Retinal Capillary Bed Research Articles

In Vivo Evaluation of Leukocyte Dynamics in the Retinal and Choroidal Circulation

We have developed a new method to visualize leukocytes and evaluate their kinetics in the chorioretinal microcirculation of the living eyes. Nuclear staining dyes and a scanning laser ophthalmoscope were used to image leukocytes in the fundus. Acridine orange was used to visualize leukocytes in the retinal microcirculation. For imaging leukocytes in the choroid, indocyanine green was injected intravenously. Dynamics of leukocytes in the capillaries of the retina and choroid were quantitatively estimated in monkeys and rats. This method also allowed evaluation of leukocyte-endothelial interactions, such as rolling or firm adhesion, in vivo. Acridine orange leukocyte fluography was used to study leukocyte dynamics in the following experimentally induced microcirculatory disturbances of the retina: (1) interferon-associated retinopathy, (2) ischemia-reperfusion injury of the retina, and (3) experimental diabetes mellitus. (1) Interferon-associated retinopathy Systemic administration of interferon alpha enhanced leukocyte-endothelial interactions in the retina, which resulted in leukocyte rolling and entrapment in the retinal capillary beds. Leukocyte accumulation was also detected in the lung. The entrapment or accumulation of leukocytes in the microcirculation was inhibited by simultaneous administration of corticosteroids or other agents. These results suggested that leukocytes play a major role in the development of adverse effects of interferon, such as retinopathy or interstitial pneumonia. (2) Ischemia-reperfusion injury of the retina During reperfusion period after transient (60 min) retinal ischemia by optic nerve ligation, the rolling of leukocytes in the retinal veins was prominent and numerous leukocytes were trapped in the retinal capillaries. The number of rolling leukocytes was at a maximum 12 hours after reperfusion. Leukocyte entrapment peaked at 24 hours after reperfusion. By blocking adhesion molecules on the vascular endothelium, these leukocyte-endothelial interactions were effectively inhibited. Postischemic retinal atrophy was also inhibited by blocking adhesion molecules. These results suggested that leukocytes may be major players in the pathophysiology of ischemia reperfusion injury of the retina. (3) Experimental diabetes mellitus Leukocyte dynamics in the retina were studied in streptozotocin-induced diabetes and spontaneous diabetes (OLETF rats). In both diabetic models, leukocyte entrapment in the retinal capillaries was increased even in the early stages of diabetes. Fluorescein angiography revealed that trapped leukocytes disturbed the regional capillary blood flow in the downstream. Enhanced expression of adhesion molecules was observed in the capillary endothelium of the retina in the diabetic rats. Leukocyte entrapment in the retinal capillaries might cause microvascular occlusions and dysfunction, in turn causing diabetic retinopathy.

Progress in measurement of ocular blood flow and relevance to our understanding of glaucoma and age-related macular degeneration.

New technologies have facilitated the study of the ocular circulation. These modalities and analysis techniques facilitate very precise and comprehensive study of retinal, choroidal, and retrobulbar circulations. These techniques include: 1. Vessel caliber assessment; 2. Scanning laser ophthalmoscopic fluorescein angiography and indocyanine green angiography to image and evaluate the retinal circulation and choroidal circulation respectively; 3. Laser Doppler flowmetry and confocal scanning laser Doppler flowmetry to measure blood flow in the optic nerve head and retinal capillary beds; 4. Ocular pulse measurement; and 5. color Doppler imaging to measure blood flow velocities in the central retinal artery, the ciliary arteries and the ophthalmic artery. These technique have greatly enhanced the ability to quantify ocular perfusion defects in many disorders, including glaucoma and age-related macular degeneration, two of the most prevalent causes of blindness in the industrialized world. Recently it has become clear, in animal models of glaucoma, that retinal ganglion cells die via apoptosis. The factors that initiate apoptosis in these cells remain obscure, but ischemia may play a central role. Patients with either primary open-angle glaucoma or normal-tension glaucoma experience various ocular blood flow deficits. With regard to age-related macular degeneration, the etiology remains unknown although some theories include primary retinal pigment epithelial senescence, genetic defects such as those found in the ABCR gene which is also defective in Stargardt's disease and ocular perfusion abnormalities. As the choriocapillaris supplies the metabolic needs of the retinal pigment epithelium and the outer retina, perfusion defect in the choriocapillaris could account for some of the physiologic and pathologic changes in AMD. Vascular defects have been identified in both nonexudative and exudative AMD patients using new technologies. This paper is a comprehensive update describing modalities available for the measurement of all new ocular blood flow in human and the clinical use.

