Caveolae are 50 to 100 nm large, sac-like indentations of the plasma membrane. They are visible by electron microscopy and are found on the surface of most cells. Caveolae are found in particularly large numbers, for example, in vascular endothelium, on fat cells and on smooth muscle cells. Caveolae are special lipid rafts with a typical composition of proteins and lipids. The protein caveolin is the most important structural element.