Photoreceptor and glial markers in human embryonic retina and in human embryonic retinal transplants to rat retina

Human photoreceptors switch from autonomous axon extension to cell-mediated process pulling during synaptic marker redistribution

Photoreceptors (PRs) are the primary visual sensory cells, and their loss leads to blindness that is currently incurable. Although cell replacement therapy holds promise, success is hindered by our limited understanding of PR axon growth during development and regeneration. Here, we generate retinal organoids from human pluripotent stem cells to study the mechanisms of PR process extension. We find that early-born PRs exhibit autonomous axon extension from dynamic terminals. However, as PRs age from 40 to 80 days of differentiation, they lose dynamic terminals on 2D substrata and in 3D retinal organoids. Interestingly, PRs without motile terminals are still capable of extending axons but only by process stretching via attachment to motile non-PR cells. Immobile PR terminals of late-born PRs have fewer and less organized actin filaments but more synaptic proteins compared with early-born PR terminals. These findings may help inform the development of PR transplantation therapies.

Sheets of human retinal progenitor transplants improve vision in rats with severe retinal degeneration

Loss of photoreceptors and other retinal cells is a common endpoint in retinal degenerate (RD) diseases that cause blindness. Retinal transplantation is a potential therapy to replace damaged retinal cells and improve vision. In this study, we examined the development of human fetal retinal sheets with or without their retinal pigment epithelium (RPE) transplanted to immunodeficient retinal degenerate rho S334ter-3 rats. Sheets were dissected from fetal human eyes (11-15.7 weeks gestation) and then transplanted to the subretinal space of 24-31 d old RD nude rats. Every month post surgery, eyes were imaged by high-resolution spectral-domain optical coherence tomography (SD-OCT). SD-OCT showed that transplants were placed into the subretinal space and developed laminated areas or rosettes, with clear development of plexiform layers first seen in OCT at 3 months post surgery. Several months later, as could be expected by the much slower development of human cells compared to rat cells, transplant photoreceptors developed inner and later outer segments. Retinal sections were analyzed by immunohistochemistry for human and retinal markers and confirmed the formation of several retinal subtypes within the retinal layers. Transplant cells extended processes and a lot of the cells could also be seen migrating into the host retina. At 5.8-8.6 months post surgery, selected rats were exposed to light flashes and recorded for visual responses in superior colliculus, (visual center in midbrain). Four of seven rats with transplants showed responses to flashes of light in a limited area of superior colliculus. No response with the same dim light intensity was found in age-matched RD controls (non-surgery or sham surgery). In summary, our data showed that human fetal retinal sheets transplanted to the severely disturbed subretinal space of RD nude rats develop mature photoreceptors and other retinal cells, integrate with the host and induce vision improvement.

Cell Proliferation in Retinal Transplants

The MIB-1 antibody against a nuclear protein Ki-67 was used to study the proliferation of cells in the rabbit retinal transplants. Fragmented pieces of embryonic day 15 rabbit retinas were transplanted into the subretinal space of adult rabbits and allowed to survive for different times. Fragmented donor tissue starts organizing in rosettes 1 day after transplantation. The transplanted cells continue to proliferate in the host eye and their pattern of proliferation resembles that of normal developing retina, suggesting that the factors responsible for the proliferation pattern are preserved after transplantation. The dividing cells in metaphase line up in the luminal layers of the rosettes. Certain cells become postmitotic in the regions corresponding to the inner retina first, followed by the cells in the luminal layers of rosettes. Cells in the regions between the rosettes, corresponding to the inner nuclear layer, presumably the Müller cells, proliferate significantly for the equivalent age of postnatal day 2. Few cells in these regions proliferate for at least the equivalent age of postnatal day 11 in transplants. There is a layer of nonproliferating, degenerating cells in the transplant situated close to the host retina. However, some cells in this layer, situated at the host-graft interface, proliferate. These cells proliferate for a long time possibly indicating gliosis.