Retinal blood flow measurements in branch retinal vein occlusion using scanning laser doppler flowmetry

PURPOSE: To determine capillary blood flow measurements in eyes with branch retinal vein occlusion using a scanning laser Doppler flowmeter. METHODS: Retinal capillary blood flow in branch retinal vein occlusion areas and corresponding ipsilateral nonbranch retinal vein occlusion areas, 11 equivalent areas of the contralateral fellow eye of 12 consecutive untreated branch retinal vein occlusion patients, and 16 eyes of 11 age-matched normal control subjects were measured with scanning laser Doppler flowmetry. A template consisting of eight squares, each with a field of 100 × 100 μm (10 × 10 pixel) with space interval of 500 μm equidistant horizontally and vertically was used to obtain blood flow measurements in all subjects. Mean blood volume, flow, and velocity were obtained by averaging the mean values measured in each field. We avoided measurement over large retinal vessels to prevent the aliasing artifact of blood cells from moving faster than the sampling frequency. RESULTS: Branch retinal vein occlusion areas have significantly decreased microvascular blood volume ( P = .0009), flow ( P = .02), and velocity ( P = .016) compared with ipsilateral nonbranch retinal vein occlusion areas in the same eye. Branch retinal vein occlusion areas also have decreased blood volume ( P = .001), flow ( P = .0042), and velocity ( P = .0044) compared with areas of contralateral fellow eyes of branch retinal vein occlusion subjects. Branch retinal vein occlusion areas have significantly decreased blood volume ( P = .0012), flow ( P = .008), and velocity ( P = .02) compared with age-matched normal areas. CONCLUSION: Average retinal blood volume, flow, and velocity in areas of branch retinal vein occlusion are significantly lower than in healthy retinas. The ability to noninvasively measure hemodynamic changes in the retinal capillary bed may be relevant to development of new therapies for retinovascular disease.

