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Ultrastructure of an infantile hemangiopericytoma

A tumor in the back of a 12‐year‐old Japanese girl is reported. Although the exact onset of its occurrence was unknown, the tumor conformed in all the other respects to the criteria of infantile hemangiopericytoma; namely, it was subcutaneous, multinodular, with prominent fibrosis, and with features of both hemangiopericytoma and hemangioendothelioma. Electron microscopic study demonstrated the perivascular tumor cells showing differentiations to various vasoformative elements. Most cells appeared to differentiate toward mature pericytes; however, some cells did show characteristics of fibroblasts, endothelial cells, and infrequently, smooth muscle cells. This supports the concept that the pericytes are poorly differentiated cells with potentials of transformation into other cell types, and that infantile hemangiopericytomas are better differentiated and benign, compared with adult‐type tumors. In addition, fibrillar inclusion bodies were found in the tumor cell nuclei; their significance is discussed briefly.