Electron microscopic features of experimental choroidal neovascularization

Persistence of vascular empty sleeves in choroidal neovascularization after VEGF therapy in both animal models and humans

PURPOSE

Choroidal neovascularization (CNV) often recurs during anti-vascular endothelial growth factor (VEGF) therapy; however, little is known about the mechanism of vascular regrowth. Vascular regrowth along the empty sleeves of basement membranes was proposed as a mechanism for recurrence after the reversal of VEGF inhibition in tumors. This study investigated whether the proposed mechanism is involved in CNV during VEGF therapy.

METHODS

We made two observations using a mice model, as well as patients with CNV. Laser-induced CNV mice were used to examine the vascular empty sleeves of the basement membrane and CNV with the immunohistochemistry of type IV collagen and CD31, respectively. A retrospective cohort study included 17 eyes from 17 patients with CNV treated with anti-VEGF treatment. Vascular regrowth during anti-VEGF treatment was assessed using optical coherence tomography angiography (OCTA).

RESULTS

In the CNV mouse model, the CD31+ vascular endothelium area was decreased during anti-VEGF treatment compared with the IgG control (33516.7 ± 10864.7 vs. 10745.9 ± 5755.9 μm2, P < 0.05), whereas a significant difference was not observed in the area of type IV collagen+ vascular empty sleeve after the treatment compared with the control (29135.0 ± 7432.9 vs. 24592.0 ± 5935.3 μm2, P = 0.7). The proportions of CD31+ to type IV collagen+ areas were significantly decreased after the treatment (38.7 ± 7.4% vs. 17.1 ± 5.4%, P < 0.05). In the OCTA observations, the follow-up period in the retrospective cohort study was 58.2 ± 23.4 months. CNV regrowth was observed in 682 neovessels of the 17 eyes. In group 1, CNV regression and regrowth are in the same form (129 neovessels, 18.9%). In group 2, CNV regression and regrowth are in a different form (170 neovessels, 24.9%). In group 3, CNV regrowth is with a different form without the regression (383 neovessels, 56.2%).

CONCLUSIONS

Parts of CNV regrowth may occur along the vascular empty sleeve, which remain after anti-VEGF treatment.

Aging of the human choriocapillaris: Evidence that early pericyte damage can trigger endothelial changes

The choriocapillaris (CC), the capillary bed in the choroid, essentially nourishes the photoreceptor cells. Its damage in aging and age-related diseases significantly influences the survival of the photoreceptor cells. Earlier reports implicated endothelial loss in aged and diseased CC; however, age-related pericyte changes and their contribution in CC death remain unknown. We examined human donor eyes (age: 56-94 years; N = 24), and found that CC pericyte damage preceded endothelial changes. With aging (>70 years), the sub-macular choroid accumulated debris in Bruch's membrane (BM). Of the debris content, the long-spaced collagens had a tendency to settle over the capillary basal lamina (BL), and this often resulted in endothelial projection into capillary lumen. Between 75 and 83 years, pericytes contained dark mitochondria, and their processes facing the BM debris showed partial loss of BL and intermediate filaments (IFs), when the endothelium remained unaltered. The endothelial changes appeared beyond 83 years, the abundance of IFs and autophagy reinforced their survival until late aging. TUNEL⁺ pericytes, and immunoreactivity to carboxymethyl lysine and 4-hydroxy 2-nonenal, but no nitro-tyrosine, was detected in aged CC walls. Iba-1⁺ dystrophic microglia were present in the vicinity of the CC. Our data indicate that (1) BM debris exerts pressure on the CC, leading to the damage of the capillary BL and pericyte processes (2) loss of IFs results in early pericyte destabilization (3) capillary wall undergoes lipid peroxidative and glycative damage, and (4) pericyte damage leads to late endothelial changes and ultimately CC loss. Future research should explore the normal ways of pericyte maintenance in the aging nervous system.

