Bone marrow, scanning electron micrograph (SEM). Bone marrow is the site of blood cell production. Differentiating white and red blood cells (cream) are visible surrounded by bone tissue. The white blood cells form part of the body's immune system while the red blood cells carry oxygen around the body. Reticular fibres make up the connective tissue framework of the bone marrow. Compact (or cortical) bone forms the hardened perimeter. Magnification: x300 when printed 10 centimetres wide.