Intraocular Transplantation of E1A-Immortalized Retinal Precursor Cells

The purpose of this study was to examine the effect of the ocular environment on the survival, tumorigenicity, and phenotypic marker expression of immortalized retinal precursor cells transplanted into immunocompetent adult and neonatal Sprague–Dawley rats. E1A-NR.3, a rat immortalized retinal precursor cell culture, was used as an inexhaustible source of experimental graft material. These cells were prelabeled with the fluorescent marker diI (1,1′-dioctadecyl-3,3,3′,3′-tetramethylindocarbocyanine perchlorate) and transplanted intravitreally (50,000 cells per μL) into 11 adult and 31 neonatal Sprague–Dawley rat eyes. At 1 mo posttransplant, animals were sacrificed and retinal tissue sections examined histologically for the presence of grafted cells, signs of tumor formation, and retinal phenotypic marker expression. No obvious signs of tumor formation or rejection were seen in a total of 42 eyes in the immunocompetent hosts. Our results indicate that E1A-NR.3 cells survive at least 1 month in vivo, and can migrate from the vitreous into neuroretinal cell layers. Subpopulations of surviving grafted cells were seen to express photoreceptor markers rhodopsin and recoverin comparably between in vitro and in vivo conditions. However, the number of cells immunoreactive for vimentin and E1A decreased significantly under in vivo conditions. This report represents the first experimental intravitreal transplantation of E1A-immortalized retinal precursor cells into adult and neonatal rats. The intraocular location and environment appears to affect phenotypic expression of surviving grafted cells, especially with respect to vimentin and E1A expression. The fact that E1A-NR.3 cells survived intraocularly at least 1 mo without tumor formation suggests that the cells may continue to be useful for further in vivo studies of experimental retinal transplantation, and effects of histological location on retinal cell phenotype and histogenesis in immunocompetent hosts. © 1998 Elsevier Science Inc.

Histologic development of the human fovea from midgestation to maturity

PURPOSE

To describe the histologic development of the human central retina from fetal week (Fwk) 22 to 13 years.

DESIGN

Retrospective observational case series.

METHODS

Retinal layers and neuronal substructures were delineated on foveal sections of fixed tissue stained in azure II-methylene blue and on frozen sections immunolabeled for cone, rod, or glial proteins. Postmortem tissue was from 11 eyes at Fwk 20-27; 8 eyes at Fwk 28-37; 6 eyes at postnatal 1 day to 6 weeks; 3 eyes at 9 to 15 months; and 5 eyes at 28 months to 13 years.

RESULTS

At Fwk 20-22 the fovea could be identified by the presence of a single layer of cones in the outer nuclear layer. Immunolabeling detected synaptic proteins, cone and rod opsins, and Müller glial processes separating the photoreceptors. The foveal pit appeared at Fwk 25, involving progressive peripheral displacement of ganglion cell, inner plexiform, and inner nuclear layers. The pit became wider and shallower after birth, and appeared mature by 15 months. Between Fwk 25 and Fwk 38, all photoreceptors developed more distinct inner and outer segments, but these were longer on peripheral than foveal cones. After birth the foveal outer nuclear layer became much thicker as cone packing occurred. Cone packing and neuronal migration during pit formation combined to form long central photoreceptor axons, which changed the outer plexiform layer from a thin sheet of synaptic pedicles into the thickest layer in the central retina by 15 months. Foveal inner and outer segment length matched peripheral cones by 15 months and was 4 times longer by 13 years.

CONCLUSIONS

These data are necessary to understand the marked changes in human retina from late gestation to early adulthood. They provide qualitative and quantitative morphologic information required to interpret the changes in hyper- and hyporeflexive bands in pediatric spectral-domain optical coherence tomography images at the same ages.

Transplantation of photoreceptor and total neural retina preserves cone function in P23H rhodopsin transgenic rat

Background

Transplantation as a therapeutic strategy for inherited retinal degeneration has been historically viewed to restore vision as a method by replacing the lost retinal cells and attempting to reconstruct the neural circuitry with stem cells, progenitor cells and mature neural retinal cells.

Methods and Findings

We present evidence for an alternative strategy aimed at preventing the secondary loss of cones, the most crucial photoreceptors for vision, by transplanting normal photoreceptors cells into the eye of the P23H rat, a model of dominant retinitis pigmentosa. We carried out transplantation of photoreceptors or total neural retina in 3-month-old P23H rats and evaluated the function and cell counts 6 months after surgery. In both groups, cone loss was significantly reduced (10%) in the transplanted eyes where the cone outer segments were found to be considerably longer. This morphological effect correlated with maintenance of the visual function of cones as scored by photopic ERG recording, but more precisely with an increase in the photopic b-wave amplitudes by 100% and 78% for photoreceptor transplantation and whole retinal transplantation respectively.

Conclusions

We demonstrate here that the transplanted tissue prevents the loss of cone function, which is further translated into cone survival.

Patterns of cell proliferation and rod photoreceptor differentiation in shark retinas

