## Definition and Reversibility **Key Point:** Metaplasia is a reversible adaptive response in which one differentiated cell type is replaced by another differentiated cell type, typically in response to chronic irritation or altered environmental conditions. If the stimulus is removed, the original cell type may be restored. **Key Point:** Dysplasia is a pre-malignant change characterized by loss of cellular uniformity, increased nuclear-to-cytoplasmic ratio, hyperchromasia, and increased mitotic activity. Dysplasia is NOT reversible and represents a step on the continuum toward malignancy. ## Comparison Table | Feature | Metaplasia | Dysplasia | | --- | --- | --- | | **Reversibility** | Reversible (if stimulus removed) | Irreversible | | **Cellular uniformity** | Maintained | Lost (pleomorphism) | | **Nuclear changes** | Normal | Hyperchromatic, enlarged nuclei | | **Malignant potential** | None | Yes (pre-malignant) | | **Mitotic activity** | Normal | Increased, abnormal mitoses | | **Response to stimulus** | Adaptive | Maladaptive | **High-Yield:** The critical distinction is that metaplasia is a **reversible adaptation** to environmental stress, whereas dysplasia is an **irreversible pre-malignant state** that cannot revert to normal even if the stimulus is removed. **Clinical Pearl:** Classic example of metaplasia: Barrett's esophagus (columnar metaplasia of squamous epithelium due to chronic GERD). If GERD is controlled early, some reversal may occur. However, once dysplasia develops in Barrett's, it is considered irreversible and carries malignant potential.
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