Myeloproliferative disorders (MPDs) are a subset of bone marrow disorders. They are a group of four diseases characterized by an overproduction of one or more types of blood or fiber cells in the bone marrow. Bone marrow is a soft fatty tissue that is located in the center of the body's larger bones. It contains a fibrous network, a population of undifferentiated pluripotent stem cells, and a mixture of red blood cells (RBC), white blood cells (WBC), and platelets in various stages of maturity.
Normally, the body maintains a dynamic but relatively stable number of blood cells in circulation. As cells age, die, or are removed from circulation, new ones are made in the marrow to replace them. When a particular kind of blood cell is needed, some of the stem cells in the bone marrow begin to change, becoming the immature “blast” forms of whatever cell type is in short supply. These blasts mature to become white blood cells, red blood cells, or platelets. Usually only fully mature cells are released into circulation.
With an MPD, excessive production of a cell's precursor leads to an increased number of that type of mature cell and to a corresponding increase or decrease in the number of other blood cells, which may be inhibited and crowded out. This results in symptoms related to blood cell overproduction, shortages, and dysfunction throughout the body.
The four MPDs include:
- Chronic myelogenous (myeloid, myelocytic) leukemia (CML), a disease that leads to an overproduction of myelocytes, precursor to granulocytes - white blood cells that digest and kill invading microorganisms. Often, these overproduced cells do not function normally.(For more information on this disorder, see Leukemia.)
- Polycythemia vera, a disease in which too many red blood cell precursors and erythrocytes (red blood cells) are made in the bone marrow. When RBCs build up in the blood stream, they cause the spleen to swell and the patient to itch all over. They also make the blood thicker, which can increase the risk of inappropriate clotting. Polycythemia vera is also called primary polycythemia. A variety of other factors can cause increased red blood cell production; for instance, long-term exposure to low concentrations of oxygen. These increases in RBCs are referred to as secondary polycythemia.
- Agnogenic myeloid metaplasia (also known as chronic idiopathic myelofibrosis and primary myelofibrosis), a disease characterized by overproduction of fiber-producing cells, leading to excess fibrous tissue in the marrow. The dense network of fiber inhibits the normal formation and maturation of RBCs and myelocytes. The red blood cells that do enter the bloodstream are often malformed, looking like teardrops instead of circles. There may be too few normal mature red blood cells to carry oxygen, causing anemia. Fiber cells may also spread to the spleen, causing it to swell.
- Essential Thrombocythemia, characterized by an increased number of megakaryocytes, precursor to platelets (also called thrombocytes). Platelets change shape and become "sticky" during the clotting process. This characteristic allows them to plug holes in leaking blood vessels, but excess platelets and platelet clumping can make it hard for the blood to flow normally and can increase a person's risk of developing inappropriate blood clots or of having a stroke. Essential thrombocythemia must be distinguished from secondary thrombocytosis – increased numbers of platelets caused by non-marrow disorders such as iron deficiency, rheumatoid arthritis, bleeding, or removal of the spleen.
MPDs are not curable, but their slow progression can usually be controlled and their symptoms alleviated. For each, there is a slight chance that the disease will develop into an acute leukemia. If this occurs, the course of the disease will be accelerated, the symptoms will intensify, and more aggressive treatment will be required.