We studied the pattern of cell proliferation and its relation with photoreceptor differentiation in the embryonic and postembryonic retina of two elasmobranchs, the lesser spotted dogfish (Scyliorhinus canicula) and the brown shyshark (Haploblepharus fuscus). Cell proliferation was studied with antibodies raised against proliferating cell nuclear antigen (PCNA) and phospho-histone-H3, and early photoreceptor differentiation with an antibody raised against rod opsin. As regards the spatiotemporal distribution of PCNA-immunoreactive cells, our results reveal a gradual loss of PCNA that coincides in a spatiotemporal sequence with the gradient of layer maturation. The presence of a peripheral growth zone containing pure-proliferating retinal progenitors (the ciliary marginal zone) in the adult retina matches with the general pattern observed in other groups of gnathostomous fishes. However, in the shark retina the generation of new cells is not restricted to the ciliary marginal zone but also occurs in retinal areas that contain differentiated cells: (1) in a transition zone that lies between the pure-proliferating ciliary marginal zone and the central (layered) retina; (2) in the differentiating central area up to prehatching embryos where large amounts of PCNA-positive cells were observed even in the inner and outer nuclear layers; (3) and in the retinal pigment epithelium of prehatching embryos. Rod opsin immunoreactivity was observed in both species when the outer plexiform layer begins to be recognized in the central retina and, as we previously observed in trout, coincided temporally with the weakening in PCNA labelling.

Regulation of prenatal human retinal neurosphere growth and cell fate potential by retinal pigment epithelium and Mash1

During development of the central nervous system, stem and progenitor cell proliferation and differentiation are controlled by complex inter- and intracellular interactions that orchestrate the precise spatiotemporal production of particular cell types. Within the embryonic retina, progenitor cells are located adjacent to the retinal pigment epithelium (RPE), which differentiates prior to the neurosensory retina and has the capacity to secrete a multitude of growth factors. We found that secreted proteinaceous factors in human prenatal RPE conditioned medium (RPE CM) prolonged and enhanced the growth of human prenatal retinal neurospheres. The growth-promoting activity of RPE CM was mitogen-dependent and associated with an acute increase in transcription factor phosphorylation. Expanded populations of RPE CM-treated retinal neurospheres expressed numerous neurodevelopmental and eye specification genes and markers characteristic of neural and retinal progenitor cells, but gradually lost the potential to generate neurons upon differentiation. Misexpression of Mash1 restored the neurogenic potential of long-term cultures, yielding neurons with phenotypic characteristics of multiple inner retinal cell types. Thus, a novel combination of extrinsic and intrinsic factors was required to promote both progenitor cell proliferation and neuronal multipotency in human retinal neurosphere cultures. These results support a pro-proliferative and antiapoptotic role for RPE in human retinal development, reveal potential limitations of human retinal progenitor culture systems, and suggest a means for overcoming cell fate restriction in vitro.

MIO-M1 cells and similar muller glial cell lines derived from adult human retina exhibit neural stem cell characteristics

Growing evidence suggests that glial cells may have a role as neural precursors in the adult central nervous system. Although it has been shown that Müller cells exhibit progenitor characteristics in the postnatal chick and rat retinae, their progenitor-like role in developed human retina is unknown. We first reported the Müller glial characteristics of the spontaneously immortalized human cell line MIO-M1, but recently we have derived similar cell lines from the neural retina of several adult eye donors. Since immortalization is one of the main properties of stem cells, we investigated whether these cells expressed stem cell markers. Cells were grown as adherent monolayers, responded to epidermal growth factor, and could be expanded indefinitely without growth factors under normal culture conditions. They could be frozen and thawed without losing their characteristics. In the presence of extracellular matrix and fibroblast growth factor-2 or retinoic acid, they acquired neural morphology, formed neurospheres, and expressed neural stem cell markers including betaIII tubulin, Sox2, Pax6, Chx10, and Notch 1. They also expressed markers of postmitotic retinal neurons, including peripherin, recoverin, calretinin, S-opsin, and Brn3. When grafted into the subretinal space of dystrophic Royal College of Surgeons rats or neonatal Lister hooded rats, immortalized cells migrated into the retina, where they expressed various markers of retinal neurons. These observations indicate that adult human neural retina harbors a population of cells that express both Müller glial and stem cell markers and suggest that these cells may have potential use for cell-based therapies to restore retinal function. Disclosure of potential conflicts of interest is found at the end of this article.

Structure and cell composition of spheres cultured from human fetal retina

The structure and cell composition of spheres obtained by culturing human fetal retinal cells after 15, 18, 22-23, and 24 weeks of gestation were studied. The cells were cultured as neurospheres: in serum-free medium with growth factors, in which they formed floating spheres. Immunocytochemical analysis showed that cell proliferation in the spheres decreased with increasing fetal age. Stem/progenitor cells, neuroblasts, and photoreceptors were detected in the spheres. Glial cells were detected only in spheres originating from 22- and 24-week fetuses. All spheres, irrespective of age and duration of culturing, consisted of numerous cell rosettes, each histotypically similar to the neuroblastic layer of the developing retina.