Age-Related Macular Degeneration: Role of Oxidative Stress and Blood Vessels

Age-related macular degeneration (AMD) is a common irreversible ocular disease characterized by vision impairment among older people. Many risk factors are related to AMD and interact with each other in its pathogenesis. Notably, oxidative stress and choroidal vascular dysfunction were suggested to be critically involved in AMD pathogenesis. In this review, we give an overview on the factors contributing to the pathophysiology of this multifactorial disease and discuss the role of reactive oxygen species and vascular function in more detail. Moreover, we give an overview on therapeutic strategies for patients suffering from AMD.

Pericytes in Vascular Development

Purpose of review

Pericytes are essential components of capillaries in many tissues and organs, contributing to vessel stability and integrity, with additional contributions to microvascular function still being discovered. We review current and foundational studies identifying pericyte differentiation mechanics and their roles in the earliest stages of vessel formation.

Recent findings

Recent advances in pericyte-focused tools and models have illuminated critical aspects of pericyte biology including their roles in vascular development.Pericytes likely collaborate with endothelial cells undergoing vasculogenesis, initiating direct interactions during sprouting and intussusceptive angiogenesis. Pericytes also provide important regulation of vascular growth including mechanisms underlying vessel pruning, rarefaction, and subsequent regrowth.

RF/6A Chorioretinal Cells Do Not Display Key Endothelial Phenotypes

Purpose

The misuse of inauthentic cell lines is widely recognized as a major threat to the integrity of biomedical science. Whereas the majority of efforts to address this have focused on DNA profiling, we sought to anatomically, transcriptionally, and functionally authenticate the RF/6A chorioretinal cell line, which is widely used as an endothelial cell line to model retinal and choroidal angiogenesis.

Methods

Multiple vials of RF/6A cells obtained from different commercial distributors were studied to validate their genetic, transcriptomic, anatomic, and functional fidelity to bona fide endothelial cells.

Results

Transcriptomic profiles of RF/6A cells obtained either de novo or from a public data repository did not correspond to endothelial gene expression signatures. Expression of established endothelial markers were very low or undetectable in RF/6A compared to primary human endothelial cells. Importantly, RF/6A cells also did not display functional characteristics of endothelial cells such as uptake of acetylated LDL, expression of E-selectin in response to TNF-α exposure, alignment in the direction of shear stress, and AKT and ERK1/2 phosphorylation following VEGFA stimulation.

Conclusions

Multiple independent sources of RF/6A do not exhibit key endothelial cell phenotypes. Therefore, these cells appear unsuitable as surrogates for choroidal or retinal endothelial cells. Further, cell line authentication methods should extend beyond genomic profiling to include anatomic, transcriptional, and functional assessments.

Animal models of ocular angiogenesis: from development to pathologies

Pathological angiogenesis in the eye is an important feature in the pathophysiology of many vision-threatening diseases, including retinopathy of prematurity, diabetic retinopathy, and age-related macular degeneration, as well as corneal diseases with abnormal angiogenesis. Development of reproducible and reliable animal models of ocular angiogenesis has advanced our understanding of both the normal development and the pathobiology of ocular neovascularization. These models have also proven to be valuable experimental tools with which to easily evaluate potential antiangiogenic therapies beyond eye research. This review summarizes the current available animal models of ocular angiogenesis. Models of retinal and choroidal angiogenesis, including oxygen-induced retinopathy, laser-induced choroidal neovascularization, and transgenic mouse models with deficient or spontaneous retinal/choroidal neovascularization, as well as models with induced corneal angiogenesis, are widely used to investigate the molecular and cellular basis of angiogenic mechanisms. Theoretical concepts and experimental protocols of these models are outlined, as well as their advantages and potential limitations, which may help researchers choose the most suitable models for their investigative work.-Liu, C.-H., Wang, Z., Sun, Y., Chen, J. Animal models of ocular angiogenesis: from development to pathologies.

Evaluation of cardiovascular biomarkers in patients with age-related wet macular degeneration

AIM

To evaluate levels of homocysteine, asymmetric dimethylarginine (ADMA), and nitric oxide (NO), as well as activity of endothelial NO synthase (eNOS), in patients with age-related macular degeneration (AMD).

METHODS

The levels of homocysteine, ADMA, and NO and activity of eNOS in patients who were diagnosed with wet AMD by fundus fluorescein angiography (n=30) were compared to a control group with no retinal pathology (n=30).

