Intracerebral Grafting Research Articles

Enhanced survival and neuronal differentiation of adrenal chromaffin cells cografted into the striatum with NGF-producing fibroblasts

Although adrenal medullary chromaffin cells have been used extensively for intracerebral grafting, their survival has generally been poor. Improved survival of the implanted cells has been achieved by exposing the chromaffin cells to NGF in vivo. Culture studies have shown, however, that chromaffin cells are converted into sympathetic neurons when NGF is included in the medium. The degree to which such a transdifferentiation may occur in vivo has not been determined. We assessed the effects of cografting chromaffin cells with primary fibroblasts genetically engineered to express NGF. Chromaffin cells from 10 d old rats were implanted with NGF-producing or beta-galactosidase-producing primary fibroblasts (control fibroblasts) into the striatum of 6-hydroxydopamine treated adult rats of the same strain. Eight weeks postgrafting, chromaffin cells cografted with NGF-producing fibroblasts displayed many of the features of mature sympathetic neurons such as large somata, long processes, transmitter vesicles similar to those found in neurons, and positive immunolabeling for the neuronal markers neurofilament, MAP2 and SCG10. Chromaffin-derived neuron number was also significantly enhanced in the presence of NGF-producing fibroblasts. While control fibroblasts were also found to increase chromaffin cell number above that of chromaffin cells grafted alone, the control fibroblasts did not induce neuronal transdifferentiation. These results demonstrate that chromaffin cells cografted with NGF-producing fibroblasts undergo transdifferentiation in vivo and express many characteristics of mature sympathetic neurons. The consequences of this transdifferentiation on the long term survival and function of the transplanted cells in vivo remain to be clarified.

Prevention of mouse-rat brain xenograft rejection by a combination therapy of cyclosporin A, prednisolone and azathioprine

Embryonic mouse hippocampal tissue was grafted as tissue blocks to the hippocampal region of adult rats and the effect of two different immunosuppressive treatments compared. Immunosuppression with cyclosporin A, prednisolone and azathioprine or with cyclosporin A alone was compared with placebo treatment. Eight weeks' postgrafting medication with cyclosporin A, prednisolone and azathioprine had resulted in survival of 14 out of 15 grafts (93%), compared with 11 out of 14 (79%) in the group treated with cyclosporin A alone. Only 2 out of 13 grafts (15%) survived in placebo-treated animals. Transplants in the trimedication group displayed distinct cell and neuropil layers and only minimal cellular infiltration by leukocyte common antigen-expressing cells, whereas grafts in cyclosporin A- and placebo-treated groups were densely infiltrated. The results are discussed in relation to the need for extended immunosuppressive and antiinflammatory therapies after intracerebral grafting of histoincompatible tissues.

Behavioral assessment of the ability of intracerebral embryonic neural tissue grafts to ameliorate the effects of brain damage in marmosets.

The transplantation of neuronal tissue into the brains of patients with Parkinson's disease is already being assessed as an experimental treatment for the symptoms of this disease, and the possibility of using similar graft tissue to ameliorate the symptoms of other neurodegenerative diseases is being considered. In this context, a small number of transplant experiments have been carried out in monkeys with lesions of the central dopamine and cholinergic systems. These experiments make it possible to determine the optimum methods of transplantation in an animal whose brain is structurally more closely related to the human than that of the rat and to assess the behavioral consequences of transplantation on symptoms that either resemble very closely the symptoms seen in patients, or are of a complex cognitive nature and are therefore more difficult to measure in the rat. It is intended that these experiments will contribute to the development of better treatments for the neurodegenerative diseases, either by the use of transplantation as a clinical treatment, or by contributing to a better understanding of the mechanisms that normally maintain neuronal function and that fail in these diseases.

