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

Retinal Explant Culture from Mouse, Human, and Nonhuman Primates and Its Applications in Vision Research

The retinal explant culture system is a valuable tool for studying the pharmacological, toxicological, and developmental aspects of the retina. It is also used for translational studies such as gene therapy. While no photoreceptor-like cell lines are available for in vitro studies of photoreceptor cell biology, the retinal explant culture maintains the laminated retinal structure ex vivo for as long as a month. Human and nonhuman primate (NHP) postmortem retinal explants cut into small pieces offer the possibility of testing multiple conditions for safety and adeno-associated viral (AAV) vector optimization. In addition, the cone-enriched foveal area can be studied using the retinal explants. Here, we present a detailed working protocol for retinal explant isolation and culture from mouse, human, and NHP for testing drug efficacy and AAV transduction. Future applications of this protocol include combining live imaging and multiwell retinal explant culture for high-throughput drug screening systems in rodent and human retinal explants to identify new drugs against retinal degeneration.

Viability of mitochondria-labeled retinal ganglion cells in organotypic retinal explant cultures by two methods

Retinal explant cultures provide a valuable system to study retinal function in vitro. This study established a new retinal explant culture method to prolong the survival of retinal ganglion cells (RGCs). Explants were prepared in two different ways: with or without optic nerve. Retinas from newborn mice that had received an injection of MitoTracker Red into the contralateral superior colliculus to label axonal mitochondria were cultured as organotypic culture for 7 days in vitro. At several time points during the culture, viability of RGCs was assessed by multi-electrode array recording, and morphology by immunohistochemical methods. During the culture, the thickness of the retinal tissue in both groups gradually decreased, however, the structure of the layers of the retina could be identified. Massive apoptosis in the retinal ganglion cell layer (GCL) appeared on the first day of culture, thereafter the number of apoptotic cells decreased. Glial activation was observed throughout the culture, and there was no difference in morphology between the two groups. RGCs loss was exacerbated on 3rdday of culture, and RGCs loss in retinal explants with preserved optic nerve was significantly lower than in retinas that did not preserve the optic nerve. More and longer-lasting mitochondrial signals were observed in the injured area of the optic nerve-preserving explants. Retinal explants provide an invaluable tool for studying retinal function and developing treatments for ocular diseases. The optic nerve-preserving culture helps preserve the integrity of RGCs. The higher number of mitochondria in the nerve-preserving cultures may help maintain viability of RGCs.

Large-scale phenotypic drug screen identifies neuroprotectants in zebrafish and mouse models of retinitis pigmentosa

Retinitis pigmentosa (RP) and associated inherited retinal diseases (IRDs) are caused by rod photoreceptor degeneration, necessitating therapeutics promoting rod photoreceptor survival. To address this, we tested compounds for neuroprotective effects in multiple zebrafish and mouse RP models, reasoning drugs effective across species and/or independent of disease mutation may translate better clinically. We first performed a large-scale phenotypic drug screen for compounds promoting rod cell survival in a larval zebrafish model of inducible RP. We tested 2934 compounds, mostly human-approved drugs, across six concentrations, resulting in 113 compounds being identified as hits. Secondary tests of 42 high-priority hits confirmed eleven lead candidates. Leads were then evaluated in a series of mouse RP models in an effort to identify compounds effective across species and RP models, that is, potential pan-disease therapeutics. Nine of 11 leads exhibited neuroprotective effects in mouse primary photoreceptor cultures, and three promoted photoreceptor survival in mouse rd1 retinal explants. Both shared and complementary mechanisms of action were implicated across leads. Shared target tests implicated parp1-dependent cell death in our zebrafish RP model. Complementation tests revealed enhanced and additive/synergistic neuroprotective effects of paired drug combinations in mouse photoreceptor cultures and zebrafish, respectively. These results highlight the value of cross-species/multi-model phenotypic drug discovery and suggest combinatorial drug therapies may provide enhanced therapeutic benefits for RP patients.

Newly Identified Chemicals Preserve Mitochondrial Capacity and Decelerate Loss of Photoreceptor Cells in Murine Retinal Degeneration Models

Purpose: Metabolic stress and associated mitochondrial dysfunction are implicated in retinal degeneration irrespective of the underlying cause. We identified seven unique chemicals from a Chembridge DiverSET screen and tested their protection against photoreceptor cell death in cell- and animal-based approaches. Methods: Calcium overload (A23187) was triggered in 661W murine photoreceptor-derived cells, and changes in redox potential and real-time changes in cellular metabolism were assessed using the MTT and Seahorse Biosciences XF assay, respectively. Cheminformatics to compare structures, and biodistribution in the living pig eye aided in selection of the lead compound. In-situ, retinal organ cultures of rd1 mouse and S334ter-line-3 rat were tested, in-vivo the light-induced retinal degeneration in albino Balb/c mice was used, assessing photoreceptor cell numbers histologically. Results: Of the seven chemicals, six were protective against A23187- and IBMX-induced loss of mitochondrial capacity, as measured by viability and respirometry in 661W cells. Cheminformatic analyses identified a unique pharmacophore with 6 physico-chemical features based on two compounds (CB11 and CB12). The protective efficacy of CB11 was further shown by reducing photoreceptor cell loss in retinal explants from two retinitis pigmentosa rodent models. Using eye drops, CB11 targeting to the pig retina was confirmed. The same eye drops decreased photoreceptor cell loss in light-stressed Balb/c mice. Conclusions: New chemicals were identified that protect from mitochondrial damage and lead to improved mitochondrial function. Using ex-vivo and in-vivo models, CB11 decreased the loss of photoreceptor cells in murine models of retinal degeneration and may be effective as treatment for different retinal dystrophies.

