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.     

Functional recovery following transplants of embryonic brain tissue in rats with lesions of visual, frontal and motor cortex: problems and prospects for future research

In animals, fetal brain tissue grafts into damaged adult host brain reduce some of the functional deficits caused by brain lesions. Although neurons from transplants survive and develop reciprocal connections with host brain tissue, such connections are generally not enough to replace damaged fibers completely and support behavioral recovery observed. Moreover, grafts never exhibit a normal morphological appearance as compared to adult tissue, but some metabolic activity is occasionally detected within the transplant. Release and/or diffusion of trophic substances from the transplant, in addition to those from the damage host brain, may partially restore neuronal and behavioral functions especially after lesions of the visual cortex. In this case, it can be hypothesized that fetal transplants serve as "living mini-pumps". In addition, there is evidence that the combination of trophic substances (e.g. GM1 ganglioside) and fetal brain transplants may provide a better opportunity for recovery than either treatment given by itself.     

Fetal neocortical transplants grafted into cortical lesion cavities made in newborn rats receive multiple host afferents. A retrograde fluorescent tracer analysis

Several reports have demonstrated efferent projections from fetal neocortical transplants placed in the cerebral cortex of newborn rats. Fewer studies have examined transplant afferents, and these have primarily used techniques based on the axonal transport of horseradish peroxidase. In the present study, we extend these initial findings on transplant afferent connections by using retrogradely transported fluorescent dyes to demonstrate a topographic and more extensive pattern of cortical transplant afferents than has been previously reported. Fetal neocortical tissue was grafted into frontal cortical lesion cavities made by aspiration in newborn rats. At 1.5-10 months later, the fluorescent dyes Fast blue and Diamidino yellow were injected into the transplants. Subsequent histological analysis demonstrated numerous retrogradely labeled fluorescent neurons within the host thalamus and cerebral cortex as well as several other areas of the host brain. The neurons were primarily single-labeled and generally found in areas that normally project to the ablated area of the cortex. The topographic distribution of retrograde labeling in several animals with non-overlapping dye injections confined to the transplants suggests that the host projections were distributed selectively within the grafts. These results support and extend previous studies suggesting the use of fetal neocortical tissue in repair of the neonatally damaged central nervous system.     

Transplantation of various regions of embryonic brain tissue into the brain of adult rats

Brain tissue from the cortex, midbrain (corpora quadrigemina ) or cerebellum was transplanted into the lateral ventricle or parenchyma of the right brain hemisphere of adult Wistar rats. Brain tissue transplants consisted of undifferentiated matrix cells and few neuroblasts. 30 and 110 days after operation transplants showed good development both in the lateral ventricle and inside parenchyma. They differentiated into organotypical and histotypical structures and cells similar to those formed in the normal development. Cortical structures developed from the cortex tissue, cerebellar structures from the cerebellum tissue, and nuclei white substance of corpora quadrigemina where formed from the midbrain tissue. Nerve and glial cells of transplants are well differentiated, tightly connected with the surrounding nervous tissue of the recipient, and remain viable by the end of the experiment. The immune response of the host to the transplant is not expressed. The behaviour of animals remains normal. The present experiments are the beginning of studies on grafting brain embryonal tissue to mammals with some nonhereditary and hereditary changes of the central nervous system (CNS) for our further investigations.     

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 within 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 other 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 connections with the host brain.     

Striatal, ventral mesencephalic and cortical transplants into the intact rat striatum: a neuroanatomical study.

