Heterogeneous endothelial cell structure along the porcine retinal microvasculature

The pivotal role of the endothelial cell in the regulation of vascular tone has been well demonstrated in many vascular beds, including the retina. However, in the retina, little is known about how the structural elements of the endothelial cells are arranged along the arborisation pathway from artery to vein, the nature of which has been linked to functional heterogeneity in other vascular beds. The relative vulnerability of the retina to vascular based diseases, and the heavy reliance on local regulation of the retinal vasculature makes an improved understanding of such local regulatory mechanisms of significant clinical importance. The present study focuses on identifying differences in endothelial cells along the arborisation pathway in the porcine retinal vasculature. Enucleated pig eyes were arterially cannulated and perfused with fixative followed by double staining for F-actin microfilaments (rhodamine phalloidin) and nucleic acid (YO-PRO-1). The intact retina was then viewed by confocal microscopy. The distribution of F-actin, vessel diameter, endothelial cell size and shape, nucleus size and shape, and position within the cell were determined as a function of location along the vascular tree. The main retinal arterioles (A1) contained full length F-actin internal stress fibers which lay parallel to the long axis of the endothelial cell. Subsequent branches from the A1 arteriole (A2 and A3) showed fewer, shorter fibers, with none visible in the A4 and A5 branches, the capillaries, or in the venous side of the vasculature. All endothelial cells showed peripheral border staining of F-actin microfilaments which allowed the shape of the cell to be determined. All endothelial cells were elongated with the long axis parallel to the vessel, but the mean aspect ratio decreased from 10.9+/-0.5, s.e.m. in the A1 arterioles to 3.2+/-0.2 in the major veins (V1). The position of the endothelial cell nucleus relative to the cell was eccentric in the downstream direction in the A2-A5 arterioles, whilst centrally placed in the A1 arterioles and veins. The structural heterogeneity of endothelial cells along the pig retinal circulation suggests that functional heterogeneity of the endothelium may be involved in regulation of retinal blood flow.