Just heard an interesting talk on endothelial dysfunction and its relation to nephrology. One thing I learned is that the vascular endothelium is actually "hairy", as demonstrated from this electron micrograph (taken from this website). These little hairs are the endothelial glycocalyx, a gel-like layer of negatively charged proteoglycans and membrane glycoproteins which helps serve as a protective barrier from blood flow for the endothelial cells. You can't see it on standard light microscopy since it apparently requires special preservation techniques not normally used. There is some data to suggest that vasculopaths and diabetics have a less developed and overall thinner endothelial glycocalyx compared to healthy controls, suggesting the importance of this structure in maintaining a healthy vasculature.