Ocular Neovascularization: Clarifying Complex Interactions

Neovascularization within the eye contributes to visual loss in several ocular diseases, the most common of which are proliferative diabetic retinopathy, neovascular age-related macular degeneration, and retinopathy of prematurity. Together, these three diseases afflict persons in all stages of life from birth through late adulthood and account for most instances of legal blindness. Retinopathy of prematurity (ROP) occurs in premature neonates. Normally, the retina becomes completely vascularized at full term. In the premature baby, the retina remains incompletely vascularized at the time of birth. Rather than continuing in a normal fashion, vasculogenesis in the premature neonatal retina becomes disrupted. Abnormal new proliferating vessels develop at the juncture of vascularized and avascular retina. These abnormal new vessels grow from the retina into the vitreous, resulting in hemorrhage and tractional detachment of the retina. Although laser ablation of avascular peripheral retina may halt the neovascular process if delivered in a timely and sufficient manner, some premature babies nevertheless go on to develop retinal detachment. Surgical methods for treating ROP-related retinal detachments in neonates have limited success at this time because of unique problems associated with this surgery, such as the small size of the eyes and the extremely firm vitreoretinal attachments in neonates. Diabetic retinopathy is the leading cause of blindness in adults of working age. In persons with diabetes mellitus, retinal capillary occlusions develop, creating areas of ischemic retina. Retinal ischemia serves as a stimulus for neovascular proliferations that originate from pre-existing retinal venules at the optic disk or elsewhere in the retina posterior to the equator. Severe visual loss in proliferative diabetic retinopathy (PDR) results from vitreous hemorrhage and tractional retinal detachment. Again, laser treatment (panretinal photocoagulation to ischemic retina) may arrest the progression of neovascular proliferations in this disease but only if delivered in a timely and sufficiently intense manner. Some diabetic patients, either from lack of ophthalmic care or despite adequate laser treatment, go on to sustain severe visual loss secondary to PDR. Vitrectomy surgery can reduce but not eliminate severe visual loss in this disease. Age-related macular degeneration is the leading cause of severe visual loss in persons over 65 years old. In contrast to ROP and PDR, in which neovascularization emanates from the retinal vasculature and extends into the vitreous cavity, AMD is associated with neovascularization originating from the choroidal vasculature and extending into the subretinal space. Choroidal neovascularization causes severe visual loss in AMD patients because it occurs in the macula, the area of retina responsible for central vision. The stimuli which lead to choroidal neovascularization are not understood. Laser ablation of the choroidal neovascularization may stabilize vision in selected patients. However, only 10% to 15% of patients with neovascular AMD have lesions judged to be appropriate for laser photocoagulation according to current criteria. Retinopathy of prematurity, proliferative diabetic retinopathy, and neovascular age-related macular degeneration are but three of the ocular diseases which can produce visual loss secondary to neovascularization. Others include sickle cell retinopathy, retinal vein occlusion, and certain inflammatory diseases of the eye. These, however, account for a much smaller proportion of visual loss caused by ocular neovascularization. Additional treatments beyond laser photocoagulation and vitrectomy surgery are needed to improve outcomes in these patients. Pharmacological antiangiogenic therapy can potentially assist in prevention of the onset or progression of ocular neovascularization and is a current goal of many research laboratories and pharmaceutical companies.

Fundus fluorescein angiography of patients with severe hypertensive nephropathy.

To analyze the fluorescein angiograms of patients with hypertensive renal failure. Fluorescein angiograms were obtained of 34 patients with hypertensive renal failure. The glomerular filtration was less than 20 ml/min in each patient. Capillary nonperfusion and a coarse retinal capillary bed are the hallmark of the hypertensive retinopathy. Tortuous retinal arterioles or tortuous retinal arteriovenous anastomoses are observed. It is suggested that capillary collaterals shunt arteriolar obstructions or short-cut nonperfused capillary beds; these remodel into tortuous arterioles or arteriovenous anastomoses with regular caliber. Elschnig spots and delayed choroidal filling are signs of hypertensive choroidopathy. Depigmentation spots of the retinal pigment epithelium without a central pigment clump are considered atypical Elschnig spots. Retinal pigment epithelial changes are permanent scars of hypertensive choroidopathy, while coarse retinal capillary bed and tortuous retinal arterioles or arteriovenous anastomoses are the result of hypertensive retinopathy in these patients with hypertensive renal failure.

Toxic action of advanced glycation end products on cultured retinal capillary pericytes and endothelial cells: relevance to diabetic retinopathy.

The toxic effects of advanced glycation end products (AGEs) on bovine retinal capillary pericytes (BRP) and endothelial cells (BREC) were studied. AGE-modified bovine serum albumin (AGE-BSA) was toxic to BRP. At a concentration of 500 micrograms/ml it reduced the BRP number to 48 +/- 3% (p < 0.05) of untreated controls, as determined by cell counting with haemocytometer. AGE-BSA was also toxic to bovine aortic endothelial cells (BAEC) reducing cell number to 84 +/- 3.1% of untreated controls. Under similar conditions, low concentrations (62.5 micrograms/ml) of AGE-BSA were mitogenic to BREC increasing the cell proliferation to 156 +/- 11% (p < 0.05) above that of untreated controls. At a higher dose of 500 micrograms/ml AGE-BSA decreased the proliferation of BREC to 85 +/- 6% of untreated controls. Immunoblot analysis demonstrated that BRP and BREC express the p60 AGE-receptor. Retinal capillary bed from the human also stained positively for the p60 AGE-receptor. Addition of 0.25 micrograms/ml of p60 AGE-receptor antibody was able to block the effects of AGE-BSA on BRP and BREC. The level of binding of [125I]-labelled AGE-BSA to the cell surface was small but significant among the three cell types. There was also an increase in the internalized pool of radioligand in BRP and BREC but this was very much lower than in BAEC. In all the cell types the internalized pool of [125I]-labelled AGE-BSA was much larger than the amount associated with the cell surface. Degradation products were not detected in the media over the 24-h incubation of the cells with [125I]AGE-BSA. The binding of [125I]-labelled AGE-BSA to the cell surface was prevented by the addition of p60 AGE-receptor. These results suggest that the interaction of AGE-modified proteins with the membrane-bound AGE-receptor may play an important role in the pathogenesis of diabetic retinopathy.

