Angiogenesis in normal human retinal development: the involvement of astrocytes and macrophages

More than meets the eye: The role of microglia in healthy and diseased retina

Microglia are the main resident immune cells of the nervous system and as such they are involved in multiple roles ranging from tissue homeostasis to response to insults and circuit refinement. While most knowledge about microglia comes from brain studies, some mechanisms have been confirmed for microglia cells in the retina, the light-sensing compartment of the eye responsible for initial processing of visual information. However, several key pieces of this puzzle are still unaccounted for, as the characterization of retinal microglia has long been hindered by the reduced population size within the retina as well as the previous lack of technologies enabling single-cell analyses. Accumulating evidence indicates that the same cell type may harbor a high degree of transcriptional, morphological and functional differences depending on its location within the central nervous system. Thus, studying the roles and signatures adopted specifically by microglia in the retina has become increasingly important. Here, we review the current understanding of retinal microglia cells in physiology and in disease, with particular emphasis on newly discovered mechanisms and future research directions.

Quantitative modeling of regular retinal microglia distribution

Microglia are resident immune cells in the central nervous system, showing a regular distribution. Advancing microscopy and image processing techniques have contributed to elucidating microglia’s morphology, dynamics, and distribution. However, the mechanism underlying the regular distribution of microglia remains to be elucidated. First, we quantitatively confirmed the regularity of the distribution pattern of microglial soma in the retina. Second, we formulated a mathematical model that includes factors that may influence regular distribution. Next, we experimentally quantified the model parameters (cell movement, process formation, and ATP dynamics). The resulting model simulation from the measured parameters showed that direct cell–cell contact is most important in generating regular cell spacing. Finally, we tried to specify the molecular pathway responsible for the repulsion between neighboring microglia.

Microglia Contribution to the Regulation of the Retinal and Choroidal Vasculature in Age-Related Macular Degeneration

The retina is a highly metabolically active tissue with high-level consumption of nutrients and oxygen. This high metabolic demand requires a properly developed and maintained vascular system. The retina is nourished by two systems: the central retinal artery that supplies the inner retina and the choriocapillaris that supplies the outer retina and retinal pigment epithelium (RPE). Pathological neovascularization, characterized by endothelial cell proliferation and new vessel formation, is a common hallmark in several retinal degenerative diseases, including age-related macular degeneration (AMD). A limited number of studies have suggested that microglia, the resident immune cells of the retina, have an important role not only in the pathology but also in the formation and physiology of the retinal vascular system. Here, we review the current knowledge on microglial interaction with the retinal vascular system under physiological and pathological conditions. To do so, we first highlight the role of microglial cells in the formation and maintenance of the retinal vasculature system. Thereafter, we discuss the molecular signaling mechanisms through which microglial cells contribute to the alterations in retinal and choroidal vasculatures and to the neovascularization in AMD.

Microglia in the developing retina

Microglia are increasingly shown to be key players in neuron development and synapse connectivity. However, the underlying mechanisms by which microglia regulate neuron function remain poorly understood in part because such analysis is challenging in the brain where neurons and synapses are intermingled and connectivity is only beginning to be mapped. Here, we discuss the features and function of microglia in the ordered mammalian retina where the laminar organization of neurons and synapses facilitates such molecular studies. We discuss microglia origins and consider the evidence for molecularly distinct microglia subpopulations and their potential for differential roles with a particular focus on the early stages of retina development. We then review the models and methods used for the study of these cells and discuss emerging data that link retina microglia to the genesis and survival of particular retina cell subtypes. We also highlight potential roles for microglia in shaping the development and organization of the vasculature and discuss cellular and molecular mechanisms involved in this process. Such insights may help resolve the mechanisms by which retinal microglia impact visual function and help guide studies of related features in brain development and disease.

Anti-angiogenic Therapy for Retinal Disease

Recent breakthroughs in our understanding of the molecular pathophysiology of retinal vascular disease have allowed us to specifically target pathological angiogenesis while minimizing damage to the neurosensory retina. This is perhaps best exemplified by the development of therapies targeting the potent angiogenic growth factor and vascular permeability mediator, vascular endothelial growth factor (VEGF). Anti-VEGF therapies, initially introduced for the treatment of choroidal neovascularization in patients with age-related macular degeneration, have also had a dramatic impact on the management of retinal vascular disease and are currently an indispensable component for the treatment of macular edema in patients with diabetic eye disease and retinal vein occlusions. Emerging evidence supports expanding the use of therapies targeting VEGF for the treatment of retinal neovascularization in patients with diabetic retinopathy and retinopathy of prematurity. However, VEGF is among a growing list of angiogenic and vascular hyperpermeability factors that promote retinal vascular disease. Many of these mediators are expressed in response to stabilization of a single family of transcription factors, the hypoxia-inducible factors (HIFs), that regulate the expression of these angiogenic stimulators. Here we review the basic principles driving pathological angiogenesis and discuss the current state of retinal anti-angiogenic pharmacotherapy as well as future directions.

