Histotypic organization of the rat retina in vitro

Retina in a dish: Cell cultures, retinal explants and animal models for common diseases of the retina

For many retinal diseases, including age-related macular degeneration (AMD), glaucoma, and diabetic retinopathy (DR), the exact pathogenesis is still unclear. Moreover, the currently available therapeutic options are often unsatisfactory. Research designed to remedy this situation heavily relies on experimental animals. However, animal models often do not faithfully reproduce human disease and, currently, there is strong pressure from society to reduce animal research. Overall, this creates a need for improved disease models to understand pathologies and develop treatment options that, at the same time, require fewer or no experimental animals. Here, we review recent advances in the field of in vitro and ex vivo models for AMD, glaucoma, and DR. We highlight the difficulties associated with studies on complex diseases, in which both the initial trigger and the ensuing pathomechanisms are unclear, and then delineate which model systems are optimal for disease modelling. To this end, we present a variety of model systems, ranging from primary cell cultures, over organotypic cultures and whole eye cultures, to animal models. Specific advantages and disadvantages of such models are discussed, with a special focus on their relevance to putative in vivo disease mechanisms. In many cases, a replacement of in vivo research will mean that several different in vitro models are used in conjunction, for instance to analyze and validate causative molecular pathways. Finally, we argue that the analytical decomposition into appropriate cell and tissue model systems will allow making significant progress in our understanding of complex retinal diseases and may furthermore advance the treatment testing.

Molecular Pathway to Protection From Age-Dependent Photoreceptor Degeneration in Mef2 Deficiency

Purpose

Photoreceptor degeneration in the retina is a major cause of blindness in humans. Elucidating mechanisms of degenerative and neuroprotective pathways in photoreceptors should afford identification and development of therapeutic strategies.

Methods

We used mouse genetic models and improved methods for retinal explant cultures. Retinas were enucleated from Mef2d^+/+ and Mef2d^−/− mice, stained for MEF2 proteins and outer nuclear layer thickness, and assayed for apoptotic cells. Chromatin immunoprecipitation (ChIP) assays revealed MEF2 binding, and RT-qPCR showed levels of transcription factors. We used AAV2 and electroporation to express genes in retinal explants and electroretinograms to assess photoreceptor functionality.

Results

We identify a prosurvival MEF2D-PGC1α pathway that plays a neuroprotective role in photoreceptors. We demonstrate that Mef2d^−/− mouse retinas manifest decreased expression of PGC1α and increased photoreceptor cell loss, resulting in the absence of light responses. Molecular repletion of PGC1α protects Mef2d^−/− photoreceptors and preserves light responsivity.

Conclusions

These results suggest that the MEF2-PGC1α cascade may represent a new therapeutic target for drugs designed to protect photoreceptors from developmental- and age-dependent loss.

Organotypic culture of neural retina as a research model of neurodegeneration of ganglion cells

Organotypic models deserve special attention among the large variety of methods of vertebrate retina cultivation. The purpose of this study was to make a detailed qualitative and quantitative characterization of a model employing roller organotypic cultivation of the neural retina of rat eye posterior segment, with special attention to morphological and functional characteristics of retinal ganglion cells. The study included morphological analysis of retina histological preparations as well as estimation of RNA synthesis and evaluation of neuron survival by the Brachet and TUNEL methods, respectively. Retina has been shown to display normal morphofunctional characteristics for the first 12 h of cultivation. After 24 h, a substantial number of ganglion cells underwent pyknosis and stopped RNA synthesis. Almost all the cells of the retinal ganglion layer became apoptotic by 3-4 days in vitro. In the course of cultivation, neural retina is detached from the underlying layers of the posterior eye segment and undergoes significant cytoarchitectonic changes. The causes of ganglion cell death during organotypic cultivation of eye posterior segment are discussed. This method can serve as a suitable model for the screening of new retinoprotectors and for research on ganglion cell death resulting from retina degenerative diseases, e.g. glaucoma.

Proteomic analysis of the retina: removal of RPE alters outer segment assembly and retinal protein expression

The mechanisms that regulate the complex physiological task of photoreceptor outer segment assembly remain an enigma. One limiting factor in revealing the mechanism(s) by which this process is modulated is that not all of the role players who participate in this process are known. The purpose of this study was to determine some of the retinal proteins that likely play a critical role in regulating photoreceptor outer segment assembly. To do so, we analyzed and compared the proteome map of tadpole Xenopus laevis retinal pigment epithelium (RPE)-supported retinas containing organized outer segments with that of RPE-deprived retinas containing disorganized outer segments. Solubilized proteins were labeled with CyDye fluors followed by multiplexed two-dimensional separation. The intensity of protein spots and comparison of proteome maps was performed using DeCyder software. Identification of differentially regulated proteins was determined using nanoLC-ESI-MS/MS analysis. We found a total of 27 protein spots, 21 of which were unique proteins, which were differentially expressed in retinas with disorganized outer segments. We predict that in the absence of the RPE, oxidative stress initiates an unfolded protein response. Subsequently, downregulation of several candidate Müller glial cell proteins may explain the inability of photoreceptors to properly fold their outer segment membranes. In this study, we have used identification and bioinformatics assessment of proteins that are differentially expressed in retinas with disorganized outer segments as a first step in determining probable key molecules involved in regulating photoreceptor outer segment assembly.

