Definition of Bone Marrow and Function

Bone marrowBone marrow – the semigelatinous tissue within the center of the bone.

Though its presence is imperceptible in health, the bone marrow is the foundation of the blood and its circulation and plays a fundamental role in immune function.

Red bone marrow is the primary source of new blood cells and is most abundant within the sternum, ribs, vertebrae, and pelvis in an adult.

In childhood the bones of the skull and face, and the long bones of the arms and legs, also contain red bone marrow. As the body matures the red bone marrow in these sites transitions to yellow bone marrow, which contains mostly connective tissue and fat. The healthy adult has less than half as much red bone marrow as blood.

Red Bone Marrow

About 99 percent of the red bone marrow’s output is erythrocytes; erythropoiesis is the process of producing erythrocytes. The red bone marrow also produces platelets (clotting cells) and granulocytes, a type of leukocyte (white blood cell).

The cells that make up the bone marrow are blood stem cells, which continuously replicate to replenish the bone marrow and differentiate into three parent lines, or precursors, that produce blood cells. The parent lines are

  • erythroblasts, which produce erythrocytes
  • megakaryoblasts, which produce platelets
  • myeloblasts, which produce neutrophils, basophils, and eosinophils (the three subtypes of granulocytes)

The red bone marrow also warehouses plasma cells, which are integral to immune function (not related to the plasma that forms the fluid base of blood). The red bone marrow is the location where B-cell lymphocytes, which migrate to the marrow from the lymph tissues that produce them, come to maturity.

Yellow bone marrow produces a few leukocytes in adulthood though primarily functions as a reserve resource for new blood cell production when the red bone marrow cannot meet the body’s needs.


LeukopeniaMetastatic cancer
Multiple MyelomaMyelodysplasia Syndrome
Polycythemia VeraRadiation toxicity

For further discussion of blood and lymph structure and function please see the overview section “The Blood and Lymph.”


Bone Marrow – Red Bone Marrow and Function
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