Intraretinal oxygen distribution in rats as a function of systemic blood pressure

Differential responses to induced changes in systemic blood pressure (BP) at different layers of both the retinal and choroidal vasculature were observed, by monitoring localized PO2 as a function of depth, in the retina and choroid of the rat eye using oxygen-sensitive recessed microelectrodes. Visual and electrophysiological localization of the microelectrode tip allowed the oxygen distribution to be related to the positions of the vascular beds of the retina and choroid. Highly reproducible intraretinal PO2 profiles were achieved. The relationship between PO2 and systemic BP was linear in the deep capillary layer of the retina (PO2 = 0.17 x BP - 2.63) and in the choriocapillaris (PO2 = 0.21 x BP + 2.95), whereas it was nonlinear in the superficial retinal capillary layer [PO2 = 40.01/[1 + (BP/66.22)-1.22]] and deep choroid [PO2 = 83.82/[1 + (BP/124.61)-0.87]]. The minimum PO2 occurred between the two retinal capillary beds, and a PO2 gradient was evident in the choroid. The contrasting responses of different layers of the two circulations reflect different blood flow control mechanisms not evident when studying the circulations as a whole.

Amelioration of Diabetes-like Retinal Changes in Galactose-Fed Dogs

Diabetic retinopathy, the leading cause of blindness among young adults in the developed world, is characterized by vascular changes of the retinal capillary bed. Beagles fed a diet containing 30% galactose develop retinal vascular lesions that are similar to those observed in diabetics. These progress from initial retinal changes which include aldose-reductase-linked formation of pericyte ghosts and the subsequent development of acellular capillaries, microaneurysms, and intraretinal hemorrhages to the appearance of occluded vessels, areas of nonperfusion, and intraretinal microvascular abnormalities (IRMA) and in the final stages, the formation of fibrovascular membranes on both the retinal surface and the posterior hyaloid membrane. In prevention studies utilizing 0.5, 5.0, 10, and 16 mg/kg/day of the aldose reductase inhibitor M79175 (2-methyl-6-fluoro-spiro-chroman-4-5′-imidazolidine-2′,4′-dione), pericyte ghost formation, and the subsequent appearance of microaneurysms, intraretinal hemorrhages, acellular capillaries associated with background retinopathy were arrested in a dose-dependent manner. Similar dose-dependent changes in the appearance of cataracts were also observed. The dog represents the first animal model to demonstrate all of the clinical and histological retinal vessel changes observed in human diabetics.

Peripheral retinal vasculature in normal Jamaican children.

A prospective study of the peripheral retinal vasculature in a Jamaican cohort of subjects with sickle cell disease has been in progress over a period of 12 years using fluorescein angiography. Various vascular patterns were identified but their significance was unclear since no comparable records were available in subjects of a similar age with normal (AA) haemoglobin genotype. Fluorescein retinal angioscopy and angiography have been performed in 76 haemoglobin AA controls participating in the cohort study. The peripheral retinal capillary bed could be seen and photographed in a limited portion of the temporal peripheral fundus in a majority of this group, and there was considerable variation in the vascular pattern which could be characterised. These observations allow deviations from normal to be identified in the retinal vasculature in subjects with sickle cell disease.

