Fetal Brain Transplants Research Articles

Characterization of neural cell lines derived from SV40 large T-induced primitive neuroectodermal tumors.

We recently reported intriguing properties of neural tumors generated by retrovirus-mediated transfer of the SV40 large T antigen into fetal rat brain transplants. Histopathologically, these neoplasms displayed characteristic features of primitive neuroectodermal tumors (PNET) and exhibited a striking potential for migration into the host brain. In the present study, four cell lines were derived from these PNETs and characterized. Two lines with an immature phenotype expressed the embryonal form of the neural cell adhesion molecule and nestin. They showed spheroid formation and delicate cell processes. The remaining cell lines had a flat, epitheloid appearance and were immunoreactive for synaptophysin, neurofilament proteins and glial fibrillary acidic protein. These cells constitute valuable tools to study the cellular origin(s) and molecular basis of PNETs, differentiation of neural progenitors and tumor cell migration in the brain.

Plaque formation in brain transplants exposed to human beta-amyloid precursor protein 695.

Abnormal processing of beta-amyloid precursor protein (APP) and precipitation of amyloidogenic beta A4 peptide have been implicated in the pathogenesis of Alzheimer's disease (AD). In an attempt to generate an in vivo model of APP-associated pathology, we have introduced recombinant retroviral vectors harboring normal and mutant cDNAs for human neuron-specific APP 695 into fetal rat brain transplants. A minor population of recombinant neural cells with expression of APP was identified immunohistochemically in the grafts. The expression was maintained during an observation period of 9 months. Neurons with strong immunoreactivity for human APP exhibited markedly swollen perikarya and showed signs of cell degeneration. At 6 months post-transplantation, recombinant grafts developed APP-positive neuropil deposits with morphological features of neuritic plaques, but without beta A4 peptide immunoreactivity. No difference was observed between wild-type APP and the mutant APP construct derived from a family with autosomal dominant AD. These observations indicate that long-term neuronal overexpression of human APP has the potential to generate APP plaques in the rodent brain.

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.

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.

Impairment in memory function and neurodegenerative changes in the cholinergic basal forebrain system induced by chronic intake of ethanol.

Chronic intake of ethanol both in human and rat results in a substantial impairment in memory function associated with a reduction in the number of cholinergic neurons in the basal forebrain which give rise to the cholinergic afferentation of the cortical mantle. Degenerative changes in the basal forebrain are paralleled by the concomitant reduction of presynaptic cholinergic markers (synthesis, content and release of acetylcholine) in the neocortex and hippocampus. Cognitive dysfunction in rat induced by ethanol treatment can be ameliorated by the pharmacological manipulation of central cholinergic neurotransmission by physostigmine, arecoline or nicotine. Furthermore, fetal brain transplants rich in cholinergic neurons are able to restore cognitive function which argues in favour of a cholinergic aspect of alcohol-induced behavioural dysfunction. The observation, that implantation of purified astrocytes results in a similar restoration of learning and memory abilities associated with a recovery of cholinergic function, further indicates that behavioural sequelae of alcohol intake can largely be ameliorated by a trophic stimulation directed towards the cholinergic basal forebrain system. Regarding the cholinergic basal forebrain system as a component of the ascending reticular activation system, the involvement of the cholinergic afferentation of the cortical mantle in the mediation of memory processes and their dysfunction under neurodegenerative conditions can be explained on the basis of the "Hippocampal Memory Indexing Theory" of Teyler and DiScenna. The hypothesis is formulated in the present paper that mental dysfunction observed after chronic ethanol consumption can largely be attributed to a degeneration of the cholinergic pathway of the ascending activation system resulting in an impairment of cortical activation, clinically appearing as the "syndrome of partial cholinergic deafferentation of the cortical mantle".

Tumor induction by ras and myc oncogenes in fetal and neonatal brain: modulating effects of developmental stage and retroviral dose.