Roller Culture of Free-Floating Retinal Slices: A New System of Organotypic Cultures of Adult Rat Retina

No experimental system exists to date for the in vitro study of retinal ganglion cell populations in a three-dimensional organotypic tissue environment. Here, we describe such a novel method for roller cultivation of adult retinas. Retinas of adult (1-3 months old) rats were cut into rectangular slices of approximately 1 mm(2). Free-floating slices were cultured on a horizontal rotating roller drum (50-60 rpm) in a dry incubator at 36.5 degrees C. During the first days of cultivation, primary flat retinal slices changed their configuration and transformed into ball-shaped tissue spheres (retinal bodies). Histological and immunocytochemical studies showed that the outer wall of the retinal bodies was formed by cell and fibre layers typical of mature retina with photoreceptors located on the outside. Initially, retinal bodies contained an inner cavity which later was completely obliterated and filled with glial cells, sprouting nerve fibres, and vascular structures. This culture system was further developed into a robust model of glutamate-induced neurotoxicity. Using a novel culture method of adult rat retina, preservation of the three-dimensional organotypic retinal cytoarchitecture was achieved, including survival of neurons in the ganglion cell layer and sprouting of nerve fibres of the axotomized retinal ganglion cells. This novel culture model promises to facilitate studies of retinal physiology and pathology.

Structural changes in the developing retina maintained in vitro

The present study examined the emergence of structural remodeling in explanted neonatal rat retina. Immunohistochemical analysis demonstrated signs of glial and neuronal remodeling after 11 days in vitro and included the activation of Müller cells, the formation of ectopic neuropil areas and sprouting of photoreceptor terminals. We also observed that cholinergic and GABA-ergic amacrine cells displayed signs of disorganized laminations. These results demonstrate that retinal culturing initiates structural changes that show morphological similarities to glial and neuronal remodeling identified in retinitis pigmentosa retinas and experimentally detached retinas.

Cell differentiation, synaptogenesis, and influence of the retinal pigment epithelium in a rat neonatal organotypic retina culture

This study was focused on the analysis of cell type differentiation and synaptogenesis as well as outer segment formation in an organotypic culture of the neonatal rat retina during a 6-14 day period of in vitro development. Moreover, the effects of the retinal pigment epithelium (RPE) on these processes were investigated. The in vitro development resulted in a retinal architecture and lamination comparable to that of in vivo retinas. The RPE influences the proper alignment of photoreceptors as well as the formation of the outer limiting membrane (OLM), but not processes of cell differentiation, synaptogenesis and inner retinal lamination.

Requirement for the zebrafish mid-hindbrain boundary in midbrain polarisation, mapping and confinement of the retinotectal projection

The organizer at the midbrain-hindbrain boundary (MHB organizer) has been proposed to induce and polarize the midbrain during development. We investigate the requirement for the MHB organizer in acerebellar mutants, which lack a MHB and cerebellum, but retain a tectum, and are mutant for fgf8, a candidate inducer and polarizer. We examine the retinotectal projection in the mutants to assay polarity in the tectum. In mutant tecta, retinal ganglion cell (RGC) axons form overlapping termination fields, especially in the ventral tectum, and along both the anterior-posterior and dorsal-ventral axis of the tectum, consistent with a MHB requirement in generating midbrain polarity. However, polarity is not completely lost in the mutant tecta, in spite of the absence of the MHB. Moreover, graded expression of the ephrin family ligand Ephrin-A5b is eliminated, whereas Ephrin-A2 and Ephrin-A5a expression is leveled in acerebellar mutant tecta, showing that ephrins are differentially affected by the absence of the MHB. Some RGC axons overshoot beyond the mutant tectum, suggesting that the MHB also serves a barrier function for axonal growth. By transplanting whole eye primordia, we show that mapping defects and overshooting largely, but not exclusively, depend on tectal, but not retinal genotype, and thus demonstrate an independent function for Fgf8 in retinal development. The MHB organizer, possibly via Fgf8 itself, is thus required for midbrain polarisation and for restricting axonal growth, but other cell populations may also influence midbrain polarity.