Intrastriatal transplantation of fetal striatal (STR), cortical (CTX), or ventral mesencephalic (VM) tissue into the normal striatum has been shown to produce behavioral deficits (38). Here, we have examined the cellular elements of the transplants and their connectivity with the host using histochemistry for cytochrome oxidase (CO) and acetylcholinesterase (AChE), immunocytochemistry for glial fibrillary acidic protein (GFAP), OX42, tyrosine hydroxylase (TH), dopamine beta-hydroxylase (DBH), serotonin (5-HT), and cholecystokinin (CCK). Autoradiography for dopamine D1 and D2, muscarinic cholinergic, and serotonin 5-HT1 and 5-HT2 receptors at 5-15 months after transplantation was also investigated. CO staining showed that all transplants were metabolically active. The STR and VM transplants contained AChE-positive neurons and fibers. The CTX transplants exhibited AChE terminals with an appearance similar to that of the host cortex. AChE staining within the STR transplants was patchy. 5-HT-, TH-, and DBH-immunoreactive (IR) fibers were found in the STR and CTX transplants. In two of six CTX transplants, many TH-IR neurons were present. The VM transplants contained many TH-IR, 5-HT-IR, and DBH-IR cell bodies and fibers. CCK-IR stain was found in the VM transplant and was coextensive with regions containing TH-IR cell bodies. Fibers stained by all markers crossed the transplant and host border. Receptor autoradiography revealed that muscarinic cholinergic and 5-HT2 receptors were present in the STR, CTX, and VM transplants. In addition, dopamine D1 and D2 receptors were present in the STR transplants. Intermittent heavy staining for GFAP and OX42 were observed along the border of most transplants and the hosts. It was noted that high densities and hypertrophy of GFAP- or OX42-stained astrocytes or microglia, respectively, were present in the transplants and adjacent host. OX42-stained macrophages were found in many transplants. The present results indicate that intrastriatal transplants into the intact normal brain express numerous histochemical, immunocytochemical, and receptor features characteristic of the appropriate adult tissues. The afferents from the host extend into the STR and CTX transplants, and neural fibers from the VM transplants grew into surrounding host tissue, suggesting possible anatomical connection. Ultrastructural evidence is needed to determine if these fibers form synaptic connections. The results from GFAP and OX42 immunocytochemical staining support the possibility suggested by behavioral studies that damage to the host brain is induced by neural transplantation.     

Innervation of embryonic hippocampal implants by regenerating 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.     

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 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.     

Intracephalic transplants of freeze-stored rat hippocampal tissue

The survival and cellular and connective organization of intracephalic transplants of developing, freeze-stored rat hippocampal tissue were examined. Blocks of tissue containing the hippocampus and fascia dentata were obtained from late embryonic (E16-E22) and early postnatal rats (P0-P4) and immersed in a tissue culture medium with 10% of the cryoprotective agent DMSO, frozen at a cooling rate of approximately 1 degree C/minute, and stored for 1-226 days in liquid nitrogen. After quick thawing and washing out of the DMSO the tissue blocks were transplanted to the brain of adult rats. From 2 weeks to 3 months later the recipient brains were processed histologically. The cellular and connective organization of the transplants and their interaction with the host brains were analyzed after thionin cell staining, Timm's staining for hippocampal and dentate afferents, immunohistochemical staining for enkephalin-, CCK-, and somatostatin-reactive neurons and afferents, AChE staining for cholinergic afferents, and silver stains for fiber architectonics and tracing of connections by anterograde axonal degeneration. Freeze-storage narrowed the range of donor ages with good transplant survival. The best surviving hippocampal and dentate transplants thus came from 17-21-day-old embryos. There was no correlation between the length of storage and survival. Structurally the transplants of stored tissue were more frequently fragmented than the transplants of fresh tissue when located outside the brain parenchyma in the brain ventricles. This was in accordance with the results of a previous study of grafts of freeze-stored and fresh hippocampal tissue placed in the anterior eye chamber. Despite the decrease in survival and the tendency for fragmentation many well-structured and organotypically organized hippocampal and dentate transplants were recovered corresponding to the donor ages E19-E21. In addition to the main cell types (granule cells and pyramidal cells) the freeze-stored transplants also contained peptidergic nerve cells reacting for CCK, somatostatin, and enkephalin. The organization of the intrinsic nerve connections and the exchange of connections with the host brain were similar for transplants of stored and fresh tissue. Besides the consistent innervation of the hippocampal and dentate transplants by host cholinergic afferents monitored by AChE staining, several appropriately located dentate transplants thus sent mossy fibers to the host CA3. Others received host perforant path projections. A CA3-associated transplant projection to the denervated perforant path zones in the host fascia dentata was also observed.(ABSTRACT TRUNCATED AT 400 WORDS)     

