## Definition and Mechanism of Metaplasia **Key Point:** Metaplasia is a reversible adaptive response in which one fully differentiated cell type is replaced by another differentiated cell type, typically in response to chronic irritation, inflammation, or altered environmental conditions. ### Fundamental Mechanism Metaplasia occurs through: 1. **Altered gene expression** — existing stem/progenitor cells in the tissue respond to new environmental signals 2. **Reversibility** — if the causative stimulus is removed, the original cell type may be restored 3. **No genetic mutation** — the change is epigenetic and phenotypic, not genotypic 4. **Adaptive purpose** — the new cell type is often better suited to the altered microenvironment ### Classic Examples | Trigger | Original Epithelium | Metaplastic Change | Location | |---------|-------------------|-------------------|----------| | Chronic smoking, reflux | Stratified squamous | Columnar (intestinal-type) | Esophagus (Barrett's) | | Chronic irritation | Respiratory epithelium | Squamous epithelium | Bladder (cystitis cystica) | | Vitamin A deficiency | Columnar epithelium | Stratified squamous | Respiratory tract | | Chronic gastritis (H. pylori) | Gastric mucosa | Intestinal-type mucosa | Stomach | **High-Yield:** Metaplasia is **NOT dysplasia** — it is a benign adaptive response, though some metaplasias (e.g., Barrett's esophagus, intestinal metaplasia of stomach) carry increased cancer risk if the stimulus persists. **Clinical Pearl:** The presence of metaplasia indicates chronic tissue injury and should prompt investigation of the underlying cause (e.g., reflux in Barrett's, H. pylori in gastric intestinal metaplasia). **Mnemonic:** **MARC** — Metaplasia is Adaptive, Reversible, and involves Change in gene expression (not mutation).
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