Bone marrow. Coloured scanning electron micrograph of red (disc-shaped) and white (round bottom) blood cells and a bone trabecula (brown) in the bone marrow. The bone marrow is a tissue filling the intertrabecular spaces of spongy (cancellous) bone. It contains stem cells (not seen here) which, through a process known as haemopoiesis, generate the three types of cells found in the blood: red (erythrocytes), white (leucocytes) and platelets (thrombocytes). Flattened osteoblastic cells (centre) are seen covering the trabecula. The dark areas are normally occupied by blood vessels. Magnification: x2400 at 6x7cm size.