Metaplasia

Metaplasia is a general pathology term that refers to process when one cell type is replaced by another. It usually occurs in the context of a changed cellular environment to which the new cell type is better adapted . Examples include :

• Barrett esophagus: normal squamous epithelium replaced by columnar epithelium in the distal esophagus
• Helicobacter pylori infection: normal duodenal epithelium is replaced by patches of gastric-type epithelium
• smoking causes the normal columnar bronchial epithelium to be replaced by a squamous epithelium
• osseous metaplasia occurs in myositis ossificans
• in schistosomiasis, the normal transitional cell epithelium of the bladder is replaced by a squamous epithelium
• squamous epithelial metaplasia of the cervical transformation zone
• fatty nodal metaplasia
• apocrine metaplasia of the breast
• lipomatous metaplasia of the myocardium
• leukoplakia of the urinary tract: squamous metaplasia of the urothelium
• dendritic calcification of the lung

While the metaplastic epithelium can be adaptive in the short term, it predisposes to dysplasia and malignant change with continued irritation. It can also result in reduced function of the normal epithelium, such as loss of cilia of the respiratory epithelium.

Metaplasia occurs due to induction of an alternate differentiation pathway of stem cells. External stressors induce different gene expression promoting differentiation to a different cellular pathway.