Hematopoiesis is the process of blood cell formation. In normal, healthy adults, blood cells are manufactured in the red marrow of relatively few bones, notably the sternum, ribs, vertebral bodies, pelvic bones, and proximal portions of the humerus and the femur. This production is in contrast to that taking place in the embryo, in which blood cells are derived from the yolk sac mesenchyme. As the fetus develops, the liver, the spleen, and the marrow cavities of nearly all bones become active hematopoietic sites (Fig. 1–1). In the newborn, hematopoiesis occurs primarily in the red marrow, which is found in most bones at that stage of development. Beginning at about age 5 years, the red marrow is gradually replaced by yellowish fat-storage cells (yellow marrow), which are inactive in the hematopoietic process. By adulthood, blood cell production normally occurs in only those bones that retain red marrow activity.