Pinocytosis. Coloured atomic force micrograph (AFM) overlayed with a fluorescent light micrograph of pinocytosis in an endothelial cell. Pinocytosis, or cellular drinking, is the intake of extracellular fluid by a cell. It occurs when the plasma membrane folds inwards and the fluid flows into the channel created. The membrane then closes around the fluid forming a small vesicle (red) that is taken into the cell cytoplasm. Endothelial cells are flat and line all of the body's blood vessels. An atomic force microscope is used to study surfaces at an atomic level. An extremely fine spring-mounted probe is moved across the surface at a constant height. Any deflections are recorded and converted into a computer map of the surface.