Grafting of Dopaminergic Ventral Mesencephalic Slice Cultures to the Striatum of Adult Rats

Live storage of dopaminergic neurons before intracerebral grafting will allow pregrafting examination and manipulation of the cells, as well as pooling and mixing of cells from several donors. In this study we examined whether mesencephalic dopaminergic neurons, grown in organotypic cultures for 1 week, would survive subsequent grafting to the adult rat striatum. Slices of ventral mesencephalon from neonatal rats were grown by the Roller drum method for 1 week and then grafted into the striatum of adult rats, with and without preceding 6-hydroxydopamine lesions of their nigrostriatal pathway. Using immunocytochemical staining for tyrosine hydroxylase, cultured dopaminergic neurons were found to survive and to extend fibers into the host striatum when examined 4, 7, 14, 28, and 87 days after grafting. When compared with slices of noncultured mesencephalic tissue from 1-week-old rats the slice culture period did not significantly reduce the number of surviving tyrosine hydroxylase positive neurons. From this we conclude that slice cultures can be used for transient storage of dopaminergic donor tissue before intracerebral grafting. The surviving tyrosine hydroxylase positive neurons in control grafts from 1-week-old rats, furthermore, extends the time frame of possible donor ages used for grafting.

Intracerebral transplantation: basic and clinical applications to the neostriatum

Many studies have used the intracerebral transplantation technique to study the neostriatum. Most of this work has been conducted in two well-characterized animal models of striatal dysfunction: the rat model of Huntington's disease (striatal damage) and the rat model of Parkinson's disease (damage of dopaminergic nigrostriatal afferents). In animals with striatal damage, fetal striatal tissue implanted into the neostriatum (homotypic transplants) displays a remarkable anatomical and functional incorporation into the host brain. These homotypic grafts also induce a wide range of behavioral improvements in experimental animals. In contrast, fetal substantia nigra neurons implanted into the dopamine-depleted neostriatum (heterotypic transplants) generally show a more restricted integration into the host brain and elicit fewer behavioral improvements. Nonetheless, the ability of grafted fetal neurons to survive, differentiate, and partially reconstruct an appropriate and functional neurocircuitry with host systems indicates that there are factors within the adult brain that promote neuronal development and regeneration. Such results have encouraged the clinical use of intracerebral grafts for the treatment of Parkinson's disease. Recent studies have emphasized the use of genetically modified cells and neural cell lines as alternative populations to study and repair the central nervous system.

Allografts of CNS tissue possess a blood‐brain barrier: III. Neuropathological, methodological, and immunological considerations

Development of a blood-brain barrier (BBB) within mammalian CNS grafts, placed either intracerebrally or peripherally, has been controversial. Published data from this laboratory have emphasized the presence or the absence of a BBB within solid mammalian tissue or cell suspension grafts is determined intrinsically by the graft and not by the surrounding host parenchyma (e.g., brain, kidney, testis, etc.). Nevertheless, correctly interpreting whether or not a BBB exists within brain grafts is manifested by methodologies employed to answer the question and by ensuing neuropathological and immunological consequences of intracerebral grafting. The present study addresses these issues and suggests misinterpretation for the absence of a BBB in brain grafts can be attributed to: (1) rupture of interendothelial tight junctional complexes in vessels of CNS grafts fixed by perfusion of the host; (2) damage to host vessels and BBB during the intracerebral grafting procedure; (3) graft placement in proximity to inherently permeable vessels (e.g., CNS sites lying outside the BBB) supplying the subarachnoid space/pial surface and circumventricular organs such as the median eminence, area postrema, and choroid plexus; and (4) graft rejection associated with antigen presenting cells and the host immune response. The latter is prevalent in xenogeneic grafts and exists in allogeneic grafts with donor-host mismatch in the major and/or minor histocompatibility complex. CNS grafts between non-immunosuppressed outbred donor and host rats of the same strain (e.g., Sprague Dawley or Wistar rats) can be rejected by the host; these grafts exhibit populations of immunohistochemically identifiable major histocompatibility complex class I+ and class EE+ cells (microglia, macrophages, etc.) and CD4+ T-helper and CD8+ T-cytotoxic lymphocytes. PC12 cell suspension grafts placed within the CNS of non-immunosuppressed Sprague Dawley rats are rejected similarly. Donor cells from solid CNS grafts placed intracerebrally and stained immunohistochemically for donor major histocompatibility complex (MHC) class I expression are identified within the host spleen and lymph nodes; these donor MHC expressing cells may initiate the host immune response subsequent to the cells entering the general circulation through host cerebral vessels damaged during graft placement. Rapid healing of damaged cerebral vessels is stimulated with exogenously applied basic fibroblast growth factor, which may prove helpful in reducing the potential entry of donor cells to the host circulation. These results have implication clinically for the intracerebral grafting of human fetal CNS cell suspensions.

The use of adenovirus vectors for intracerebral grafting of transfected nervous cells.

