Bone marrow. Scanning electron micrograph (SEM) of bone marrow in cancellous bone tissue. Bone tissue can be either cortical (compact) or cancellous (spongy). Cancellous bone is found in the centre of the bone and is characterised by a honeycomb arrangement, comprising a network of trabeculae (fibrous tissue). These structures provide support and strength to the bone. The spaces between the trabeculae are filled with bone marrow (red/orange), which produces blood cells. Differentiating white blood cells (green), part of the body's immune system, are seen here. Reticular fibres make up the connective tissue framework of the bone marrow. Magnification: x500 when printed 10 centimetres wide.