Angiosarcoma of the heart: ultrastructural study.

Two angiosarcomas of the heart, both arising from the right atrium, were studied with the electron microscope. Ultrastructural characteristics of angiomatous development were demonstrated both at the better differentiated vascular areas and at the less differentiated, spindle-cell foci. Both tumors were dominated by immature endothelial cells. These cells were associated with primitive mesenchymal cells and transitional forms of pericytes. Intracytoplasmic lumen formation and junctional specialization with neovascular development are unique features of angiogenesis in these tumors. Ultrastructural studies may provide useful diagnostic features separating a true malignant vascular neoplasm of endothelial cell origin from other forms of mesenchymal neoplasia that may have a prominent stromal vascular component.