A reliable method for organ culture of neonatal mouse retina with long-term survival

Organ culture systems of the central nervous system have proven to be useful tools for the study of development, differentiation, and degeneration. Some studies have been limited by the inability to maintain the cultures over an extended period. Here we describe an organ culture technique for the mouse retina. This method uses commercially available supplies and reproducible procedures to maintain healthy retinas with normal architecture for 4 weeks in vitro. The system is amenable to quantitative analysis. It can be used with both normal and retinal degeneration (rd) retinas to study of the role of various factors in photoreceptor degeneration in retinal cell fate determination and development.

Differentiation and morphogenesis in pellet cultures of developing rat retinal cells

We previously developed a reaggregate cell culture system (pellet cultures) in which retinal neuroepithelial cells proliferate and give rise to rod photoreceptor cells (rods) in vitro (Watanabe and Raff, 1990, Neuron 4:461-467). In the present study, we analyzed cell differentiation and morphogenesis in pellet cultures by using both cell-type-specific markers with immunofluorescence and electron microscopy. We demonstrated that, in addition to rods, the other major retinal cell types, including amacrine cells, bipolar cells, Müller cells, and ganglion cells were all present in the pellets, where most were able to develop from dividing precursor cells in vitro. The different cell types in the pellets became organized into two distinct structures: dark rosettes and pale rosettes. The cellular composition of these structures indicated that the dark rosettes correspond to the outer nuclear layer and the pale rosettes to the inner nuclear layer of the normal retina. Ultrastructural studies have indicated that the thin layer of neuronal processes surrounding the dark rosettes correspond to the outer plexiform layer, and the central region of the pale rosettes correspond to the inner plexiform layer of the normal retina. Other features of normal retinal development also occurred in the pellets, including programmed cell death and the formation of inner and outer rod cell segments and synapses. Thus, pellet cultures provide a convenient way to study different aspects of retinal development where one can control the size and the cellular composition of the initial reaggregate.

Synaptic organization of an organotypic slice culture of the mammalian retina

Vertical slices of postnatal day 6 (P6) rat retina were cut and cultured using the roller-tube technique. The organotypic differentiation during a culture period of up to 30 days has been described in a previous study (Feigenspan et al., 1993a). Here we concentrated on the synaptic organization in the retinal slice culture. Electron microscopy revealed the presence of ribbon synapses in the outer plexiform layer and conventional and ribbon synapses in the inner plexiform layer. Immunofluorescence with antibodies that recognize specific subunits of GABAA or glycine receptors revealed a punctate distribution of the receptors. They were aggregated in "hot spots" that correspond to a concentration of receptors at postsynaptic sites. Different isoforms of GABAA and glycine receptors occurred in the slice cultures. The experiments show that there is a differentiation of synapses and a diversity of transmitter receptors in the slice cultures that is comparable to the in vivo retina.

Outer segment disc membrane assembly in the absence of the pigment epithelium: the effect of exogenous sugars

When retinal rudiments of Xenopus laevis embryos are allowed to differentiate in vitro, membrane precursors destined for the outer segment do not form well-ordered stacks of membrane leaflets if the pigment epithelium is removed from the outer retinal surface. However, addition of certain saccharides permits the organization of outer segment membranes into stacked membranous saccules, much like that demonstrated during in vivo or in vitro development in the presence of an investing pigment epithelium. Sugars that support the formation of well-organized photoreceptor outer segments include lactose, galactose and their derivatives, while numerous other carbohydrates failed to support outer segment organization.

Presynaptic fibres of spiral neurons and reciprocal synapses in the organ of Corti in culture

Isolated segments of the newborn mouse organ of Corti were explanted together with the spiral ganglion components. Within the innervation provided by the spiral neurons, we observed presynaptic vesiculated nerve endings that form reciprocal ribbon-afferent/efferent synapses with inner hair cells. These intracochlear presynaptic fibres are characteristically located between adjoining inner hair cells, on the modiolar side, low and close to the supporting cells. The presynaptic fibres display different modes of synaptic connectivity, forming repetitive reciprocal synapses on single inner hair cells or on adjoining hair cells, or connecting adjoining inner hair cells through simultaneous efferent synapses. Many presynaptic fibres exhibit a distinctive ultrastructure: defined clusters of synaptic vesicles, dense core vesicles, coated vesicles, and mitochondria. These organelles occur focally at the synaptic sites; beyond the efferent synaptic specializations, the endings appear quite nondescript and afferent-like. We believe that the reciprocal synapses, although observed in cultures of the organ of Corti, represent real intracochlear synaptic arrangements providing a feedback mechanism between the primary sensory receptors and a special class of spiral ganglion cells that have yet to be recognized in the organ in situ.

