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Soft tissue inside bones; red marrow makes blood cells (hematopoiesis) and yellow marrow stores fat.
Medically reviewed & updated
Bone marrow is the soft, highly vascular tissue that fills the spaces inside bones, particularly within the mesh-like spongy (trabecular) bone and the central cavities of long bones. Though it weighs only a few kilograms in total, marrow is one of the body's most active tissues because it is where most blood cells are made.
There are two types of bone marrow. Red marrow is hematopoietic—meaning it actively produces blood cells—and gets its color from developing red blood cells. Yellow marrow is composed mainly of fat (adipose) cells and serves as an energy reserve; its yellow color comes from stored fat. At birth, nearly all marrow is red. With age, much of it is gradually replaced by yellow marrow, so in adults red marrow is concentrated in the axial skeleton (skull, vertebrae, sternum, ribs, and pelvis) and the proximal ends of the femur and humerus. In times of severe blood loss or anemia, yellow marrow can convert back to red marrow to boost production.
The blood-forming process is called hematopoiesis. Within the marrow, hematopoiesis occurs in islands of tissue surrounded by vascular sinuses and interspersed with trabecular bone. It begins with multipotent hematopoietic stem cells, which can either renew themselves or mature into more specialized progenitor cells. These progenitors eventually give rise to all the cellular components of blood: red blood cells (erythrocytes) that carry oxygen, several kinds of white blood cells (granulocytes, lymphocytes, and monocytes) that defend against infection, and platelets that help blood clot.
Because marrow is the source of blood cells, marrow disorders have body-wide effects. Leukemia is a cancer of blood-forming cells; aplastic anemia occurs when marrow fails to make enough cells; and chemotherapy or radiation can suppress marrow function. Bone marrow (or stem cell) transplantation can replace diseased marrow with healthy hematopoietic stem cells. A bone marrow biopsy, often taken from the pelvis, lets clinicians examine the tissue directly. This content is educational and not medical advice.