RESULTS

Levels of homocysteine and ADMA were found to be significantly higher in the wet AMD group than in the control group (P<0.001), whereas NO levels and eNOS activity were higher in the control group (P<0.001). In the wet AMD group, we detected a 2.64- and 0.33-fold increase in the levels of ADMA and homocysteine, respectively, and a 0.49- and 2.41-fold decrease in the eNOS activity and NO level, respectively.

CONCLUSION

Elevated levels of homocysteine and ADMA were observed in patients with wet AMD. Increased ADMA may be responsible for the diminished eNOS activity found in these patients, which in turn contributes to the decrease in NO levels, which likely plays a role in the pathogenesis of AMD.

Review of the ocular angiogenesis animal models

Increasing interest in developing reliable and reproducible models to study angiogenesis has emerged due to recent advances in the treatment of eye disease with pathologic angiogenesis. This review provides a summary of the principal ocular animal models for angiogenesis. Models of anterior segment neovascularization include the corneal micropocket assay, used to study the influence of specific molecules/proteins in angiogenesis, and corneal chemical and suture induced injury, which mimic more closely the complex nature of the human disease. Angiogenesis models of the posterior segment include the well-known laser-induced injury of the choroid/Bruch's membrane, as well as the oxygen induced retinopathy and models of injections of pro-angiogenic/inflammatory molecules. In addition, knockout or knock-in transgenic mice provide powerful tools in studying the role of specific proteins in angiogenesis.

Receptor tyrosine kinase inhibitors AG013764 and AG013711 reduce choroidal neovascularization in rat eye

Age-related macular degeneration (AMD) is the major cause of blindness for people over 60. In the "wet" form of AMD compounds targeting growth factor signaling pathways such as VEGF have been a major focus for therapeutic interventions. In a previously developed rat model of CNV, we utilized two receptor tyrosine kinase inhibitors (RTKi) to block VEGFR-1, VEGFR-2 and PDGFR signaling following the establishment of CNV. AAV-VEGF(165) was injected into the subretinal space of rats at postnatal days 15-17. Six weeks later, a suspension of RTK inhibitors, AG013764 or AG013711, was injected intraperitoneally (IP, twice daily) or intravitreally (every five days) over a two week period. FITC-dextran whole-mounts of RPE-choroid-sclera were prepared after the animals were sacrificed. CNV area was quantified using Neurolucida to measure the hyperfluorescence on FITC-dextran whole-mounts. Histology and immunohistochemistry were performed as described previously. VEGF expression in control and treated eyes was confirmed by immunohistochemistry and histological sections indicated recovery of retinal morphology and CNV reduction in treated eyes. In the animals IP injected with AG013764 or AG013711 the mean CNV level was reduced by 25 to 33% compared to control, but this effect did not achieve statistical significance. Intravitreal injections of AG013764 or AG013711 reduced the level of CNV by approximately 60% compared to control (p<0.005 or p<0.05, respectively). These data show that two RTK inhibitors, AG013764 or AG013711, delivered intravitreally, significantly reduce blood vessel proliferation in this AAV-VEGF(165) model of CNV.

Plasma homocysteine, vitamin B12 and folate levels in age-related macular degeneration

PURPOSE

The purpose of this study was to investigate the association of age-related macular degeneration (AMD) with plasma homocysteine, vitamin B12, and folate levels.

METHODS

Sixty patients diagnosed with AMD at our clinic between March 2004 and September 2004 were assessed in a prospective cross-sectional study. Plasma homocysteine, vitamin B12, and folate levels taken after 8 h of fasting from 30 patients with exudative AMD and 30 patients with dry AMD were compared with the results of 30 age- and sex-matched healthy participants.

RESULTS

Patients with both exudative and dry types of AMD had significantly higher plasma homocysteine levels (mean 14.19+/-3.11 micromol/l; 13.07+/-2.90 micromol/l respectively) compared with the controls (mean 10.79+/-2.56 micromol/l; (p=0.000 and p=0.008 respectively). Homocysteine levels were higher in the exudative AMD group compared with the dry AMD group, but the difference was not statistically significant (p=0.290). Plasma vitamin B12 levels were found to be significantly lower in the exudative AMD group (289.14+/-113.44 pg/l) compared with the controls (436.17+/-204.12 pg/l) and dry AMD group (443.47+/-190.83 pg/l; (p=0.000). Plasma folate levels were comparable among groups (p=0.106).