Grafting genetically-modified cells into the brain is a promising approach to address fundamental and clinical issues in neurobiology. Despite recent substantial progress, most of the methods used for introducing DNA sequences into donor cells result in weak efficacy or transient gene expression after transplantation. We tested whether the use of adenovirus as the vector for foreign genes avoided these problems. A replication-defective adenovirus vector carrying a reporter gene encoding for beta-galactosidase was used to transfect primary astrocytes. After grafting into various brain structures, transfected cells exhibited robust survival and expressed the transgene for at least five months. These results demonstrate the advantage of adenovirus-mediated gene transfer for prolonged transgene expression in grafted primary cells.

Effects of adrenal medulla grafts on memory capacities of rats after hippocampal lesions

Behavioral and immunocytochemical techniques were used to study the effects of adrenal medulla grafts implanted in hippocampus—after lesion of this structure—on the memory capacities of rats. Performances of the grafted rats in the radial maze test were significantly improved and, in some aspects, fully restored. On the other hand, grafts had no significant effects on a one-trial spatial recognition test and impaired object recognition. Immunocytochemical identification showed that the grafts contained chromaffin cells with a choline acetyltransferase stainings while, in parallel, phenylethanolamine- N-methyltransferase stainings seemed to be decreased. Cholinergic innervation was established between the graft and the host hippocampus. A likely interpretation of this complex pattern of results is that the functional effects of the grafts depended on the arousal level induced by the behavioral task. At the neurobiological level, these effects probably set into play an interaction between opioid, catecholaminergic and cholinergic factors. Our results may contribute to the clarification of the problem of specificity of functional effects of intracerebral grafts as well as the problem of hippocampal role in learning and memory.

Autoantibodies against H- and M-subunits of neurofilaments are induced by PC12 cell grafts or lesions into different sites of rat brain

Since autoantibodies against neurofilaments (NF) were frequently found in neurodegenerative disorders, this work is an attempt to investigate whether the same phenomenon occurs after intracerebral grafting or lesioning. We have thus either grafted PC12 cells or injected culture medium alone into three sites of rat central nervous system (CNS): olfactory bulb (OB), olfactory anterior nucleus (OAN) and hippocampus (HC), all three sites being impaired in Alzheimer's disease. At day 15, rat sera were collected and tested against NF by western blotting. Sera from grafted rats recognized the H- and M-subunits of NF; we have then quantified the autoantibody response by using an ELISA technique. We show that, in all cases of grafts, the autoantibody response against NF significantly increased when compared to controls (normal rats without grafts or lesions) for total immunoglobulin (Ig) amount. In contrast, concerning the Ig isotypes, some differences appeared depending on the implantation site: for grafts into OB, the immune response was of both the IgG and IgM isotypes, into OAN it was mainly of the IgM isotype and into HC, the isotype of antibodies against NF was mostly IgG. In the case of lesions alone into OAN and HC, no significant enhancement of autoantibody response was observed; in contrast, lesions into OB induced an increase in autoantibody response against NF which significantly differed from controls for all Ig isotypes tested. These data point out the diversity of the autoantibody responses following lesions or grafts according to the rat brain areas.

In vivo production and release of acetylcholine from primary fibroblasts genetically modified to express choline acetyltransferase.

Primary rat fibroblasts genetically modified to express Drosophila choline acetyltransferase (dChAT) synthesize and release acetylcholine (ACh) in vitro. The ACh produced from the transduced fibroblasts was found to be enhanced by increasing amounts of choline chloride in the culture media. These dChAT-expressing cells were then implanted into the intact hippocampus of adult rats and in vivo microdialysis was performed 7-10 days after grafting to assess the ability of the cells to produce ACh and respond to exogenous choline in vivo. Samples collected from anesthetized rats revealed fourfold higher levels of ACh around dChAT grafts than from either non-grafted or control-grafted hippocampi. Localized choline infusion (200 microM) through the dialysis probes was found to induce a selective twofold increase in ACh release only from the dChAT-expressing fibroblasts. These results indicate not only that dChAT-expressing fibroblasts continue to synthesize and secrete ACh for at least 10 days after intracerebral grafting, but that the levels of ACh can be manipulated in vivo. The ability to regulate products within genetically modified cells in vivo may provide a powerful avenue for exploring the role of discrete substances within the CNS.