Selective Histone Deacetylase 6 Inhibitors Restore Cone Photoreceptor Vision or Outer Segment Morphology in Zebrafish and Mouse Models of Retinal Blindness

Blindness arising from retinal or macular degeneration results in significant social, health and economic burden. While approved treatments exist for neovascular (‘wet’) age-related macular degeneration, new therapeutic targets/interventions are needed for the more prevalent atrophic (‘dry’) form of age-related macular degeneration. Similarly, in inherited retinal diseases, most patients have no access to an effective treatment. Although macular and retinal degenerations are genetically and clinically distinct, common pathological hallmarks can include photoreceptor degeneration, retinal pigment epithelium atrophy, oxidative stress, hypoxia and defective autophagy. Here, we evaluated the potential of selective histone deacetylase 6 inhibitors to preserve retinal morphology or restore vision in zebrafish atp6v0e1^–/– and mouse rd10 models. Histone deacetylase 6 inhibitor, tubastatin A-treated atp6v0e1^–/– zebrafish show marked improvement in photoreceptor outer segment area (44.7%, p = 0.027) and significant improvement in vision (8-fold, p ≤ 0.0001). Tubastatin A-treated rd10/rd10 retinal explants show a significantly (p = 0.016) increased number of outer-segment labeled cone photoreceptors. In vitro, ATP6V0E1 regulated HIF-1α activity, but significant regulation of HIF-1α by histone deacetylase 6 inhibition in the retina was not detected. Proteomic profiling identified ubiquitin-proteasome, phototransduction, metabolism and phagosome as pathways, whose altered expression correlated with histone deacetylase 6 inhibitor mediated restoration of vision.

Particle-Mediated Gene Transfection and Organotypic Culture of Postmortem Human Retina

Purpose

Particle-mediated gene transfer has been used in animal models to study the morphology and connectivity of retinal ganglion cells. The aim of the present study was to apply this method to transfect ganglion cells in postmortem human retina.

Methods

Postmortem human eyes from male and female donors aged 40 to 76 years old were obtained within 15 hours after death. In addition, two marmoset retinas were obtained immediately after death. Ganglion cells were transfected with an expression plasmid for the postsynaptic density 95 protein conjugated to green or yellow fluorescent protein. Retinas were cultured for 3 days, fixed and then processed with immunohistochemical markers to reveal their stratification in the inner plexiform layer.

Results

The retinas maintained their morphology and immunohistochemical properties for at least 3 days in culture. Bipolar and ganglion cell morphology was comparable to that observed in noncultured tissue. The quality of transfected cells in human retina was similar to that in freshly enucleated marmoset eyes. Based on dendritic field size and stratification, at least 11 morphological types of retinal ganglion cell were distinguished.

Conclusions

Particle-mediated gene transfer allows efficient targeting of retinal ganglion cells in cultured postmortem human retina.

Translational Relevance

The translational value of this methodology lies in the provision of an in vitro platform to study structural and connectivity changes in human eye diseases that affect the integrity and organization of cells in the retina.

Laminin β2 Chain Regulates Retinal Progenitor Cell Mitotic Spindle Orientation via Dystroglycan

Vertebrate retinal development follows a pattern during which retinal progenitor cells (RPCs) give rise to all retinal cell types in a highly conserved temporal sequence. RPC proliferation and cell cycle exit are tightly coordinated to ensure proper and timely production of each of the retinal cell types. Extracellular matrix (ECM) plays an important role in eye development, influencing RPC proliferation and differentiation. In this study, we demonstrate that laminins, key ECM components, in the inner limiting membrane, control mitotic spindle orientation by providing environmental cues to the RPCs. In vivo deletion of laminin β2 in mice of both sexes results in a loss RPC basal processes and contact with the ECM, leading to a shift of the mitotic spindle pole orientation toward asymmetric cell divisions. This leads to decreased proliferation and premature RPC pool depletion, resulting in overproduction of rod photoreceptors at the expense of bipolar cells and Müller glia. Moreover, we show that deletion of laminin β2 leads to disruption and mislocalization of its receptors: dystroglycan and β1-integrin. Addition of exogenous β2-containing laminins to laminin β2-deficient retinal explants stabilizes the RPC basal processes and directs their mitotic spindle orientation toward symmetric divisions, leading to increased RPC proliferation, as well as restores proper receptor localization at the retinal surface. Finally, functional blocking of dystroglycan in wild-type retinal explants phenocopies laminin β2 ablation. Our data suggest that dystroglycan-mediated signaling between RPCs and the ECM is of key importance in controlling critical developmental events during retinogenesis.SIGNIFICANCE STATEMENT The mechanisms governing retinogenesis are subject to both intrinsic and extrinsic signaling cues. Although the role of intrinsic signaling has been the subject of many studies, our understanding of the role of the microenvironment in retinal development remains unclear. Using a combination of in vivo and ex vivo approaches, we demonstrate that laminins, key extracellular matrix components, provide signaling cues that control retinal progenitor cell attachment to the basement membrane, mitotic axis, proliferation, and fate adoption. Moreover, we identify, for the first time, dystroglycan as the receptor responsible for directing retinal progenitor cell mitotic spindle orientation. Our data suggest a mechanism where dystroglycan-mediated signaling between the cell and the extracellular matrix controls the proliferative potential of progenitors in the developing CNS.

Transplantation of Cultured Adult Porcine Full-Thickness Retina

In this study we wanted to examine how an adult neuroretina from an animal with an eye similar to the human one survives in vitro. We also wanted to investigate how the culture process affects the adult retina when used in a transplantation paradigm. Full-thickness neuroretinal sheets from adult porcine eyes were dissected into pieces measuring 3 mm in diameter. These were kept in culture for 1–3 days. After this time, the explants were fixed or transplanted subretinally to adult pigs, which were killed after 72–74 days. Transplanted eyes, as well as tissue kept in culture only, were processed for hematoxylin and eosin staining and immunohistochemistry. Explants kept 1 day in vitro (DIV) displayed the normal morphology. In these specimens, single pyknotic cells were evident in the outer nuclear layer (ONL) and ganglion cell layer, but were more frequent in the inner nuclear layer (INL). After longer times in vitro, severe degenerative changes appeared. Transplanted explants kept 1 DIV prior to transplantation exhibited normal retinal lamination in two out of four specimens. Transducin and recoverin labeling revealed photoreceptors with inner segments in these grafts. Rod bipolar cells displayed a normal morphology. Vertically arranged Mùller cells were also seen in the laminated grafts. Two of the three transplants kept 2 DIV displayed minimal lamination. Eyes with transplants kept 3 DIV prior to transplantation displayed degenerated grafts in all eyes. This study shows that adult porcine neuroretinal explants kept in culture for 1 day display a normal morphology in their major part. Additionally, 1-day explants can survive transplantation with retained morphology even after several months. This indicates the possibility of storing adult donor tissue between harvest and transplantation. The culture system may also be used in the future as a tool for manipulating retinal donor tissue prior to transplantation.

