Here are some links to pages with more information about endothelial cells:

http://hemeteam.com/endothelium/safari3/index.html?dhtmlActivation=inplace

http://www.jci.org/articles/view/62589

http://www.youtube.com/watch?v=0pnpoEy0eYE

and of course Wikipedia
http://en.wikipedia.org/wiki/Endothelium