Physical exercise increases GFAP expression and induces morphological changes in hippocampal astrocytes

Physical exercise has an important influence on brain plasticity, which affects the neuron-glia interaction. Astrocytes are susceptible to plasticity, and induce and stabilize synapses, regulate the concentration of various molecules, and support neuronal energy metabolism. The aim of our study was to investigate whether physical exercise is capable of altering the morphology, density and expression of glial fibrillary acidic protein (GFAP) in astrocytes from the CA1 region of rat hippocampus. Thirteen male rats were divided in two groups: sedentary (n = 6) and exercise (n = 7). The animals in the exercise group were submitted to a protocol of daily physical exercise on a treadmill for four consecutive weeks. GFAP immunoreactivity was evaluated using optical densitometry and the morphological analyses were an adaptation of Sholl's concentric circles method. Our results show that physical exercise is capable of increasing the density of GFAP-positive astrocytes as well as the regional and cellular GFAP expression. In addition, physical exercise altered astrocytic morphology as shown by the increase observed in the degree of ramification in the lateral quadrants and in the length of the longest astrocytic processes in the central quadrants. Our data demonstrate important changes in astrocytes promoted by physical exercise, supporting the idea that these cells are involved in regulating neural activity and plasticity.

From blood islands to blood vessels: morphologic observations and expression of key molecules during hyaloid vascular system development

PURPOSE

The mode of development of the human hyaloid vascular system (HVS) remains unclear. Early studies suggested that these blood vessels formed by vasculogenesis, while the current concept seems to favor angiogenesis as the mode of development. We examined embryonic and fetal human HVS using a variety of techniques to gain new insights into formation of this vasculature.

METHODS

Embryonic and fetal human eyes from 5.5 to 12 weeks gestation (WG) were prepared for immunohistochemical analysis or for light and electron microscopy. Immunolabeling of sections with a panel of antibodies directed at growth factors, transcription factors, and hematopoietic stem cell markers was employed.

RESULTS

Light microscopic examination revealed free blood islands (BI) in the embryonic vitreous cavity (5.5-7 WG). Giemsa stain revealed that BI were aggregates of mesenchymal cells and primitive nucleated erythroblasts. Free cells were also observed. Immunolabeling demonstrated that BI were composed of mesenchymal cells that expressed hemangioblast markers (CD31, CD34, C-kit, CXCR4, Runx1, and VEGFR2), erythroblasts that expressed embryonic hemoglobin (Hb-ε), and cells that expressed both. Few cells were proliferating as determined by lack of Ki67 antigen. As development progressed (12 WG), blood vessels became more mature structurally with pericyte investment and basement membrane formation. Concomitantly, Hb-ε and CXCR4 expression was down-regulated and von Willebrand factor expression was increased with the formation of Weibel-Palade bodies.

CONCLUSIONS

Our results support the view that the human HVS, like the choriocapillaris, develops by hemo-vasculogenesis, the process by which vasculogenesis, erythropoiesis, and hematopoiesis occur simultaneously from common precursors, hemangioblasts.

Gene networks: dissecting pathways in retinal development and disease

During retinal neurogenesis, diverse cellular subtypes originate from multipotent neural progenitors in a spatiotemporal order leading to a highly specialized laminar structure combined with a distinct mosaic architecture. This is driven by the combinatorial action of transcription factors and signaling molecules which specify cell fate and differentiation. The emerging approach of gene network analysis has allowed a better understanding of the functional relationships between genes expressed in the developing retina. For instance, these gene networks have identified transcriptional hubs that have revealed potential targets and pathways for the development of therapeutic options for retinal diseases. Much of the current knowledge has been informed by targeted gene deletion experiments and gain-of-functional analysis. In this review we will provide an update on retinal development gene networks and address the wider implications for future disease therapeutics.

Cellular and physiological mechanisms underlying blood flow regulation in the retina and choroid in health and disease

We review the cellular and physiological mechanisms responsible for the regulation of blood flow in the retina and choroid in health and disease. Due to the intrinsic light sensitivity of the retina and the direct visual accessibility of fundus blood vessels, the eye offers unique opportunities for the non-invasive investigation of mechanisms of blood flow regulation. The ability of the retinal vasculature to regulate its blood flow is contrasted with the far more restricted ability of the choroidal circulation to regulate its blood flow by virtue of the absence of glial cells, the markedly reduced pericyte ensheathment of the choroidal vasculature, and the lack of intermediate filaments in choroidal pericytes. We review the cellular and molecular components of the neurovascular unit in the retina and choroid, techniques for monitoring retinal and choroidal blood flow, responses of the retinal and choroidal circulation to light stimulation, the role of capillaries, astrocytes and pericytes in regulating blood flow, putative signaling mechanisms mediating neurovascular coupling in the retina, and changes that occur in the retinal and choroidal circulation during diabetic retinopathy, age-related macular degeneration, glaucoma, and Alzheimer's disease. We close by discussing issues that remain to be explored.

Sorafenib protects human optic nerve head astrocytes from light-induced overexpression of vascular endothelial growth factor, platelet-derived growth factor, and placenta growth factor

OBJECTIVES

Growth factors, such as vascular endothelial growth factor (VEGF), platelet-derived growth factor (PDGF), and placenta growth factor (PlGF) are key players in the development of diabetic retinopathy, age-related macular degeneration, and other retinal neovascular diseases. Glial cells provide a significant source of retinal growth factor production under physiologic and pathologic conditions. Cumulative light exposure has been linked to increased retinal growth factor expression. Previous reports indicate that sorafenib, an oral multikinase inhibitor, might have a beneficial effect on retinal neovascularization. This study was designed to investigate the effects of sorafenib on light-induced overexpression of growth factors in human retinal glial cells.

METHODS

Primary human optic nerve head astrocytes (ONHAs) were exposed to white light and incubated with sorafenib. Viability, expression, and secretion of VEGF-A, PDGF-BB, and PlGF and their mRNA were determined by reverse transcription-polymerase chain reaction, immunohistochemistry, and enzyme-linked immunosorbent assay.

