Which histological cell type is the neoplastic component of giant cell tumor of bone?

Explanation

## Histology of Giant Cell Tumor **Key Point:** The neoplastic component of GCT is the *mononuclear stromal cells* (spindle-shaped fibroblast-like cells), NOT the multinucleated giant cells. The giant cells are reactive, derived from osteoclast precursors. ### Histological Composition | Component | Origin | Character | Significance | | --- | --- | --- | --- | | Mononuclear stromal cells | Neoplastic | Spindle/oval, mitotically active | **True tumor component** | | Multinucleated giant cells | Reactive | Osteoclast-like, 10–100+ nuclei | Derived from precursor cells, not neoplastic | | Hemosiderin-laden macrophages | Reactive | From hemorrhage/necrosis | Incidental finding | **High-Yield:** This distinction is crucial: the *mononuclear stromal cells* are the proliferating neoplastic population; the giant cells are a reactive response to the stromal cells. This is why GCT is sometimes called "stromal cell tumor with osteoclastic giant cells." **Mnemonic:** **MGCS** = Mononuclear Stromal cells = Genuine neoplastic component (the "M" is the malignant/neoplastic part). **Clinical Pearl:** The presence of multinucleated giant cells is what gives GCT its name and makes it histologically distinctive, but they are not the tumor cells themselves—they are recruited by the stromal cells. This explains why GCT can recur (the stromal cells persist) even if the giant cells are removed. ### Immunohistochemistry Support - Stromal cells express p63, β-catenin, and show increased mitotic activity - Giant cells are CD51+ (osteoclast marker) but are not clonal