Xenografts of mouse hippocampal tissue. Formation of nerve connections between the graft fascia dentata and the host rat brain

The dentate mossy fibers: structural organization, development and plasticity

Hippocampal mossy fibers are the axons of the dentate granule cells and project to hippocampal CA3 pyramidal cells and mossy cells of the dentate hilus (CA4) as well as a number of interneurons in the two areas. Besides their role in hippocampal function, studies of which are still evolving and taking interesting turns, the mossy fibers display a number of unique features with regard to axonal projections, terminal structures and synaptic contacts, development and variations among species and strains, as well as to normal occurring and lesion-induced plasticity and neural transplantation. These features are the topic of this review, which will use the mossy fiber system of the rat as basis and reference in its aim to provide an up-to-date, yet historically based guide to students in the field.

Different primary target cells are important for fiber lamination in the fascia dentata: a lesson from reeler mutant mice

The factors determining the lamina-specific termination of entorhinal and commissural afferents to the fascia dentata are poorly understood. Recently it was shown that early generated Cajal-Retzius (CR) cells in the outer molecular layer and reelin, synthesized by CR cells, play a role in the lamina-specific termination of entorhinal fibers which form transient synapses with CR cells before establishing their definite contacts with granule cell dendrites (J. A. del Rio et al., 1997, Nature 385, 70-74). By using anterograde tracing with Phaseolus vulgaris leukoagglutinin we show that the normal, sharply delineated entorhinal projection to the outer molecular layer is retained in reeler mutant mice lacking reelin. This coincides with the regular presence of CR cells, the primary, transient target cells of entorhinal fibers. In contrast, the commissural fibers were found to terminate in an abnormal broad, not clearly defined area. This widespread projection coincides with the distribution of granule cells which in the mutant do not form a dense cell layer but are scattered all over the hilus due to a migration defect. Unlike the entorhinal fibers, the commissural fibers arrive in their target layer late in development, when granule cell dendrites are already there. We hypothesize from these results that the presence of the adequate postsynaptic element at the time of fiber ingrowth, CR cells for the early ingrowing entorhinal fibers and granule cells for the late-arriving commissural fibers, is crucial for the normal formation of these layer-specific projections.

The hippocampus of the reeler mutant mouse: fiber segregation in area CA1 depends on the position of the postsynaptic target cells

Area CA1 of the rodent hippocampus is characterized by a highly lamina-specific and nonoverlapping termination of afferent fiber tracts. Entorhinal fibers terminate in stratum lacunosum-moleculare and commissural/associational fibers terminate in strata radiatum and oriens. It has been hypothesized that this fiber lamination depends on specific signals for the different afferent fiber tracts that are located on distinct dendritic segments of the postsynaptic neuron. In order to test this hypothesis, entorhinal and commissural/associational afferents to Ammon's horn were investigated in the adult reeler mutant mouse, in which developmental cell migration defects have disrupted the normal array of cells. Golgi staining revealed a deep and a superficial principal cell layer in the mutant. The morphology of the cells located in the deep pyramidal cell layer was considerably abnormal, whereas most cells located in the superficial pyramidal cell layer showed an almost normal cellular and dendritic morphology. Anterograde tracing with Phaseolus vulgaris leukoagglutinin revealed that the duplication and disorganization of the pyramidal cell layer in area CA1 are mirrored by the duplication and disruption of afferent fiber termination zones. In the zone above the abnormal deep pyramidal cell layer, i.e., between the two cell layers, entorhinal fibers as well as commissural/associational fibers terminate and intermingle. In contrast, in the zone above the fairly normal superficial pyramidal cell layer, entorhinal and commissural/associational fibers retain their normal fiber segregation. These data suggest that the normal laminar organization of the murine hippocampus depends on positional cues presented by their target cells.