A quick, simple method for detecting circulating fluorescent advanced glycation end-products: Correlation with in vitro and in vivo non-enzymatic glycation

OBJECTIVE

Advanced glycation end-products (AGEs) constitute a highly heterogeneous family of compounds, relevant in the pathogenesis of diabetic complications, which could represent efficient biomarkers of disease progression and drug response. Unfortunately, due to their chemical heterogeneity, no method has been validated to faithfully monitor their levels in the course of the disease. In this study, we refine a procedure to quantitatively analyze fluorescent AGEs (fAGEs), a subset considered remarkably representative of the entire AGE family, and measure them in in vitro glycated BSA (gBSA) and in plasma and vitreous of diabetic rats, for testing its use to possibly quantify circulating AGEs in patients, as markers of metabolic control.

METHODS

fAGE levels were evaluated by spectrofluorimetric analysis in in vitro and in vivo experimental models. BSA was glycated in vitro with increasing D-glucose concentrations for a fixed time or with a fixed D-glucose concentration for increasing time. In in vivo experiments, streptozotocin-induced diabetic rats were studied at 1, 3, 6 and 12weeks to analyze plasma and vitreous. To confirm the presence of AGEs in our models, non-diabetic rat retinal explants were exposed to high glucose (HG), to reproduce short-term effects, or in vitro gBSA, to reproduce long-term effects of elevated glucose concentrations. Rat retinal explants and diabetic retinal tissues were evaluated for the receptor for advanced glycation end-product (RAGE) by Western blot analysis.

RESULTS

In in vitro experiments, fluorescence emission showed glucose concentration- and time-dependent increase of fAGEs in gBSA (p≤0.05). In streptozotocin-induced diabetic rats, fAGE in plasma and vitrei showed an increase at 6 (p≤0.005) and 12 (p≤0.05) weeks of diabetes, with respect to control. RAGE was time-dependently upregulated in retinas incubated with gBSA, but not with HG, and in diabetic retinal tissue, substantiating exposure to AGEs.

CONCLUSIONS

Applying the proposed technique, we could show that fAGEs levels increase with glucose concentration and time of exposure in vitro. Furthermore, in diabetic rats, it showed that circulating fAGEs are similarly upregulated as those in vitreous, suggesting a correlation between circulating and tissue AGEs. These results support the use of this method as a simple and reliable test to measure circulating fAGEs and monitor diabetes progression.

Photoreceptor Outer Segment-like Structures in Long-Term 3D Retinas from Human Pluripotent Stem Cells

The retinal degenerative diseases, which together constitute a leading cause of hereditary blindness worldwide, are largely untreatable. Development of reliable methods to culture complex retinal tissues from human pluripotent stem cells (hPSCs) could offer a means to study human retinal development, provide a platform to investigate the mechanisms of retinal degeneration and screen for neuroprotective compounds, and provide the basis for cell-based therapeutic strategies. In this study, we describe an in vitro method by which hPSCs can be differentiated into 3D retinas with at least some important features reminiscent of a mature retina, including exuberant outgrowth of outer segment-like structures and synaptic ribbons, photoreceptor neurotransmitter expression, and membrane conductances and synaptic vesicle release properties consistent with possible photoreceptor synaptic function. The advanced outer segment-like structures reported here support the notion that 3D retina cups could serve as a model for studying mature photoreceptor development and allow for more robust modeling of retinal degenerative disease in vitro.

A Method for the Isolation and Culture of Adult Rat Retinal Pigment Epithelial (RPE) Cells to Study Retinal Diseases

Diseases such as age-related macular degeneration (AMD) affect the retinal pigment epithelium (RPE) and lead to the death of the epithelial cells and ultimately blindness. RPE transplantation is currently a major focus of eye research and clinical trials using human stem cell-derived RPE cells are ongoing. However, it remains to be established to which extent the source of RPE cells for transplantation affects their therapeutic efficacy and this needs to be explored in animal models. Autotransplantation of RPE cells has attractions as a therapy, but existing protocols to isolate adult RPE cells from rodents are technically difficult, time-consuming, have a low yield and are not optimized for long-term cell culturing. Here, we report a newly devised protocol which facilitates reliable and simple isolation and culture of RPE cells from adult rats. Incubation of a whole rat eyeball in 20 U/ml papain solution for 50 min yielded 4 × 10(4) viable RPE cells. These cells were hexagonal and pigmented upon culture. Using immunostaining, we demonstrated that the cells expressed RPE cell-specific marker proteins including cytokeratin 18 and RPE65, similar to RPE cells in vivo. Additionally, the cells were able to produce and secrete Bruch's membrane matrix components similar to in vivo situation. Similarly, the cultured RPE cells adhered to isolated Bruch's membrane as has previously been reported. Therefore, the protocol described in this article provides an efficient method for the rapid and easy isolation of high quantities of adult rat RPE cells. This provides a reliable platform for studying the therapeutic targets, testing the effects of drugs in a preclinical setup and to perform in vitro and in vivo transplantation experiments to study retinal diseases.

Primary retinal cultures as a tool for modeling diabetic retinopathy: an overview

Experimental models of diabetic retinopathy (DR) have had a crucial role in the comprehension of the pathophysiology of the disease and the identification of new therapeutic strategies. Most of these studies have been conducted in vivo, in animal models. However, a significant contribution has also been provided by studies on retinal cultures, especially regarding the effects of the potentially toxic components of the diabetic milieu on retinal cell homeostasis, the characterization of the mechanisms on the basis of retinal damage, and the identification of potentially protective molecules. In this review, we highlight the contribution given by primary retinal cultures to the study of DR, focusing on early neuroglial impairment. We also speculate on possible themes into which studies based on retinal cell cultures could provide deeper insight.

Physical cues of cell culture materials lead the direction of differentiation lineages of pluripotent stem cells

Differentiation methods of hPSCs into specific cell lineages. Differentiation of hPSCsviaEB formation (types AB, A–D) or without EB formation (types E–H).

Generation of three-dimensional retinal tissue with functional photoreceptors from human iPSCs

Many forms of blindness result from the dysfunction or loss of retinal photoreceptors. Induced pluripotent stem cells (iPSCs) hold great potential for the modelling of these diseases or as potential therapeutic agents. However, to fulfill this promise, a remaining challenge is to induce human iPSC to recreate in vitro key structural and functional features of the native retina, in particular the presence of photoreceptors with outer-segment discs and light sensitivity. Here we report that hiPSC can, in a highly autonomous manner, recapitulate spatiotemporally each of the main steps of retinal development observed in vivo and form three-dimensional retinal cups that contain all major retinal cell types arranged in their proper layers. Moreover, the photoreceptors in our hiPSC-derived retinal tissue achieve advanced maturation, showing the beginning of outer-segment disc formation and photosensitivity. This success brings us one step closer to the anticipated use of hiPSC for disease modelling and open possibilities for future therapies.