CONCLUSION

This study suggests an association between elevated plasma homocysteine and AMD regardless of the subtype. Further controlled prospective studies are needed to investigate the possible role of homocysteine in AMD and the effect of vitamin B12 and folate supplementation in this process.

Association of neovascular age-related macular degeneration and hyperhomocysteinemia

PURPOSE

To assess the relationship between plasma homocysteine levels and exudative neovascular age-related macular degeneration (AMD).

DESIGN

Cross-sectional study.

METHODS

A prospective comparative cross-sectional study was conducted in outpatient ophthalmology clinics in a university-affiliated medical institution. The cohort consisted of 59 patients (25 male, 34 female) with a mean age of 78 years (standard deviation [SD] = 8.4) with neovascular AMD who were candidates for photodynamic treatment. Patients were compared for plasma homocysteine levels with 58 patients who had dry AMD (24 male, 34 female) with a mean age of 76.3 years (SD = 8.4) and with a control group of 56 age-matched subjects (27 male, 29 female), with a mean age of 77.3 years (SD = 8.2). A 3-ml venous blood sample was obtained from each participant after an 8-hour fast. Levels of plasma homocysteine were measured by high performance liquid chromatography. The main outcome measure was hyperhomocysteinemia, defined as a plasma homocysteine level above 15 micromol/l.

RESULTS

Homocysteine levels were higher by 27.9% in the neovascular AMD than in the dry AMD group, and by 21.9% than in the control group (P <.02). Hyperhomocysteinemia was found in 44.1% of the study group, in 22.4% of the dry AMD group, and in 21.4% of the control group (P =.011).

CONCLUSIONS

This study suggests an association between an elevated plasma level of homocysteine and exudative neovascular AMD but not dry AMD.

Tissular insemination of progenitor endothelial cells: the problem, and a suggested solution

The contribution of circulating precursor endothelial cells (CPEC) to adult angiogenesis is now well established. However, the mechanism of their tissular engrafting remains poorly understood. The classical paradigm of "sprouting" cannot accommodate the main features of the CPEC-based angiogenic process. Additionally, vasculogenesis based on the differentiation of angioblasts, as defined in the embryonic stages, is not applicable to adult neo-vascularization either. In search for a solution to this dilemma, I suggest that the ability of monocytes/macrophages to produce tunnels, as effect of their protease-dependent migration in the extracellular matrices, is instrumental for the tissular insemination of CPEC. Here I present in vivo and in vitro experimental evidence for the existence of tunnels, and for their colonization by monocytes/macrophages and by other cells, including CPEC. As a paradigm of CPEC behavior, the tunneling model (in an extended sense) may also explain the propagation of the endothelium with arteriolar phenotype within the pre-existent downstream capillary network. Thus, the sprouting mechanism might be a valid explanation for the formation of new capillaries and venules, whereas CPEC would contribute mostly, if not exclusively, to the extension of arteriolar branches of microvasculature. Adult angiogenesis occurs therefore as a multifunctional process based on intercellular cooperation, in which there are involved endothelial cells (EC) or their precursors, as well as other cell types. In specific circumstances, the lumen (i.e. the tunnel) may occur before the "definitive" microvessel. Therefore the very notion of microvessel may need to be extended, to include the tunnels.

Role of monocytes and macrophages in adult angiogenesis: a light at the tunnel's end

In spite of sustained efforts, there are still gaps in our understanding of angiogenesis as it takes place in vivo. Older observations and a number of recent developments strongly involve the blood mononuclear cell population, collectively known as monocytes (MC), in the normal and pathological adult angiogenesis. An emerging paradigm should eventually incorporate the established biochemical cross talk between MC and their descendents, tissular macrophages (Mph), and the endothelial cells (EC); additionally, it should account for both the intercellular cooperation at the morphological level and the phenotypic overlap between the two cell populations. This focused review puts together the pieces of this puzzle in such a way as to suggest an alternative angiogenic model applicable to adult animals, and particularly to pathological conditions. A working hypothesis is put forward, which is centered on the preformation of capillary lumen as a "tunnel" drilled by penetrating MC/Mph. The tunnels may be colonized in a later stage by sprouts, circulating progenitor endothelial cells (CPEC) or transdifferentiated EC. Thus, MC/Mph are suggested to be included among the targets of therapeutic manipulation of angiogenesis.