Sequential intracerebral transplantation of allogeneic and syngeneic fetal dopamine-rich neuronal tissue in adult rats: will the first graft be rejected?

The immune response against intracerebral grafts of allogeneic fetal dopamine-rich tissue was assessed in adult rats. Sprague-Dawley rats, now outbred, but originating from an inbred stock, were given unilateral 6-hydroxy-dopamine lesions of the mesostriatal pathway, and grafted intrastriatally with mechanically dissociated ventral mesencephalic tissue (embryonic day 13-15) obtained from an inbred Lewis strain. Graft survival was assessed by functional recovery of amphetamine-induced rotational behavior on four different occasions postsurgery, and histologically using catecholamine histofluorescence and tyrosine hydroxylase immunohistochemistry. The following groups were analysed: long-term survival of a single allogeneic graft; survival of a first allogeneic graft with a syngeneic second graft; survival of a first allograft combined with a second allogeneic graft; the survival of bilateral allogeneic grafts following a subsequent orthotopic allogeneic skin graft. Evidence for recipient immunization was obtained using an indirect fluorescent antibody detection technique, Simonsen's Spleen Index (S I) test. Viable grafts, giving rise to behavioral compensation, were present after 40 wk in rats from all groups. The "first" allograft always displayed good survival and function, even following a second intracerebral allograft. However, five of nine "second" allogeneic intracerebral grafts survived poorly. In contrast, all secondary syngeneic grafts survived well. Following the application of a subsequent orthotopic allogeneic skin graft in a subgroup of rats, there was a significantly lower survival of grafted dopamine neurons in the "first" graft.

The activity of an analogue of MPF (β-endorphin 28–31) in a rat model of Parkinson's disease

A single intrastriatal injection of a slow release formulation of a metabolically-stable MPF analogue was given to rats with lesioned right nigrostriatal pathways. After 6 weeks the turning behaviour of the rats in response to d-amphetamine began to decline, and after 12 weeks the reduction was marked and consistent. The implication of our results in the use of intracerebral grafts in parkinsonian patients is discussed.

The biology and behaviour of intracerebral adrenal transplants in animals and man.

The catecholamine containing chromaffin cells of the adrenal medulla have recently been employed as intracerebral grafts in man and animals with lesions of the nigrostriatal dopaminergic system. This review outlines the basic biology of the chromaffin cell with reference to its efficacy as a source of dopamine in the grafted state. This is followed by an evaluation of the use of these grafts in experimentally lesioned animals and in patients with Parkinson's disease.

1207 Intracerebral grafting of genetically modified cells to produce basic fibroblast growth factor

1207 Intracerebral grafting of genetically modified cells to produce basic fibroblast growth factor

Amphetamine-induced rotational behavior in rats: relationship to hypothalamic and striatal degeneration.

When lesions are placed unilaterally in the nigrostriatal system of experimental animals, rotational behavior occurs in response to peripheral administration of dopamine (DA) agonists. In spite of considerable evidence to the contrary, it is assumed that in order for this rotation to occur, an almost complete depletion of striatal DA must be achieved. To test this hypothesis further, 20 male Sprague-Dawley rats were injected unilaterally with 2 microL of 8 micrograms/microL of 6-hydroxydopamine (6-OHDA) via acute injection needles or chronically indwelling cannulae. Acute injection of 6-OHDA resulted in a rotation rate of 7.2 to 18.9 revolutions per minute in response to peripheral amphetamine injection (5 mg/Kg) while injection of 6-OHDA through chronically indwelling cannulae produced rotation ranging from 1.4 to 9.9 rotations per minute. Under the conditions of either method of injection, the animals displaying the most severe rotation still showed partial denervation of striatal DA as revealed by catecholamine fluorescence histochemistry. Conversely, numerous animals demonstrating very low rates of amphetamine-induced rotation often displayed a complete loss of striatal, accumbens, and olfactory tubercle catecholamine fluorescence. Moreover, large quantities of lateral hypothalamic amine accumulation were observed in rotating rats indicating that this neurochemical change may be of functional significance for rotational responses. The present results, when taken into consideration with previous work, indicate that the routine selection of rotating animals for pharmacological testing for potential antiParkinsonian medication or intracerebral grafting purely on the basis of their rotational behavior does not necessarily imply that complete striatal denervation has occurred. Moreover, these findings demonstrate that amine accumulation in the lateral hypothalamus of rotating animals with DA depleting lesions is an important phenomenon implicated in the expression of rotational behavior in animals and possibly in the pathophysiology of Parkinson's disease.