Organotypic slice culture of the mammalian retina

Vertical slices of 6-day postnatal (P6) rat retina were cut at a thickness of 100 microns and cultured using the roller-tube technique. After 14-21 days in vitro there was significant distortion of normal retinal architecture, but localized areas of the slices showed the typical pattern of layering of mature retina. The following immunocytochemical markers were used to characterize the different retinal cell types: antibodies against protein kinase C (PKC), calcium binding protein (CabP 28kD), neurofilaments (NF), glia-specific antibodies (GFAP, vimentin), and transmitter-specific antibodies (GABA, TH). The expression of these markers was compared in P6 retina, adult retina, and slice culture. To further characterize the cultured cells, patch-clamp recordings were performed in combination with intracellular injection of Lucifer Yellow (LY). Transmitter- and voltage-gated membrane currents were recorded from morphologically identified neurons. The experiments show that a mammalian slice culture can be used to study differentiation and function of retinal cell types.

Cell commitment and differentiation in explants of embryonic rat neural retina. Comparison with the developmental potential of dissociated retina

The differentiation of presumptive neural retina following its isolation from rat embryos and growth in explant and monolayer culture has been studied to obtain information regarding the extent to which factors extrinsic and intrinsic to the retina participate in determining molecular and cytological differentiation. Explanted retinal epithelium retained the capacity for mitosis, as shown by [3H]thymidine incorporation, and from the undifferentiated neuroepithelium, retinal cell-types emerged and acquired a laminar organization resembling that in vivo. Characterization of rod photoreceptor cells at both the light and electron microscopic level showed that these cells exhibit differentiated structural features including inner segments, connecting cilia and membranous expansions suggestive of forming outer segments. Immunofluorescent labeling with an antibody to a synaptic vesicle protein, and electron microscopic identification of synaptic elements showed formation of synapses by the photoreceptor cells within the explant. Neurites extending from the explants exhibited growth on laminin, fibronectin and collagen substrates. Since the neurites immunolabeled with antibodies to the 140 kDa subunit of neurofilament and with antibodies to Thy-1, they could be identified as axons of ganglion cells. Antibodies to a variety of cell-type specific antigens showed that the cells expressed molecules associated with the fully differentiated cell. Furthermore, since our approach has been to explant embryonic retina at an age when the antigens are not yet expressed in vivo, the appearance of the antigens in culture represented de novo expression. In contrast, neural retinal cells in dissociated cultures did not exhibit de novo expression of differentiated molecular properties.(ABSTRACT TRUNCATED AT 250 WORDS)

Histotypic differentiation of neonatal mouse retina in organ culture

Retinae from neonatal mice were explanted in toto, with or without the retinal pigment epithelium (RPE) and adjoining mesenchymal cells, and maintained in organ culture for up to 3 weeks. The explants remained flat, rosette formation was minimal and histogenetic changes followed in the normal sequence. After 11, 14 and 21 days in vitro the three cellular layers--the outer nuclear layer including well differentiated rod and cone perikarya, the inner nuclear layer and the ganglion cell layer--with the intervening plexiform layers were comparable to those of the in vivo eyes. Electron microscopic analysis revealed that in the explants without RPE the nuclear layers developed as in vivo, but receptor outer segments (ROS) were not formed. When the RPE was present, receptor inner segments appeared normal and ROS including profuse disc structures were developed. Presence of synaptic elements was also recognized. Mesenchymal cells, when present differentiated into choroidal and scleral tissues and appeared to play a supportive role for the RPE cells. The system is described in detail and its suitability for the analysis of various cellular and metabolic factors in the development of the retina is discussed.

Retinal transplants into the anterior chamber of the rat eye

Developing retinas from 13-18-day fetuses and 2-day neonatal Long-Evans rats transplanted into the anterior chamber of adult eyes of the same or different strain (Lewis) survive and differentiate. Light and electron microscopic studies show that the transplants undergo histogenetic differentiation, resulting in the development of neurons and Müller glial cells and formation of nuclear and plexiform layers. Vascular connections develop between the host iris and the retinal transplant. Vessels and nerves, presumably of iridal origin, were seen on the surface of some transplants. Possible manifestations of graft rejection were monitored; signs of tissue rejection in transplants performed in the Long-Evans rats, an outbred strain, were rare and if present they were mild, at least during the survival periods of up to 91 days allowed in these experiments. Transplants into the eyes of Lewis rats were also well tolerated during the survival period. These observations indicate that retinal transplantation to the adult eye of a genetically different host can be successfully achieved and that both embryonic and perinatal retinas are suitable as donor tissue for ocular transplants. The procedure offers ample opportunities for the study of problems related to retinal plasticity.

