Bone marrow, the soft fatty tissue inside bone cavities, is the source of all the red blood cells and platelets and most of the white blood cells. In adults, blood cells are formed mainly in the bone marrow of flat bones, such as the shoulder blades, ribs, breastbone, and pelvis. All the blood cells are derived from a single type of cell called a stem cell. Production of stem cells is controlled by hormones, including erythropoietin, which is produced by the kidneys; thyroid hormones; corticosteroids secreted by the adrenal glands; and growth hormones from the pituitary gland.
From the 2010 revision of the Complete Home Medical Guide © Dorling Kindersley Limited.