Disruption of human and murine BM sinusoids after irradiation and chemotherapy. Transmission electron microscope images of murine (A,B) and human (C,D) BM sinusoids. Elongated endothelial cells (EC) form the luminal layer of the sinusoid, surrounded by discontinuous cytoplasmic protrusions of reticular adventitial cells (RAC). Erythrocytes (E), platelets (P), and nucleated hematopoietic cell (H) are seen within the sinusoid lumen (L). The continuous murine endothelial cell layer is damaged after irradiation, resulting in gap formation due to rupture of the EC cytoplasm (B, arrow). Tight junctions between adjacent EC (C inset, arrow) are frequently seen in normal human sinusoids (C). Chemotherapy and irradiation cause significant narrowing of EC cytoplasm (D, arrow) together with rupture of the cell membrane and EC necrotic appearance (D). Images were acquired with a Techai-12 microscope (Philips, Eindhoven, The Netherlands) and Megaview III (Soft Imaging System, Munster, Germany). Samples were counterstained in uranyl acetate and lead citrate. Original magnification, × 3700 (panels A, C, D), × 5900 (panel B), and × 37 000 (panel C, inset).