Newborn rat retinal cells transplanted into a retinal lesion site in adult host eyes

We report the successful grafting of embryonic (newborn) rat retina into a lesion site (die-back zone) of an adult retina with a corresponding 90-100% survival rate. A penetrating lesion was made through the sclera, choroid and retina on the superior surface of the host eye and closed with microsutures. The lesion site was either allowed to stabilize for 5 weeks or immediately received a retinal graft. Retinas were removed from 1-day-old neonate donors, drawn through a small gauge needle and injected into the fresh or stabilized lesion site. Host animals were sacrificed and the eyes processed for light, scanning- and transmission-electron microscopic analysis at 1, 2, and 4 weeks after grafting. Analysis of sections through grafted tissue within the lesion site revealed that the neonatal cells not only survived at all times examined but also continued their development reminiscent of normal littermate controls. Examination of 4-week grafts revealed a laminar pattern similar to adult ganglion cell, inner plexiform, inner nuclear, and outer plexiform layers as well as developing photoreceptor neurons. The grafted tissue could easily be delivered into the retinal lesion site where it established a pattern of retinal layers within the die-back zone. In addition, the plexiform areas of the graft appeared to integrate with those of the host. The age of the adult retina lesion transplantation site had little effect on the graft/host integrative phenomenon. These studies show for the first time a method utilizing immature retinal grafts to fill and/or bridge the wound area of the lesioned adult mammalian retina. These observations also demonstrate the utility of using this model for the study of numerous retinal developmental phenomena.

Neurotrophic stimulation of fetal rat retinal explant neurite outgrowth and cell survival: age-dependent relationships

Serum-free tissue culture conditions have been defined where stimulation of neurite outgrowth from fetal rat retinal explants occurred only in the presence of an active fraction (BE) prepared from a pig brain extract purification procedure. Under these conditions, 18-20-day fetal retinal explants survived and continued to extend long radial neurites for at least 3 weeks in the presence of BE. However, if fibronectin was not equilibrated onto the basic collagen/poly-L-lysine substrate the neurite outgrowth was restricted to a short halo about the circumference of the explant. In addition, a dose-response relationship was demonstrated in the presence of increasing concentrations of BE with respect to the neurite growth index. The half-maximal response for BE was estimated to be between 5 and 10 micrograms/ml. In addition a number of important age-dependent relationships were observed with respect to BE stimulation of retinal neurite outgrowth and cell survival. An inverse relationship was demonstrated between increased developmental age and responsiveness to BE. After 1 week in culture, there was a 3-fold reduction in retinal neurite length measured from the 2-day neonatal explant when compared to that of the 18-day fetus. There was also a significant inverse relationship demonstrated between the length of time before BE was added to the culture medium and the ability of 20-day fetal explants to extend neurites onto the culture substrate. If BE was added as late as 2 weeks after initial explant culture, the various neurite outgrowth indices were significantly lower than in those situations where BE was added at the time of culture or 1 week later. These results imply that BE not only is required for stimulating neurite outgrowth from fetal rat retinal explants, but may be important in survival and maturation of developing retinal neurons. This hypothesis was confirmed when morphometric analysis was performed on 16- and 20-day explants cultured for a week in the presence or absence of BE. The number of necrotic cells in the developing retinal ganglion plexiform-cell layer of 20-day fetal explants was significantly lower when treated with BE. Conversely, the density of identifiable differentiating retinal ganglion-like cells was significantly greater in response to BE treatment in both 16- and 20-day retinal explants.

Promotion of neurite outgrowth and cell survival in dissociated fetal rat retinal cultures by a fraction derived from a brain extract

We have reported previously that a fraction (BE) derived from a pig brain extract stimulated neurite outgrowth and cell survival from fetal rat retinal explants. The BE effects were dose dependent and could not be altered by NGF or its antiserum. In the present study we have observed that under similar culture conditions BE was also capable of stimulating neurite outgrowth and cell survival from fetal rat dissociated retinal cells. More specifically, the neurite-promoting activity of BE was found to be dose dependent over a concentration range of 0-50 micrograms/ml with a half-maximal response between 5 and 10 mg/ml. The ability of BE to stimulate neurite outgrowth was also age-related. There was a progressive decrease in the BE-mediated response between fetal day 17 and the second neonatal day. Viable process bearing cells could also be maintained in culture for at least two weeks in the presence of BE (25 micrograms/ml). In contrast, after 1 day in culture control cells began to rapidly degenerate and by days 3-5 no process-bearing cells were observed. The BE was found to exert its action primarily through a soluble factor(s) in the culture medium. However, we also report evidence for a substrate bound component of the BE which may aid in the attachment and/or neurite outgrowth phenomena.

