Myelodysplasia refers to a set of syndromes (also called myelodysplastic syndromes, or MDS) in which the normal process of making mature blood cells (red blood cells, white blood cells, and platelets) – known as hematopoiesis – is impaired. Hematopoiesis begins with a hematopoietic stem cell (HSC) present in the bone marrow. The HSC is capable of differentiating into two more specialized stem cells: lymphoid stem cells and myeloid stem cells. Lymphoid stem cells differentiate into a type of white blood cell called a lymphocyte, while myeloid stem cells can differentiate into red blood cells, platelets, and a group of white blood cells called granulocytes and monocytes.
In myelodysplasia, the stem cells do not differentiate completely; they remain as immature “blast cells” instead of maturing into normal red blood cells, white blood cells and platelets. This results in a disproportionately low number of healthy mature blood cells, a condition known as cytopenia. When there is a shortage of red blood cells, this is called anemia. The corresponding deficiencies in the other cell types are called leukocytopenia (white blood cells) and thrombocytopenia (platelets). Each of these deficiencies is associated with a host of health problems such as bleeding (caused by low platelet counts) and infection (due to low white blood cell counts).
Besides the effects caused by a deficiency of normal blood cells, myelodysplasia often produces increased numbers of immature blast cells in the bone marrow. The accumulation of excess blast cells may result in some of the blasts becoming abnormal (their morphology, or form, is defective). This process is known as malignant transformation, and leads to leukemia. Hence, myelodysplasia is often considered to be a premalignant, or preleukemic condition, necessitating careful monitoring and intervention.