Light micrograph showing squamous metaplasia of the human bronchus. Metaplasia describes an alteration in the appearance of cells brought about by a change in environmental factors. In the bronchi, the airways of the lung, respiratory epithelium becomes replaced by stratified squamous epithelium due to the chronic irritation of cigarette smoke. Here, the upper layer of purple-stained cells resembles the type of native stratified squamous epithelium found, for example, in the vagina. Metaplasia is believed to be an adaptive response that produces cells which are better equipped to withstand an altered environment.