Platelet. Coloured Scanning Electron Micrograph (SEM) of a blood platelet (thrombocyte) among red blood cells. Here, the platelet is coloured white. It has dendritic structures (pseudopodia) which project from the cell surface and in this state is activated" (in the process of causing the blood to clot). Unactivated platelets are smooth and oval in shape. Platelets are cells without nuclei, formed in the bone marrow, which survive for about 9 days. They clump together to plug defects in blood vessels and so prevent bleeding; they also assist in clot formation. Mature red blood cells (erythrocytes) are the biconcave cells visible. Magnification: x6,800 at 6x6cm size. x10,500 at 4x4""