Anatomical and behavioral correlates of a xenograft-mediated pupillary reflex

Retinae from mouse embryos were transplanted to the midbrain of neonatal rats which were unilaterally enucleated at the time of transplantation. At maturity the host's remaining optic nerve was cut and the skull over the transplant was removed. Illumination of the transplant caused pupilloconstriction of the host eye, a response abolished by removal of the transplant or damage to the olivary pretectal nucleus which is the pupilloconstriction center of the brain stem. The degree of constriction correlated with the intensity of transplant illumination. Using mouse-specific monoclonal antibodies, we demonstrated that the grafts projected only to areas of the brain stem normally innervated by the eye. The density of label in the olivary pretectal nucleus did not, however, correlate with the briskness of the graft-mediated pupillary reflex. In summary, this study shows that ectopic neural xenografts are capable of making specific connections which can drive an appropriate response to a natural stimulus in the host animal. While important in showing the ability of transplants to make specific connections, this study also provides a simple assay system for examining conditions which may influence the efficacy of host innervation by the transplant.     

Neural tissue transplants rescue axotomized rubrospinal cells from retrograde death

Rubrospinal tract cells undergo massive retrograde degeneration following spinal cord damage in newborn rats (Prendergast and Stelzner, J. Comp. Neurol. 166:163-172, '76b). In the current study, fetal spinal cord tissue (E12-14) was grafted into midthoracic spinal cord lesions in newborn rats (less than 72 hours old) in order to determine whether such transplants could modify the response of the immature host central nervous system (CNS) to axotomy. These transplants grew, differentiated, and formed extensive areas of apposition with the recipient spinal cords. Counts of red nucleus (RN) neurons indicated a significant loss of RN neurons in animals with lesion alone, but a rescuing of most of these cells if a transplant was placed into the lesion site. In fact, the number of neurons in animals with lesions and transplants was not significantly different from control animals. Horseradish peroxidase injected 10-15 mm caudal to the transplant (at 1-12 months post-transplantation) labeled neurons within the transplant and RN neurons contralateral to the spinal cord lesions and transplant. In animals with spinal cord lesion but no transplant, only the unaxotomized RN was labeled. Thus, spinal cord transplants prevented the massive retrograde cell death of immature axotomized rubrospinal neurons. Some of these rescued neurons projected to the host spinal cord caudal to the transplant.     

Transplantation of tectal tissue in rats. II. Distribution of host neurons which project to transplants

Tectal tissue was dissected from fetal rats and transplanted adjacent to the superior colliculus of newborn rats. The recipient animals were then allowed to survive for 6 or more weeks. Subsequent examination revealed that the transplants generally lay over the host inferior colliculus and rostral part of the cerebellum and had substantial fiber connections with the host superior colliculus. To determine which host areas projected to the transplants, horseradish peroxidase (HRP) was injected into the transplants, and the host brain was examined for the presence of retrogradely filled neurons. Labeled cells were found in nearly 50 host areas. Most of these areas are known to project to normal superior colliculus. There was a consistency between one animal and another in the frequency and density of cell label in the various areas. The projection from host cortex (particularly from visual cortical areas) was the densest and most consistent projection. Other areas which commonly projected into the transplants included pretectum, parabigeminal nucleus, superior colliculus, and the brachial region of the inferior colliculus. Sparse and infrequent projections were found from ventral lateral geniculate nucleus, substantia nigra, zona incerta, and catecholaminergic nuclei. No unequivocally labeled retinal ganglion cells were found. The results indicate that the host projection into the transplants is limited to those areas with axons in the vicinity of the host/transplant interconnection. However, the data also suggest that (1) the relative maturity of particular host pathways at the time of transplantation and (2) some form of preferential or absolute affinity expressed between host axons and transplant cells are also factors which influence the pattern of connections formed between host and transplant.     

Fetal Neocortical Tissue Blocks Implanted in Brain Infarcts of Adult Rats Interconnect with the Host Brain

