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.
In the kidney, the glomerular endothelium is a little different than most other endothelia in that they contain fenestrae: actual pores in the endothelial cells which are large enough to let most proteins pass through, but small enough to exclude circulating blood cells. The glomerular fenestrated endothelium makes up a component of the glomerular filtration barrier (along with the glomerular basement membrane and the podocyte layers), and the classic teaching is that the negatively charged proteoglycans on this layer help form part of the charge barrier keeping albumin from getting filtered (though this is now being debated by some). Here are some pretty pictures of the glomerular fenestrated endothelium:
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