Myelofibrosis is a disorder in which fibrous tissue replaces the blood-producing cells in the bone marrow, resulting in abnormally shaped red blood cells, anemia, and an enlarged spleen.
In normal bone marrow, cells called fibroblasts produce fibrous (connective) tissue that supports the blood-producing cells. In myelofibrosis, the fibroblasts produce too much fibrous tissue, which crowds out the blood-producing cells. Consequently, red blood cell production decreases, fewer red blood cells are released into the bloodstream, and anemia develops, becoming progressively more severe. In addition, many of these red blood cells are immature or misshapen. Variable numbers of immature white blood cells and platelets also may be seen in the blood. As myelofibrosis progresses, the number of white blood cells may increase or decrease, and the number of platelets typically decreases.
Myelofibrosis is rare, affecting less than 2 of 100,000 people in the United States. It occurs most commonly among people between the ages of 50 and 70.
Myelofibrosis may develop on its own (in which case it is also called idiopathic myelofibrosis or agnogenic myeloid metaplasia) or may accompany other blood disorders, such as chronic myelocytic leukemia, polycythemia vera, thrombocythemia, multiple myeloma, lymphoma, and myelodysplasia; tuberculosis; or bone infections. People who have been exposed to certain toxic substances, such as benzene and radiation, are at increased risk of developing myelofibrosis.
