Bone marrow: structure, function, and clinical significance

Types and components

Two broad types are commonly described: red marrow, rich in blood-forming (hematopoietic) cells, and yellow marrow, dominated by fat cells and connective tissue. The marrow contains stem and progenitor cells, supporting stromal cells, blood-forming cells and developing precursors.

• Hematopoietic stem and progenitor cells (give rise to all mature blood cells)
• Developing erythrocytes, leukocytes and platelets
• Stromal cells, fat (adipocytes) and blood vessels

Function

The marrow produces most of the body's circulating blood cells, including red blood cells that carry oxygen, white blood cells that participate in immunity, and platelets that help with clotting. It also provides a microenvironment, or niche, that regulates stem cell maintenance and differentiation, and it acts as a reservoir of fat and growth factors.

Medical importance and uses

Bone marrow is central to many clinical practices: samples obtained by biopsy or aspiration help diagnose infections, anemias, and cancers such as leukemia; transplanted marrow or hematopoietic stem cells can restore blood formation after chemotherapy or bone marrow failure. Common disorders involving marrow range from marrow suppression to infiltrative diseases.

Development, distribution and notable facts

During development hematopoiesis shifts from yolk sac and liver to the marrow. In adults red marrow is concentrated in the pelvis, skull, vertebrae, ribs and proximal long bones, while yellow marrow predominates in the shafts of long bones and increases with age. Bone marrow constitutes a small fraction of body mass—often cited as about 4% of an adult human's weight, roughly about 2.6 kg—and remains a key focus of biomedical research and clinical care.