RESULTS

Light exposure decreased cell viability and increased VEGF-A, PDGF-BB, and PlGF expression and secretion. These light-induced effects were significantly reduced when cells were treated with sorafenib at a concentration of 1 microg/ml.

CONCLUSION

Sorafenib significantly reduced light-induced overexpression of VEGF-A, PDGF-BB, and PlGF in primary human ONHAs. Sorafenib has promising properties as a potential supportive treatment for retinal neovascularization.

Vascular endothelial growth factor in eye disease

Collectively, angiogenic ocular conditions represent the leading cause of irreversible vision loss in developed countries. In the US, for example, retinopathy of prematurity, diabetic retinopathy and age-related macular degeneration are the principal causes of blindness in the infant, working age and elderly populations, respectively. Evidence suggests that vascular endothelial growth factor (VEGF), a 40kDa dimeric glycoprotein, promotes angiogenesis in each of these conditions, making it a highly significant therapeutic target. However, VEGF is pleiotropic, affecting a broad spectrum of endothelial, neuronal and glial behaviors, and confounding the validity of anti-VEGF strategies, particularly under chronic disease conditions. In fact, among other functions VEGF can influence cell proliferation, cell migration, proteolysis, cell survival and vessel permeability in a wide variety of biological contexts. This article will describe the roles played by VEGF in the pathogenesis of retinopathy of prematurity, diabetic retinopathy and age-related macular degeneration. The potential disadvantages of inhibiting VEGF will be discussed, as will the rationales for targeting other VEGF-related modulators of angiogenesis.

Age-related macular degeneration and the immune response: implications for therapy

PURPOSE

To review the available information concerning the immune mediation of age-related macular degeneration (AMD) and to speculate on proposed mechanisms and immunotherapy.

DESIGN

Interpretative essay.

METHODS

Literature review and interpretation.

RESULTS

An ever-growing body of evidence is gathering concerning the role of the immune system in AMD. Evidence to date suggests that the underlying mechanism leading to AMD is the decline of the ocular downregulatory immune environment. The subsequent activation of the immune system would lead to T-cell sensitization. When combined with local antiangiogenic therapy, several existing immunotherapies may be used to downregulate the immune response, potentially leading to a more efficient inhibition of choroidal neovascularization.

CONCLUSIONS

The loss of the downregulatory immune environment is central to the development of AMD, permitting activation of the immune system. If so, immunotherapy could positively alter the course of the disease.

Potentiation of neurogenesis and angiogenesis by G-CSF after focal cerebral ischemia in rats

Recently, granulocyte colony-stimulating factor (G-CSF) is expected to demonstrate beneficial effects on cerebral ischemia. Here, we showed the potential benefit of G-CSF administration after transient middle cerebral artery occlusion (tMCAO). Adult male Wistar rats received vehicle or G-CSF (50 microg/kg) subcutaneously after reperfusion, and were treated with 5-bromodeoxyuridine (BrdU, 50 mg/kg) once daily by the intraperitoneal route for 3 days after tMCAO. Nissl-stained sections at 7 days after tMCAO showed significant reduction of the infarction area (31%, P<0.01). At 7 days after tMCAO, BrdU plus NeuN double-positive cells increased by 43.3% in the G-CSF-treated group (P<0.05), and BrdU-positive endothelial cells were increased 2.29 times in the G-CSF-treated group, to a level as high as that in the vehicle-treated group (P<0.01), in the periischemic area. Our results indicate that G-CSF caused potentiation of neuroprotection and neurogenesis and is expected to have practical therapeutic potential in treating individuals after ischemic brain injury.

Macroglial and retinal changes in hypercholesterolemic rabbits after normalization of cholesterol levels

This study evaluates hypercholesterolemic rabbits, examining the retinal changes in Müller cells and astrocytes as well as their variations after a period of normal blood-cholesterol values induced by a standard diet. New Zealand rabbits were divided into three groups: G0, fed a standard diet; G1A, fed a 0.5% cholesterol-enriched diet for 8 months; and G1B, fed as G1A followed by standard diet for 6 months. Eyes were processed for transmission electron microscopy and immunohistochemistry (GFAP). While G1B resembled G0 more than did G1A, they shared alterations with G1A: a) as in G1A, Müller cells were GFAP+, filled spaces left by axonal degeneration, formed glial scars and their nuclei were displaced to the nerve-fibre layer. The area occupied by the astrocytes associated with the nerve-fibre bundles (AANFB) and by perivascular astrocytes (PVA) in G1A and G1B was significantly lower than in controls. However, no significant differences in PVA were found between G1A and G1B. In G1B, type I PVA was absent and replaced by hypertrophic type II cells; b) Bruch's membrane (BM) was thinner in G1B than in G1A; c) the retinal pigment epithelium (RPE) cytoplasm contained fewer lipids in G1B than in G1A; d) in G1A and G1B choriocapillaris and retinal vessel showed alterations with respect to G0; e) cell death and axonal degeneration in the retina were similar in G1A and G1B. The substitution of a hyperlipemic diet by a standard one normalizes blood-lipid levels. However, the persistence of damage at retinal vessels and BM-RPE could trigger chronic ischemia.