Introduction into fetal rat brain cells of a replication-defective retroviral vector harboring v-Ha-ras and v-gag-myc rapidly causes the induction of highly malignant undifferentiated neuroectodermal tumors following transplantation into the brains of syngeneic hosts [Wiestler, et al. (1992) Cancer Res. 52: 3760-3767]. In the present study, we have investigated the modulating effect of the developmental stage of neural target cells and of the dose of the retroviral vector used in the grafting experiments. Exposure of fetal cells from embryonic day (E)12 or E14 produced a 100% incidence of malignant neuroectodermal tumors which led to the death of recipient animals after a median latency period of 32 days. A 100-fold reduction of the virus dose from 2.062 x 10(6) to 2.062 x 10(4) focus-forming units/ml resulted in a lower tumor incidence of 25%. Of six neural grafts exposed to v-Ha-ras and v-myc at E16, only one showed evidence of tumorigenesis (low-grade astrocytoma and hemangioma). All other transplants were morphologically normal for observation periods of 26 weeks, indicating a marked loss of transforming activity of ras and myc in more advanced stages of brain development. In retrovirus-exposed donor cells which caused the development of neural tumors in recipient rats, malignant transformation was also evident during culture in vitro, usually after 9-12 days. Oncogene complementation was also studied in the newborn rat brain. After microinjection of the retroviral vector into the brain at postnatal day (P)0, P1 and P3, 5 out of 20 animals (25%) developed a total of seven brain tumors. Histopathologically, three of these neoplasms were malignant neuroectodermal tumors which, in contrast to those induced in fetal brain transplants showed evidence of focal glial and/or neuronal differentiation. In addition, we observed one oligodendroglioma, two hemangiomas and a malignant hemangioendothelioma. These data indicate that neural precursor cells and endothelia of the rat brain represent the major target cells for the complementary action of ras and myc and that the use of target cells from later developmental stages (E16 and postnatal) leads to the induction of both primitive and more differentiated neoplasms.

Complementary tumor induction in neural grafts exposed to N-ethyl-N-nitrosourea and an activated myc gene.

Using a combination of transplacental carcinogen exposure and retrovirus-mediated oncogene transfer into fetal brain transplants, we have studied complementary transformation by N-ethyl-N-nitrosourea (NEU) and the v-myc oncogene in the nervous system. Previous experiments had demonstrated that both agents will not induce tumors independently whereas simultaneous expression of v-H-ras and v-gag/myc exerted a powerful transforming potential in neural grafts. In order to identify other genetic alterations that co-operate with an activated myc gene, the neurotropic carcinogen NEU was used to generate mutations of cellular genes. On embryonic day 14 (ED14), pregnant donor animals (F344 rats) received a single i.v. dose of NEU (50 mg/kg). Twenty-four hours later (ED15), the fetal brains were removed, triturated and incubated with a retroviral vector carrying the v-gag/myc oncogene. Subsequently, these primary cell suspensions were transplanted stereotactically into the caudate-putamen of syngenic adult recipients. After latency periods of 3-6 months, 5 of 10 recipients harboring ED15 fetal brain transplants developed malignant, poorly differentiated neuroectodermal tumors in the grafts. No tumor development was observed in seven recipients harboring ED16 neural grafts. Cell lines were established from three tumors and the 110 kd gag/myc fusion protein encoded by the retroviral construct was identified in the tumors by Western blotting. Several candidate genes for mutational activation by NEU including the H-ras, K-ras and neu oncogenes were analyzed for specific point mutations by polymerase chain reaction (PCR) and direct DNA sequencing of the PCR products. However, no mutations were found in any of these genes. These findings lend further support to the multistep hypothesis of neoplastic transformation in the brain. The tumors induced in this model provide an interesting tool for the identification of genes that co-operate with an activated myc gene in neurocarcinogenesis.

A new approach to the molecular basis of neoplastic transformation in the brain.

Gene transfer into living organisms has evolved as a powerful approach to study in vivo effects of specific genes and to devise animal models of hereditary disorders. We have been particularly interested in an approach to introducing transforming genes into the nervous system. Since specific promoter sequences for targeting the expression of a transgene to many cell types of the brain are not yet isolated, a suitable transgenic mouse model was not available for these experiments. This has prompted us to develop an alternative strategy for gene transfer into the brain. The rationale is to introduce foreign genes into fetal brain transplants using embryonic CNS as donor tissue and replication-defective retroviral vectors as genetic vehicles. This technique relies on the extraordinary organotypic differentiation capacity of neural grafts and the expression of retrovirally transmitted genes in different cell types of CNS transplants. In contrast to transgenic animals but analogous to sporadic tumour formation, target cells for the retroviral vector will develop in an environment of unmodified neural tissue. We have introduced a number of neurotropic oncogenes into fetal brain transplants to study potential effects of such genes on the brain. This review will summarize some of the findings which have emerged from this experimental study including the tropism of several genes for endothelial cells, attempts to identify cooperating combinations of transforming genes and an experimental model for primitive neuroectodermal tumours in neural grafts.

