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Myelodysplastic syndromes (MDS) are conditions that can occur when the blood-forming cells in the bone marrow become abnormal. This leads to low numbers of one or more types of blood cells. MDS is considered a type of cancer.

Normal bone marrow

Bone marrow is found in the middle of certain bones. It is made up of blood-forming cells, fat cells, and supporting tissues. A small fraction of the blood-forming cells are blood stem cells. Stem cells are needed to make new blood cells.

There are 3 main 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 a person 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. There are different types of white blood cells:

• Granulocytes are white blood cells that have granules that can be seen under the microscope. In the bone marrow, granulocytes develop from young cells called myeloblasts. The most common type of granulocyte is the neutrophil. When the number of neutrophils in the blood is low, the condition is called neutropenia. This can lead to severe infections.
• Monocytes are also important in protecting the body against germs. The cells in the bone marrow that turn into monocytes are called monoblasts.
• Lymphocytes make proteins called antibodies that help the body fight germs. They can also directly kill invading germs. Lymphocytes are not usually abnormal in MDS.

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 called the megakaryocyte. Pieces of this cell break off and enter the bloodstream as platelets. You need platelets 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.

Myelodysplastic syndromes

In MDS, some of the cells in the bone marrow are abnormal (dysplastic) and have problems making new blood cells. Many of the blood cells formed by these bone marrow cells are defective. Defective cells often die earlier than normal cells, and the body also destroys some abnormal blood cells, leaving the person without enough normal blood cells. Different cell types can be affected, although the most common finding in MDS is a shortage of red blood cells (anemia).

There are several different types of MDS, based on how many types of blood cells are affected and other factors.

In about 1 in 3 patients, MDS can progress to a rapidly growing cancer of bone marrow cells called acute myeloid leukemia (AML). In the past, MDS was sometimes referred to as pre-leukemia or smoldering leukemia. Because most patients do not get leukemia, MDS used to be 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.