Age-associated neovasculopathy with recurrent bleeding

We hypothesize the existence of an age-associated neovasculopathy with recurrent bleeding. It could be the result of age-related defects in the regulation of the angiogenetic process creating pathologic small vessels with a bleeding tendency. Conditions like subdural hematoma, ocular angiopathies, intestinal angiodysplasia, coronary atherosclerotic plaques and others, may be the clinical expression of such a neovasculopathy.

Age-related macular degeneration is associated with increased vascular endothelial growth factor, hemorheology and endothelial dysfunction

OBJECTIVE

To investigate laboratory evidence of abnormal angiogenesis, hemorheologic factors, endothelial damage/dysfunction, and age-related macular degeneration (ARMD).

DESIGN

Comparative cross-sectional study.

PARTICIPANTS

We studied 78 subjects (26 men and 52 women; mean age 74 years; standard deviation [SD] 9.0) with ARMD attending a specialist referral clinic. Subjects were compared with 25 healthy controls (mean age, 71 years; SD, 11).

INTERVENTION AND OUTCOME MEASURES

Levels of vascular endothelial growth factor (VEGF, an index of angiogenesis), hemorheologic factors (plasma viscosity, hematocrit, white cell count, hemoglobin, platelets), fibrinogen (an index of rheology and hemostasis), and von Willebrand factor (a marker of endothelial dysfunction) were measured.

RESULTS

Median plasma VEGF (225 vs. 195 pg/ml, P = 0.019) and mean von Willebrand factor (124 vs. 99 IU/dl, P = 0.0004) were greater in ARMD subjects than the controls. Mean plasma fibrinogen and plasma viscosity levels were also higher in the subjects (both P < 0.0001). There were no significant differences in other indices between cases and controls. When "dry" (drusen, atrophy, n = 28) and "exudative" (n = 50) ARMD subjects were compared, there was no significant differences in VEGF, fibrinogen, viscosity, or von Willebrand factor levels. There were no significant correlations between the measured parameters. Stepwise multiple regression analysis did not demonstrate any significant clinical predictors (age, gender, smoking, body mass index, history of vascular disease, or hypertension) for plasma VEGF or fibrinogen levels, although smoking status was a predictor of plasma von Willebrand factor levels (P < 0.05).

CONCLUSIONS

This study suggests an association between markers of angiogenesis (VEGF), hemorheologic factors, hemostasis, endothelial dysfunction, and ARMD. The interaction between abnormal angiogenesis and the components of Virchow's triad for thrombogenesis may in part contribute to the pathogenesis of ARMD.

Expression of vascular endothelial growth factor in experimental choroidal neovascularization

BACKGROUND

Although the choroidal neovascularization (CNV) is a common pathologic feature of a number of different eye diseases, its pathological mechanisms have not been fully elucidated. We investigated the expression of vascular endothelial growth factor (VEGF) in CNV using an experimental primate model.

METHOD

CNV was induced by intense laser photocoagulation in four monkey eyes. Single eyes were enucleated at 1, 3, 7 or 14 days after photocoagulation and examined immunohistochemically for VEGF, macrophage antigen, von Willebrand factor and glial fibrillary acidic protein (GFAP). Expression of VEGF mRNA was examined by in situ hybridization.

RESULTS

One day after photocoagulation, the normal structure of the outer portion of the retina and the inner portion of the choroid was destroyed. Three days after photocoagulation, choroidal vascular endothelial cells migrated into the subretinal space through the defect in Bruch's membrane. Increased expression of VEGF was detected in the accumulating macrophages, migrating retinal pigment epithelial (RPE) cells and Müller cells. Maximal expression of VEGF was observed between 3 and 7 days after wounding, and many newly formed vessels extended into the subretinal space 7-14 days after photocoagulation.