The purpose of the present study was to study if the connectivity of fetal neocortical tissue blocks placed in ischemic brain infarcts of adult rats would be enhanced in rats housed in an enriched environment. We also investigated whether the enriched housing conditions could enhance the postischemic and postgrafting functional outcome, in terms of motor behavior. This part of the study has been published recently. The middle cerebral artery was ligated on the right side in 37 inbred, adult male spontaneously hypertensive rats. The rats were placed at random either in an enriched environment (groups A and B) or in standard laboratory cages (group C). Three weeks after the artery occlusion, blocks of fetal sensorimotor cortex (Embryonic Day 17) were transplanted into the infarct cavity of rats from groups B and C. After 9 weeks all transplanted rats received an injection, into the graft, of a mixture containing the two tracers Fluoro-Gold and biotinylated Dextran amine. The transplants revealed a structured morphology with whorls and bands of cells reminiscent of normal neocortex. Tracing of efferent transplant to host fibers with biotinylated Dextran amine showed pronounced intrinsic transplant projections, as well as fibers, although significantly fewer, to the host ipsilateral sensorimotor cortex, striatum, and thalamus. Host to transplant projections were revealed by Fluoro-Gold-labeled cells found in the ipsilateral host sensorimotor cortex, the basal nucleus of Meynert, the thalamic ventrobasal, ventrolateral and posterior nuclei, and in the dorsal raphe nuclei. We conclude that fetal frontal neocortical block grafts placed in brain infarcts of adult rats develop a morphology reminiscent of normal neocortex and that both afferent and efferent neural connections, although sparse, are established with the host brain, whether the rats are reared under enriched housing conditions or not.     

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-;menth-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 of 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.     

Neocortical transplants in the micrencephalic rat brain: Morphology and behavior

Normal fetal (E18) neocortical tissue transplanted into the hypoplastic posterior neocortex of infant (10 +/- 2-day-old) rats with transplacentally induced micrencephaly developed into very large, healthy, and permanent transplants. Although the cellular organization within the transplants rarely resembled that of normal rat neocortex, the transplants formed a broad area of interface with the host brain and established fiber connections with it. When tested at 2 months and 1-year-of-age, the presence of the transplant had no significant effect on the typically abnormal performance of micrencephalic rats on two tests of unspecific function, open field activity and maze learning. However, a small group of micrencephalic rats in whom the transplant tissue had failed to fill in the small brain lesions inescapably inflicted during surgery, showed greater behavioral deficits than the micrencephalic controls, suggesting that the transplant had corrected the lesion effect.     

Morphological, biochemical and physiological changes in brain nervous tissue of adult intact and hypoxia-subjected rats after transplantation of embryonic nervous tissue

Data of morphological, biochemical and physiological investigations of brain nervous tissue in adult intact and hypoxia-subjected rats after transplantation of rat embryonic nervous tissue are presented. It has been established that transplants survive and develop successfully in the brain of both intact and hypoxia-subjected rats. The Golgi technique reveals that transplanted neurons form dendrites and axons. They also establish afferent and efferent connections with neurons in different brain regions of the recipient, as has been demonstrated by the horseradish peroxidase (HRP) tract tracing technique. Hypoxia induces reversible as well as irreversible dystrophy of host brain neurons, and transplantation of embryonic brain tissue leads to considerable normalization of the state of reversibly dystrophic neurons. Biochemical investigations have revealed that the spectrum of cytoplasmic proteins of neurons and proteins of the whole brain cortex tissue changed after hypoxia are significantly normalized after subsequent transplantation. It should be also pointed out that there is a considerable increase in the number of proteins in certain fractions which is correlated with regeneration of reversibly dystrophic neurons and other cortical cells. Electrophysiological experiments revealed distinct but short-term EEG changes induced by hypoxia. 1-2 days after hypoxia all EEG parameters were back to normal and displayed no changes during the 2-3-month registration period.     

Patterns of angiogenesis in neural transplant models: II. Fetal neocortical transplants

Vascular integration between transplanted fetal CNS tissues and host brain is essential for long-term transplant survival. This study compares the time course and mechanism of vascularization in allografts of fetal cerebral cortex inserted either into the fourth ventricle or directly into the parietal cortex or hippocampus of perinatal rats. Recipient animals were administered 3H-thymidine after various postoperative time periods. The tissues were processed for light microscopic autoradiography to determine the temporal pattern of endothelial proliferation at the graft sites. Correlative electron microscopy depicted the morphological changes in transplant vasculature. Some recipients were prelabelled with 3H-thymidine prior to transplantation to determine if host vessels invaded the grafts; conversely, some donor tissues were prelabelled in utero to ascertain if the intrinsic vascular anlagen survived. Intraventricular transplants contained patent vessels, probably originating from the host pia mater, as early as 24 hours postoperative. Intraparenchymal transplants had patent vessels by 72 hours and a more complete network by 5 days. Prelabelling experiments and ultrastructural observations demonstrated that adjacent host pial vessels became incorporated into the perimeter of the intraventricular transplants and later grew centrally into the grafts. Intraparenchymal transplants also contained host vessels that exhibited a similar growth pattern. Intrinsic graft vessels remained viable and continued their development, and presumably anastomosed with the ingrowing host vasculature. Temporal labelling studies revealed that both vessel populations attained their highest proliferative rates within 72 hours after transplantation. This study demonstrates that the vasculature which develops within both intraventricular and intraparenchymal fetal CNS transplants is chimeric, consisting of intrinsic fetal vasculature and proliferating host vessels. The mechanism of transplant vascularization may be significant with regard to astrocytic, immunological, or blood-brain-barrier characteristics at these transplantation sites.     

