What is myelodysplastic syndrome?
Myelodysplastic syndrome (MDS) is the name of a group of conditions that occur when the blood-forming cells in the bone marrow are damaged. This damage leads to low numbers of one or more type of blood cells.
Normal bone marrow
Bone marrow is found inside certain bones, including the skull, ribs, pelvis, and spine. It is made up of blood-forming cells, fat cells, and supporting tissues that help the blood-forming cells grow. A small fraction of the blood-forming cells are a special type of cell known as blood stem cells. Stem cells are needed to make new cells. When a stem cell divides it makes 2 cells: one cell that stays a stem cell, and another cell that can keep changing and dividing to make blood cells. There are 3 types of blood cells: red blood cells, white blood cells, and platelets.
Red blood cells pick up oxygen in the lungs and carry it to the rest of the body. These cells also bring carbon dioxide back to the lungs. Having too few red blood cells is called anemia. It can make people feel tired and weak and look pale. Severe anemia can cause shortness of breath.
White blood cells (also known as leukocytes) are important in defending the body against infection. The 2 major types of white blood cells are lymphocytes and granulocytes.
Lymphocytes are immune cells that are found in the bone marrow, the blood, and in lymph nodes. They make the antibodies that help the body fight germs. They can also directly kill invading germs by producing toxic substances that damage the cells. Lymphocytes are not usually abnormal in MDS.
Granulocytes are a group of white blood cells that destroy bacteria. They are called granulocytes because they contain granules that can be seen under the microscope. These granules are made up of enzymes and other substances that can destroy germs that cause infections. In the bone marrow, granulocytes develop from young cells called myeloblasts. The most common type of granulocyte is the neutrophil; this cell is crucial in fighting bacteria. Other types of granulocytes are basophils and eosinophils. When the number of neutrophils in the blood is low, it is called neutropenia. This can lead to severe infections.
Monocytes, which are related to the granulocyte family, are also important in protecting the body against bacteria. The cells in the bone marrow that turn into monocytes are called monoblasts. Monocytes can leave the bloodstream to become macrophages in some of the body’s organs. Macrophages can destroy germs by surrounding and digesting them. They are also important in helping lymphocytes recognize germs and begin producing antibodies to fight them.
Platelets are thought of as a type of blood cell, but they are actually small pieces of a cell. They start as a large cell in the bone marrow cell called the megakaryocyte. Pieces of this cell break off and enter the bloodstream as platelets. Platelets are needed for your blood to clot. They plug up damaged areas of blood vessels caused by cuts or bruises. A shortage of platelets, called thrombocytopenia, can result in abnormal bleeding or bruising.
In MDS, some of the cells in the bone marrow are damaged and have problems making new blood cells. Many of the blood cells that are formed by the damaged bone marrow cells are defective. The body destroys many of these abnormal blood cells, leaving the patient with low blood counts because there aren’t enough normal blood cells.
In about one-third of patients, MDS can progress to a rapidly growing cancer of bone marrow cells called acute myeloid leukemia. Because most patients do not get leukemia, MDS was previously classified as a disease of low malignant potential. Now that doctors have learned more about MDS, it is considered to be a form of cancer. The major reason is that MDS is a clonal disease, which means that there is a large population of abnormal cells that all came from a single, abnormal cell. These abnormal cells have the same genes – just like identical twins – and they share abnormal growth properties. Clonal growth is typical of cancer.
In the past, MDS was referred to as pre-leukemia and smoldering leukemia. Since most MDS patients do not get leukemia, these terms are not accurate and are no longer used.
Our document, Leukemia: Acute Myeloid (Myelogenous), provides more information about the leukemia that develops in some MDS patients.
Last Medical Review: 11/08/2012
Last Revised: 11/08/2012