Development of fetal hippocampal grafts in intact and lesioned hippocampus

Functional recovery observed in Parkinson's disease patients following grafting of fetal substantia nigra has encouraged the development of similar grafting therapy for other neurological disorders. Fetal hippocampal grafting paradigms are of considerable significance because of their potential to treat neurological disorders affecting primarily hippocampus, including temporal lobe epilepsy, cerebral ischemia, stroke, and head injury. Since many recent studies of hippocampal transplants were carried out with an aim of laying the foundation for future clinical applications, an overview of the development of fetal hippocampal transplants, and their capability for inducing functional recovery under different host conditions is timely. In this review, we will summarize recent developments in hippocampal transplants, especially the anatomical and/or functional integration of grafts within the host brain under specific host conditions, including a comparison of intact hippocampus with various types of hippocampal lesions or injury. Improvements in grafting techniques, methods for analysis of graft integration and graft function will be summarized, in addition to critical factors which enhance the survival and integration of grafted cells and alternative sources of donor cells currently being tested or considered for hippocampal transplantation. Viewed collectively, hippocampal grafting studies show that fetal hippocampal tissue/cells survive grafting, establish both afferent and efferent connections with the host brain, and are also capable of ameliorating certain learning and memory deficits in some models. However, the efficacy of intracerebral fetal hippocampal grafts varies considerably in different animal models, depending on several factors: the mode of donor tissue preparation, the method of grafting, the state of host hippocampus at the time of grafting, and the placement of grafts within the hippocampus. Functional improvement in many models appeared to be caused partially by re-establishment of damaged circuitry and partially by a trophic action of grafts. However, exact mechanisms of graft-mediated behavioral recovery remain to be clarified due to the lack of correlative analysis in the same animal between the degree of graft integration and behavioral recovery. Issues of mechanisms of action, degree of restoration of host circuitry and amelioration of host pathological conditions will need to be sorted out clearly prior to clinical use of fetal hippocampal transplants for susceptible neurological conditions.

Neural xenotransplantation: reconstruction of neuronal circuitry across species barriers

Selective replacement of degenerated neurons in the adult brain with allogeneic fetal neuroblasts is a promising therapeutic modality for human neurodegenerative diseases, but is confounded with practical and potential ethical problems. To evaluate the potential of xenogeneic donors as a cell source for neural transplantation, we have critically examined the available experimental evidence in animal models pertaining to the survival, integration and function of xenogeneic fetal neuroblasts in the host brain. A statistical meta-analysis across multiple studies revealed that immunologically-related transplantation parameters (immunosuppression and donor-host phylogenetic distance) were the main determinants of neural xenograft survival. The immunological basis for xenograft rejection is reviewed in the context of novel immunoprotection strategies designed to enhance xenograft survival. Furthermore, the evidence for behavioral recovery based on anatomical and functional integration of neural xenografts in the host brain is examined with an awareness of developmental considerations. It is concluded that neural xenotransplantation offers a unique opportunity for effective neuronal replacement with significant potential for clinical use.

Traumatic lesions and transplants of granule cells in the dentate gyrus alter the distribution of afferent fibers in the molecular layer

The present experiments determined whether traumatic lesions of the dentate gyrus granule cells had a different effect on the afferents in the molecular layer (ML) than nontraumatic lesions. Nontraumatic lesions of the granule cells induced by colchicine, ibotenic acid, x-radiation, and adrenalectomy have been reported to reduce both the acetylcholinesterase (AChE)-positive fibers and entorhinal afferents in the ML. After the nontraumatic granule cell lesions, the laminar distribution of the entorhinal afferents was maintained in the ML, whereas the AChE laminar pattern was lost. In the present study, dentate granule cells were traumatically lesioned by a fluid injection into the infragranular cleavage plane (IGCP) of the dentate gyrus. The traumatic lesion resulted in an altered distribution of the afferents in the ML. The perforant path fibers, shown by injection of wheat germ agglutinin horseradish peroxidase into the entorhinal cortex, occupied a greater proportion of the ML in lesioned animals than in control animals. The normal laminar pattern of AChE-positive afferents was not present after the granule cell lesion. There was an initial increase in AChE-positive fibers in the ML that lasted several weeks but eventually returned to near normal levels. The altered distribution of afferents could in part be due to uneven shrinkage of the molecular layer and/or sprouting of the afferents. Granule cell suspension transplants into the IGCP also traumatically lesioned the host granule cells but immediately replaced the damaged host granule cells with immature granule cells. The distribution of afferents was similar to that found in lesioned-only animals. The traumatic lesion induced MAP2 immunoreactivity in the anisomorphic reactive astrocytes of the ML. At the longer survival times, MAP2 was not seen in either the astrocytes of the ML or in the isomorphic reactive astrocytes in CA3.