Neural transplantation: An historical perspective

Literature on transplantation of neural and nonneural tissues into the brains of host animals is reviewed in the perspective of various issues. The two dominant issues determining this research were elucidation of embryological processes underlying the development of the nervous system and regeneration in the host brain. A comprehensive review of studies on regeneration in the central nervous system (CNS), using this technique of transplantation, indicates that regeneration of axonal fibers is small in magnitude and extent, and that it is more directly related to the trauma caused to the brain than to any other variable. This literature review attempts to provide a perspective to the contemporary research on neural transplantation and on regeneration in the CNS.     

The lamination and connectivity of embryonic cerebral cortex transplanted into newborn rat cortex

Sheets of frontal or occipital cerebral cortex were taken from embryonic day (E) 15 rat embryos and placed in shallow depressions made in the occipitoparietal region of newborn rats. These transplants developed normal patterns of lamination, which could be in an inverted orientation if the transplant itself was placed upside down. Irrespective of the cortical area of origin of the grafted tissue, the transplants consistently received projections from those host thalamic nuclei that were normally found to innervate the adjacent host cortex. This indicates that immature cortical tissue, up to at least E15, may not contain the information necessary to define the specific thalamocortical connections characteristic of individual areas. On the contrary, the observed input pattern may be the result of sprouting of fibers that normally innervated host cortical regions adjacent to the transplant. Similarly, callosal afferents to transplants seemed to be a direct extension of the callosal input to the host cortex immediately beneath the transplant. Results from HRP studies of callosal connections indicated that transplant efferents to the contralateral cortex are smaller in magnitude than their afferents. This may be related to the superficial location of the transplants, which may limit the access transplant efferents have to the white matter. This study suggests that, while the cortical lamination is largely determined intrinsically, the innervation of the cortex is influenced by the context in which it develops.     

Normalization of protein synthesis and the structure of brain dystrophic neurons after the action of hypoxia, 10% NaCl and organ-specific RNA.

It was shown previously (Polezhaev and Alexandrova, 1986) that hypoxic hypoxia causes mass (up to 30%) diffuse dystrophy of brain cortex and hippocamp neurons in rats, disturbances in the higher nervous activity, reduction of protein, RNA synthesis in neurons and of DNA synthesis in the whole brain cortex. Transplantation of embryonic nervous tissue (ENT) in one of the hemispheres normalizes all the above abnormalities observed in some neurologic and mental diseases in humans. However, transplantation may entail injuries of parenchyma and brain blood vessels. This forces researchers to search for another biological method similar by its action but safer and simpler. ENT transplantation has a dual action: 1) formation of biologically active substances (BAS) releasing from the ENT transplant and from the host brain nervous tissue upon operation; 2) establishment of synaptic connections between the transplant and host neurons. Previously we (Vitvitsky, 1987) described the isolation of BAS from rat forebrain in the form of organ-specific RNA. The latter was injected intraperitoneally several times to post-hypoxic rats in which 30 min prior to that the blood-brain barrier (BBB) was opened by injecting intravenously and intraperitoneally 10% NaCl solution without damaging the host brain. At the beginning 10% NaCl increased the destruction of brain cortical neurons and then stimulated protein synthesis in them. RNA injections stimulated the synthesis in cortical neurons and normalized their structure. Thus, we propose a safe and simple method for normalization of dystrophic neurons which can be used after certain improvement for curing neurodegenerative and neuropsychic diseases in humans.