Human Retinal Progenitor Cells Grown as Neurospheres Demonstrate Time-Dependent Changes in Neuronal and Glial Cell Fate Potential

The spatiotemporal birth order of the seven major classes of retinal cells is highly conserved among vertebrates. During retinal development, long projection neurons (ganglion cells) are produced first from resident progenitors, followed by the appearance of retinal interneurons, photoreceptors, and Muller glia. This sequence is maintained through the complex orchestration of cell-intrinsic and cell-extrinsic events and factors, including local influences between neighboring cells. Here we asked whether cultures of human prenatal retinal cells might also yield different ratios of cell types based on gestational age and time spent in vitro, thus recapitulating in vivo development. An established chopping technique was used to passage human prenatal retinal cells as neurospheres, avoiding the use of proteases and preserving cell-cell contacts and native microenvironments present in vivo. Retinal neurospheres cultured in this manner demonstrated specific patterns of growth over a limited time period, possibly reflecting trends in normal retinal development. Upon differentiation, immunocytochemical analysis revealed that retinal neurospheres produce predominantly glial cells with increasing gestational age and time in culture. Conversely, the percentage of betaIII tubulin-positive neurons declined over time. This provides information for optimizing culture systems aimed at the study of human retinal development and the generation of specific retinal cell types for therapeutic use or drug testing.

Enhanced Neurotrophin Synthesis and Molecular Differentiation in Non-Transformed Human Retinal Progenitor Cells Cultured in a Rotating Bioreactor

One approach to the treatment of retinal diseases, such as retinitis pigmentosa, is to replace diseased or degenerating cells with healthy cells. Even if all of the problems associated with tissue transplant were to be resolved, the availability of tissue would remain an ongoing problem. We have previously shown that transformed human retinal cells can be grown in a NASA-developed horizontally rotating culture vessel (bioreactor) to form three-dimensional-like structures with the expression of several retinal specific proteins. In this study, we have investigated growth of non-transformed human retinal progenitors (retinal stem cells) in a rotating bioreactor. This rotating culture vessel promotes cell-cell interaction between similar and dissimilar cells. We cultured retinal progenitors (Ret 1-4) alone or as a co-culture with human retinal pigment epithelial cells (RPE, D407) in this system to determine if 3D structures can be generated from non-transformed progenitors. Our second goal was to determine if the formation of 3D structures correlates with the upregulation of neurotrophins, basic fibroblast growth factor (bFGF), transforming growth factor alpha (TGFalpha), ciliary neurotrophic factor (CNTF), and brain-delivered neurotrophic factor (BDNF). These factors have been implicated in progenitor cell proliferation, commitment, differentiation, and survival. We also investigated the expression of the following retinal specific proteins in this system: neuron specific enolase (NSE); tyrosine hydroxylase (TH); D(2)D(3), D(4) receptors; protein kinase-C alpha (PKCalpha), and calbindin. The 3D structures generated were characterized by phase and scanning transmission electron microscopy. Retinal progenitors, cultured alone or as a co-culture in the rotating bioreactor, formed 3D structures with some degree of differentiation, accompanied by the upregulation of bFGF, CNTF, and TGFalpha. Brain-derived neurotrophic factor, which is expressed in vivo in RPE (D407), was not expressed in monolayer cultures of RPE but expressed in the rotating bioreactor-cultured RPE and retinal progenitors (Ret 1-4). Upregulation of neurotrophins was noted in all rotating bioreactor-cultured cells. Also, upregulation of D(4) receptor, calbindin, and PKCalpha was noted in the rotating bioreactor-cultured cells. We conclude that non-transformed retinal progenitors can be grown in the rotating bioreactor to form 3D structures with some degree of differentiation. We relied on molecular and biochemical analysis to characterize differentiation in cells grown in the rotating bioreactor.

Isolation of retinal progenitor cells from post-mortem human tissue and comparison with autologous brain progenitors

The goal of the present study was threefold: to determine whether viable human retinal progenitor cells (hRPCs) could be obtained from cadaveric retinal tissue, to evaluate marker expression by these cells, and to compare hRPCs to human brain progenitor cells (hBPCs). Retinas were dissected from post-mortem premature infants, enzymatically dissociated, and grown in the presence of epidermal growth factor and basic fibroblast growth factor. The cells grew as suspended spheres or adherent monolayers, depending on culture conditions. Expanded populations were banked or harvested for analysis by RT-PCR, immunocytochemistry, and flow cytometry. hBPCs derived from forebrain specimens from the same donors were grown and used for RT-PCR. Post-mortem human retinal specimens yielded viable cultures that grew to confluence repeatedly, although not beyond 3 months. Cultured hRPCs expressed a range of markers consistent with CNS progenitor cells, including nestin, vimentin, Sox2, Ki-67, GD2 ganglioside, and CD15 (Lewis X), as well as the tetraspanins CD9 and CD81, CD95 (Fas), and MHC class I antigens. No MHC class II expression was detected. hRPCs, but not hBPCs, expressed Dach1, Pax6, Six3, Six6, and recoverin. Minority subpopulations of hRPCs and hBPCs expressed doublecortin, beta-III tubulin, and glial fibrillary acidic protein, which is consistent with increased lineage restriction in subsets of cultured cells. Viable progenitor cells can be cultured from the post-mortem retina of premature infants and exhibit a gene expression profile consistent with immature neuroepithelial cells. hRPCs can be distinguished from hBPC cultures by the expression of retinal specification genes and recoverin.