Somatic gene transfer of nerve growth factor promotes the survival of axotomized septal neurons and the regeneration of their axons in adult rats

Intracerebral grafts consisting of primary fibroblasts genetically engineered to express NGF were used to assess the regenerative capacity of cholinergic neurons of the adult rat septum. Our data reveal that NGF-producing grafts sustain a significantly higher proportion of NGF receptor-immunoreactive septal neurons following axotomy (approximately 65-75%) than do grafts of noninfected fibroblasts. In addition, NGF promotes the regeneration of septal axons. Following the ablation of cholinergic septal projections to the hippocampus, NGF-producing grafts placed within the lesion cavity contain large numbers of AChE-positive axons; control grafts, on the other hand, lack such cholinergic axons. Ultrastructural examination reveals that unmyelinated axons within NGF-producing grafts use many different substrates for growth, including astrocytes and components of the extracellular matrix. Grafts of control fibroblasts possess the same cellular and matrix substrates but contain only a small population of axons, probably of peripheral origin. AChE-positive axons growing through NGF-producing grafts provide a new topographically organized input to the deafferented hippocampal dentate gyrus. Furthermore, regenerating septal axons terminate predominantly on the dendritic processes of granular neurons. The dentate gyrus ipsilateral to grafts of noninfected fibroblasts, on the other hand, remains devoid of AChE-positive fibers. From these results, we conclude that the availability of NGF is a necessary requirement to sustain axotomized cholinergic septal neurons and to promote axon regeneration and cholinergic reinnervation of dentate granular neurons by these lesioned neurons. The presence of many permissive substrates (e.g., astrocytes, basal lamina, and collagen) alone, however, is not sufficient to induce axon regrowth from adult septal neurons.

Cryopreservation of embryonic cerebral tissue of rat

Embryonic cerebral tissues (ECT) either fresh or frozen-stored, were cultured and transplanted into the cerebella of neonatal host rats. Many variables including composition of the freezing medium, freezing and thawing rates, and storage time in liquid nitrogen were studied systematically. The results indicated that the following conditions yielded good results for tissue culture: using 1 M Me 2SO as the cryoprotectant, freezing the brain tissues at a rate of 1 °C/min until it reached −70 °C, storing the frozen samples in liquid nitrogen and thawing them fast in a 37 °C water bath. The viability of the frozen-thawed tissues was assessed by their abilities to grow and differentiate in vitro and in vivo after intracerebral grafting. In tissue culture, growth and differentiation were similar to those of the fresh ECT. Cell morphology and staining reactions were normal in supravital methylene blue staining, cresyl violet staining, and acetylcholinesterase staining. Neurons had well-developed Nissl bodies, and cholinergic neurons also differentiated. Autoradiographic studies showed that more than 50% of the neurons had the ability to uptake γ-aminobutyric acid with high affinity. In brain tissue transplantation, 9 of 12 transplants survived subsequent grafting after cryopreservation. Moreover, the grafts of surviving cryopreserved tissue displayed cytological and cytoarchitectural characteristics identical to those of fresh grafts. All grafts were integrated with the surrounding host neural tissue. This suggested that there may be synaptic connections between the transplants and the host brain tissues. From this and similar studies on the subject by others we conclude that cryopreservation is a feasible method for storage of embryonic brain tissue to be used later for intracerebral grafting.

Morphological and neurochemical features of cultured primary skin fibroblasts of Fischer 344 rats following striatal implantation.

