A 32-year-old woman presents to the gynaecology clinic with a 2-year history of heavy menstrual bleeding and lower abdominal pain. On examination, the uterus is enlarged, firm, and irregularly nodular. Transvaginal ultrasound reveals multiple well-defined hypoechoic lesions within the myometrium, the largest measuring 4 cm. Histopathology shows smooth muscle cells arranged in interlacing bundles with increased cellularity but no atypia or mitotic activity. Which cellular adaptation is most prominently demonstrated in this lesion?

Explanation

## Diagnosis: Uterine Leiomyoma (Fibroid) ### Pathological Basis **Key Point:** Uterine fibroids are benign smooth muscle tumours that demonstrate **hyperplasia** — an increase in the NUMBER of smooth muscle cells — rather than hypertrophy (increase in cell SIZE). ### Distinguishing Hyperplasia from Hypertrophy | Feature | Hyperplasia | Hypertrophy | |---------|-----------|----------| | **Definition** | ↑ Number of cells | ↑ Size of individual cells | | **Mechanism** | Increased mitotic activity (controlled) | Increased protein synthesis per cell | | **Cell count** | Elevated | Normal | | **Individual cell size** | Normal | Enlarged | | **Reversibility** | Reversible if stimulus removed | Reversible if stimulus removed | | **Example** | Leiomyoma, endometrial hyperplasia | Cardiac hypertrophy, skeletal muscle | **High-Yield:** Leiomyomas show: - Increased NUMBER of smooth muscle cells (hyperplasia) - Normal mitotic rate (benign, not dysplastic) - Absence of atypia or increased mitotic figures - Well-demarcated, encapsulated lesions ### Clinical Pearl The distinction matters: **hyperplasia = benign proliferation with normal cell morphology**, whereas dysplasia = abnormal morphology with increased mitotic rate and atypia (premalignant). This fibroid shows none of the latter features. ### Pathophysiology Uterine fibroids arise from: 1. Monoclonal proliferation of a single smooth muscle cell (somatic mutation) 2. Altered response to oestrogen and progesterone 3. Abnormal growth factor signalling (TGF-β, FGF pathways) 4. Result: controlled but excessive hyperplasia **Mnemonic: HyPERplasia = PERcentage of cells ↑**