurements as well as computerized image analysis showed that the total amount of GFA-like immunoreactivity was much higher in both types of grafts than in corresponding host cortex cerebri. No differences in amount of S-100-like immunoreactivity could be demonstrated. As S-100 is thought to be a more general astrocytic marker than GFA, this suggests that the difference in GFA-like immunoreactivity is due mainly to an increased amount of GFA within the individual astrocytes. It is concluded that grafts of fetal cortex cerebri pieces to the CNS of young hosts develop a profound astrocytic reaction characterized by an increased amount of GFA-like immunoreactivity.

Mossy fibre projections into and out of hippocampal transplants

Hippocampal primordia taken from one day old postnatal rats and grown for 1 month in the hippocampus of adult rat hosts developed hippocampal pyramids, dentate granule cells, and specific patterns of mossy fibres. In 26 cases where certain boundary conditions were precisely met, the transplant mossy fibres crossed into the host and ramified for a distance of from 0.5 mm to no more than 1 mm in the stratum oriens of the host field CA1. They formed a layer 35 micron thick adjacent to the host CA1 pyramidal cell layer. The necessary boundary conditions were: (1) direct (and "unscarred") contact between the neuropil of the transplant and the host field CA1, (2) that the hilar (and not the molecular) aspect of the transplanted dentate granule cell lamina faced the host CA1 pyramids, and (3) that some of the interface was devoid of transplant hilar cells or CA3-type pyramids interposed between the transplant dentate granules and the host CA1 pyramids. In 3 cases a converse connection was found--viz. the host mossy fibres entered the transplant. In these cases the transplants consisted entirely of pyramidal cells (with no dentate granule cells of their own), and the part of the transplant receiving the host mossy fibres was embedded directly in the host mossy fibre pathway. For the dentato-hippocampal mossy fibre system, therefore, it is shown that postsynaptic targets in the adult mammalian brain can receive specific patterns of innervation from growing axons derived from a transplant, and that cut central axons of the same type can grow and form target-specific terminal arborizations in a transplant.

Cellular, histochemical and connective organization of the hippocampus and fascia dentata transplanted to different regions of immature and adult rat brains

The aims of the present study were to examine the survival and the cellular and connective differentiation of intracerebral transplants of fascia dentata and hippocampus. Pieces of immature dentate and hippocampal tissue were taken from late embryonic (E18) and early postnatal (1-9 days old) rats and transplanted into the brains of 1- to 13-day-old and adult rats. After survival times from 4 days to 2 years the cellular and connective organization of the transplants was monitored in parallel series of sections stained with thionin (cell bodies), Timm's sulphide silver method (terminal fields). Nauta and Fink-Heimer methods (normal and degenerating fibers) and a method for AChE activity (cholinergic afferents). The transplants survived well in all combinations of donor and recipient ages used, and they survived and differentiated in all parts of the recipient brains, although relations to pial and ventricular surfaces appeared to be optimal. Cell differentiation continued after transplantation, and a characteristic laminar organization was retained, although least in embryonic donor tissues. The distribution of intrinsic connections was determined by the types of subfields present in the transplants and interaction with ingrown host afferents. All aberrant intrinsic connections observed corresponded to aberrant connections formed in the hippocampus and fascia dentata denervated in situ and included supragranular mossy fibers in the fascia dentata, aberrant infrapyramidal mossy fibers in CA3, spread of CA4-associated afferents beyond the normal commissural-associational zone in the dentate molecular layer together with ingrowth of CA3-associated and CA1-subiculum-associated afferents. Most transplants received a cholinergic input of host origin irrespective of the localization in the host brain, but also non-cholinergic host pathways innervated the transplants, in particular when the transplants were in close contact with host fiber tracts, and when the recipients were immature. At various transplant locations the non-cholinergic host afferents belonged to the commissural hippocampo-dentate system, the commissural hippocampal system and the callosal system. Other cases suggested innervation of dentate transplant by host entorhinal afferents. The formation and distribution of intrinsic transplant connections and connections between transplant and host appeared to be regulated by the same factors that regulate the development and reorganization of fiber connections in the normal and the in situ denervated hippocampus and fascia dentata. As a special variety of this, the distribution of cholinergic afferents adjusted to the distribution of the major intrinsic and extrinsic non-cholinergic pathways.