Müller cells in allogeneic adult rabbit retinal transplants

Müller cell morphology and degree of activation in adult retinal transplants have, to our knowledge, never been reported previously. We transplanted adult rabbit neuroretinal full-thickness sheets, prepared under strict control, to the subretinal space of adult rabbits. After surviving 6-174 days, eyes were examined in the light microscope, and grafts displaying the normal laminated morphology were labeled with antibodies against vimentin and glial fibrillary acidic protein (GFAP). Müller cells in the grafts displayed the normal vertical arrangement, from outer limiting membrane to vitread endfeet. They showed an initial degree of activation, evident by GFAP upregulation, which diminished with increasing survival times, and was absent in the oldest specimens. In the host retina, Müller cells in the transplant area became progressively more disorganized with increasing survival times, and their degree of activation increased. Our results suggests that adult full-thickness neuroretinal grafts are structurally stable, even in long-term specimens, and thrive in spite of their allogeneic environment. The gliotic change seen in the host retina covering the graft is identical to the one seen in earlier reported eyes receiving embryonic grafts, and is due to the merangiotic nature of the rabbit neuroretina.

Transplanted sheets of human retina and retinal pigment epithelium develop normally in nude rats

This study investigated whether transplanted sheets of human fetal retina together with its retinal pigment epithelium (RPE) could develop and maintain their cytoarchitecture after long survival times. Transplant recipients were nine albino athymic nu/nu rats with a normal retina. The donor tissue was dissected from fetuses of 12-17 weeks gestational age. Transplants were analyzed at 5-12 months after surgery by light and electron microscopy, and immunohistochemistry with various antibodies specific for rhodopsin, S-antigen, transducin, neurofilament and synaptophysin. In 4 of 11 transplants, the RPE stayed as a monolayer sheet and supported the development of the retinal sheet with a normal lamination, including photoreceptor inner and outer segments. Cones and rods in the organized transplants were labeled with different photoreceptor markers. Inner and outer plexiform layers, containing cone pedicles and rods spherules, were immunoreactive for synaptophysin. As the recipients had a normal retina, transplant/host integration was not expected. However, at the transplant/host interface, there were sometimes areas without glial barriers, and neurofilament-containing processes could be observed crossing between transplant and host. In other, more disorganized transplants, the RPE cells were partially dispersed or clumped together in clusters. Such transplants developed photoreceptors in rosettes, often with inner and outer segments. In conclusion, sheets of human fetal retina transplanted together with its RPE to the subretinal space of nude rats can develop and maintain perfectly laminated transplants after long survival times, indicating the potential of applying cotransplantation to human patients with retinal diseases.

Retinal transplantation--advantages of intact fetal sheets

Retinal transplantation aims to prevent blindness and to restore eyesight, i.e., to rescue photoreceptors or to replace damaged photoreceptors with the hope of reestablishing neural circuitry. Retinal donor tissue has been transplanted as dissociated cells or intact sheets. A promising experimental paradigm is the subretinal transplantation of sheets of fetal retina with or without its attached retinal pigment epithelium (RPE) into recipient rats with retinal degeneration. As long as healthy RPE either from the host or from the graft is present, such transplants can develop lamination resembling a normal retina. Different methods have been used to demonstrate transplant/host connectivity. In two different rat retinal degeneration models, visually evoked responses can be demonstrated in an area of the superior colliculus corresponding to the placement of the transplant in the retina. In summary, sheets of fetal retina can morphologically repair an area of a degenerated retina, and there is evidence to suggest that transplants form synaptic connections with the host and restore visual responses in blind rats.

Müller cells in long-term full-thickness retinal transplants

Müller cells are essential in creating and maintaining intricate neuroretinal architecture. The functions of this important glial cell are not limited to mere support of the retinal neurons, but also include interaction in synaptic transmission and activation in response to retinal insult. In this study, we have examined Müller cell morphology and degree of activation in embryonic full-thickness rabbit neuroretinal grafts, which were positioned under the host retina using vitrectomy technique. After surviving 3-10 months, retinal specimens were examined with hematoxylin and eosin staining and immunohistochemical analysis of vimentin and glial fibrillary acidic protein (GFAP) expression. In the host retina covering the graft, outer layers were degenerated, and vimentin-labeled Müller cells in this area appeared short, disorganized, and displayed strong GFAP labeling. In the graft, vimentin-labeled Müller cells spanning the retinal layers in the normal manner were found. Müller cells in 3-month grafts were well labeled by GFAP, whereas in older grafts, GFAP labeling was very weak or absent. Our results suggest that Müller cells in well-laminated full-thickness retinal grafts display many of the normal morphological features and retain a normal organization even after prolonged survival times. The loss of the initial degree of Müller cell activation indicates a long-term stability of the graft. The degeneration and gliosis of the host retina covering the graft is best explained by the merangiotic nature of the rabbit retina and may limit the usefulness of the rabbit in retinal transplantation experiments.

