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.
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
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