Embryonic neural transplants across a major histocompatibility barrier: survival and specificity of innervation

The ability of embryonic tissue to survive cross-transplantation between histologically incompatible rat strains was examined by transplanting septal neurons from Sprague Dawley fetuses to adult Wistar rats (Ag-B6 to Ag-B2 histocompatibility haplotype). Transplants were found to survive without rejection over a period of 3 months. Furthermore, the laminar pattern of cholinergic innervation was similar to that of homogenic septal transplants and of intrinsic septal projections. These results suggest that embryonic neural tissue transplanted across major histocompatibility barriers are capable of survival for extended periods of time, and are in support of the concept of the privileged nature of embryonic tissue as a source of material for cross-transplantation.

Neurotransmitter characteristics of brain grafts: striatal and septal tissues form the same laminated input to the hippocampus

In previous experiments we have studied the development of grafts of embryonic septal tissues implanted alongside the hippocampal formation of neonatal rats. In the present study we examined intracerebral implants of corpus striatum, a brain region that contains acetylcholinesterase-positive cells and does not normally project to the hippocampal formation, in order to evaluate the possibility that neurotransmitter identity may be involved in mechanisms guiding patterns of afferent growth and connectivity. Implant cavities were made in the entorhinal cortices of neonatal rat recipients and 3-6 days later embryonic striatal tissues were grafted to these preformed cavities. Implants were examined with acetylcholinesterase histochemistry one month after implantation. Grafts of embryonic striatal tissues did not survive implantation when the implant was introduced at the same time as the cavity was made. Grafts of corpora striata containing acetylcholinesterase-positive neurons were found in 7 of 11 rats in the delayed implant paradigm and, in all but one of these animals, acetylcholinesterase was present within those terminal laminae in the ipsilateral hippocampus and dentate gyrus that normally receive cholinergic input from the septal area. These findings suggest that cues underlying the development of specific connections between native (and implanted) septal efferents and hippocampal target neurons may be recognized by ingrowing acetylcholinesterase-reactive fibers from striatal implants.

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.

Connectivity of neural transplants in adult rats: analysis of afferents and efferents of neocortical transplants in the cerebellar hemisphere

Embryonic neocortical tissue, 3.5 mm3 in volume, obtained from 17-day-old Long-Evans rat embryos, was transplanted into the intact cerebellar hemisphere of normal adult rat hosts. Transplants examined 90-160 days later had grown to a final volume of 27.24 mm3, which reflected nearly an 8-fold increase in the initial volume of tissue transplanted. The transplants were all intraparenchymal, having replaced large parts of the cerebellar hemisphere and occasionally portions of the vermis and paramedian lobule during their course of growth and differentiation. They retained a cellular and cytoarchitectural identity characteristic of neocortical tissue. Anterograde degeneration studies and retrograde tracing methods on the light microscopic level revealed that transplants had received afferent connections from the ponto-, olivo- and spinocerebellar projection systems. In addition to these major connections, afferents from other nuclei such as the locus coeruleus and the lateral reticular nucleus were also observed with the HRP method. Efferent outgrowth as studied with degeneration methods revealed projections to the nearby host cerebellum and to the ipsilateral deep cerebellar nuclei. All transplants had developed massive intratransplant connections. Findings on the nature and magnitude of connections were analyzed in terms of different characteristics of the interface between the transplant and the host brain tissue. The surface of cortical transplants was found to consist of 7 distinct components, 5 of which were interface regions. Two types of interface regions, those between the cerebellar medullary and granular layers and transplants, were readily related to the magnitude of extrinsic afferent ingrowth, and hence were effective sprouting surfaces. Using two correlated estimates of the magnitude of afferent ingrowth to cortical transplants, volume of degeneration and surface area of degeneration of transplants resulting from lesions of host brain structures, the pontine system was found to provide more afferents to transplants than the olivary or spinal systems. Generally, extrinsic fibers were located nearer to transplant-host brain interface regions than deep within transplants. A positive correlation existed between the available effective sprouting surface area of transplants (an estimate of interactive host fibers with a suitable trajectory) and the magnitude of innervation of cortical transplants by extrinsic afferents.

Behavioral effects of CNS transplants in the rat

Embryonic forebrain tissue from 17-day embryos was transplanted into the midline cerebellum of 10-day-old rat pups. The animals were allowed to survive for behavioral testing and were compared with animals receiving aspiration lesions of midline cerebellum and with normal controls. Subsequent histology indicated that the transplanted tissue had produced a compression lesion of the host cerebellum and had become fully integrated with the neuropil of the host animal. Behavioral results revealed no significant differences between transplant and control animals. Both of these groups were discriminably different from the lesion condition. It is suggested that the transplant may establish afferent and efferent connections similar to those present in the intact animal and thus may be anatomically well integrated with the host brain.