Reversal of reproductive deficiency in the hpg male mouse by neonatal androgenization.

Some aspects of reproductive function in the GnRH-deficient hypogonadal (hpg) mutant mouse can be restored by transplanting normal fetal brain tissue containing GnRH cells into the central nervous system of adult hpg mice. However, hpg males showing physiological response to the graft fail to display sexual behavior and are infertile. We hypothesized that the reproductive deficit of these males is due to insufficient perinatal exposure to testicular androgens as a consequence of the GnRH deficiency. To test this hypothesis we androgenized hpg males by giving them neonatal injections of testosterone propionate (TP). Controls consisted of hpg males not androgenized neonatally and of normal males. All three groups received a TP implant in adulthood, and their copulatory behavior and reproductive capability were recorded. In addition, other hpg males, not androgenized neonatally, received fetal brain transplants containing GnRH neurons and were also tested for copulatory behavior and reproductive capability before and after receiving a TP implant. Three of 8 neonatally androgenized hpg males expressed the full repertoire of male sexual behavior, including intromission and ejaculation, and sired several litters. Three of 7 control hpg males that were not androgenized neonatally but received TP implants in adulthood also displayed mounting and intromission, but there was no evidence of ejaculation, and these males failed to impregnate normal females. Of the 8 hpg males that responded to a fetal transplant with testicular growth, only 1 displayed mounting behavior. However, when given a TP implant, 4 of 8 hpg males with grafts displayed mounting and intromissions.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of transplantation of fetal hippocampal or hindbrain tissue into the brains of adult rats with hippocampal lesions on water maze acquisition.

Rats were given bilateral aspiration lesions of the hippocampus. Some of these rats then received bilateral transplants of fetal hippocampal or dorsal ventricular ridge tissue that was dissected from embryonic rat brains at 16 or 17 days of gestation. The remaining rats with hippocampal lesions did not receive fetal brain transplants. Rats with neocortical aspiration lesions, but without transplants, and rats without brain damage were also included in the study. All of the rats were trained to find a submerged platform in a Morris water maze. Rats with the fetal brain transplants were more impaired in some measures of maze learning than were rats with hippocampal lesions only. The results indicate that transplants of fetal brain tissue are not always associated with recovery of behavioral function after brain damage and may even increase a lesion-induced behavioral impairment in tasks that require complex cognitive functioning.

Time-dependent recovery of taste aversion learning by fetal brain transplants in gustatory neocortex-lesioned rats

We recently showed that fetal brain transplants produced a significant recovery in the ability of gustatory neocortex-lesioned rats to learn a conditioned taste aversion. In this report we assessed the capability of gustatory neocortex fetal brain transplants to produce behavioral recovery at different times. Four groups of male Wistar rats showing disrupted taste aversions due to gustatory neocortex lesions were studied. The lesioned animals received fetal cortical grafts, obtained from 16-day-old fetuses, and were retrained in the behavioral procedure after 15, 30, 45, or 60 days postgraft. Behavioral results showed a very good functional recuperation at 60 days, slight recovery at 45 and 30 days, and a poor recovery at 15 days postgraft. Results with HRP histochemistry revealed that at 30, 45, and 60 days postgrafting there were increased connections with the ventromedial nucleus of the thalamus and with the amygdala. At 15 days postgrafting there was an absence of HRP-labeled cells. In addition, behavioral recovery was correlated with increased acetylcholinesterase activity, detected histochemically, and with morphological neuronal maturation, revealed by Golgi staining. These results suggest that morphological maturity and reconnectivity between grafts and host tissue are important for behavioral recovery in gustatory neocortex-lesioned rats.

Retrovirus‐Mediated Oncogene Transfer into Neural Transplants

A gene transfer model was developed which allows for the identification of transformation pathways in the developing nervous system. Transforming genes were introduced into fetal brain transplants using embryonic CNS as donor tissue and replication-defective retroviral vectors as genetic vehicles. This technique relies on the extraordinary organotypic differentiation capacity of neural grafts and the expression of retrovirally transmitted genes in various cell types of CNS transplants. In contrast to transgenic animals but analogous to sporadic tumor formation, target cells for the retroviral vector develop in an environment of unmodified neural tissue. We have introduced a number of neurotropic oncogenes into fetal brain transplants including genes with an associated tyrosine kinase activity (polyoma medium T, v-src), a novel member of the fibroblast growth factor (fgf) gene family and the SV40 large T antigen. These experiments have demonstrated a significant transformation potential of oncogenes in specific target cells of the brain, provided evidence for a dominant complementary transforming effect of simultaneously expressed ras and myc genes in neural precursor cells and have yielded intriguing model systems for human CNS neoplasms such as the cerebellar medulloblastoma. This review describes the transplantation model, demonstrates several striking phenotypes induced by oncogene expression in neural grafts and elaborates on future prospects of this experimental approach.

