The bone marrow is the site of hematopoiesis, or blood cell for-mation (Fig. 33-1). In a child all skeletal bones are involved, but as the child ages marrow activity decreases. By adulthood, mar-row activity is usually limited to the pelvis, ribs, vertebrae, and sternum.
Marrow is one of the largest organs of the body, making up 4% to 5% of total body weight. It consists of islands of cellular com-ponents (red marrow) separated by fat (yellow marrow). As the adult ages, the proportion of active marrow is gradually replaced by fat; however, in the healthy person, the fat can again be replaced by active marrow when more blood cell production is required. In adults with disease that causes marrow destruction, fibrosis, or scarring, the liver and spleen can also resume production of blood cells by a process known as extramedullary hematopoiesis.
The marrow is highly vascular. Within it are primitive cells called stem cells. The stem cells have the ability to self-replicate, thereby ensuring a continuous supply of stem cells throughout the life cycle. When stimulated to do so, stem cells can begin a process of differentiation into either myeloid or lymphoid stem cells. These stem cells are committed to produce specific types of blood cells. Lymphoid stem cells produce either T or B lymphocytes. Myeloid stem cells differentiate into three broad cell types: RBCs, WBCs, and platelets. Thus, with the exception of lymphocytes, all blood cells are derived from the myeloid stem cell. A defect in the myeloid stem cell can cause problems not only with WBC pro-duction but also with RBC and platelet production. The entire process of hematopoiesis is highly complex. Research has identi-fied many of the complex mechanisms involved, often at the mo-lecular level. A thorough description of these processes is beyond the scope; however, some mechanisms against which a specific treatment is targeted.