Increased astrocyte proliferation in rats after running exercise

The aim in this study was to investigate whether physical exercise could induce astroglial proliferation in the frontoparietal cortex and dorsolateral striatum where extensive angiogenesis had been found after exercise in previous studies. Adult male Sprague Dawley rats (n=48) were used in four experimental groups. Animals were exercised 30 min each day on a treadmill on which repetitive locomotor movement was required, for 0 (n=12), 3 (n=12) or 6 (n=12) weeks, as well as 3-week exercise plus 3-week rest (n=12). Brain tissues of the exercised and non-exercised rats were processed for glial fibrillary acidic protein (GFAP) immunocytochemistry (n=6 x 4) and Western blotting (n=6 x 4) to evaluate regional astrocyte proliferation in the frontoparietal cortex and dorsolateral striatum. By using GFAP immunocytochemistry and stereological methods, we compared the density of astrocytes in the animals with or without exercise. In comparison to non-exercised animals, a significant (p<0.01) increase in the number of astrocytes was observed in both cortex and striatum of rats exercised for 3 or 6 weeks. Our data also indicated that astrocytic density continued to increase up to 6 weeks either with an additional 3 weeks of exercise (p<0.01) or 3 weeks of rest (p<0.01). In addition, Western blotting analysis showed an obvious increase in GFAP protein from cortex and striatum of exercised animals. Astrocytosis after exercise, coupled with angiogenesis, is thought to provide strength to the neurovascular unit (a construct consisting of microvascular endothelium, astroglia, neurons and the extracellular matrix). Strengthening of this unit by exercise may protect blood-brain-barrier function following brain injury, such as that occurring after stroke.

A two-compartment model of the human retina

PURPOSE

In this article we question a basic concept in retinal pathology, which views the retina as composed primarily of neural elements, in a single compartment.

METHODS

We suggest an alternative approach, centering on the epithelial-glial elements of the retina, dividing the retina into two distinct compartments. The framework of these two compartments is composed of two epithelial-like monostratified cell layers facing each other by their apical surfaces. This model is in agreement with the embryological development of the retina.

RESULTS

Each compartment is composed of a monostratified cell layer in which neural elements are embedded and each is supplied by a different blood supply. The inner compartment, also referred to as the Muller cell compartment, extends between the inner and outer limiting membranes. The outer, or RPE, compartment extends between the outer limiting and Bruch's membranes. The border between the two compartments is formed by the outer limiting membrane (OLM). One simplified example utilizing the two-compartment concept is as follows: inner compartment edema (inner blood-retinal barrier breakdown) may manifest as cystoid edema, but not as serous retinal detachment, while outer compartment edema (outer blood-retinal barrier breakdown) may manifest as serous retinal detachment but not as cystoid edema, as long as the integrity of the OLM is maintained.

CONCLUSION

A two-compartment approach to the structure of the retina, centering on non-neural elements, may enhance our understanding of some retinal pathologies. Various retinal diseases, mainly of vascular origin, are limited to one of the two compartments.

Desmin ensheathment ratio as an indicator of vessel stability: evidence in normal development and in retinopathy of prematurity

We developed a measure of pericyte/endothelial interaction, the desmin ensheathment ratio (DER), using the intermediate filament desmin as an indicator of pericyte ensheathment and have examined the DER in normal retinal vascular development and in the kitten retinopathy of prematurity (ROP) model. We also examined the role of mural cells in the pathogenesis of ROP. Postnatal day 1 to 45 kitten retinae were labeled for desmin, alpha-smooth muscle actin (SMA), and isolectin-B4. Newborn kittens exposed to hyperoxia and then returned to room air for 0 to 40 days (dRA) were similarly labeled. The ratio of desmin to lectin labeling on confocal images yielded the DER. Ultrastructural studies showed that mural cells were present on even the most primitive vessels. During normal development, immature vascular beds had DERs of 0.3 to 0.6 whereas mature beds, which predominated by postnatal day 28, had DERs greater than 0.9. Immature pericytes and smooth muscle cells did not prevent hyperoxia-induced vessel regression. During the vasoproliferative stage of ROP, the DERs of intra- and preretinal vessels ranged between 0.2 and 0.5. In the recovery stage, the DER increased in parallel with regression of pathology, reaching 0.9 at 34 dRA. Stabilization of the DER by the fifth postnatal week was temporally coincident with the development of resistance to hyperoxia-induced vessel regression previously reported in the kitten. These observations lead us to suggest that a DER of 0.9 represents a vascular stability threshold and that a low DER observed during ROP raises the possibility that mural cell abnormalities play a key role in the pathogenesis of ROP.

Immunological and aetiological aspects of macular degeneration

Aetiological and immunological aspects of AMD, a leading cause of blindness in Western countries, have been reviewed. Developmental studies suggest that anatomical features unique to the fovea result in a critical relationship between metabolic demand and blood supply at the macula, which is maintained throughout life. Recent studies show a sufficient degree of consistency in the link between smoking and both dry and wet AMD to regard it as causative. Dry AMD is considered to be the natural endstage of the disease; epidemiological and morphological studies point to choroidal vascular atrophy as the causative event and it is suggested that signals associated with acute vascular compromise lead to the development of subretinal neovascularisation. The relationship between sub-pigment epithelial deposits, including basal laminar deposit, and the pathogenesis of AMD is examined. Much of the literature is consistent with a choroidal origin for the constituents of drusen. The blood-retinal barrier preserves the physiological environment of the neural retina and limits inflammatory responses. The factors, including cytokines, adhesion molecules and the presence of resident immunocompetent cells (microglia), which determine the immune status of the retina are considered. Historical descriptions of the involvement of inflammatory cells are provided, evidence implicating inflammation in the pathogenesis of AMD involving macrophages, giant cells and microglia has been derived from observations of human and animal subretinal neovascular lesions. The role of humoral factors such as anti-retinal autoantibodies and acute phase proteins together with clinical observations has been surveyed. Taken together these data demonstrate the involvement of both cellular and humoral immunity in the pathogenesis of AMD. It remains to be determined to what degree the influence of immunity is causative or contributory in both wet and dry AMD, however, the use of anti-inflammatory agents to ameliorate the condition further indicates the existence of an inflammatory component.