Two-photon microscopy reveals early rod photoreceptor cell damage in light-exposed mutant mice

Atrophic age-related and juvenile macular degeneration are especially devastating due to lack of an effective cure. Two retinal cell types, photoreceptor cells and the adjacent retinal pigmented epithelium (RPE), reportedly display the earliest pathological changes. Abca4(-/-)Rdh8(-/-) mice, which mimic many features of human retinal degeneration, allowed us to determine the sequence of light-induced events leading to retinal degeneration. Using two-photon microscopy with 3D reconstruction methodology, we observed an initial strong retinoid-derived fluorescence and expansion of Abca4(-/-)Rdh8(-/-) mouse rod cell outer segments accompanied by macrophage infiltration after brief exposure of the retina to bright light. Additionally, light-dependent fluorescent compounds produced in rod outer segments were not transferred to the RPE of mice genetically defective in RPE phagocytosis. Collectively, these findings suggest that for light-induced retinopathies in mice, rod photoreceptors are the primary site of toxic retinoid accumulation and degeneration, followed by secondary changes in the RPE.

Microglial cells in organotypic cultures of developing and adult mouse retina and their relationship with cell death

Organotypic cultures of retinal explants allow the detailed analysis of microglial cells in a cellular microenvironment similar to that in the in situ retina, with the advantage of easy experimental manipulation. However, the in vitro culture causes changes in the retinal cytoarchitecture and induces a microglial response that may influence the results of these manipulations. The purpose of this study was to analyze the influence of the retinal age on changes in retinal cytoarchitecture, cell viability and death, and microglial phenotype and distribution throughout the in vitro culture of developing and adult retina explants. Explants from developing (3 and 10 postnatal days, P3 and P10) and adult (P60) mouse retinas were cultured for up to 10 days in vitro (div). Dead or dying cells were recognized by TUNEL staining, cell viability was determined by flow cytometry, and the numbers and distribution patterns of microglial cells were studied by flow cytometry and immunocytochemistry, respectively. The retinal cytoarchitecture was better preserved at prolonged culture times (10 div) in P10 retina explants than in P3 or adult explants. Particular patterns of cell viability and death were observed at each age: in general, explants from developing retinas showed higher cell viability and lower density of TUNEL-positive profiles versus adult retinas. The proportion of microglial cells relative to the whole population of retinal cells was higher in explants fixed immediately after their dissection (i.e., non-cultured) from adult retinas than in those from developing retinas. This proportion was always higher in non-cultured explants than in explants at 10 div, suggesting the death of some microglial cells during the culture. Activation of microglial cells, as revealed by their phenotypical appearance, was observed in both developing and adult retina explants from the beginning of the culture. Immunofluorescence with the anti-CD68 antibody showed that some activated microglial cells were CD68-positive but others were CD68-negative. Flow cytometry using CD68-labeling revealed that the percentage of CD68-positive microglial cells was much higher in developing than in adult retina explants, despite the activation of microglia in both types of explants, indicating that CD68-labeling was more closely related to the maturity degree of microglia than to their activation. Some swollen activated microglial cells entered the outer nuclear layer in developing and adult cultured retinal explants, whereas this layer was devoid of microglia in non-cultured explants. There was no apparent correlation between the distribution of microglia and that of TUNEL-labeled profiles. However, some swollen activated microglial cells in the outer and inner nuclear layers engulfed clusters of cell nuclei that were negative or weakly positive for TUNEL. This engulfment activity of microglia mimicked that observed in degenerative pathologies of the retina. We conclude that organotypic cultures from developing retinas show a higher rate of cell viability and better preservation of the normal cytoarchitecture in comparison to those obtained from adult retinas. In addition, the features of microglial response in cultured retinal explants show them to be a useful model for studying interactions between microglial cells and degenerating neurons in retinal diseases.

Immunocytochemical analysis of misplaced rhodopsin-positive cells in the developing rodent retina

During the first postnatal weeks of the developing rodent retina, rhodopsin can be detected in a number of neuron-like cells in the inner retina. In the present study, we aim to characterize the morphology, number and staining characteristics of this peculiar population. Misplaced rhodopsin-positive cells (MRCs) were analyzed on retinas of four rodent species, labeled with various rhodopsin-specific antibodies. To investigate their possible relation with non-photoreceptor cells, sections were double-stained against distinct retinal cell types and proteins of the phototransduction cascade. The possibility of synapse formation and apoptosis were also investigated. In all species studied, misplaced cells comprised a few percent of all rhodopsin-positive elements. This ratio declined from the end of the second week and MRCs disappeared nearly completely from the retina by P24. MRCs resembled resident neurons of the inner retina, while outer segment-like processes were seen only rarely. MRCs expressed no other photopigment types and showed no colocalization with any of the bipolar, horizontal, amacrine and ganglion cell markers used. While all MRCs colabeled for arrestin and recoverin, other proteins of the phototransduction cascade were only detectable in a minority of the population. Only a few MRCs were shown to form synaptic-like endings. Our results showed that, during development, some rhodopsin-expressing cells are displaced to the inner retinal layers. Although most MRCs lack morphological features of photoreceptors, they contain some but not all, elements of the phototransduction cascade, indicating that they are most probably misplaced rods that failed to complete differentiation and integrate into the photoreceptor mosaic.

