Bone marrow, scanning electron micrograph (SEM). Bone marrow is the site of blood cell production. Differentiating white blood cells (yellow), part of the body's immune system, and red blood cells (red), which carry oxygen around the body, are seen amongst reticular fibres (cream). Reticular fibres make up the connective tissue framework of the bone marrow. Compact (or cortical) bone, which forms the hardened perimeter that contains the marrow, is seen at top. Magnification: x500 when printed 10 centimetres wide.