Lung bronchus. Light micrograph of a cross section through the primary bronchus wall of the lung. The bronchus is a tube that supplies the lungs with air from the windpipe. It is lined by epithelium (top, pink) which contains goblet cells producing mucus. The epithelial cells are covered in tiny hair-like cilia (just visible) which with mucus remove dust particles from the air. A smooth muscle layer (centre, pale pink) lies between two layers of connective tissue. Elastic lamina propria (black, above) and loose submucosa (pink and white, below). At bottom (blue) is cartilage which provides the structural support for the bronchus. Elastic van Gieson/Alcian blue stained.