Leukocyte infiltration and glial reactions in xenografts of mouse brain tissue undergoing rejection in the adult rat brain. A light and electron microscopical immunocytochemical study

Neural mouse xenografts undergoing rejection in the adult recipient rat brain were characterized with regard to infiltrating host leukocytes and reactions of graft and host astro- and microglial cells. Rejection occurred within 35 days with infiltration of the grafts by in particular macrophages and T-cells as well as blood-brain barrier (BBB) leakage for IgG. In the surrounding host brain microglial cells showed increased histochemical staining for nucleoside diphosphatase (NDPase) and increased immunocytochemical expression of complement receptor type 3 (CR3), while astroglial cells displayed an increased immunoreactivity for glial fibrillary acidic protein (GFAP). Light microscopic findings of rat major histocompatibility complex (MHC) antigen class I on microglial cells, endothelial cells and leukocytes were confirmed at the ultrastructural level and extended to include a few astrocytes. Rat and mouse MHC antigen class II was only detected on leukocytes and activated microglia. We suggest that host macrophages and activated host and xenograft microglial cells act in situ as immunostimulatory cells on T-helper cells, and that increased levels of donor MHC antigen class I may further enhance the killer activity exerted by host T-cytotoxic cells.

Grafting of fetal CA3 neurons to excitotoxic, axon-sparing lesions of the hippocampal CA3 area in adult rats

Hippocampal CA3 neurons from fetal rats were grafted to excitotoxic lesions in the CA3 subfield of the adult rat hippocampus and the formation of graft-host brain nerve connections examined. The excitotoxic lesions were induced by localized, stereotaxic injection of ibotenic acid (IA), a glutamic acid agonist, into CA3 of the dorsal hippocampus. The result was a so-called axon-sparing lesion with localized degeneration of nerve cells, but preservation of the extrinsic afferent fibers, now deprived of their targets. One week after the lesion a suspension of embryonic (E18-20) CA3 cells was grafted to the lesion site. Six weeks or more later the recipient brains were processed and analyzed by ordinary cell stains, histochemistry for acetylcholinesterase (AChE) and heavy metals (Timm staining), immunohistochemistry for the neuropeptides cholecystokinin and somatostatin and glial fibrillary acidic protein (GFAP) for astroglia, electron microscopy, and axonal tracing with retrogradely axonal transported fluorescent dyes or lesion-induced, anterograde degeneration combined with silver staining or electron microscopy. More than 90% of the grafts survived. They contained the normal types of CA3 neurons, which are mainly pyramidal cells, in addition to some normal, peptidergic, cholecystokinin- and somatostatin-reactive neurons. The grafts were innervated by AChE-positive, host cholinergic fibers, Timm-positive mossy fiber terminals from the host fascia dentata, and host commissural fibers traced by axonal degeneration. Efferent transplant projections were traced to the ipsilateral host CA1 (Schaffer collaterals) and the contralateral host hippocampus by retrograde axonal transport of fluorochromes injected into these host brain areas. All grafts analyzed by electron microscopy contained axonal varicosities resembling axonal growth cones even after long survival times. The results demonstrate that fetal rat hippocampal neurons, grafted to excitotoxic, axon-sparing lesions in the adult brain, can become both structurally and connectively well incorporated in the mature host central nervous system.

Xenografting of fetal mouse hippocampal tissue to the brain of adult rats: effects of cyclosporin A treatment

This study examines the effect of the immunosuppressive drug Cyclosporin A (CyA) on the survival and differentiation of solid grafts of fetal (E16-17) mouse hippocampi transplanted to the brain of adult rats. The CyA was given as daily subcutaneous injections of 20 mg/kg from the day before transplantation with reduction of the dose to 15 mg/kg after 14 days. Five weeks after transplantation neuron containing xenografts were recovered in 11 out of 17 CyA-treated recipients (65%). After 8 weeks 9 out of 21 grafts were found (43%). In the control groups, treated only with the vehicle olive oil, 8 out of 14 xenografts were recovered after 5 weeks survival (36%) and only 3 out of 17 after 8 weeks (18%). All xenografts were infiltrated with mononuclear lymphocytic-like cells, but the infiltration was least extensive and least dense in the CyA treated animals. An observed correlation between this cellular infiltration and the gliosis in the xenografts suggested that CyA also directly or indirectly influenced the glial reaction. Most surviving xenografts were located next to the lateral ventricles or the choroid fissure. They were organotypically organized with identifyable cell and neuropil layers, and their connectional organization was similar to rat and mouse allografts grafted to adult recipients. In the absence of major extrinsic afferents the intrinsic pathways observed with Timm staining had reorganized according to known principles for aberrant growth and collateral sprouting. Ingrowth of extrinsic host afferents was only demonstrated for AChE positive host fibers. We conclude that CyA treatment of adult rat recipients can increase the survival of intracerebral fetal mouse hippocampal xenografts and reduce the histological signs of rejection. Xenografting combined with CyA treatment thereby permits the use of a wider spectrum of donor neurons for studies of neuronal interaction and repair.