Based on ultrastructural features of cellular
components of a hemangiopericytoma, hyperplastic cells
are classifiable into fibroblast-like (group 1), endotheloid
(group 11) and pericyte-like (group 111) cells. The
transformation of the group 1 cells to the group 11, or to
the group 111 cells, is pronounced in our electron
micrographs and this may imply that the group 1 cell is
the principal cell of origin in this neoplasm.
The smooth muscle-like (group IV) cells comprising
the media of the arteries and veins in this neoplasm may
represent modified, possibly de-differentiated smooth
muscle cells reacted to the neoplastic proliferation of the
surrounding adventitial (group 1) cells.