Retinal Vascular Changes in Congenital Hypertrophy of the Retinal Pigment Epithelium

Congenital hypertrophy of the retinal pigment epithelium (CHRPE) is a well-defined clinical entity with usually well-delineated, pigmented oval or round lesion with smooth or scalloped margins. Occasional retinal vascular changes have been reported previously. To assess the prevalence of these changes, the authors performed a retrospective analysis of 12 patients with CHRPE, for whom fluorescein angiography allowed visualization of the entire lesion and of the retinal vascular capillary bed. Retinal vascular changes were found in 11 (91%) of these 12 patients. The changes consisted of capillary rarefaction in all 11 patients, with areas of capillary nonperfusion exceeding 1 disc diameter (DD) in three patients (25%), micro-aneurysmal capillary dilatations in three (25%), and chorioretinal anastomosis in one. These results suggest that the above changes could constitute clinical and angiographic characteristics of CHRPE and allow easy corroboration of its diagnosis, thus avoiding the need for further clinical investigations.

Evidence and implication of a local hemodynamic regulation in retinal capillary beds

Evidence and implication of a local hemodynamic regulation in retinal capillary beds

Assessment of the eye at risk of neovascularisation

The causal linkage between retinal ischaemia and ocular neovascularisation has been recog­ nised and widely accepted by the ophthalmic community since the early pioneering work of Michaelson/ Ashton,2 Patz,3 Wise,4 and many others. In the initial stages of this research, the presence of retinal vascular occlusion in eyes with neovascular retinopath­ ies was surmised primarily on the basis of his­ tological specimens of the retinal vessels obtained from autopsy eyes of patients with diabetic retinopathy. Trypsin digestion prep­ aration of the retinal vasculature showed focal areas of capillary acellularity, which were interpreted as being nonperfused capillaries in life. 5.6 Subsequently, the development of fluorescein angiography of the retinal vessels allowed assessment of their perfusion status in the living eye. In the case of diabetic ret­ inopathy, focal nonperfusion of the retinal capillary bed, as well as of arterioles in the more advanced stages of the disease, was demonstrated angiographically. In a few cases in which both in vivo fluorescein angiography and post mortem trypsin digestion prep­ arations were available, clinicopathological correlation studies showed a topographical correlation between capillary acellularity and angiographic nonperfusion. 7,8,9 Numerous clinical studies over the years support the notion that retinal ischaemia pro­ vokes ocular neovascularisation. Both cross­ sectional and longitudinal studies have demonstrated the following: (1) retinal vascular growth in diabetic retino­ pathy and in sickle cell retinopathy pref­ erentially occur at the border between perfused and nonperfused retinal zones,lO,11 (2) disc neovascularisation in diabetic retino­ pathy occurs mainly in eyes with exten­ sive mid-peripheral or generalised capillary nonperfusion,12 (3) iris neovascularisation in central retinal vein occlusion (CRVO) is found primarily in eyes with widespread retinal vascular nonperfusion. 13,14,15,16 In a recent large-scale longitudinal study of eyes with non proliferative diabetic retino­ pathy (NPDR), the extent of capillary non­ perfusion, arteriolar abnormalities, and fluorescein leakage present at the baseline visit, (as measured by a standardised angio. graphic grading system) were significant risk factors for the subsequent development of proliferative retinopathy (PDR). 17 The differ­ ence in rates of developing new vessels within one year between eyes with and without these abnormalities ranged from two to threefold, depending upon the subgroup of NPDR sev­ erity considered. On the basis of this study, it is clear that fluorescein angiography is a useful tool in assessing the risk for neovascularisa­ tion in eyes with NPDR. Besides determining the anatomic exten� of nonperfusion in eyes with vascular retina· pathy, evaluating the effects of ischaemia on various functions of the eye can also be useful. The major test, in this respect, has been elec­ troretinography, although other measures such as perimetrl8 and pupillary lighf response18 have been used. Conventional electroretinography (ERG) is a non-invasive test that reflects the light-induced electrical activity of the outer and middle layers of the, retina throughout its entire extent; therefore, it can relatively easily detect extensive 01 generalised ischaemia. As example, ampli.