The expression and function of netrin-4 in murine ocular tissues

Netrin-4, a member of the netrin family, is a potent regulator of embryonic development. It promotes neurite extension and regulates pulmonary airway branching, vasculogenesis patterning, and endothelial proliferation in pathological angiogenesis. The initial characterization of netrin-4 expression was focused on epithelial-derived organs (kidney, lung and salivary gland) and the central nervous system. Ocular development is an ideal system to study netrin-4 expression and function, as it involves both ectodermal (cornea, lens and retina) and mesodermal (sclera and choroid) derivatives and has an extensive and well-characterized angiogenic process. Netrin-4 is expressed in all ocular tissues. It is a prominent component of the basement membranes of the lens and cornea, as well as all three basement membranes of the retina: the inner limiting membrane, vascular basement membranes, and Bruch's membrane. Netrin-4 is differentially deposited in vascular basement membranes, with more intense anti-netrin-4 reactivity on the arterial side. The retinal microcirculation also expresses netrin-4. In order to test the function of netrin-4 in vivo, we generated a conventional mouse lacking Ntn4 expression. Basement membrane formation in the cornea, lens and retina is undisrupted by netrin-4 deletion, demonstrating that netrin-4 is not a major structural component of these basement membranes. In the Ntn4 homozygous null (Ntn4-/-) cornea, the overall morphology of the cornea, as well as the epithelial, stromal and endothelial stratification are normal; however, epithelial cell proliferation is increased. In the Ntn4-/- retina, neurogenesis appears to proceed normally, as does retinal lamination. In the Ntn4-/- retina, retinal ganglion cell targeting is intact, although there are minor defects in axon fasciculation. In the retinal vasculature of the Ntn4-/- retina, the distribution patterns of astrocytes and the vasculature are largely normal, with the possible exception of increased branching in the deep capillary plexus, suggesting that netrin-4 may act as a negative regulator of angiogenesis. These data, taken together, suggest that netrin-4 is a negative regulator of corneal epithelial cell proliferation and retinal vascular branching in vivo, whereas netrin-4 may be redundant with other members of the netrin family in other ocular tissue development. Ntn4-/- mice may serve as a good model in which to study the role of netrins in vivo of the pathobiologic vascular remodeling in the retina and cornea.

Guiding the morphogenesis of dissociated newborn mouse retinal cells and hES cell-derived retinal cells by soft lithography-patterned microchannel PLGA scaffolds

Embryonic stem (ES) cell-derived photoreceptors are a promising cell source for enhanced in vitro models of retinal degenerative diseases, but the more differentiated characteristics of retinal cells do not typically develop in dissociated cell cultures. Therefore, we have reconstructed organized retinal tissue by seeding dissociated cells into an array of aligned units that more faithfully mimics the retina. We solvent-processed poly(lactic-co-glycolic acid) (PLGA) into a microchannel scaffold format to achieve this geometric constraint. We compared the effect of PLGA concentration on channel morphology and, along with other culture conditions, on the infiltration of dissociated newborn mouse retinal cells into the channels. Culturing scaffolds at the gas-liquid interface with low serum media increased infiltrated rod photoreceptor viability 18-fold over submerged, high serum cultures when evaluated after seven days. Rod photoreceptors and Müller glia aligned processes parallel to the microchannel walls. Otx2+ and Pax6+ subpopulations recapitulated lamination behavior. Further, we constructed scaffold/retinal pigment epithelium (RPE) co-cultures and observed rods extending rhodopsin-positive processes toward RPE cells, mimicking normal rod polarization and morphology. Finally, human embryonic stem cell-derived photoreceptors exhibited infiltration and morphological characteristics similar to mouse retinal cells inside the scaffolds. These findings constitute an important advance in generating tissue-level retinal models from dissociated cells for use as drug screening platforms and in regenerative medicine.

Human retinal development in an in situ whole eye culture system

Phenotypic characterization of human retinogenesis may be facilitated by use of the tissue culture system paradigm. Traditionally, most culture protocols involve isolation of retinal tissue and/or cells, imposing various degrees of trauma, which in many cases leads to abnormal development. In this paper, we present a novel culture technique using whole embryonic eyes to investigate whether the retina in situ can develop normally in vitro. All procedures were carried out in accordance with the Declaration of Helsinki. Human embryos were obtained from elective abortions with the informed consent of the women seeking abortion. A total of 19 eyes were enucleated. The ages of the embryonic retinas were 6-7.5 weeks. Eyecups from 2 eyes were fixed immediately, to be used as controls. The remaining 17 eyes were placed on culture plates and divided into 3 groups kept for 7 (n = 4), 14 (n = 7) and 28 (n = 6) days in vitro (DIV). After fixation, the specimens were processed for hematoxylin and eosin staining, immunohistochemistry and terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL). Antibodies against recoverin (rods and cones), protein kinase C (PKC; rod bipolar cells) and vimentin (Müller cells) were used. TUNEL was used to detect apoptotic cells. In hematoxylin- and eosin-stained sections, the control retinas displayed a neuroblast cell layer (NBL) and an inner marginal zone. Specimens kept 7-14 DIV had a similar appearance, while 28-day specimens consisted of an NBL with almost no marginal zone. Thirteen of the 17 cultured retinas displayed completely normal lamination without rosettes or double folds. Pyknotic cells were found at the inner margin of the retinas, and the proportion of these cells increased with time in vitro. TUNEL staining revealed a few scattered cells in 7-DIV specimens, and the amount of stained cells in the inner part of the retinas progressively increased in 14- and 28-DIV specimens. Vimentin labeling showed cells arranged in a vertical pattern in all retinas. Labeling with recoverin revealed photoreceptors in 4 of the retinas kept for 14 DIV, and in all retinas kept for 28 DIV. After 28 DIV, 2 of the eyes labeled with PKC contained rod bipolar cells with minimal axons. Here we showed that human embryonic retinas can be kept in culture in situ within the eye for at least 4 weeks. Abnormal lamination is not as frequent as in isolated full-thickness retinas, indicating that physical and biochemical contact with surrounding tissues is vital for proper development. Several types of the retina-specific neuronal and glial cells were seen to differentiate according to the in vivo schedule. The results are important for future studies of retinal development, and the technique can also be used for testing the effects of various drugs on the immature retina.

Self-organizing optic-cup morphogenesis in three-dimensional culture

Balanced organogenesis requires the orchestration of multiple cellular interactions to create the collective cell behaviours that progressively shape developing tissues. It is currently unclear how individual, localized parts are able to coordinate with each other to develop a whole organ shape. Here we report the dynamic, autonomous formation of the optic cup (retinal primordium) structure from a three-dimensional culture of mouse embryonic stem cell aggregates. Embryonic-stem-cell-derived retinal epithelium spontaneously formed hemispherical epithelial vesicles that became patterned along their proximal-distal axis. Whereas the proximal portion differentiated into mechanically rigid pigment epithelium, the flexible distal portion progressively folded inward to form a shape reminiscent of the embryonic optic cup, exhibited interkinetic nuclear migration and generated stratified neural retinal tissue, as seen in vivo. We demonstrate that optic-cup morphogenesis in this simple cell culture depends on an intrinsic self-organizing program involving stepwise and domain-specific regulation of local epithelial properties.

