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Marie N O'Connor, Sussan Nourshargh, John Greenwood; Regional heterogeneity in the endothelial glycocalyx and basement membrane of the retinal vasculature. Invest. Ophthalmol. Vis. Sci. 2014;55(13):2866.
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The vascular blood-retinal barrier is composed of endothelial cells expressing a thick gel-like glycocalyx which projects into the lumen and is surrounded abluminally by a sheath of pericytes and basement membrane. Whilst leukocyte recruitment during inflammation is enabled by changes in the expression of adhesion molecules and chemokines, the role of the glycocalyx and basement membrane in this process remain poorly characterised.
We have used retinal whole-mounts and state-of-the-art imaging technology to examine the distribution of basement membrane components in normal and inflamed retina, as well as examining the endothelial glycocalyx of the retina by perfusion of fluorescently-labelled lectins.
We demonstrate that the endothelial basement membrane of the retinal vasculature is heterogeneous, exhibiting low expression regions (LERs) for type IV collagen, laminin-411, and nidogen. These LERs coincide with gaps in the pericyte layer and, unlike peripheral vessels, are situated adjacent to, but not overlapping with, endothelial cell junctions. In inflamed retina, laminin-411 LERs are greater in size than in non-inflamed retina, though the number remain constant. The endothelial glycocalyx is also heterogeneous, displaying both regional and local differences in lectin binding; Griffonia simplicifolia I isolectin-B4 preferentially binds to arterioles and arteriolar branch points, and Lycopersicon esculentum agglutinin concentrates near endothelial cell junctions and in patches across the apical surface.
LERs have previously been observed in the peripheral vasculature, where they can be permissive sites for leukocyte extravasation. Here we show that they exist in the retina, and are potentially modified during inflammation. We have also identified heterogeneity in the composition of the endothelial glycocalyx which is especially intriguing considering emerging evidence for the importance of the endothelial glycocalyx in leukocyte recruitment.
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