Vitreal macrophages express vascular endothelial growth factor in oxygen-induced retinopathy

PURPOSE

The possibility of vitreal macrophages playing an angiogenic role in oxygen-induced retinopathy (OIR) was investigated. Oxygen-induced retinopathy was produced in newborn animals with the purpose of modeling the proliferative phase of human retinopathy of prematurity (ROP).

MATERIALS AND METHODS

To produce OIR in neonatal mice, litters at postnatal day 7 were placed in 80-90% oxygen for a period of 5 days and then returned to room air. Pups were killed on days 7, 12, 15, 17 and 20 over the postnatal period and were perfusion-fixed using a saline wash-out, followed by 4% paraformaldehyde and then India Ink. Eyes were enucleated and either whole-mounted, or snap-frozen and cryosectioned. Immunostaining procedures were used to visualize macrophages and vascular endothelial growth factor (VEGF) protein. The primary antibodies used were anti-F4/80 and antimouse VEGF, respectively. Vitreal macrophages closely associated with the vitreo-retinal interface (within 25 microm of the inner limiting membrane) were counted. In situ hybridization procedures were used to analyse for the presence of VEGF mRNA transcript in vitreal macrophages.

RESULTS

Macrophage numbers were found to significantly increase (P < 0.05) in eyes from oxygen-treated animals compared with those from age-matched controls. A close spatial relationship was observed between macrophages and vitreal neovascular sprouts. In addition, vitreal macrophages were also found to transcribe and express VEGF in the oxygen-treated animals during the vasoproliferative phase.

CONCLUSIONS

Our results raise the possibility that vitreal macrophages play a role in the pathogenesis of OIR and by inference, ROP.

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

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

The human hyaloid system: cellular phenotypes and inter-relationships

We have investigated the expression of leucocyte markers, phenotypic characteristics and cellular relationships of the normal human fetal hyaloid vasculature using immunohistochemistry, light and electron microscopy. Antibodies against von Willebrand Factor, alpha-smooth muscle actin, glial fibrillary acidic protein, vimentin, major histocompatibility complex classes-I and -II, CD45 (leucocyte-common antigen) and calcitonin gene-related peptide were used to identify the cellular constituents of the hyaloid vasculature in whole mounts. Additional morphological features were described at the ultrastructural level. Endothelial cells throughout the hyaloid system were immunoreactive to von Willebrand Factor and major histocompatibility complex class-I antibodies. Pericytes were immunoreactive to alpha-smooth muscle actin antibody; labeled cells were distributed along large branches of the hyaloid artery, vasa hyaloidea propria, tunica vasculosa lentis and pupillary membrane but no immunoreactivity was detected on small connecting capillaries. Vessel and non-vessel-associated hyalocytes on the hyaloid artery, vasa hyaloidea propria, tunica vasculosa lentis, pupillary membrane and vitreous were immunoreactive to major histocompatibility complex classes-I and -II, CD45 and calcitonin gene-related peptide antibodies. Anti-glial fibrillary acidic protein reactivity was detected on Bergmeister's papilla but not on the hyaloid artery. Cells immunoreactive for vimentin were present throughout the hyaloid vasculature including small connecting capillaries. Ultrastructural observations of the hyaloid vasculature revealed junctional complexes, including zonulae adherens, macula adherens and possible zonulae occludens, between adjacent endothelial cells. Fenestrae were not observed in the gestational ages included in the present study. The use of whole mounts in conjunction with specific antisera has provided novel immunohistochemical definitions of the structure and cellular constituents of the human hyaloid. The results indicate that hyalocytes are a heterogeneous population of leucocyte-lineage cells.

Modulation of the resistance of a human endothelial cell line by human retinal glia

BACKGROUND

Human umbilical endothelial cells were used to model the vascular component of the blood-retinal barriers and to examine the capacity of glial cultures to modulate endothelial cell resistivity in vitro.

METHODS

Endothelial cell resistivity was monitored with and without cocultured human retinal glia. Immunohistochemistry indicated that both macroglia and microglia were present in one culture, while only macroglia were detectable in the second culture.

RESULTS

Both cocultures produced increased resistivity in the target endothelial cells; however, a further significant increase in resistivity was noted with the glial coculture containing microglia. The results suggest that the presence of microglia significantly increases the capacity of astrocytes and Müller cells to modulate endothelial cell resistivity.

Development of the human retinal vasculature: cellular relations and VEGF expression