In order to assess the feasibility of using primary skin fibroblasts as a donor population for genetic modification and subsequent intracerebral grafting, the present study examines the structural and neurochemical characteristics of intrastriatal grafts of isogeneic primary fibroblasts over a period of 6 months. In culture, primary skin fibroblasts obtained from a female Fischer 344 rat display robust growth, but once confluent these cells exhibit contact inhibition. Following the implantation of cultured primary cells within the striatum of other adult rats from the same inbred strain, isologous grafts stain immunohistochemically for fibronectin at 1 week, and this immunostaining persists up to 6 months. Immunoreactivity for laminin is intense within the grafts from 1 to 8 weeks, but decreases by 6 months. Astrocytes within the striatum respond dramatically to the implantation of primary fibroblasts, such that immunohistochemical staining for glial fibrillary acidic protein increases markedly from 1 to 8 weeks after implantation. Although the intensity of immunostaining for glial fibrillary acidic protein diminishes among striatal astrocytes between 8 weeks and 6 months, the astrocytic border between the grafts and striatal neuropil remains intensely immunoreactive. Capillaries within the grafts stain immunohistochemically for glucose transporter (a facilitated glucose uptake carrier) as early as 3 weeks after implantation. Following intravenous infusions of peroxidase, capillaries within fibroblast grafts do not permit the extravasation of this macromolecule at 8 weeks and 6 months. Thus, capillaries formed within intracerebral grafts of primary skin fibroblasts exhibit a functional impermeable barrier to macromolecules similar to those capillaries of the host striatum. At the ultrastructural level, grafts possess numerous fibroblasts and have an extracellular matrix filled with collagen. Reactive astrocytic processes filled with intermediate filaments are found throughout the grafts. Hypertrophied astrocytes and their processes also appear to form a continuous border between the grafts and striatal neuropil. Grafts of primary fibroblasts also possess an extensive vasculature that is composed of capillaries with nonfenestrated endothelial cells; the occurrence of reactive astrocytic processes closely associated with or enveloping capillaries is variable. These results provide direct morphological and neurochemical evidence for the long-term survival of isologous fibroblasts after implantation within the rat striatum. From these data, we propose that isologous skin fibroblasts can be considered as donor candidates for successful intracerebral grafting following gene transfer.

Factors influencing structure and function of intracerebral grafts in the mammalian brain: a review

After twenty years of intensive research, the possibility to induce recovery from various disorders in brain damaged mammals by means of intracerebral grafts of fetal CNS tissue is well documented and largely accepted by the scientific community. However, there are several reports on animal research suggesting that intracerebral grafts may fail to induce the expected recovery after brain injury or even that they may cause deficits which are actually more pronounced than those induced by the lesions alone. In addition, attempts to produce functional benefits with catecholamine-releasing tissue grafts in the brain of Parkinsonian patients have given limited and variable results; graft-induced deleterious effects have also been occasionally reported in a few clinical cases. One way to progress towards a better understanding of such disappointing, although informative, discrepancies between successful and less successful experimental studies and clinical trials would be to consider that there are several factors which may influence, in one direction or the other, the survival, development, integration and functional expression of intracerebral fetal CNS grafts. The present review considers the following factors: (i) some of the technical factors such as the constraints of transplantation surgery, the origin of donor tissue, the implantation site, the age of both the donor and the recipient, and tissue manipulations prior to grafting (i.e., cryopreservation, culture, genetic modification); (ii) exogenous and endogenous neurotrophic factors, the latter being distinguished by whether they may be host- or graft-derived; (iii) immunological factors (from the particular immunological status of the brain to some effects of immunosuppression in the case of xenografting)', (iv) pharmacological factors, with a particular focus on experimental data suggesting that administration of drugs may or might contribute to elicit, enhance or block some functional effects of grafts. It is concluded that all these factors may become simultaneously operative and interacting, thereby presiding over the functional outcome of intracerebral grafting in both experimental research and clinical trials.

Chapter 16: Intracerebral grafting of solid tissues and cell suspensions: the blood-brain barrier and host immune response

Publisher Summary Neither the immunological privilege of the central nervous system (CNS) nor the blood–brain barrier (BBB) is absolute. This belief is exemplified by solid tissue and cell suspension grafts placed intracerebrally. With the exception of homografts, grafts allogeneic or xenogeneic to the adult host are likely to be the target of a host immune response in the absence of immunosuppression. Whether a BBB or a “mini” circumventricular organ permeable to blood-borne protein develops within an intracerebral graft is determined intrinsically by the grafted tissue/cells and not by the surrounding host CNS. These statements affect upon experimental and clinical intracerebral grafting of solid tissues and cells of peripheral or CNS origin. The presence or absence of a BBB within solid tissue or cell suspensions grafted intracerebrally is determined by the graft and not by the surrounding host parenchyma. Solid CNS grafts are supplied with a mosaic of host and donor vessels, whereas solid peripheral tissue grafts are supplied solely with donor vessels. Homogeneous cell suspension grafts necessarily are supplied with host BBB vessels that may surrender their BBB properties, depending upon the type of cell suspension graft.