Bone Marrow

I

Introduction

Bone Marrow, soft, pulpy tissue that fills the cavities of bones, occurring in two forms, red and yellow. One of the largest tissues in the body, bone marrow accounts for 2 to 5 percent of an adult’s weight. Red marrow, present in all bones at birth, serves as the blood manufacturing center. As an infant matures, most of the red marrow in the shaft of long bones, such as the arm and leg bones, is gradually replaced by yellow marrow. Yellow marrow is composed primarily of specialized fat cells.

II

Structure

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.

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III

Marrow Function

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.

Within red bone marrow, all blood cells originate from a single type of cell, called a hematopoietic stem cell. Stimulated by hormones and growth factors, these stem cells divide to produce immature, or progenitor blood cells. Most of these progenitor cells remain in the stroma and rapidly undergo a series of cell divisions, producing either red blood cells or white blood cells. At any one time, the stroma consists largely of progenitor cells in various stages of development. At the appropriate developmental stage, the fresh, new cells squeeze through the walls of the capillaries. From there, the cells leave the bone and enter the body’s circulatory system. Some progenitor cells migrate to the sinusoids, where they produce platelets, which also travel to the circulatory system via the capillaries.

Although stem cells are relatively rare—about 1 in every 10,000 marrow cells is a stem cell—they typically produce the forerunners of an estimated 2 million red cells per second and 2 billion platelets per day. However, if significant amounts of blood are lost or other conditions reduce the supply of oxygen to tissues, the kidneys secrete the hormone erythropoietin. This hormone stimulates stem cells to produce more red blood cells. To fight off infection, hormones collectively termed colony stimulating growth factors are released by the immune system. These hormones stimulate the stem cells to produce more infection-fighting white blood cells. And in severe cases, the body converts yellow marrow into red marrow to help produce needed blood cells.

IV

Bone Marrow Diseases

Diseases of the bone marrow can be life threatening because they disrupt blood cell production, which is essential for survival. Inadequate production of blood cells results in aplastic anemia. The causes of this relatively rare disease are often unknown, although some cases result from exposure to toxic chemicals, such as lead, benzene, or arsenic. Radiation from nuclear explosions or X rays can also damage the marrow because the radioactive elements involved have a strong affinity for bone marrow.

Leukemias are cancers that affect bone marrow (as well as other tissues). A cell can become cancerous at any state during the series of divisions that produce red or white blood cells. If a progenitor cell becomes cancerous at the beginning of a series of cell divisions, the leukemia is termed acute. Chronic leukemia results when cells in later stages of division become cancerous.

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