Perivascular and intervascular reticular fibers of the retina

Histopathology of diabetic retinopathy in man

The retinal changes associated with diabetes mellitus are a consequence of the systemic microangiopathy with modifications related to the intraocular environment. The vascular disorders underlying background retinopathy are arteriolar hyalinosis (which together with abnormalities in the circulating blood can give rise to focal capillary closure), venular dilatation, and capillaropathy in the form of pericyte degeneration, basement membrane thickening and microaneurysm formation. Retinal complications consist of plasma exudation and punctate haemorrhages. Maculopathy is due to cystoid oedema. Increasing closure of capillaries is linked with cotton-wool spots and intraretinal microvascular anomalies, the former reflecting a consequence and the latter a response to increased ischaemia. Vascular proliferation in front of the retina originates from venules close to areas of ischaemia; the endothelium may be fenestrated initially and fibrosis may accompany the new vessels.

Clinicopathologic correlation and pathogenesis of ocular and central nervous system manifestations in Hallervorden-Spatz syndrome

We have correlated the clinical and histopathologic features of the eyes and central nervous system in a patient with Hallervorden-Spatz syndrome who died at age 11 years. The main ocular findings included degeneration of photoreceptors, marked thinning of the outer nuclear and outer plexiform layers, retinal gliosis, narrowing and obliteration of blood vessels with a perivascular cuffing of pigment cells, and degenerative changes in the retinal pigment epithelial cells with accumulation of melanolipofuscin. The positive findings in the brain included a symmetrical, partially destructive lesion of the globus pallidus, especially in its internal fibers and neurons; in addition, we noted gliosis, widely disseminated axonal spheroidal bodies, which were most numerous in the globus pallidus and pars reticulata, as well as deposits of iron. Our histopathologic findings implicate three possible mechanisms, namely, lipid peroxidation, a deficiency of fatty acid membrane components, and increased cGMP which, either singly or in combination, are responsible for a pathogenesis that is common to the eye and brain in Hallervorden-Spatz syndrome.

Human retinal capillary basement membrane leaflets are morphologically distinct: a correlated TEM and SEM analysis

Because retinal capillaries and their associated basement membranes (BMs) are significantly altered in a number of diseases (most notably diabetic retinopathy), the human retinal microvasculature is of interest to both basic scientists and clinicians. Consequently, numerous TEM studies centered primarily on cellular elements of retinal microvessels have been carried out. Ultrastructural studies emphasizing retinal capillary extracellular matrix (ECM) materials including BMs however, are nearly non-existent. Accordingly, the current correlated TEM/SEM investigation was undertaken. The study shows that retinal capillary walls are comprised of a continuous layer of endothelial cells and a discontinuous layer of intramural pericytes which are in frequent contact. These are underlain and/or surrounded by a retinal capillary BM complex which includes pericytic matrix, fibrillar collagen, and subendothelial, pericytic and Müller cell BM leaflets. Following sequential detergent treatment, all retinal cells are solubilized. Vessel ECM components, however, maintain their in vivo histoarchitectural relationships. Moreover, on the basis of substructure, susceptibility to non-specific proteases and anionic site density, BM leaflets are morphologically distinct. In addition, high-resolution SEM studies show that BM surface characteristics are tissue specific. It is concluded that retinal capillary BM complexes are comprised of structurally unique subcomponents the characteristics of which should be considered in future studies of retinal capillary BM structure, composition and function and particularly in investigations in which retinal capillaries are pathologically altered.

Ultrastructural evidence for morphological specificity in isolated bovine retinal capillary basement membranes

Because of its relative availability, large size, and presumed similarity to human, the bovine retina has been used by numerous investigators as a source of vessels, cells, and basement membranes (BMs) for biochemical analyses and in vitro studies of cells and extracellular matrix. Careful morphological studies of these vessels and their associated BMs, however, have not been done. Accordingly, we carried out experimental ultrastructural studies in an effort to show their cellular composition, their histoarchitectural relationships within retinal capillary walls, and the disposition and features of their isolated BMs. Our study shows that these vessels are complex, multicomponent structures composed of endothelial cells and intramural pericytes, which frequently communicate via direct cell/cell contacts, and a system of BMs. The latter includes continuous Muller cell BMs, interrupted subendothelial BMs, and pericytic BMs with masses of pericytic matrix (PCM) intervening. Isolated subendothelial BMs are remarkable for fenestrations, selective susceptibility to nonspecific proteases, and high density of ruthenium red (RR)-positive anionic sites. On the contrary, Muller cell BMs are continuous (completely surrounding retinal capillaries), relatively refractory to proteases, and show significantly fewer anionic sites by RR. Acellular capillary BMs frequently show ghost-like "pockets" previously occupied by pericytes. These are surrounded by pericytic BMs and interstitial spaces are "filled-in" by a BM-like material (PCM) which frequently contains striated collagen fibrils and is positionally and morphologically homologous to glomerular mesangial matrix. These data indicate that tissue specificity of BMs may be far more precise than previously thought and that each capillary BM leaflet may possess a peculiar macromolecular architecture commensurate with its specific function.

The architecture of the most peripheral retinal vessels

The vascular bed of the extreme periphery of the human retina is characterized by the occurrence of relatively wide arcades and bridging vessels connecting the peripheral portions of arteries and veins. These vessels differ from the more posterior capillaries not only by their larger diameter but also by the presence of a perivascular halo, which consists of an electron-translucent ground substance containing fine collagen fibrils, basement membrane material, dense bodies, vesicles, and portions of macrophages. The appearance of the external portion of the halo and the relationship of its collagen fibrils to the basement membrane of the surrounding Müller cells is indistinguishable from that of the cortical vitreous and the inner limiting membrane located at the vitreoretinal interface. Toward the periphery the thickness of the halo increases from vessel to vessel. As a rule, one or more of the vessel loops closet to the ora serrata are occluded. The structure of these occluded vessels is identical with the structure of the halo.

Reticulum fibres in relation to retinal vessels

Argyrophilic perivascular and intervascular fibres in the mammalian retina are shown by specific antireticulin immunofluorescence to consist of reticulin. The possible significance of these findings is briefly discussed.