A scanning electron micrograph (SEM) showing the inner surface (epithelium) of the trachea or windpipe. The many tiny hair-like cilia and microvilli which can be seen on most of the cells are capable of active movement. They work in coordination to produce waves in one direction which sweep away any fluids or particles in contact with the surface of the trachea. Supplied by Giorgio Gabella, courtesy of Wellcome Images.