Caveolae (also known as plasmalemmal vesicles) are 50-100 nm membrane domains that represent a subcompartment of the plasma membrane. Caveolae are most abundant in simple squamous epithelia, such as endothelial cells, fibroblasts, smooth muscle cells and adipocytes, but are believed to exist in most cell types. Three types of caveolins, designated caveolin-1, caveolin-2 and caveolin-3, are the principal components of caveolae. Caveolin-1 is a substrate for v-Src protein tyrosine kinase.