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.
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.
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.