Differential expression of syntaxin-1 and synaptophysin in the developing and adult human retina

Synaptophysin and syntaxin-1 are membrane proteins that associate with synaptic vesicles and presynaptic active zones at nerve endings, respectively. The former is known to be a good marker of synaptogenesis; this aspect, however, is not clear with syntaxin-1. In this study, the expression of both proteins was examined in the developing human retina and compared with their distribution in postnatal to adult retinas, by immunohistochemistry. In the inner plexiform layer, both were expressed simultaneously at 11-12 weeks of gestation, when synaptogenesis reportedly begins in the central retina. In the outer plexiform layer, however, the immunoreactivities were prominent by 16 weeks of gestation. Their expression in both plexiform layers followed a centre-to-periphery gradient. The immunoreactivities for both proteins were found in the immature photoreceptor, amacrine and ganglion cells; however, synaptophysin was differentially localized in bipolar cells and their axons, and syntaxin was present in some horizontal cells. In postnatal-to-adult retinas, synaptophysin immunoreactivity was prominent in photo-receptor terminals lying in the outer plexiform layer; on the contrary, syntaxin-1 was present in a thin immunoreactive band in this layer. In the inner plexiform layer, however, both were homogeneously distributed. Our study suggests that (i) syntaxin-1 appears in parallel with synapse formation; (ii) synaptogenesis in the human retina might follow a centre-to-periphery gradient; (iii) syntaxin-1 is likely to be absent from ribbon synapses of the outer plexiform layer, but may occur at presynaptic terminals of photoreceptor and horizontal cells, as is apparent from its localization in these cells, which is hitherto unreported for any vertebrate retina.

Immunohistochemical markers in full-thickness embryonic rabbit retinal transplants

PURPOSE

To examine immunohistochemical markers in straight, well-laminated retinal transplants with special attention paid to the interphotoreceptor matrix, the Müller cells and the ganglion cells as these three retinal components have been abnormal in transplants produced by previous methods.

METHODS

Nine rabbits underwent subretinal transplantation of a complete full-thickness embryonic neuroretina. After 31 or 49 days, the transplants were stained for light microscopy and processed for immunohistochemistry.

RESULTS

Six of 9 eyes contained transplants with straight, well-laminated regions with all light-microscopic characteristics of a normal retina. In the outer segment region, the expression of peanut agglutinin showed segmental labeling of cone domains in the interphotoreceptor matrix, and interphotoreceptor retinoid binding protein immunoreactivity was found. Glial fibrillary acidic protein and vimentin immunoreactivity revealed normal Müller cell morphology. In 3 transplants the AB5-antibody-labeled ganglion cells in the ganglion cell layer and all transplants contained nerve fibers in the nerve fiber layer labeled by an antibody against neurofilament of 160 kD. The latter also labeled fibers connecting the transplant with the host.

CONCLUSIONS

Full-thickness embryonic retinal transplants develop the normal retinal appearance and display several of the retinal components necessary for normal function which are not found in transplants produced by previous methods.

Human retinal astroglia. A comparative study of adult and the 18 month postnatal developmental stage

The immunohistochemical location of glial fibrillary acidic protein (GFAP) was used to study the state of maturation of retinal astrocytes from an 18-mo-old infant and to compare it with the situation in the adult. Infant astrocytes showed intense GFAP immunoreactivity in the perikarya and possessed spindle-like enlargements in their processes, while in the adult immunoreactivity in the perikarya was scarce and the spindle-like enlargements were not evident. Two types of astrocyte were observed in adult and child retinas: elongated and star-shaped. In the adult, the star-shaped type tend to be more stylised and to have longer processes than in the infant. In the infant, numerous astrocyte cell bodies were observed over vessels, while in the adult these were scarce. In the infant, the star-shaped astrocytes made up a honeycomb plexus, but this was not fully developed. These results suggest that at 18 mo of postnatal development the retinal astrocytes are still increasing and growing into the astroglial structure found in adults.