CONCLUSION

VEGF derived from RPE cells, macrophages and Müller cells may play a role in the formation of CNV.

Iatrogenic choroidal neovascularisation following argon laser photocoagulation for choroidal malignant melanoma

BACKGROUND

Choroidal neovascularisation is known to occur following photocoagulation for choroidal melanomas. Its occurrence rate, possible causes and clinical impact were studied.

METHODS

Post-treatment fluorescein angiograms were reviewed from 18 patients who had received argon laser photocoagulation as sole treatment of their small choroidal melanomas, to look for choroidal neovascularisation. Where it was found an assessment of its clinical impact was made.

RESULTS

choroidal neovascularisation was found in 50% of cases. Choroidoretinal neovascularisation, found in five patients, caused vitreous haemorrhage in one patient but was otherwise benign. Choroidovitreal neovascularisation was found in four patients. It occurred early and altered their clinical management. Three of these patients had a vitreous haemorrhage, one of whom also suffered a retinal detachment. The three diabetic patients in the series all developed aggressive choroidovitreal neovascularisation. Tumour size, tumours location and number of treatment sessions did not appear to affect the occurrence of choroidal reovascularisation, nor did other medical or ocular conditions except for diabetes.

CONCLUSION

Choroidal neovascularisation occurs commonly after melanoma photocoagulation. Although sometimes benign, it can be aggressive, particularly in diabetic patients, in whom it might be better to consider different forms of tumour treatment.

Development of pericyte-like cells during angiogenesis in quail chick chimeras as detected by combined Feulgen reaction and immunohistochemistry

Perivascular fibroblasts have been proposed as possible precursor cells for microvascular pericytes. To investigate the development of pericytes during angiogenesis we examined interspecific grafts between chick and quail embryos. Limb buds of three-day old quail embryos were transferred to the chorioallantoic membrane (CAM) of ten to fourteen day-old chick embryos. Six days after grafting, the limb buds were explanted and histologically examined by combined Feulgen reaction and immunohistochemistry using an antibody to quail endothelial and hemopoietic cells (QH-1). Limb buds were found to be vascularized by a network of capillaries which were partially derived from sprouts of the chick CAM microvasculature. Numerous hybrid capillaries were detected, consisting of host endothelial cells (chick) and graft pericytes (quail). These results provide further support for the idea that microvascular pericytes can evolve from perivascular fibroblasts.

A microcarrier-based cocultivation system for the investigation of factors and cells involved in angiogenesis in three-dimensional fibrin matrices in vitro

Angiogenesis in situ includes coordinated interactions of various microvascular cell types, i.e., endothelial cells, pericytes and perivascular fibroblasts. To study the cellular interactions of microvascular cells in vitro, we have developed a microcarrier-based cocultivation system. The technical details of this method include seeding of endothelial cells on unstained cytodex-3 microcarriers and seeding of pericytes, fibroblasts or vascular smooth muscle cells on microcarriers which have been labeled by trypan blue staining. A mixture of both unstained and trypan blue-stained microcarriers was subsequently embedded in a three-dimensional fibrin clot. The growth characteristics of each cell type could be conveniently observed since the majority of cells left their supporting microcarriers in a horizontal direction to migrate into the transparent fibrin matrix. As differently stained microcarriers were randomly arranged in the fibrin matrix, the characteristic patterns of the microcarriers allowed location of particular points of interest at different developmental stages, facilitating the observation of cellular growth over the course of time. One further advantage of this microcarrier-based system is the possibility of reliably quantifying capillary growth by determination of average numbers of capillary-like formations per microcarrier. Thus, this model allows convenient evaluation of the effects of non-endothelial cells on angiogenesis in vitro. By using this coculture system, we demonstrate that endothelial capillary-like structures in vitro do not become stabilized by contacting vascular smooth muscle cells or pericytes during the initial stages of capillary formation.

