Chondroid and Osseous Metaplasia of the Endometrium: A Tale of Two Rare Mesenchymal Metaplasia

Mesenchymal metaplasia of the endometrium is a rare phenomenon, characterized by the nonneoplastic cytomorphologic transformation of the uterine endometrial lining to ectopic bone/cartilage/fat/smooth muscle tissue/synovial tissue in the endometrium. Osseous metaplasia has an estimated incidence of 3/10,000, with around 100 cases documented in the literature. Most of the cases, it occurs following a pregnancy/abortion. The two most acceptable theories to account for its origin are first, bone metaplasia occurs from fetal bones that continue to develop leading to ossification, and second, multipotent stromal cells developing into chondroblasts/osteoblasts. Several factors play a role in the pathogenesis namely chronic endometritis, pyometra, tuberculosis, hyperkalcemia, etc., An accurate diagnosis of this lesion is crucial for surgical pathologists, as potential misdiagnosis as a malignant mixed Müllerian tumor, could result in overzealous treatment for a completely benign entity.