Abstract
A large, well-demarcated solitary mass appeared in the chest of an intact female Sprague-Dawley rat. The tumor cells were generally small and round, with scant eosinophilic cytoplasm, and arranged in an alveolar pattern. In some areas, the tumor cells contained fine vacuoles, which were confirmed to be lipid droplets. Immunohistochemistry analysis revealed that most of the tumor cells were strongly positive for vimentin and weakly positive for S100 protein, neuron-specific enolase, and CD34. The cells were negative for desmin, myogenin, α-smooth muscle actin, keratin, CD68, and von Willebrand factor. Electron microscopy revealed that there were no cellular organelles showing specific differentiation, other than mitochondria and rough endoplasmic reticulum. The tumor had no characteristic features for determining the cell of origin and was diagnosed as an undifferentiated mesenchymal tumor and classified as a sarcoma, NOS (not otherwise specified).