Optimal conditions required for the creation of an iatrogenic chorioretinal venous anastomosis in the dog using argon green laser photocoagulation

We have previously reported iatrogenic retinal to choroidal vein anastomosis, developed as a potential method of by-passing the site of obstruction to venous outflow in retinal venous occlusion in dogs (1), and in rats (2). In order to minimise tissue damage to the retina and choroid and increase the rate of success in these experiments, we investigated in the dog model the factors that would promote an anastomosis and compare the effects of three different power levels. A small spot size (50 microns) argon green laser beam of 514 nm at power levels of 0.5w, 1.5w and 2.5w were used. Spaced serial sections from each lesion were examined by light and by transmission electron microscopy. Morphometric measurements of the corresponding retinal pigment epithelium (RPE)/glial scar was computed using IBMPC digitising pad and sigma scan software and the extent of tissue damage at the different power levels assessed. At the lowest power level of 0.5w the damage to the retina was mild and there was an absence of anastomosis formation. At the 1.5w power level an anastomosis formed in 40% of the lesions. At the highest power level of 2.5w a 71% rate of success was obtained however, the damage to the retina tended to be severe. The results of this study also indicate that disruption of Bruch's membrane and vein rupture at the time of irradiation are essential for anastomosis formation, which may be further enhanced by necrotic tissue, retinal pigment epithelial and glial scar formation and inflammation. These findings are useful in establishing optimal conditions for the creation of a chorioretinal venous anastomosis, for consideration in human trials.

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

Recent studies have suggested a role for mononuclear phagocytes series (MPS) cells in neovascularisation associated with retinal pathology and experimentally induced subretinal neovascularisation. The present study is concerned with the normal development of the human retinal vasculature. Morphological details are provided of developing vascular structures including the formation of tight junctions and canalisation of angioblast cords. The relationships of astrocytes and pericytes to developing structures and the presence of a perivascular collagenous matrix are described. Ultrastructural and histochemical analyses reveal an association between MPS cells and developing vascular structures. It is suggested that MPS cells may influence angiogenesis in normal retinal development, as well as in retinal pathology.

Fibrovascular proliferation and retinal detachment after intravitreal injection of activated macrophages in the rabbit eye

Injection of activated macrophages into the posterior vitreous of the rabbit induced vigorous fibrovascular proliferation over the optic disk and medullary rays, as demonstrated by 3H-thymidine autoradiography. One week after injection, endothelial cells and pericytes of the capillaries near the inner surface of the optic disk and rays were labeled; fibroblast-like cells, which were also labeled, migrated and formed vitreous strands. By the second week after injection, the fibrovascular tissue proliferated most actively, and traction medullary ray detachment and peripapillary retinal fold formation were observed. The cellular proliferation was accompanied by inflammatory cell infiltration. Glial cells within the optic disk, as well as retinal pigment epithelial cells beneath the detached retina, were labeled by 3H-thymidine. These results demonstrate that the fibrovascular proliferation originates from the vessel complex of the optic disk and medullary rays in this experimental model of retinal detachment.

The effect of transscleral laser cyclophotocoagulation on rabbit ciliary body vascularization

Long-term changes in the vascular network of rabbit ciliary processes induced by Nd:YAG laser cyclophotocoagulation were examined both from a morphological viewpoint and with respect to three-dimensional organization (in vascular casts). Relative changes in intraocular pressure (IOP), monitored within eyes irradiated across one-quarter, one-half, three-quarters, or all of the ciliary body circumference, are discussed in relation to morphological changes. The clinical implications of these findings are considered.

Radiation retinopathy as an experimental model for ischemic proliferative retinopathy and rubeosis iridis

We produced radiation retinopathy in capuchin monkeys and studied them with fluorescein angiography and light and electron microscopy. The animals were followed up from ten days to 3 1/2 years after radiation in order to determine whether this could provide an experimental model for other chronic ischemic-proliferative retinopathies, such as diabetes. The first change detected after radiation was the focal loss of capillary endothelial cells and pericytes. As the areas of acellular capillaries became confluent, cotton-wool spots became visible ophthalmoscopically. These increased in number and then faded away, leaving large areas of retinal capillary perfusion. Histologic studies showed occlusion first of the deeper, smaller retinal vessels and then gradually of the larger vessels. Intraretinal neovascularization as well as apparent recanalization then developed, but no new vessels extended through the internal limiting lamina into the vitreous. Rubeosis iridis with neovascular glaucoma developed 2 1/2 to 3 1/2 years postirradiation, and vitreous aspirate demonstrated a high level of angiogenic factor.