Ganglion cells in retinae transplanted to newborn rats

Cells projecting out of retinal transplants placed over the tectum of newborn rats were studied by labelling with horseradish peroxidase 1 month or more after transplantation. Using this technique, it was found that only cells with the dendritic characteristics of ganglion cells were labelled and, furthermore, that the major classes of ganglion cells seen in normal retinae were also present in the transplants. The cell body size histograms of ganglion cells in normal and transplanted retinae compared closely with each other. Dendritic trees were closely confined by the limits of the inner plexiform layer, and if that layer was folded or distorted, they were themselves frequently abnormal. While axons usually coursed over the surface of the retinal transplants, they quite often followed an anomalous course crossing the individual layers. It appears, therefore, that this transplantation procedure has relatively little impact on the ability of ganglion cells to develop many of their characteristic morphological features. Whether the different functional responses of the various ganglion cell classes are also preserved after transplantation is a matter for further investigation.

Immunocytochemical study on photoreceptor cell differentiation in the cultured retina of the chick

Photoreceptor cell differentiation was investigated in a dissociated monolayer culture of chick embryonic retinas with electron microscopic immunohistochemistry. The antibody was raised against bovine rhodopsin purified on SDS-polyacrylamide gel electrophoresis. In the developing retina, immunoreactivity was first recognized on the 14th day of incubation and was localized on the plasma membrane of the growing inner segment. On the 16th day, immunoreactivity was observed on some differentiating outer segments but not on inner segments. In the culture from 6 1/2-day-old embryonic retinas, immunoreactivity was found on the 7th day of culturing on the plasma membrane of large-sized neurons. Electron microscopic observations confirmed that such stained cells showed reaction product on the plasma membrane, and that they displayed fine structures characteristic of intact photoreceptor cells. They had a number of microvillous processes and often one thick process, both of which were intensely stained. Outer segment formation, however, was not observed in the present monolayer culture. These results indicate that opsin synthesis and its transport to the plasma membrane begins prior to and probably independently of outer segment formation.

Ectopic photoreceptor cells and cell death in the developing rat retina

To confirm the identity of presumed photoreceptor-like neurones displaced from their normal location in the developing retina we have examined their morphology and extent of occurrence in the Long-Evans hooded rat aged one to six weeks postnatally. Displaced photoreceptor cells (PR) in the inner nuclear layer showed changing nuclear chromatin patterns during retinal development akin to those occurring in PR cells in the outer nuclear layer. PR cell cytoplasmic specializations included outer segments in various stages of formation and presynaptic terminal features including synaptic ribbons and vesicles. Processes abutting on PR cell terminals did not have postsynaptic specializations. Displaced PR cells may have arisen from PR progenitors which failed to retain a foothold at the retina's ventricular surface. The incidence of displaced PR cells determined from quantification of their planimetric densities decreased from 18% of the INL cell population at day 9 postnatally to less than 2% at day 15. A few such cells remained even at 36 days. Their fate appeared to be migration to the ONL and, or, in situ degeneration. Counts of necrotic cells carried out at ages preceding, during, and following the period during which ectopic PR cells were most numerous indicated that the decline in numbers of displaced PR cells coincided temporally with the period during which cell degeneration in the INL was most prominent. Degeneration of cells in the INL, including ectopic PR cells, was sufficient to account for a considerable proportion of the retinal thinning that occurred during development. Results suggest that future studies of retinal development in genetically or experimentally manipulated animals should consider abnormalities in cell migration and death.

Radiosensitivity and differentiation of ganglion cells within fetal mouse retinal explants in vitro

Fetal mouse retinas were explanted at 13-14 days of gestation, and exposed to gamma radiation in vitro. Not all regions of the retina were equally susceptible to radiation-induced necrosis; when exposed to 5000 rads soon after explanation, each explant had a single small radioresistant nubbin of apparently intact tissue, located near the optic nerve-head. This region of radioresistant tissue was larger when the dose of radiation was reduced and when the explants were exposed at later times in vitro, indicating the existence of a gradient of radioresistance across retinal explants which spread outward through at least the first week in vitro, the period examined. Based upon the extensive in situ literature which has correlated the emergence of radioresistance with the differentiation of retinal neurons, we conclude that the in situ central-to-peripheral sequence of cellular differentiation continues in vitro within our retinal explants. Whereas the ganglion cell axonal outgrowth from control retinas grown in isolation on collagen substrates underwent a gradual disintegration over 3 weeks in vitro, the sparse axonal outgrowth from explants exposed to 5000 rads disintegrated abruptly at 5-7 days in vitro. This did not appear to be due to direct damage from radiation, but instead reflected the fact that axons in irradiated cultures arose from central retinal regions only, while many axons in control cultures emerged from later-differentiating peripheral regions. We suggest that disintegration of individual axons in the outgrowth may occur rapidly and in a central-to-peripheral sequence. These findings should be useful in designing assays for trophic factors which may prevent ganglion cell axon disintegration in this in vitro model system.