Fluorescein angiogram in diabetic dogs.

Fluorescein angiography was carried out see retinal vascular changes of the ocular fundus in 4 spontaneously occurring diabetic dogs. In all 4 cases, several hypofluorescent areas were determined to be filling defects of the retinal capillary bed of the tapetal fundi during the arteriovenous phase. Around these hypofluorescent areas, many dots of hyperfluorescence were also observed to be microaneurysms. However, no leaking of fluorescein was detected during angiography. These findings of 4 diabetic dogs were similar to those reported in man with non-proliferative diabetic retinopathy.

Retinal and Choroidal Blood Flows in Hypoxic and Hypercarbic Newborn Lambs

We studied the effect of changes in the arterial tensions of oxygen and carbon dioxide on blood flow to the retinal (RBF) and choroidal ( ChBF ) capillary beds in 20 lambs. One to three days after placement of catheters in the left ventricle, abdominal aorta, and brachiocephalic artery, different gas mixtures were delivered to a bag enclosing the lamb's head. One group of lambs was studied at normal and low oxygen tensions while normocarbic , and another group was studied at normal and high carbon dioxide tensions while normoxic. RBF and ChBF were measured using the radioactive microsphere technique. RBF increased as PaO2 and, thus, arterial oxygen content [( O2]a) fell; in contrast, ChBF was not related to [O2]a. Oxygen delivery to the capillary bed of the retinal artery (i.e., RBF X [O2]a) was independent of arterial oxygen content because the change in [O2]a was balanced by a reciprocal change in RBF. Oxygen delivery to the choroidal capillary bed, however, rose with [O2]a because there was no reciprocal decrease in ChBF . Both RBF and ChBF increase as arterial carbon dioxide tension rose. Although an increase in arterial carbon dioxide tension produced an increase in RBF with no change in [O2]a, oxygen delivery to the retinal capillaries rose. Similarly, oxygen delivery to the capillaries of the choroid rose with carbon dioxide tension. Oxygenation of retinal tissue, which depends on oxygen diffusion from both the retinal and choroidal capillary beds, may change with variations in arterial oxygen content (increase in oxygen delivered to the choroidal capillary bed) or arterial carbon dioxide tension (increase in oxygen delivered to both the retinal and choroidal capillary beds).

Experimental carotid occlusion: funduscopic and fluorescein angiographic findings.

A characteristic fundus picture was consistently produced following acute bilateral common carotid artery ligation in mature rats, reminiscent of human carotid occlusive disease. Two days after ligation it consisted of dilatation and tortuosity of retinal veins, blurring and swelling of the optic disc, retinal whitening primarily along the venous distribution, and straightening of retinal arteries. Fluorescein angiography showed hyperfluorescence of the disc, delay in the rate of retinal arterial and venous filling, venous dilatation, disc oedema, disruption of the retinal capillary bed pattern, and late peripapillary staining/leakage. This pattern was not seen in rats which underwent acute unilateral ligation, although some mild changes were seen on fluorescein angiography. The vascular alterations seemed to regress spontaneously within one week. A peripapillary 'halo' and a granular-appearing nerve fibre layer developed later, exclusively in bilaterally ligated animals.

Fluorophotometric evaluation of capillary permeability in gram-negative shock.

We developed a new method by which capillary permeability changes secondary to gram-negative sepsis can be easily monitored. In a preliminary trial, ten dogs were injected intravenously with live Escherichia coli. Progressive extravasation of fluorescein from the retinal capillary bed was demonstrated by retinal angiography, thus indicating "capillary leak." In a subsequent trial, fluorophotometry facilitated direct quantitation of this leak. Three control and three septic dogs underwent continuous hemodynamic and physiologic monitoring. Hourly retinal fluorophotometric measurements were recorded. The control group demonstrated no capillary leak nor any change in hemodynamic and physiologic status. In the septic group, there was a dramatic increase in extravascular fluorescein concentration two to four hours after E coli infusion, correlating well with hemodynamic and physiologic parameters typical of gram-negative shock.

