Paper I
Question
Define metaplasia and give examples
Answer
Definition
- Metaplasia is a reversible change in which one adult (differentiated) cell type is replaced by another adult cell type, usually as an adaptive response to chronic irritation/stress, allowing the tissue to better withstand the altered environment
Examples
- Squamous metaplasia of respiratory epithelium: Normal ciliated columnar epithelium of the bronchus is replaced by stratified squamous epithelium in chronic smokers
- Barrett oesophagus: Normal squamous epithelium of the distal oesophagus is replaced by columnar epithelium with goblet cells (intestinal metaplasia), in response to chronic gastroesophageal reflux
- Squamous metaplasia of the endocervix: Columnar epithelium replaced by squamous epithelium, commonly seen at the transformation zone
- Osseous metaplasia: Formation of bone in soft tissue at sites of chronic injury (e.g., myositis ossificans)
Clinical significance
- Generally reversible if the inciting stimulus is removed
- The replacing cell type, though better adapted to the abnormal environment, usually has reduced/lost specialized function
- Persistent metaplasia can predispose to dysplasia and subsequent malignant transformation (e.g., squamous metaplasia of bronchial epithelium predisposing to squamous cell carcinoma of the lung; Barrett oesophagus predisposing to adenocarcinoma)