An ultrastructural analysis of the development of foetal rat retina transplanted to the occipital cortex, a site lacking appropriate target neurons for optic fibres

Foetal retina was removed from donor rats at 15 days of gestation and transplanted to the occipital cortex of neonatal host rats. The purpose of this procedure was to examine the development of retinal neurons and photoreceptors, and document synaptic patterns during maturation of the transplanted retina in an environment lacking a normal target for optic axons. Host animals were sacrificed at 5, 10, 15, 20 and 30 days and samples of cortex containing the transplant were subjected to a light and electron microscopic analysis. During early stages of development, (5 days) the retina assumes a radial orientation with the scleral (outer) surface located centrally and the vitreal (inner) surface occupying the periphery. Numerous mitotic figures are found at the centre of the transplant and columns of primitive neuroblasts appear to radiate out from this zone. By 10 to 15 days after transplantation the retinal tissue contains numerous small rosettes each of which displays a histotypic organization with recognizable layers of sensory cells and their centrally-projecting processes, an outer limiting membrane, made up of a network of zonulae adherentes, and a rudimentary outer and inner plexiform layer which delineate the cells of the inner nuclear layer. Ultrastructural analysis of such rosettes confirmed the presence of typical bipolar, amacrine, horizontal and ganglion cells, but revealed that while the plexiform layers were occupied by numerous processes from these neurons, few if any, of these exhibited synaptic vesicles. By 20 to 30 days following transplantation sensory cells have completely differentiated, giving rise to prominent inner and outer segments which display typical cilia, centrioles and basal bodies, together with numerous stacked lamellae of photoreceptors which were contorted, presumably due to growth in an abnormal site. It should be further emphasized that these structures developed in the absence of pigment cells. Synaptic development ensues during this period to form characteristic dyads within the outer and inner plexiform layers. Additionally, clusters of amacrine to amacrine contacts occurred in the inner plexiform layer and were found to be increased relative to other types of junctions. In general, synaptogenesis takes place in the outer and inner plexiform layers and all categories of retinal synapses are established, but the process was found to be significantly delayed in comparison to normal retina at the same stage of development. Quantitative analysis revealed a reduced number of presumptive ganglion cells in proportion to the other categories of neurons. Optic fibres remained small and failed to myelinate. It is suggested that lack of an appropriate target for optic axons induced this alteration and may be indirectly related to the delay in the onset of synaptic development.

Transplantation of reaggregates of embryonic neural retinae to neonatal rat brain: differentiation and formation of connections

Embryonic day 14 neural retinae were dissociated into single cells and reaggregated prior to transplantation over the superior colliculi of newborn rats. One month after transplantation, the brains of the host rats were examined for transplant differentiation and the formation of projections from the transplant to the host brain. All reaggregated retinal transplants differentiated in the host brain, showing normal lamination and cellular morphology. Electron microscopic examination demonstrated normal synaptology with the transplanted retinal neuropil. Horseradish peroxidase injections into the host superior colliculus retrogradely filled cells within the transplant. These cells were found in the lamina corresponding to the ganglion cell layer and displayed a morphology characteristic of normal ganglion cells. Lesions of the transplants confirmed the projection of the reaggregated retinal transplants to superior colliculus. Degeneration was also traced into a number of ther nuclei that are normally retinorecipient. The presence of degenerating synaptic terminals resembling those seen after eye removal in control rats was confirmed by electron microscopic examination. It appears that despite disruption of initial cell-cell associations early in retinal development and prior to transplantation, the reaggregated transplants retain the ability to differentiate and form appropriate connection with the host brain.

Cultured embryonic retinae transplanted to rat brain: differentiation and formation of projections to host superior colliculus

Retinae of rats on embryonic day 14 were placed in explant culture for 2-14 days prior to transplantation adjacent to the superior colliculus of newborn rats. In explant culture cell division and neuronal differentiation continued unabated. One month after transplantation host brains were examined for transplant survival, differentiation and formation of projections to the host brain. The cultured retinal transplants survived and developed a morphology typical of mature retina, with normal cell and fiber laminae present. HRP injections into the host superior colliculus labeled neurons in the ganglion cell layer of the transplant which closely resembled ganglion cells in vivo. A small number of transplants received lesions. Degeneration material was traced into the superior colliculus and pretectal nuclei confirming that the cultured transplants had projections appropriate for retina entering the host brain. These results correlate closely with those seen after transplantation of embryonic rat retinae that had not been cultured. Thus, a period of explant culture of up to two weeks does not affect the ability of embryonic retinal transplants to differentiate and form projections into the host brain.