Gene therapy in ophthalmology: validation on cultured retinal cells and explants from postmortem human eyes

Gene therapy studies in primates can provide important information regarding vector tropism, specific cellular expression, biodistribution, and safety prior to clinical trials. In this study, we report the assessment of transduction efficiency of recombinant adeno-associated virus (rAAV) vectors using human postmortem retina. Transductions were performed using two in vitro models prepared from human tissue: dissociated cell cultures and retinal explants. These models were used to assess cellular tropism and selectivity of rAAV vectors encoding for fluorescent proteins under the control of different promoters. These promoters were a ubiquitous cytomegalovirus promoter and a cell type-specific promoter targeting expression in ON bipolar cells. The results demonstrate that this in vitro approach can limit the use of living primates for the validation of gene therapy in vision and ophthalmology.

Rod bipolar cells in the retina of the capuchin monkey (Cebus apella): Characterization and distribution

Rod bipolar cells in Cebus apella monkey retina were identified by an antibody against the alpha isoform of protein kinase C (PKCα), which has been shown to selectively identify rod bipolars in two other primates and various mammals. Vertical sections were used to confirm the identity of these cells by their characteristic morphology of dendrites and axons. Their topographic distribution was assessed in horizontal sections; counts taken along the dorsal, ventral, nasal, and temporal quadrants. The density of rod bipolar cells increased from 500 to 2900 cells/mm2 at 1 mm from the fovea to reach a peak of 10,000–12,000 cells/mm2 at 4 mm, approximately 5 deg of eccentricity, and then gradually decreased toward retinal periphery to values of 5000 cells/mm2 or less. Rod to rod bipolar density ratio remained between 10 and 20 across most of the retinal extension. The number of rod bipolar cells per retina was 6,360,000 ± 387,433 (mean ± s.d., n = 6). The anti-PKCα antibody has shown to be a good marker of rod bipolar cells of Cebus, and the cell distribution is similar to that described for other primates. In spite of the difference in the central retina, the density variation of rod bipolar cells in the Cebus and Macaca as well as the convergence from rod to rod bipolar cells are generally similar, suggesting that both retinae stabilize similar sensitivity (as measured by rod density) and convergence.

Early effects of high glucose in retinal tissue cultures Renin-Angiotensin system-dependent and -independent signaling

The early effects of the diabetic milieu on retinal tissue and their relation to the Renin-Angiotensin system (RAS) activation are poorly known. Here we investigated RAS signaling in retinas explanted from adult rats exposed for 48 h to high glucose (HG), with or without the Angiotensin Converting Enzyme inhibitor enalaprilat, which blocks RAS. HG was observed to i) initiate a phosphotyrosine-dependent signaling cascade; ii) up-regulate Angiotensin(1) Receptor (AT(1)R); iii) activate src tyrosine kinase and increase phosphorylation of Pyk2, PLCgamma1 and ERK1/2; and iv) activate Akt and the transcription factor CREB. In the presence of enalaprilat, tyrosine phosphorylation signal and AT(1)R upregulation decreased and activation of PLCgamma1 and CREB reverted, showing their relation to RAS signaling. In line with Akt activation, no apoptosis or synapse degeneration was found. Müller glia was activated, but in a RAS-independent manner. Our results suggest that, in early phases of HG exposure, a pro-survival cell program may be induced in the retina.

Regulation of cytokine signaling components in developing rat retina correlates with transient inhibition of rod differentiation by CNTF

Ciliary neurotrophic factor (CNTF) is known to inhibit the differentiation of rod photoreceptors from postmitotic precursor cells. During early postnatal development, photoreceptor precursors lose their responsiveness to CNTF. The underlying events causing this change in responsiveness are unknown. Moreover, whether rods express CNTF receptor alpha, a prerequisite for a direct response to the factor, is controversial. Since morphological studies have previously produced conflicting results, we have analyzed the expression of cytokine receptor components and potential ligands in the rat photoreceptor layer by real-time reverse transcription with the polymerase chain reaction after laser microdissection and by immunoblotting. Cytokine effects on rods were studied in explant cultures from newborn rat retina. CNTF receptor alpha (CNTFR alpha) and leukemia inhibitory factor receptor ss (LIFRss) were expressed in immature photoreceptors. Expression of the CNTF-specific alpha-subunit (but not of LIFRss) was downregulated specifically in the photoreceptor layer in parallel with the appearance of opsin-positive rods. The decrease of CNTFR alpha levels in explant cultures was closely correlated with the loss of precursor cell responsiveness to CNTF. Increasing the CNTF concentration in the culture medium led to prolonged CNTFR alpha expression and, concomitantly, to persistent inhibition of rod differentiation. Application of CNTF and LIF in vitro induced phosphorylation of STAT3. Inducibility of STAT3 activation by CNTF decreased with photoreceptor maturation, whereas the LIF effect persisted. Our results thus indicate that CNTF acts directly on photoreceptor precursors inhibiting their differentiation via activation of the JAK/STAT3 signal transduction pathway, and that this effect is temporally limited because of the downregulation of CNTFR alpha.

Survival, excitability, and transfection of retinal neurons in an organotypic culture of mature zebrafish retina

Over the last 20 years, the zebrafish has become an important model organism for research on retinal function and development. Many retinal diseases do not become apparent until the later stages of life. This means that it is important to be able to analyze (gene) function in the mature retina. To meet this need, we have established an organotypic culture system of mature wild-type zebrafish retinas in order to observe changes in retinal morphology. Furthermore, cell survival during culture has been monitored by determining apoptosis in the tissue. The viability and excitability of ganglion cells have been tested at various time points in vitro by patch-clamp recordings, and retinal functionality has been assessed by measuring light-triggered potentials at the ganglion cell site. Since neurogenesis is persistent in adult zebrafish retinas, we have also monitored proliferating cells during culture by tracking their bromodeoxyuridine uptake. Reverse genetic approaches for probing the function of adult zebrafish retinas are not yet available. We have therefore established a rapid and convenient protocol for delivering plasmid DNA or oligonucleotides by electroporation to the retinal tissue in vitro. The organotypic culture of adult zebrafish retinas presented here provides a reproducible and convenient method for investigating the function of drugs and genes in the retina under well-defined conditions in vitro.

Roller organ cultures of the retina from postnatal rats

Whole retinas of 2-14-day-old rats were cultured in a roller device for 2-14 days. Floating retinas of 7-14-day-old rats formed hole spheroid structures (spheroids) with the wall completely retaining the linear structure and layer-by-layer cellular and fibrous architecture, including the outer nuclear, outer plexiform, inner nuclear, inner plexiform layers, layers of ganglion cells and nerve fibers. The retina obtained at earlier terms of development often formed folds, with pyknotic nuclei of dead neurons in their deep compartments. In organ cultures of the retina isolated from rats at early postnatal periods, rosettes were formed in sites of local injury to the outer nuclear layer and pigmented epithelium. Roller organ cultures can be used for in vitro studies of the development and experimental diseases of the retina.

