The structure of caveolae. (A) Scheme showing a caveola and the principle membrane curvatures of the caveolar domain. Curvature is described in two perpendicular directions. The bulb of the caveola is positive in both directions (x and y), but the neck shows negative and positive curvature, as indicated. (B) Electron micrograph showing the bulblike morphology of caveolae. Bar, 100 nm. (C) Higher-order rosette organization of caveolae in a cultured adipocyte. Bar, 100 nm. (D) The protein composition of caveolae. (E) The neck domain accounts for the overall mean negative curvature of the caveolar domain. Highlighted are the lipids of the inner leaflet of the PM. Blue, cone-shaped lipids. (F) Average dimensions of neck and bulb domains from endothelial caveolae preserved by high-pressure freezing and freeze substitution published previously (Richter et al., 2008). Curvature calculations were based on these dimensions. Diagram, including bilayer thickness, is to scale.