Differentiation of a unique macroglial cell type in the pecten oculi of the chicken

Glial cells in the CNS of vertebrates serve specialized functions in close interaction with surrounding neurons and blood vessels. In the avian eye, the neural tissue (retina) and the supporting vascular structure (pecten oculi) are spatially separated and comprise distinct glial cell types, i.e., the Müller glia and the pecteneal glia, respectively. In the present study we combined morphological and immunocytochemical investigations on the differentiation of the pecteneal glia in comparison to the retinal Müller glia, the retinal pigment epithelium, and the astrocytic cells of the optic nerve head in order to elucidate the nature, origin, and function of the pecteneal glia. Conventional transmission electron microscopy and freeze-fracture imaging revealed striking similarities between the pecteneal glia and retinal pigment epithelial cells at the transition zone to the optic nerve head. Immunofluorescence investigation identified specific labeling for vimentin and glutamine synthetase (GS) but not for glial fibrillary acidic protein (GFAP) in the mature pecteneal glia. Immunogold labeling confirmed the cellular specificity. GS labeling was weak during embryonic development but increasingly strong after hatching. Surprisingly, the intraneuroectodermal endothelial cells were highly immunopositive for GS throughout embryonic development and lost GS expression after hatching. GS expression in the pecteneal glia may participate in pH-regulation of the avian eye. Endothelial GS expression in the developing CNS may detoxify detrimental ammonium concentrations resulting from egg yolk degradation.

Photoreceptor function of retinal transplants implicated by light-dark shift of S-antigen and rod transducin

The aim was to demonstrate functional properties of transplanted histologically normal photoreceptors. Subretinal intact-sheet transplants of fetal E17-E20 rat retinas to light-damaged albino rat eyes were fixed in light or dark, 2 to 42 weeks after transplantation, and stained immunohistochemically for certain phototransduction proteins. In light adapted transplants, transducin was predominantly found in inner segments of parallel-organized photoreceptors. Transducin shifted to the outer segments with dark-adaptation. S-antigen distribution was opposite to transducin. Rhodopsin distribution did not change. The shift of signal transduction proteins correlated to the light conditions indicates that normal phototransduction processes were established in photoreceptors of transplanted retinal sheets.

Pigmented epithelium induces complete retinal reconstitution from dispersed embryonic chick retinae in reaggregation culture

Reaggregation of dispersed retinal cells of the chick embryo leads to histotypic retinospheroids in which the laminar organization remains incomplete: photoreceptors form rosettes which are surrounded by constituents of the other retinal layers. Here, for the first time, a complete arrangement of layers is achieved in cellular spheres (stratoids), provided that fully dispersed retinal cells are younger than embryonic day E6, and are reaggregated in the presence of a monolayer of retinal pigmented epithelium (RPE). A remarkable mechanism of stratoid formation from 1 to 15 days in vitro is revealed by the establishment of a radial Müller glia scaffold and of photoreceptors. During the first two days of reaggregation on RPE, rosettes are still observed. At this stage immunostaining with vimentin and F11 antibodies for radial Müller glia reveal a disorganized pattern. Subsequently, radial glia processes organize into long parallel fibre bundles which are arranged like spokes to stabilize the surface and centre of the stratoid. The opsin-specific antibody CERN 901 detects photoreceptors as they gradually build up an outer nuclear layer at the surface. These findings assign to the RPE a decisive role for the genesis and regeneration of a vertebrate retina.

Expression of GABA in the fetal, postnatal, and adult human retinas: an immunohistochemical study

The expression of GABA in the human fetal (12-25 weeks of gestation), postnatal (five-month-old), and adult (35-year-old) retinas was investigated by immunohistochemistry. GABA expression was seen as early as 12 weeks in the undifferentiated cells of the inner neuroblast zone; a few optic nerve fiber layer axons were clearly labeled, suggesting that some of the stained cell bodies were prospective ganglion cells, others could be displaced amacrine cells. From 16-17 to 24-25 weeks, intense labeling was found in the amacrine, displaced amacrine, and some ganglion cells. During this time period, horizontal cells (identified by calbindin immunohistochemistry), undergoing migration (periphery) and differentiation (center), expressed GABA prominently. In the postnatal retina, some horizontal cells were moderately labeled, but very weakly in a few cells, in the adult. The Müller cells developed immunoreactivity first weakly at 12 weeks and then moderately from 16-17 weeks onward. The staining was also evident in the postnatal and adult retinas, showing labeled processes of these glial cells. Virtually no axons in the adult optic nerve and nerve fiber layer were stained; the staining was restricted to a few, large ganglion cells and displaced amacrine cells: Some amacrines were also labeled. The possibility that GABA might play a role in horizontal cell differentiation and maturation is highlighted. Other evidences suggest that GABA might play a role in metabolism during retinal development.

Influence of technique and transplantation site on rosette formation in rabbit retinal transplants

In order to determine mechanical and host-graft related interactions in the histogenesis of retinal transplants, a new technique for transplanting flat and comparatively large pieces of embryonic rabbit retina into adult rabbit eyes was elaborated. With the procedure, free-floating grafts in the epiretinal space survive, develop and differentiate largely without rosette formation, suggesting that the dissection and transplantation procedure is adequate for obtaining a normal development. On the other hand, subretinal transplants mature at an apparently faster pace than epiretinal transplants, but do not become regularly laminated. Outer segments do not develop well in the epiretinal transplants, whereas they do so in the subretinal ones, suggesting host-graft interactions by means of yet unknown diffusible factors.

