<P> Metaplasia occurs when a differentiated cell of a certain type is replaced by another cell type, which may be less differentiated . It is a reversible process thought to be caused by stem cell reprogramming . Stem cells are found in epithelia and embryonic mesenchyme of connective tissue . A prominent example of metaplasia involves the changes associated with the respiratory tract in response to inhalation of irritants, such as smog or smoke . The bronchial cells convert from mucus - secreting, ciliated, columnar epithelium to non-ciliated, squamous epithelium incapable of secreting mucus . These transformed cells may become dysplasic or cancerous if the stimulus (e.g., cigarette smoking) is not removed . The most common example of metaplasia is Barrett's esophagus, when the non-keratinizing squamous epithelium of the esophagus undergoes metaplasia to become mucinous columnar cells, ultimately protecting the esophagus from acid reflux originating in the stomach . If stress persists, metaplasia can progress to dysplasia and eventually carcinoma; Barrett's esophagus, for example, can eventually progress to adenocarcinoma of the esophagus if not treated . </P> <P> Dysplasia refers to abnormal changes in cellular shape, size, and / or organization . Dysplasia is not considered a true adaptation; rather, it is thought to be related to hyperplasia and is sometimes called "atypical hyperplasia". Tissues prone to dysplasia include cervical and respiratory epithelium, where it is strongly associated with the development of cancer; it may also be involved in the development of breast cancer . Although dysplasia is reversible, if stress persists, then dysplasia progresses to irreversible carcinoma . </P>

Who is at risk for developing alterations in cellular or tissue adaptation