Bone marrow is the soft inner part of bones. New blood cells (red blood cells, white blood cells, and platelets) are made there. In infants, active bone marrow is found in almost all bones of the body, but by the teenage years it is found mainly in the flat bones (skull, shoulder blades, ribs, and hip bones) and vertebrae (the bones that make up the spine).
Bone marrow is made up of a small number of blood stem cells, more mature blood-forming cells, fat cells, and supporting tissues that help cells grow. Blood stem cells go through a series of changes to make new blood cells. During this process, the cells develop into 1 of the 3 main types of blood cell components.