Development of ganglion cells and their axons in organized cultures of fetal mouse retinal explants

Retinas from 13-15 day fetal mice were explanted alone, with adjacent eyeball tissue, or with nearby superior colliculus explants. The organotypic structure of the retina developed in situ, including photoreceptors, interneurons, plexiform layers, ganglion cells, and an optic fibre layer. Electrophysiologic recordings demonstrated that functional synaptic networks developed resembling bioelectric response patterns seen in situ. Within half-retinas, arrays of optic fibers converged to the optic nerve head; in co-cultures with tectum they could become myelinated. Large bundles of long, naked neurites--1 degree primary retinal fibers--emerged from the explant in the first few days in vitro; these could often be traced back to the optic nerve head and a detailed survey of their properties using horseradish peroxidase (HRP) tracing methods identified tham as ganglion cell axons. When growing upon collagen substrata, 1 degree fibers began to disintegrate during the second week in vitro; however, many 1 degree fibers that grew into superior colliculus explants were maintained for at least 5 weeks in vitro, where they formed elaborate, functional terminal arborizations. In a few cases, 1 degree fibers grew across skeletal muscle fibers and appeared to induce them to contract. A second type of neuritic outgrowth pattern appeared after the first week in vitro: 2 degrees retinal fibers. This was composed of a mixed population of interneuronal neurites; a small percentage was catecholaminergic. Our characterization of the morphologic properties of retinal ganglion cells and their axons in organotypic cultures provides the necessary background to interpret electrophysiologic mapping and neural-specificity analyses of retino-CNS co-cultures. This in vitro model system may have biological relevance to understanding the cues that control the development of the retinotectal projection in situ.

Specific projections of retina transplanted to rat brain

Retinae were taken from fetal rats and transplanted adjacent to the superior colliculus of neonatal rats. After 1 month survival, the transplants were surgically removed from the hosts, locally damaged or injected with horseradish peroxidase (HRP) to determine the distribution of the transplant efferents in the host brains. Histological examination of the transplants revealed cell and plexiform layers characteristic of normal retinae. Since the retinae were undifferentiated at the time of transplantation, this layering developed within the host. The only obvious differences from normal retina were that the layers were organized in rosettes or folded sheets and lacked well developed photoreceptor outer segments. In animals which had lesions or HRP injections confined to the retinal transplant, proper staining of sections of the host brain revealed transplant projections. These projections were confined to the optic tract and nuclei which are normally retinorecipient such as the superior colliculus and dorsal lateral geniculate nucleus. Projections were found along the border of non-retinorecipient nuclei such as the lateral posterior nucleus, but did not appear to enter these nuclei. It was observed that within the superior colliculus the host retinal input had an effect on the distribution of the transplant projection. In one-eyed hosts the transplant projection was distributed throughout the stratum (s.) zonale, s. griseum superficiale, and s. opticum; whereas in the two-eyed hosts, the transplant projection was confined to the s. zonale and the border between x. griseum superficiale and s. opticum. We suggest that a special affinity exists between the axons of the retinal transplants and host visual structures. Furthermore, factors, such as competition and timing may be important in determining the distribution of the transplant axons within the specific target nuclei. Transplantation appears to be a useful technique for further studies on the mechanisms underlying the development of specific neuronal connections.

Human retinal dysplasia

We studied the ultrastructure of the four types of dysplastic rosettes and compared them with retinoblastoma rosettes. Dysplastic rosettes have morphologic characteristics intermediate between the normal photoreceptor layer and retino-blastoma rosettes; Müller cells contribute to the formation of dysplastic but not neoplastic rosettes. Abnormality in the relationship between the retina and the retinal pigment epithelium is frequent in cases with spontaneously occurring retinal dsyplasia and is consistent with previous observations that the retinal pigment epithelium influences the development of retinal morphology and function. We believe the normal developmental sequence of cell death and disappearance of necrotic cells may have gone awry in retinal dysplasia.

Renewal of normal and degenerating photoreceptor outer segments in the Ozark cave salamander

To determine whether photoreceptor degeneration in the Ozark cave salamander is associated with cessation or changes in the kinetics of outer segment (OS) renewal, an autoradiographic study of 3H-leucine incorporation in photoreceptors was carried out. Six days after isotope injection rods and cones showed labeling in both inner and outer segments. Cone OS were diffusely labeled whereas rods contained a band of radioactivity at the base of the OS. At 13 and 21 days the radioactive band in rods was located progressively nearer the distal tip of the OS. The rate of rod OS renewal ranged from 0.30 to 0.38 mu of OS length per day at 18 degrees C. L-thyroxin induced metamorphosis and light increased the renewal rate compared to larvae in darkness, and adults with photoreceptors in an early stage of degeneration had a slightly higher renewal rate than larvae. Light and electron microscope autoradiographs of degenerate photoreceptors revealed that even in the final stages of degeneration when OS are reduced to small, irregular whorls of membrane, 3H-leucine labeling was present in inner segments and OS membranes. These observations demonstrate that OS renewal occurs in both larvae and adults, and suggest that photoreceptor degeneration may be due to disruption of some aspect of the OS disposal process.