We have investigated the relationships of the cellular constituents of the retinal vasculature--astrocytes, microglia and pericytes--to the differentiating endothelium in human fetal retina. The vascular endothelium was stained using NADPH-diaphorase histochemistry in 12 human fetal retinae ranging in gestational age from 15-22 weeks. Specimens were double labeled using antibodies against glial fibrillary acid protein, alpha smooth muscle actin, or major histocompatibility complex class II antigens to label astrocytes, contractile cells and microglia, respectively. In addition, specimens of 12, 14, 16 and 20 weeks gestation were hybridized in situ for VEGF expression. In retinal wholemounts the vascularized area comprised four lobes that converged on the optic disc. The vascular network was more dense in the temporal lobes than in the nasal lobes, and different growth patterns were evident. Astrocytes were distributed in two layers--one associated with the optic axons and a deeper layer associated with the developing vessels. In retinae younger than 20 weeks, astrocytes in the deep layer were only loosely associated with the developing vessels and extended as far as 150 microns ahead of the most peripheral vessels. A closer register between retinal vessels and the distribution of astrocytes was evident in the nasal region of retinas older than 20 weeks. In situ hybridization demonstrated expression of VEGF mRNA in the vascular layer, superficial to the ganglion cell layer, at the margins of the vascularized zone. Differences were evident in the density of astrocyte coverage of developing vessels and in the extent of VEGF expression in different regions of the retina: the relationship of these differences to differentiation gradients in the neural retina is discussed. Intensely immunoreactive microglia were observed in the vascular layer, associated with the vascular endothelium as far as the most peripheral loops, but not beyond. Alpha smooth muscle actin-containing cells covered the proximal parts of large arteries, but not corresponding veins; they were absent from arterial side-arm branches, as well as the newly formed and small diameter vessels in the age range studies. The results suggest that microglia, contractile cells and astrocytes have distinct temporo-spatial relationships to the differentiating vascular endothelium in human retinas and that VEGF expression at the vascular front, presumably by astrocytes, is associated with the spread of the retinal vasculature, as described in other species.

Human retinoblastoma: a morphological study of apoptotic, leukocytic, and vascular elements

Retinoblastoma (Rb), derived from retinal neuroepithelial progenitor cells, is the most common intraocular malignancy of childhood. This study examined 10 human Rb biopsy specimens with light and electron microscopy for histopathological features not previously described in detail, including cell death, leukocytic infiltration, and the tumor vasculature. Rb is a solid well-vascularized tumor with regions of viable tumor cells surrounding vessels, interspersed with zones of necrosis; apoptotic cells were seen in all specimens. Mononuclear phagocyte series (MPS) cells and lymphocytes often colocalized, adjacent to tumor vessels, and MPS cells frequently invested the perivascular space. Lymphocytes were rarely seen within areas of viable tumor. Tumor vessels at early stages of formation resembled normal developing retinal vessels. While junctions were often seen between endothelial calls, disruption of these junctions and endothelial fenestrae was sometimes evident. Müller cells and astrocytes extended processes around tumor cells and blood vessels, and contributed to the formation of the vascular glia limitans, which in some mature vessels was disrupted and discontinuous. Overall, this study provides further morphological details of cell death within Rb, particularly apoptotic involution, and describes the presence of a vascular-associated leukocytic infiltration in Rb. Evidence of compromise of the normal blood-retinal barrier (BRB) within the Rb tumor vessels is presented.

Morphologic changes in age-related maculopathy

Age-related maculopathy (ARM) is a degenerative disorder of the central part of the retina with a rising prevalence in patients 50 years of age and older, and comprises different histopathological changes. The morphologic changes in ARM are described and illustrated with light-microscopical, electron microscopical, and fundus pictures. Furthermore, the most important biochemical data are given. The most prominent aging changes in early stages of ARM are drusen and basal laminar deposit (BLD), both extracellular deposits, that are assumed to be important in the development of ARM. Drusen accumulate within Bruch's membrane, whereas BLD is present between Bruch's membrane and the retinal pigment epithelium. Although the histopathologic characteristics of the deposits are well documented, the chemical composition has only been partly resolved. Biochemical analysis of these deposits is necessary to determine the source of the deposits and to find possible ways to avoid or treat them. The late stages of ARM, geographic atrophy, and neovascular (disciform) degeneration, are called age-related macular degeneration (AMD), and result in severe and irreversible visual impairment. Since there is still no adequate therapy for the majority of people disabled by AMD, and because of the aging population resulting in even more patients with this disease, it is necessary to intensify the research on ARM in order to prevent AMD or find a therapy for it.

Fundus morphology assessed by digital image analysis in children with fetal alcohol syndrome

BACKGROUND

Fetal alcohol syndrome (FAS) is associated with anomalies of the eye ground. The aim of this study was to evaluate the development of fundus morphology in children with FAS from early childhood to adolescence.

METHODS

Funds photographs were evaluated by digital image analysis in 16 children with FAS. Age at the baseline study was between 0.25 and 14 years, and the median follow-up period was 7 years (range: 0.5 to 12). Sixteen sex- and age-matched healthy children were used as controls.

RESULTS

Children with FAS had a smaller optic disc area, lower frequency of excavations, greater tortuosity of retinal vessels, and a smaller number of vascular branching points compared with the controls. There were no significant differences in the funds abnormalities between the first and last examinations.

CONCLUSION

No interval change of retinal funds morphology was found at follow up, suggesting that the ocular abnormalities associated with FAS remain unchanged during childhood.

Structural specializations of human retinal glial cells

Electron microscopy and immunohistochemistry have been used to study the structural specializations of astrocyte and Müller glia cells in human retinas. The astrocytes and Müller cells contribute to the formation of the internal limiting membrane, the retina, the blood vessel glial limiting membranes and the glial sheaths of the ganglion cells. Two types of junctions were observed among retinal glial cells. Adherent junctions were found between astrocytes and Müller cells, and between adjacent astrocytes. Gap junctions were only observed between astrocyte processes. These similarities suggest that astrocytes and Müller cells can perform the same functions in human retinas. Finally, the "perivascular astrocyte of Wolter", related only to the blood vessels, was not found. All the retinal astrocytes have the same ultrastructural characteristics, confirming the absence of these astroglial cells in human retinas observed by immunohistochemical techniques.