Fetal brain transplants induce recovery of male sexual behavior in medial preoptic area-lesioned rats

Male rats received bilateral lesions within the medial preoptic area which completely abolished sexual behavior. Hypothalamic fetal brain transplants gradually restored sexual behavior to prelesion levels by the 6th week after the transplant. Immunocytochemical analyses revealed tyrosine hydroxylase immunoreactivity neurons within the transplanted tissue. These results demonstrate that fetal brain transplants can restore an innate complex behavior in which no spontaneous recovery is observed.

Cellular and Molecular Aspects of Neurocarcinogenesis

Although the morphology of neural tumors induced in rats by N-ethyl-N-nitrosourea (NEU) and related alkylating agents has been extensively investigated, their histogenesis and the molecular basis of malignant transformation are still largely unknown. This review gives an account of the interaction of neurocarcinogenic agents with cellular DNA, the possible role of promutagenic O6-alkyldeoxyguanine and their deficient repair by the cerebral O6-alkylguanine-DNA alkyltransferase. A new experimental model is described in which neural tumors are induced in fetal brain transplants. Pregnant rats received a single iv dose of NEU (50 mg/kg) on the 14th day of gestation. One day later, suspensions were prepared from the fetal forebrain and stereotactically injected into the caudoputamen of adult rats. After additional exposure to NEU of the host animals 8 days and 9 weeks post transplantation, all rats developed brain tumors within the neural graft. Histopathologically, all neoplasms were classified as olidogdendrogliomas. Other neoplasms typically induced by NEU transplacentally (astrocytomas, mixed gliomas, ependymomas) were absent. The selective induction of oligodendrogliomas indicates that neoplastic transformation in the nervous system can occur in a differentiated glial cell or a precursor cell committed to oligodendrocytic differentiation, and that transformation of a pluripotential stem cell is not necessary. Transplacental exposure of the donor fetuses to NEU alone, i.e., without additional postgrafting exposure, did not produce brain tumors in any of the experimental animals indicating that in the microenvironment of fetal brain transplants the multistep development of gliomas requires additional mutational events. Malignant schwanomas perinatally induced by NEU carry a point mutation in the transmembrane domain of the neu gene. The mode of oncogene activation in NEU-induced CNS gliomas has not yet been elucidated. We have used cerebral grafting techniques to study the effects of known oncogenes on the developing nervous system, taking advantage of efficient gene transfer by replication-defective retroviral vectors and of the extraordinary capacity of fetal CNS to differentiate in and fully integrate with the host brain. Rats carrying transplants exposed in vitro to the polyoma medium T-antigen developed endothelial hemangiomas in the graft which often led to fatal cerebral hemorrhage within 13-50 days after transplantation. Introduction of the viral src gene caused astrocytic and mesenchymal tumors after latency periods of 2-6 months. Following infection of fetal donor cells with a vector encoding the v-myc oncogene, only a single embryonal CNS tumor was observed whereas exposure to v-H-ras produced a low incidence of gliomas.(ABSTRACT TRUNCATED AT 400 WORDS)

Noradrenergic innervation does not affect chronic regulation of [ 125I]pindolol receptors in fetal rat brain transplants or host neocortex

