Blood cells are formed in the hematopoietic tissues, or myeloidtissues.
The two types of hematopoietic tissue in the adult are:
- red bone marrow (found primarily in the flat and irregular bones), and
- lymphatic tissue (found in the spleen, lymph nodes, and thymus gland)
Myeloid tissue generally refers to the red bone marrow, especially in the formation of red blood cells (erythropoiesis).
In the adult, red bone marrow is found in portions of the vertebrae, sternum, ribs, skull, scapulae, pelvis, and proximal limb bones, collectively known as flat and irregular bones.
Other marrow areas contain a fatty tissue known as yellow marrow. Yellow marrow is found in the hollow center of the diaphysis (the long shaft of the bone) known as the medullary cavity. Normally, yellow marrow does not have any blood-producing function. However, under certain conditions, as a hemorrhage, yellow marrow can be converted to red marrow and assume the responsibility of producing blood cells.
During infancy and early childhood, all bone marrow is red; but as one grows older and less blood cell production is needed, the fat content of the marrow increases, forming more yellow and less red. This means that the elderly are more prone to infections and cancers because there are fewer lymphocytes being produced since the red bone marrow is decreasing.
Bone marrow stem cells are the most primitive cells in the bone marrow and consist of of packed hematopoietic, reticular, and adipose cells. Various blood cells are formed from them, including non-marrow cells. The most common stem cells to develop in bone marrow are red blood cells (RBCs), white blood cells (WBCs – granular leukocytes or monocytes), and platelets. In adults, bone marrow stem cells give rise to bone, fat, cartilage, and muscle cells. As scientists now know, stem cells display remarkable flexibility, producing cell types well beyond what would seem to be their natural formation.
Bone marrow depression, or myelosuppression, is caused mainly by drugs and radiation therapy. When this happens, the bone marrow is unable to manufacture the needed RBCs, WBCs, or platelets with the following results:
- A reduction in the production of RBCs can cause a serious form of anemia called aplastic anemia.
- A reduction of WBCs causes a condition called leukopenia, which leaves a person open to many infections.
- A reduction in the production of platelets is called thrombocytopenia, leaving a person at high risk for hemorrhage.
Updated October 2012
