In myeloproliferative disorders (myelo = bone marrow, proliferative = rapid multiplication), the blood-producing cells in the bone marrow (precursor cells) develop and reproduce abnormally or are crowded out by an overgrowth of fibrous tissue.

Three major myeloproliferative disorders are polycythemia vera, myelofibrosis, and thrombocythemia. The proliferation of blood-producing cells is always clinically noncancerous (benign) when it begins. However, in a small number of people, a myeloproliferative disorder progresses or transforms to a cancerous (malignant) condition (leukemia).

Major Myeloproliferative Disorders Disorder Bone Marrow Characteristics Blood Characteristics Polycythemia vera Increased number of the cells that produce the circulating blood cells Increased number of red blood cells, often accompanied by an increased number of platelets and white blood cells Myelofibrosis Excess fibrous tissue Increased number of immature red and white blood cells and misshapen red blood cells; decrease in the overall number of red blood cells (anemia); eventually, the number of white blood cells and platelets decreases Thrombocythemia Increased number of megakaryocytes (cells that produce platelets) Increased number of platelets

Last full review/revision February 2003