Bone marrow is soft tissue found mainly inside the long bones, vertebrae, and pelvic bones of the body. It is made up of red marrow, which produces red and white blood cells and platelets, and yellow marrow, which contains fat and connective tissue and produces some white blood cells.

People are born with only red bone marrow. As a person matures, the red marrow in many of the bones is replaced by yellow marrow. By adulthood, only about half of the bone marrow is red. Red bone marrow is found mostly in the ribs, breastbone, shoulder blades, collarbones, hip bones, skull, and spine.