Spontaneous Malignant Mixed Müllerian Tumor in a Wistar Rat

This report presents a rare, spontaneous, heterologous, malignant mixed Müllerian tumor observed in a 98-week-old untreated Wistar rat. At necropsy, the right uterine horn was dilated and contained a mass of approximately 10 × 15 mm. Histopathologically the mass consisted of 2 parts with different morphologic characteristics, a polypoid mass protruding into the uterine lumen and a more solid part that replaced the adjacent uterine wall. The solid part exhibited characteristics of a highly infiltrative adenocarcinoma, whereas the composition of the polypoid mass was heterogeneous, consisting of a variety of benign and malignant epithelial and mesenchymal elements. The epithelial components included well-differentiated endometrial and squamous epithelium juxtaposed to carcinosarcomatous areas. The mesenchymal components consisted of well-differentiated cartilage and bone along with chondrosarcomatous, rhabdomyomatous, and other mesenchymal elements. Immunohistochemical analysis further supported the diagnosis of malignant mixed Müllerian tumor.