Levels of transient gap junctions between the retinal pigment epithelium and the neuroblastic retina are influenced by catecholamines and correlate with patterns of cell production

Retinal mitosis takes place at the interface between the retinal pigment epithelium (RPE) and the neural retina. Multiple studies have highlighted the essential role that gap junction-mediated communication plays in the regulation of retinal organogenesis. Here, the localization pattern and function of the gap junction protein connexin 43 were examined in vivo in the rat at the interface between the retina and RPE during the main phases of retinal cell production. Connexin 43 was expressed at this site from E15 onward, and levels were subsequently temporally regulated. When Cx43 protein levels were reduced experimentally, by using antisense oligodeoxynucleotides, mitotic activity in the retina decreased significantly. Conversely, in the hypopigmented eye elevated mitotic levels were associated with a significant increase of connexin 43. Both excess protein levels and elevated mitosis were corrected by the in vivo administration of L-DOPA (a dopamine precursor and intermediary compound in the melanin synthesis pathway). These findings suggest that connexin 43-mediated communication between the retina and RPE is essential for the correct pacing of retinal organogenesis. Furthermore, this pathway may be gated by levels of ocular catecholamines.

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.

Pals1/Mpp5 is required for correct localization of Crb1 at the subapical region in polarized Muller glia cells

Mutations in the human Crumbs homologue-1 (CRB1) gene cause retinal diseases including Leber's congenital amaurosis (LCA) and retinitis pigmentosa type 12. The CRB1 transmembrane protein localizes at a subapical region (SAR) above intercellular adherens junctions between photoreceptor and Müller glia (MG) cells. We demonstrate that the Crb1-/- phenotype, as shown in Crb1-/- mice, is accelerated and intensified in primary retina cultures. Immuno-electron microscopy showed strong Crb1 immunoreactivity at the SAR in MG cells but barely in photoreceptor cells, whereas Crb2, Crb3, Patj, Pals1 and Mupp1 were present in both cell types. Human CRB1, introduced in MG cells in Crb1-/- primary retinas, was targeted to the SAR. RNA interference-induced silencing of the Crb1-interacting-protein Pals1 (protein associated with Lin7; Mpp5) in MG cells resulted in loss of Crb1, Crb2, Mupp1 and Veli3 protein localization and partial loss of Crb3. We conclude that Pals1 is required for correct localization of Crb family members and its interactors at the SAR of polarized MG cells.

Making the gradient: thyroid hormone regulates cone opsin expression in the developing mouse retina

Most mammals have two types of cone photoreceptors, which contain either medium wavelength (M) or short wavelength (S) opsin. The number and spatial organization of cone types varies dramatically among species, presumably to fine-tune the retina for different visual environments. In the mouse, S- and M-opsin are expressed in an opposing dorsal-ventral gradient. We previously reported that cone opsin patterning requires thyroid hormone beta2, a nuclear hormone receptor that regulates transcription in conjunction with its ligand, thyroid hormone (TH). Here we show that exogenous TH inhibits S-opsin expression, but activates M-opsin expression. Binding of endogenous TH to TRbeta2 is required to inhibit S-opsin and to activate M-opsin. TH is symmetrically distributed in the retina at birth as S-opsin expression begins, but becomes elevated in the dorsal retina at the time of M-opsin onset (postnatal day 10). Our results show that TH is a critical regulator of both S-opsin and M-opsin, and suggest that a TH gradient may play a role in establishing the gradient of M-opsin. These results also suggest that the ratio and patterning of cone types may be determined by TH availability during retinal development.

Advanced glycation end products can induce glial reaction and neuronal degeneration in retinal explants

BACKGROUND/AIMS

Neuronal degeneration has been reported to occur in diabetic retinopathy before the onset of detectable microvascular abnormalities. To investigate whether advanced glycation end products (AGE) could be directly responsible for retinal neurodegeneration, retinal explants were incubated with glycated bovine serum albumin (BSA).

METHODS

Retinal explants obtained from non-diabetic adult rats were incubated 4 days with or without 200 mug/ml glycated BSA. Neural apoptosis was quantified by terminal dUTP nick end labelling (TUNEL) binding and immunostaining with anti-cleaved caspase-3 antibody. Expression of glial fibrillary acidic protein (GFAP) was localised by immunofluorescence.

RESULTS

TUNEL and cleaved caspase-3 positive cells increased significantly by 2.2-fold and 2.5-fold in retinal explants incubated in glycated BSA (p<0.05), respectively. The ganglion cell layer was the most sensitive retinal layer to the glycated BSA. Neuronal degeneration was confirmed by the increased GFAP labelling in Müller glial cells from retinal explants treated with glycated BSA.

CONCLUSION

These results suggest that AGE could induce retinal neurodegeneration in the absence of blood perfusion. Cells in the ganglion cell layer appeared to be the most sensitive as in diabetic retinopathy and its animal models. AGE toxicity could therefore contribute to the early pathological mechanisms of diabetic retinopathy.

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.

Development of the embryonic porcine neuroretina in vitro

PURPOSE

The objective of this study was to investigate the survival and morphology of embryonic porcine full-thickness neuroretina in culture.

METHODS

Porcine fetuses were taken out by cesarian section, and the eyes were enucleated. Neuroretinas were explanted on culture plate inserts and were kept for 0-42 days in vitro under standard culture conditions. Green nucleic acid (Sytox) was used for measuring the extent of cell death, and 4,6-diaminidine-2-phenylindoldihydrochloride was used as a marker for the cellular layers. The explants were examined as whole-mount preparations and vertical sections. Sectioned tissue was stained with hematoxylin-eosin and labeled for immunohistochemistry with photoreceptor-specific antibodies raised against transducin and recoverin.

RESULTS

In explants kept for 0-5 days in vitro, the developing retina consisted of multiple rows of neuroblastic cells and a more defined, but multilayered ganglion cell layer (GCL). Older explants revealed a more differentiated appearance with ultimately all normal retinal layers present, even after 42 days in vitro. Transducin- and recoverin-labeled photoreceptors were seen in these specimens, but no outer segments were found. The whole-mount preparation revealed extensively Sytox-labeled cells in the GCL at 2 days in vitro, but very few cells were labeled in older explants.