Fetal (E15–16) somatosensory cortex ( n = 15) or cerebellum ( n = 9) were placed into the somatosensory cortex (SmI) of adult rat hosts to study the relative importance of tissue origin versus host milieu on graft β-adrenoceptor regulation. Autoradiographic studies of [ 125I]pindolol ([ 125I]pin) binding in the presence of 3 μM serotonin were performed as an index of β-receptor binding in both intact hosts and those with ipsilateral locus coeruleus (LC) lesions and/or ipsilateral superior cervical ganglionectomy. [ 125I]pin binding within fetal grafts was highly variable with areas of highest specific binding in cortical grafts (( K d = 209 ± 30pM, B max = 106 ± 7fmol/mg protein) being comparable to host cortex ( K d = 211 ± 41pM, B max = 111 ± 9fmol/mg protein). Average total binding in whole cortical grafts was 73% and in cerebellar grafts was 60% of that in comparable adult cortex. Host cortex had 66–73% and cerebellum had 4–8%β 1-receptors while cortical grafts had 59% and cerebellar grafts had 43%β 1-receptors as determined by competitive binding with ICI 89406 and 118551. Noradrenergic fibers derived from both the host LC and superior cervical ganglion grew into fetal cortical grafts. Binding to high affinity uptake sites ([ 3H]desmethylimipramine, [ 3H]DMI) on noradrenergic terminals in cerebellar grafts was 28% higher than that in cortical grafts; superior cervical ganglionectomy decreased [ 3H]DMI binding in cortical grafts by 37% but had no effect on cerebellar grafts. Neither ganglionectomy nor LC lesions affected total specific binding or binding to β-receptor subtypes in the grafts or host cortex 3–6 months after removal. Therefore, anatomic site of origin appeared to be the predominant factor in determining the development of β-adrenoceptors in fetal cortical tissue. In ectopically placed cerebellar grafts, β-receptor subtypes did not develop comparably to host cerebellar receptors suggesting that host milieu may be of critical importance in receptor development in this tissue.

Fetal brain grafts induce recovery of learning deficits and connectivity in rats with gustatory neocortex lesion

Three groups of rats showing disrupted taste aversion due to gustatory neocortex lesions, were studied. One group received a transplant of homotopic cortical tissue, another of heterotopic tectal tissue, obtained from 17-day-old fetuses. The third group remained without transplant as a lesioned control group. Comparisons of the taste aversion scores before and after graft, revealed that cortical grafted animals significantly improved the taste aversion, whereas those which received tectal grafts, and the cortical-lesioned controls did not. Moreover, results with horseradish peroxidase (HRP) histochemistry revealed that the homotopic, but not the heterotopic, brain transplants were able to re-establish connections with amygdala and with the ventromedial nucleus of the thalamus areas who normally kept connectivity with the gustatory neocortex. These results support the hypothesis that fetal brain transplants can re-establish cognitive functions, as well as connectivity with its host tissue.

The Use of Bromodeoxyuridine‐Immunohistochemistry to Identify Transplanted Fetal Brain Tissue

The immunohistochemical detection of the thymidine analog, 5-bromo-2′-deoxyuridine (BrdU) is shown to be a useful and reliable method to positively identify fetal brain transplants in standard histological preparations. This technique offers several advantages over the [3H] thymidine autoradiographic method, including being much more rapid and avoiding the use of radionuclides.

Chapter 4 Fetal brain tissue transplants and recovery of locomotion following damage to sensorimotor cortex in rats

Publisher Summary Fetal brain tissue transplants have been shown to be effective in reducing motor impairments accompanying nigrostriatal damage. This chapter examines whether fetal brain transplants would be effective in reducing the motor impairment that occurs following sensorimotor cortex ablations. The chapter discusses experiments with following objective: (1) can fetal brain transplants facilitate recovery of locomotion following damage to sensorimotor cortex, and (2) will the combination of fetal brain transplants and G MI ganglioside treatments enhance recovery of locomotion following sensorimotor cortex damage. Based on the experimental results, it appears that fetal brain tissue transplants from different sites or of different fetal ages fail to produce a demonstrable enhancement of recovery after bilateral damage to sensorimotor cortex. The specific location and type of cerebral injury may be a factor to consider in deciding upon, whether to employ transplants to promote recovery from traumatic injury. The effects of the combined treatment of G MI gangliosides and fetal brain transplants are different from either of these treatments alone. These findings indicate that limited functional recovery can be enhanced by combining transplants with systemic injections of putative neurotrophic factors.

Fetal brain transplants induce recuperation of taste aversion learning

Rats showing disrupted taste aversion due to gustatory neocortex or amygdala lesions were transplanted into the lesioned area with homologous brain tissue obtained from 17-day-old fetuses. Comparisons of taste aversions scores before and after the graft, revealed that the grafted animals significantly recuperated taste aversions, whereaa cortical lesioned animals without grafts did not. Surprisingly, however, amygdala-lesioned animals without graft presented spontaneous recovery. These results not only support the hypothesis that fetal brain transplants can restore cognitive functions, but also that there are some fundamental functional differences between the gustatory neocortex and the amygdala in the regulation of the processes involved in the acquisition and retention of taste aversion.

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.