Walsh & Hoyt: Hemangiopericytomas

These tumors are composed of the pericytes of blood vessels. They may develop in virtually any location of the body where there are capillaries, including the orbit and the intracranial cavity. It was once thought that these lesions simply represented a form of angioblastic meningioma arising from the vascular elements of the pia mater. Subsequent immunohistochemical (e.g., CD34 and epithelial membrane antigen) and ultrastructural studies, however, have led to a better understanding of the histogenesis of these tumors. Meningeal hemangiopericytomas have morphologic and biologic features that are different from angioblastic meningiomas, but which are identical with those of hemangiopericytomas located within the substance of the brain. Unlike meningothelial cells, neoplastic pericytes show less cytoplasmic interdigitations and lack well-developed desmosomes. In addition, typical pericytes are irregularly invested by basement membrane material.