CONCLUSION

This study shows that cultured fetal porcine full-thickness neuroretina can survive and develop according to its intrinsic timetable for at least 6 weeks in vitro. The in vitro system for culturing of the full-thickness retina may be useful in experiments involving retinal transplantation.

Effects of brain-derived neurotrophic factor on cell survival, differentiation and patterning of neuronal connections and Muller glia cells in the developing retina

The aim of the present study was to determine the influence of brain-derived neurotrophic factor (BDNF) on survival, phenotype differentiation and network formation of retinal neurons and glia cells. To achieve a defined concentration and constant level of BDNF over several days, experiments were performed in an organotypic culture of the developing rat retina. After 6 days in vitro, apoptosis in the different cell layers was determined by TUNEL staining and cell-type-specific antibodies were used to identify distinct neuronal cell types and Müller cells. Cultured retinas treated with BDNF (100 ng BDNF/mL medium) were compared with untreated as well as with age-matched in vivo retinas. Quantitative morphometry was carried out using confocal microscopy. BDNF promoted the in vitro development and differentiation of the retina in general, i.e. the number of cells in the nuclear layers and the thickness of the plexiform layers were increased. For all neurons, the number of cells and the complexity of arborizations in the synaptic layers were clearly up-regulated by BDNF. In control cultures, the synaptic stratification of cone bipolar cells within the On- and Off-layer of the inner plexiform layer was disturbed and a strong reactivity of Müller cell glia was observed. These effects were not present in BDNF-treated cultures. Our data show that BDNF promotes the survival of retinal interneurons and plays an important role in establishing the phenotypes and the synaptic connections of a large number of neuronal types in the developing retina. Moreover, we show an effect of BDNF on Müller glia cells.

Developmental plasticity of photoreceptors

During development, retinal ganglion cells undergo conspicuous structural remodeling as they gradually attain their mature morphology and connectivity. Alterations in their dendritic organization and in their axonal projections can also be achieved following early insult to their targets or their afferents. Other retinal cell types are thought not to display this same degree of developmental plasticity. The present review will consider the evidence, drawn largely from recent experimental studies in the carnivore retina, that photoreceptors also undergo structural remodeling, extending their terminals transiently into inner plexiform layer before retracting to the outer plexiform layer. The determinants of this transient targeting to the inner plexiform layer are considered, and the role of cholinergic amacrine cells is discussed. The factors triggering this retraction are also considered, including the concurrent maturational changes in outer segment formation and in the differentiation of the outer plexiform layer. These results provide new insight into the life history of the photoreceptor cell and its connectivity, and suggest a transient role for the photoreceptors in the circuitry of the inner retina during early development, prior to the onset of phototransduction.

Vertical meridian asymmetry in spatial resolution: visual and attentional factors

We investigated whether spatial resolution would be the same in the lower and upper halves of the vertical meridian (VM) of our visual field and whether attention would affect them differentially. It has been reported that (1) attending to the target's location improves performance in a texture segregation task when the observer's spatial resolution is too low (peripheral locations) but impairs it when resolution is already too high (central locations) for the task. This finding indicates an enhanced spatial resolution at the attended location (Yeshurun & Carrasco, 1998,2000), (2) observers' contrast sensitivity is higher in the lower than in the upper VM, a phenomenon known as vertical meridian asymmetry (VMA), an asymmetry determined by visual rather than by attentional factors (Carrasco, Talgar, & Cameron, 2001). In the present texture segregation task, performance was assessed under neutral- and peripheral-cue conditions. Transient covert attention was systematically manipulated by using a peripheral cue that indicated the target's location and its onset. Observers reported the interval containing a target patch appearing at one of a number of eccentricities in a large texture pattern along the VM. We found that (1) performance peaked at farther eccentricities in the lower than in the upper visual VM, indicating that resolution was higher in the lower half, and (2) the peripheral cue affected performance along the VM uniformly, indicating that the degree of enhanced resolution brought about by transient attention was constant along the VM. Thus, we conclude that the VMA for spatial resolution is determined by visual, not transient covert attentional, constraints.

Mouse retina explants after long-term culture in serum free medium

The neonatal mouse retina remains viable as an explant in serum-supplemented growth media for more than 4 weeks. Interpretation of drug effects on this tissue is compromised by the enigmatic composition of the serum. We sought to remove this ambiguity by culturing neonatal as well as late postnatal mouse retina in serum-free nutrient medium. In this study three important observations were made, (1) there is histotypic development of neonatal as well as preservation of late postnatal mouse retinal structure during long-term culture in serum-free medium, although the late postnatal tissue tends to show some loss of cells in the outer nuclear layer. (2) Protein expression in explant photoreceptor cells was similar to that in the litter-matched ones, except for green cone opsin and interphotoreceptor retinoid-binding protein, although mRNA of the latter is present at similar amounts as in age-matched in vivo controls. (3) Cells of the inner retina stained by antibodies to calcium-binding proteins display some novel sprouting of processes. The results show that the mouse retina can be cultured as an explant for more than 4 weeks in a serum-free medium. This represents an important step forward because, (1) the possibility of interference of drug effects by unknown serum factors has been eliminated; and (2) the spent culture medium can be analyzed to investigate biomolecules released by the retina in vitro.

A comparative study of protein kinase C-like immunoreactive cells in the retina

The present study is a morphological and quantitative analysis of protein kinase C-like immunoreactive (PKC-L ir) bipolar cells in the retinas of five different vertebrate species (chicken, tench, zebrafish, goldfish and rat). The morphology of PKC-L-ir bipolar cell axon terminals in fish differs significantly from those of chicken and rat retinas. Fish have bulky terminals whereas chicken and rat have their terminals in the form of small knob-shaped branches. In tench and goldfish, PKC-L-ir bipolar cells gradually decrease in size from the medial (i.e., in tench: mean +/- SD soma area of 30.09 +/- 5.98 microm2) to the peripheral (i.e., in tench: 19.93 +/- 1.73 microm2) retinal regions. This is not observed in chicken, rat or zebrafish where there is more homogeneity in s oma and axon terminal sizes between different retinal regions. Except in chicken, cell density increases from the central (i.e., in tench: mean +/- SD 1795.88 +/- 242.35 cells/mm2) to the peripheral (i.e., in tench: 4295.41 +/- 279.23 cells/mm2) retina. This study provides data that show relevant differences in the PKC-L-ir bipolar morphology and density among birds, fish and mammals. Moreover, these structural variations could mean not only differences in the cellular physiology, but also in the patterns of development and maintenance of the retina in each species.