Metaplasia is a reversible change where one differentiated cell type is replaced by another differentiated cell type, often in response to chronic irritation or inflammation. A common example of metaplasia is the transformation of columnar epithelium (such as that found in the respiratory tract) to squamous epithelium.
This often occurs in smokers: the normal respiratory epithelium (ciliated columnar cells with goblet cells) undergoes metaplasia to stratified squamous epithelium. This change protects the underlying tissues from harmful substances in smoke but can impair the respiratory function and lead to increased risk of respiratory diseases, such as chronic bronchitis.
Another example is in the esophagus, where chronic gastroesophageal reflux disease (GERD) can lead to Barrett's esophagus, in which the normal squamous epithelium is replaced by columnar epithelium, a process known as intestinal metaplasia. This condition increases the risk of esophageal adenocarcinoma.