The first decade of continuous progress in retinal transplantation

In recent months, neural fetal retina has been transplanted into blind human patients affected by Retinitis Pigmentosa. Initial success, as documented by improved visual activity, has been reported (del Cerro et al., Neuroscience Abstract, 1996). With the rapid progress in human patients, additional questions are arising concerning transplantation issues. Additional answers and further success in treating clinical disease will necessarily come from new laboratory research in animal models as well as in vitro systems. This increases the need for evaluation of the data already gathered over the first decade of retinal transplantation. The extensive experimental background work that preceded the current wave of human retinal transplants is reviewed in this paper, with particular emphasis given to the work dealing with the transplantation of neural retina.

Co-transplantation of embryonic retina and retinal pigment epithelial cells to rabbit retina

The retinal pigment epithelium (RPE) is important for normal development of the neural retina. We sought to investigate whether cografting RPE cells affected the differentiation and survival of retinal grafts. Pigmented embryonic day 16 (E16) rabbit retina was dissected either with or without attached RPE and injected into a lesion site in retinas of young adult rabbit hosts. Each host obtained a pure retina graft in one eye and a retina/RPE cograft in the other. Animals were sacrificed after 4, 8 and 12 weeks. After 4 weeks, grafts (1-2 mm in diameter) were seen in both experimental groups at the lesion site or in the subretinal space. However, 8 and 12 weeks after transplantation, the graft survival rate decreased. The grafts developed cell layers in folded sheets and many rosettes (a rosette consists of photoreceptors and cells of other retinal layers around a central lumen defined by an outer limiting membrane). Cografts of retina with RPE had areas of more distinct cell lamination than transplants of pure retina. Grafted RPE cells were organized in clusters of cells surrounded by extracellular matrix and often associated with blood vessels. If the extracellular matrix of RPE cell clusters was outside the rosettes close to inner retinal layers in the graft, transplant Müller cell endfeet developed an inner limiting membrane. Müller cell endfeet could also be observed in subretinal transplants attached to the denuded Bruch's membrane of the host. In 12-week grafts, when RPE cell clusters were inside rosettes, the surrounded photoreceptors survived better. No RPE effect could be seen if single RPE cells were dispersed among retinal donor cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Human embryonic retinal cell transplants in athymic immunodeficient rat hosts

This study investigates the possibility to use the athymic "nude" rat as a host for the transplantation of human embryonic retinal cells without immunosuppression. The long-term development of such transplants is compared with results from our earlier study that used immunosuppressed rats, and showed transplant immunoreactivity for S-antigen. Several additional cell markers have been included: rhodopsin, rod (alpha-transducin, neuron-specific enolase (NSE), synaptophysin (SYN), cone-specific opsins, vimentin, cellular retinaldehyde binding protein (CRALBP), glial fibrillary acidic protein (GFAP), rat major histocompatibility antigen class II (MHC-II) and a rat macrophage marker (Ox-42). Human retinal cells (9-13 wk postconception) were transplanted to the eyes of 28 athymic rats. Host rats were kept in microisolator cages for up to 48 wk after surgery. Host immune response and the development of the transplants were studied using histology, immunohistochemistry and electron microscopy. When using retinas of donors 9-11 wk postconception, transplants grew to 2-3 mm in diameter with many rosettes, in 31 of 35 eyes. Transplants derived from donors 12-13 wk postconception did not survive as well (8 out of 11 eyes), were smaller and less organized. All transplants fused well with the host retina, better than corresponding transplants to immunosuppressed rat hosts. Most transplants appeared to be healthy, even after long survival times, and only occasionally were MHC-II positive macrophages observed in transplants or host retinas. All retinal layers were observed, except for an inner limiting membrane on the vitreous surface. The oldest transplants (34-57 wk total age = donor age + time after surgery) exhibited well developed photoreceptors, rods and cones, with inner and outer segments. SYN-staining showed the development of inner and outer plexiform layers. Although many cones stained for SYN and NSE, few were immunoreactive for red-green or blue opsin. Most rods became immunoreactive for S-antigen and rhodopsin. Transplant Müller cells stained for vimentin and CRALBP. Immunoreactivity for GFAP developed slowly and was not completely expressed in all transplant Müller cells until 44 wk total age. Nude rats offer an excellent model for the study of human retinal xenografts without the negative effects of immunosuppression. Compared to immunosuppressed rats, transplantation to nude rats gives consistent results and superior long-term survival of hosts and transplants.