Normal and pathological mechanisms in retinal vascular development

Angiogenesis is a complex biologic process that occurs normally in development and in turnover and remodeling of mature vascular networks. Pathological angiogenesis and neovascularization occur in association with retinal and ocular ischemic diseases, in retinopathy of prematurity and other developmental disorders, and in tumor growth and metastasis. We describe current understanding of cellular and molecular mechanisms of retinal vascular development, highlighting aspects that relate to eye diseases, that provide sites of therapeutic intervention in ophthalmology and that are potential avenues for research.

Development of microglial topography in human retina

The development of microglial topography in wholemounts of human retina has been examined in the age range 10-25 weeks gestation (WG) using histochemistry and immunohistochemistry for CD45 and major histocompatibility complex class II antigens. Microglia were present in three planes corresponding to the developing nerve fibre layer/ganglion cell layer, the inner plexiform layer and the outer plexiform layer. Distribution patterns of cells through the retinal thickness and across the retinal surface area varied with gestational age. Microglia were elongated in superficial retina, large and ramified in the middle plane, and small, rounded and less ramified in deep retina. Intensely labeled, rounded profiles seen at the pars caeca of the ciliary processes, the retinal margin and at the optic disc may represent precursors of some retinal microglia. At 10 WG, the highest densities of microglia were present in middle and deep retina in the far periphery and at the retinal margin, with few superficial microglia evident centrally at the optic disc. At 14 WG, high densities of microglia were apparent superficially at the optic disc; microglia of middle and deep retina were distributed at more central locations although continuing to concentrate in the retinal periphery. Microglia appear to migrate into the developing human retina from two mains sources, the retinal margin and the optic disc, most likely originating from the blood vessels of the ciliary body and iris, and the retinal vasculature, respectively. The data suggest that the development of microglial topography occurs in two phases, an early phase occurring prior to vascularization, and a late phase associated with the development of the retinal vasculature.

Ontogeny and cellular expression of MHC and leucocyte antigens in human retina

We have investigated the ontogeny of MHC class I, class II, CD45, and macrophage antigens in whole mounts of normal human fetal retina at 10-25 weeks gestation (WG) using monoclonal antibodies and immunogold histochemistry. MHC class I antigens were expressed on retinal vascular endothelial cells and provided a useful marker of vessel organization from 14-25 WG. Microglial cells expressed immunoreactivity to MHC class I, class II, and CD45 antigens from 10 WG (pre-vascularization) and macrophage S22 (Mac S22) antigen from 14 WG (post-vascularization), although none of the antigens tested were detected on neuronal or macroglial elements. Microglia expressing MHC, CD45, and macrophage antigens occurred in both ramified and rounded forms with no close correlation being observed between morphology and antigenicity. The numbers of immunoreactive cells labeled with each of the four markers increased steadily throughout gestation in all specimens studied. Equivalent numbers of microglia expressed MHC class I, class II, and CD45 antigens in retinae at similar gestational ages; however, our data indicate that microglia expressing Mac S22 antigen comprise approximately 40% or less of the population of MHC and CD45-immunoreactive cells during development. Topographical analyses suggest that MHC class I, class II, and CD45-positive microglia enter the retina from both the peripheral retinal margin and the optic disc from at least 10 WG; Mac S22-positive cells appear in association with the development of the retinal vasculature and enter the retina via the optic disc after 14 WG.

Exudative macular degeneration and intravitreal triamcinolone. A pilot study

PURPOSE

To study the efficacy of the anti-inflammatory agent triamcinolone (Kenacort A-40) in patients with exudative age-related macular degeneration and subfoveal and juxtafoveal choroidal new vessels, considered unsuitable for laser photocoagulation.

METHOD

Thirty eyes of 28 patients were treated with intravitreal injection of triamcinolone. The subsequent visual acuity (VA) of treated eyes was compared with published VA outcomes of untreated eyes. Patients were classified into three types according to their responses to treatment.

RESULTS

Within two weeks of receiving treatment, exudation decreased and vision improved in the majority of Types I and II patients (87%), the trend continuing in longer term follow-up. The overall VA outcome for treated eyes was significantly better than published VA data for untreated exudative macular lesions.

CONCLUSIONS

The preliminary results are encouraging and no serious side effects of a single injection of triamcinolone have been detected in patients followed for up to 18 months. The treatment should, however, continue to be regarded as unproven and only administered in the context of a prospective, case-controlled clinical trial.

Human retinal microglia: expression of immune markers and relationship to the glia limitans

The immunoreactivity, morphology and relationship to the glia limitans of microglia were investigated in flatmounts and sections of normal human retina, using immunogold histochemistry, electron microscopy (EM), and antibodies directed against CD45, major histocompatability complex class I (MHC-I), MHC-II, and human macrophage antigens. Immunoreactivity was evident for all antibodies tested, including MHC-I, which labeled both microglia and retinal vascular endothelium. Most consistent labeling was obtained using antibodies to CD45, MHC-II, and anti-human macrophage (S22) antigen. Immunoreactive cells were seen in the perivascular space (perivascular cells), where they were closely adherent to the vessel profile, and in the retinal parenchyma (microglia). Some parenchymal microglia were also vessel associated and by EM were seen to be closely related to the glia limitans (paravascular microglia). Paravascular microglia were shown by optical densitometry, to express higher levels of MHC antigens than neighboring, non-vessel associated, parenchymal microglia. In addition, paravascular microglia were macrophage (S22) antigen positive, while other parenchymal microglia did not express macrophage antigens. Quantitative data indicate that similar populations of microglia are immunoreactive to CD45, MHC-I, and MHC-II, while relatively few microglia (approximately 10%) are immunoreactive for human macrophage (S22) antigens, supporting previous suggestions that microglia are a heterogeneous population.

