Subplasmalemmal linear density: a mesodermal feature and a diagnostic aid.

Subplasmalemmal linear density (SLD), a surface structure known to occur on mononuclear phagocytes, has been considered a possible morphologic marker of mesenchymal cells or cells of mesodermal origin. A retrospective review of human surgical pathology cases studied in a diagnostic electron microscopy laboratory over a five-year period confirmed that SLDs are primarily seen in cells of mesodermal origin in a variety of neoplastic and nonneoplastic tissues. Therefore, SLD may be of value as a mesodermal marker in evaluating poorly differentiated tumors. Furthermore, the presence or absence of SLD in tissue histiocytes in diverse conditions may contribute to the understanding of heterogeneity among this group of cells.