Bronchus. Coloured scanning electron micrograph (SEM) of epithelial cells lining the bronchus. Some of the cells are covered in cilia (finger-like processes), while others have tiny microvilli which increase the surface area of the bronchus. The epithelium is covered in mucus ( removed here) which traps large dust particles and removes gaseous pollutants such as ozone and sulphur dioxide from inhaled air. The cilia beat rhythmically, filtering particles suspended in the incoming air, and sweeping dust and other foreign material back towards the trachea and larynx. Magnification: x1200 when printed 10cm wide.