Origin of microglia in the quail retina: central-to-peripheral and vitreal-to-scleral migration of microglial precursors during development

The origin, migration, and differentiation of microglial precursors in the avascular quail retina during embryonic and posthatching development were examined in this study. Microglial precursors and developing microglia were immunocytochemically labeled with QH1 antibody in retinal whole mounts and sections. The retina was free of QH1+ macrophages at embryonic day 5 (E5). Ameboid QH1+ macrophages from the pecten entered the retina from E7 on. These macrophages spread from central to peripheral areas in the retina by migrating on the endfeet of the Müller cells and reached the periphery of the retina at E12. While earlier macrophages were migrating along the inner limiting membrane, other macrophages continued to enter the retina from the pecten until hatching (E16). From E9 on, macrophages were seen to colonize progressively more scleral retinal layers as development advanced. Macrophages first appeared in the ganglion cell layer at E9, in the inner plexiform layer at E12, and in the outer plexiform layer at E14. Therefore, it seems that macrophages first migrated tangentially along the inner retinal surface and then migrated from vitreal to scleral levels to gain access to the plexiform layers, where they differentiated into ramified microglia. Macrophages appeared to differentiate shortly after arrival in the plexiform layers, as poorly ramified QH1+ cells were seen as early as E12 in the inner plexiform layer and at E14 in the outer plexiform layer. Radial migration of macrophages toward the outer plexiform layer continued until posthatching day 3, after which retinal microglia showed an adult distribution pattern. We also observed numerous vitreal macrophages intimately adhered to the surface of the pecten during embryonic development, when macrophages migrated into the retina. These vitreal macrophages were not seen from hatching onwards, when no further macrophages entered the retina.

Early stages of age-related macular degeneration: an immunofluorescence and electron microscopy study

In subretinal neovascularisation capillaries originating from the choriocapillaris must cross Bruch's membrane to reach the subretinal pigment epithelial space. Thus gaps in Bruch's membrane have to be formed before subretinal neovascularisation. Histological examination of eyes with subretinal neovascularisation or disciform scars has shown macrophages adjacent to thin areas and ruptures in Bruch's membrane. This has been interpreted as phagocytosis of Bruch's membrane. The purpose of this study was to investigate whether immune complex depositions can be detected in maculae with early stages of age-related macular degeneration and to explain the macrophage reaction before the disciform reaction. A series of 20 human maculae were examined by direct immunofluorescence light microscopy to detect the presence of immune complexes with antibodies directed against immunoglobulins, fibrinogen, and complement factors. Transmission electron microscopy on several maculae was performed to identify the macrophages. Macrophages were observed in close relation to the readily recognisable long spacing collagen, which suggested that long spacing collagen was selectively internalised by these cells. Definite immune complex depositions were not found in basal laminar deposits or drusen. Linear deposits of fibrinogen and complement were frequently found in the outer collagenous zone of Bruch's membrane. However, because of the absence of immunoglobulins, it seems unlikely that these non-specific deposits might cause chemoattraction of macrophages and play a role in the initial phase of the development of subretinal neovascularisation and disciform macular degeneration.

Human retinal microglia express phenotypic characteristics in common with dendritic antigen-presenting cells

Neural tissue has been considered to be immunologically privileged and major histocompatibility complex (MHC) class II antigens not expressed in normal human brain grey matter and retina. In the present study we compare phenotypic characteristics of human retinal microglia and dendritic Langerhans cells, including their morphologies and distribution, MHC class II and CD45 antigen expression and nucleotidase reactivity. Levels of class II expression were measured using optical densitometry in combination with standard immunohistochemical techniques applied to retinal flatmounts. The results indicate that ramified retinal microglia have features in common with dendritic antigen presenting cells of cornea and conjunctivum, including the constitutive expression of MHC class II antigens.

Antibodies to human leucocyte antigens indicate subpopulations of microglia in human retina

Monoclonal antibodies to human leucocyte antigens, including anti-CD45 and anti-CD68, have been used to describe microglia in flatmounts of normal adult human retina for the first time. Anti-CD45 (the leucocyte common antigen) intensely labeled large numbers of cells in a regular distribution across the retina; anti-CD68 and anti-macrophage antibodies labeled fewer cells with distinctive morphologies, suggesting the presence of subpopulations of microglia in the human retina expressing leucocyte antigens.

Development of retinal vasculature in the cat: processes and mechanisms

Two principal processes can be distinguished in the development of the retinal circulation in the cat. One process, which forms most of the inner layer of vasculature, involves three stages. First, beginning prior to E (embryonic day) 26, spindle cells of mesenchymal origin spread over the inner surface of the retina. Second, beginning at approximately E48, a network of coarse capillaries forms, apparently derived from spindle cells. Third, major vessels differentiate from the capillary plexus, and the capillaries become thinner and more widely spaced. All three stages begin at the optic disc and spread towards the margin of the retina. The other process involves budding of capillary sized vessels from existing vasculature. This process forms the inner layer of vasculature at the area centralis, the outer layer of vasculature, and the radial peripapillary capillaries. It begins between P (postnatal day) 7 and P10 at the area centralis and spreads to the margins of the retina. The radial peripapillary capillaries form at a later stage (P20). The different topographies of the two processes suggest that they are controlled by distinct mechanisms. In the first process, the formation of vessels follows a pattern set by the early migration of spindle cells. In the second process, the vessels form in a pattern determined by the metabolic needs of the developing retina.