## Epithelioid Cell Morphology and Differentiation ### Definition and Origin **Key Point:** Epithelioid cells are activated macrophages that undergo morphologic and functional transformation in response to persistent antigenic stimulation and IFN-γ. ### Morphologic Features of Epithelioid Cells | Feature | Epithelioid Cells | Ordinary Macrophages | | --- | --- | --- | | **Cytoplasm** | Increased volume, pale, eosinophilic | Abundant, basophilic | | **Nucleus** | Elongated, vesicular, pale-staining | Round to oval, darker | | **Phagocytic capacity** | **Reduced** | High | | **Organelles** | Prominent RER and Golgi | Moderate | | **Appearance** | Resembles epithelial cells (hence "epithelioid") | Macrophage-like | ### Functional Changes in Epithelioid Differentiation 1. **Loss of phagocytic function** — reduced ability to engulf pathogens 2. **Enhanced secretory capacity** — increased cytokine and enzyme production 3. **Increased cell-cell adhesion** — facilitates granuloma cohesion 4. **Reduced mobility** — cells become stationary and organized **High-Yield:** The name "epithelioid" refers to the resemblance to epithelial cells, not to their origin. They are still macrophages, but activated and differentiated. **Mnemonic:** **E**pithelioid = **E**longated nucleus, **E**nhanced secretion, **E**nlarged cytoplasm, **E**ncircled arrangement. **Clinical Pearl:** Epithelioid cells are the hallmark of granulomatous inflammation and indicate a chronic, organized immune response rather than acute inflammation.
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