Undifferentiated ("embryonal") sarcoma of the liver: ultrastructure of a case presenting as a primary intracardiac tumor.

We report the ultrastructure of an undifferentiated ("embryonal") sarcoma of the liver that presented as a right-sided intracardiac tumor in an 8-year-old boy. The patient eventually died because of repeated embolization of tumor to the lungs. At autopsy the hepatic primary was composed of primitive mesenchymal cells with occasional small cysts and ducts lined by benign-appearing epithelium. Electron microscopy of the mesenchymal cells indicated a limited capacity for pluripotential differentiation along fibroblastic, lipoblastic, and even myogenous lines. Epithelial elements closely resembled those lining normal bile ducts but contained increased fibrillar material with haphazard orientation. The histogenesis of this rare hepatic tumor is still unknown, but the occurrence of both epithelial and frankly sarcomatous elements within the primary tumor again raises the possibility of malignant transformation within a pre-existing mesenchymal hamartoma.