Intravascular papillary endothelial hyperplasia: light and electron microscopic observations of a case.

The ultrastructure of a case of intravascular papillary endothelial hyperplasia is presented. The papillae and septa within the lesion showed a similar organization. The surface was lined by a nonstratified layer of plump endothelial cells. Immediately beneath lay cells and processes of pericytes rich in thin filaments with fusiform densities, resembling smooth muscle cells. Primitive mesenchymal cells were observed to lie among the pericytes. The stroma beyond the subendothelial layer was collagenous and contained fibroblasts. The ultrastructure of the cells and their organization closely resembles human granulation tissue.