Reactive hyperplasia of vascular pericytes in meningioma: immunohistochemical and ultrastructural studies of two cases.

An isolated proliferation of pericytes is a unique vascular reaction seen almost exclusively in the stroma of secretory meningioma. We report the results of immunohistochemical and ultrastructural studies of a pericytic proliferation that was found in two cases of meningioma (a secretory meningioma of the sphenoid ridge and a parasagittal atypical meningioma showing predominantly fibroblastic features). Pericytes had hyperchromatic nuclei and scant cytoplasm, and showed stratification or formed small clusters within the walls of small blood vessels. They occasionally showed close contact with pseudopsammoma bodies in secretory meningioma. Pericytes showed immunoreactivity for α-smooth muscle actin but were not immunoreactive for desmin. They also exhibited characteristic ultrastructural features of pericytes, including the presence of microfilaments and abundant pinocytotic vesicles, and investment by the basal lamina. This isolated pericytic proliferation is likely a peculiar response of the vascular wall, probably induced by some cytokines secreted from neoplastic meningothelial cells. The close contact of proliferating pericytes with pseudopsammoma bodies suggests a close pathogenetic association between them. The occurrence of pericytic proliferation that was found in our second case (atypical meningioma with predominantly fibroblastic features) is exceptional and has not been documented to date.