Electron microscope studies on the ellipsoid of the cat spleen with special reference to the filaments in the endothelial cell.

The fine structure of the ellipsoid of the cat spleen was studied by electron microscopy. The ellipsoid consists of closely packed reticular cells and fibers, and macrophages around a capillary. The macrophage cytoplasm contains phagocytized red blood cells within cytoplasmic vacuoles. The endothelial cells are specifically high in the lumen surrounded by an incomplete basement membrane. Two types of filaments are revealed within the endothelial cell cytoplasm. Thick filaments, 100A in diameter, are found in almost any part of the cytoplasm and show a tubular structure in cross section. They are thought to be cytoskeletal in function. Thin filaments, 60A diameter, are seen as electron-dense patches in the basal area of the cytoplasm. Their possible contractile function is discussed.