Age-related macular degeneration: ultrastructural studies of the relationship of leucocytes to angiogenesis

We describe the ultrastructural features of subretinal neovascularisation associated with the pathogenesis of age-related macular degeneration (AMD). The choroidal origin of new vessels was confirmed, and ultrastructural details are presented. Serial sectioning of new vessels revealed a relationship between leucocytes and neovascular structures. The results are discussed in the context of the previously established role of leucocytes in angiogenesis. Our results provide circumstantial evidence, based on morphological observations, for the involvement of leucocytes in the promotion of neovascular proliferation and exudation from new vessels.

Choroidal neovascularization

Choroidal neovascularization is now considered to be a most important feature in the pathogenesis and treatment of a number of chorioretinal disorders. Since the recognition of its importance, choroidal neovascularization has been described in clinical and pathologic studies of a variety of different ophthalmologic conditions. We have summarized the diseases in which choroidal neovascularization occurs and have reviewed studies on the natural history, clinical findings, and therapy of choroidal neovascularization that provide the basis for the current management of this condition.

Cellular processes causing defects in Bruch's membrane following krypton laser photocoagulation

Krypton laser photocoagulation of the posterior fundus of the rat created white retinal lesions comparable with therapeutic burns in the human. Twenty-four hours after injury, the retinal pigment epithelium was destroyed and the majority of the endothelial cells survived. The choriocapillary lumen was occluded. Bruch's membrane was not fractured nor had it undergone thermal lysis in any area studied but choriocapillary endothelial cell buds invaded the choroidal side of Bruch's membrane. After seven days, cellular protrusions arising from retinal pigment epithelial cells traversed Bruch's membrane through full-thickness defects and entered the choroid. Although intense laser burns may destroy Bruch's membrane, we conclude from this data that many so-called "breaks" in Bruch's membrane following laser therapy are formed by cellular processes and are not caused primarily by laser coagulation.

Subretinal neovascularization after experimental ocular histoplasmosis in a subhuman primate

Subretinal neovascularization was demonstrated in a subhuman primate eye (Macaca speciosa) one year after the internal carotid injection of yeast-phase Histoplasma capsulatum. In this animal model of ocular histoplasmosis, initial injection of viable H. capsulatum results in the development of self-limiting acute multifocal choroiditis. Acute lesions resolve within six months, forming chorioretinal scars that are clinically similar to human histo spots. Detailed ultrastructural study of a peripapillary scar 30 months after the injection showed the presence of subretinal neovascularization located between Bruch's membrane and degenerated retinal inner segments. These vessels appeared to be continuous, had tight junctional complexes, and a well-developed basal lamina with occasional pericytes. Failure of fluorescein angiography to demonstrate the presence of this neovascular net may be explained by the presence of tight junctions within the subretinal capillaries.

A transmission and scanning electron microscopic study of surgically excised preretinal membrane proliferations in diabetes mellitus

Thirty preretinal membranes excised from diabetic patients during pars plana vitrectomy were examined by transmission electron microscopy, scanning electron microscopy, or both. All the specimens were still correctly orientated after removal from the eyes. All the specimens contained collagen in varying amounts and most had blood vessels with a thickened basal lamina and adjacent pericytes. Fenestrated capillaries were identified in only four of the 21 membranes containing blood vessels. A wide variety of cell types were scattered within the membranes, including polymorphonuclear leukocytes, lymphocytes, plasma cells, macrophages (containing melanin and hemosiderin), glial cells, retinal pigment epithelium, and cells of uncertain derivation. Scanning electron microscopy showed the retinal side of the membranes to be smoother than the vitreal side and disclosed preretinal cells on the vitreal side of the membranes, either as individual stellate cells or as a monolayer of flattened cells with microvilli. Membranes stripped from the surface of the retina had a smooth, generally acellular surface on the retinal side.