Tracheal epithelial cell. Coloured Transmission Electron Micrograph of a section through a healthy epithelial cell of the human trachea (windpipe). The cell has many cilia, which have internal supporting microtubules. Microvilli increase the cell's surface area. The cell's cytoplasm contains some mitochondria. The epithelium of the trachea is normally covered in mucus which traps dust and foreign particles and helps to remove gaseous pollutants from inhaled air. The cilia beat rhythmically helping to move glandular secretions and mucus carrying foreign particles towards the pharynx and away from the lungs. Magnification: x7000 when printed 10 centimetres wide.