Endothelial actin cytoskeleton in rat mesentery microvasculature.

The actin cytoskeleton plays a key role in maintaining and modulating cellular morphology and thus is important in various aspects of endothelial cell function, yet few studies to date have studied the endothelial actin cytoskeleton in the microvasculature, where many of the specialized endothelial functions take place. We developed a method to fix and stain the microvasculature of the rat mesentery under physiological conditions and used a confocal microscope to observe the pattern of actin filaments in individual endothelial cells throughout the intact vascular network. We observed several distinct patterns of actin filaments in the endothelial cells depending on the caliber and type of vessel. The endothelial cells of large arterioles and venules (> 50 microns in diameter) displayed a prominent circumferential rim of actin, with few central actin fibers. Endothelium of midsized (20-50 microns) arterioles (and some midsized venules) displayed prominent abluminal actin fibers across the central portion of the cell, aligned parallel to the longitudinal axis of the vessel, in addition to a peripheral rim of actin. The endothelium of capillaries contained few actin fibers but instead typically displayed diffuse staining throughout the cell. The endothelial cells in the postcapillary venules generally displayed a thin peripheral rim of actin and occasional short central fibers. These results indicate that the local environment dictates the actin pattern in endothelial cells and suggests that specialized endothelial function is correlated with specialized endothelial actin architecture.