Morphology and enzyme activities of the retinal capillaries in streptozotocin-diabetic mice.

Male lean mice belonging to the obese-hyperglycemic strain were made diabetic by intravenous injection of streptozoticin. The retinal capillary bed freed by trypsin digestion was studied with regard to morphology and the activity of some enzymes. There was a significant increase in the ratio between the endothelial and mural cells which was interpreted as indicating mural pericyte disappearance. The activities of adenylate kinase, aspartate-aminotransferase and hydroxyacyl-CoA-dehydrogenase in the retinal vessels of the diabetic animal were significantly higher than in vessels from the control animals. No differences were found in the activities of glucose-6-phosphate dehydrogenase, glutathione reductase and phosphofructokinase between the two animal groups. It is suggested that these results reflect early morphological and metabolic changes of the retinal vessels, preceding the well known clinical picture of diabetic retinopathy.

Aging changes in the retinal capillary bed of the rat

Thirty normal albino rats of ages from 2 to 32 months were sacrificed, their globes removed, and their retinas prepared by the pepsin-trypsin digest technique. The mounted preparations were stained with hematoxylin and PAS. The retinal capillaries were then analyzed with a light microscope in the peripapillary, midretinal, and peripheral areas. The following results were obtained: (1) The ratio of endothelial cells to intramural pericytes ranged from 1.57 to 2.18. No definite changes were found between different age groups or between different retinal regions. (2) The number of endothelial cells and intramural pericytes per high-power field decreased with age, with the decrement most marked between 8 and 12 months. (3) Two types of acellular strands were noted: thin, threadlike strands and thick, acellular capillary strands. The number of thin strands per high-power field averaged 4.86, with no obvious increase with aging. The number of thick strands increased markedly between 12 and 24 months: Up to the age of 12 months an average of only 0.02 thick strands were found per high-power field, whereas in rats 24 months or older an average of 0.38 thick strands were found per high-power field. (4) In all age groups the capillary cells appeared more ragged and their nuclei more weakly stained in the periphery than in the other retinal regions; this was more marked in older rats.

MORPHOLOGY AND ENZYME ACTIVITIES OF THE RETINAL CAPILLARIES IN MICE WITH THE OBESE‐HYPERGLYCAEMIC SYNDROME (GENE SYMBOL OB)

The retinal capillary bed from 67 obese-hyperglycaemic mice and 64 lean litter mates was isolated by trypsin digestion and investigated with respect to structure and enzyme activities. There was no significant difference in the ratio between numbers of endothelial and mural cells. The capillary walls did not show any obvious structural differences and microaneurysms were not observed. The retinal vessels from the obese-hyperglycaemic mice, however, displayed significantly higher activities of the enzymes hydroxyacyl-CoA-dehydrogenase, asparate aminotransferase (ASAT) and adenylate kinase than their lean litter mates. The activities of glutathione reductase, glucose-6-phosphate dehydrogenase (G-6-PDH) and phosphofructokinase were similar in the two experimental groups. It is suggested that the present data reflect early metabolic disturbances related to diabetic retinopathy.

Methyl Alcohol Poisoning

The ocular toxicity of methyl alcohol has been investigated in six rhesus monkeys. All the animals developed fundus changes within 43 to 171 hours after its ingestion. The only fundus lesion seen was optic disc edema and associated changes, usually of a marked degree. Fluorescein fundus angiography confirmed the findings. The retinal and choroidal circulations, including the retinal capillary bed, were normal. Ophthalmoscopically and angiographically, optic disc edema in methyl alcohol poisoning was indistinguishable from that seen in raised intracranial pressure, except that no increased intracranial pressure was observed. It is postulated that optic disc edema in methyl alcohol poisoning is due to an axoplasmic flow stasis.