The Human Body

Bone Marrow

Bone marrow, the soft, pulpy tissue that fills bone cavities, contains a network of blood vessels and fibers surrounded by fat and blood-producing cells. In children, the cells that give rise to blood cells can be found throughout the marrow. In adults, these cells are found mostly in the red marrow of the bones of the chest, hips, back, skull, and of the upper arms and legs. The marrow in the long shafts of bones gradually loses its ability to manufacture blood. This marrow, which is dominated by fat cells and takes on a yellowish color, is called yellow marrow. This cross section of a long bone shows yellow bone marrow in the shaft of a long bone.

Red marrow consists primarily of a loose, soft network of blood vessels and protein fibers interspersed with developing blood cells. The blood vessels are termed the vascular component, and the protein fibers and developing blood cells collectively are referred to as the stroma, or the extravascular component. The protein fibers crisscross the marrow, forming a meshwork that supports the developing blood cells clustered in the spaces between the fibers.

Red marrow contains a rich blood supply. Arteries transport blood containing oxygen and nutrients into the marrow, and veins remove blood containing carbon dioxide and other wastes. The arteries and veins are connected by capillaries, blood vessels that branch throughout the marrow. In various places, the capillaries balloon out, forming numerous thin, blood-filled cavities. These cavities are called sinusoids, and they assist in blood-cell production.

Yellow marrow is so named because it is composed of yellow fat cells interspersed in a rich mesh of connective tissue that also supports many blood vessels. While not usually actively involved in blood formation, in an emergency yellow marrow is replaced by blood-forming red marrow when the body needs more blood.

Red marrow produces all of the body’s blood cells—red blood cells, white blood cells, and platelets. Red blood cells in the circulatory system transport oxygen to body tissues and carbon dioxide away from tissues. White blood cells are critical for fighting bacteria and other foreign invaders of the body (see Immune System). Platelets are essential for the formation of blood clots to heal wounds.