Myelodysplastic (myelo – bone marrow, dysplastic – abnormal growth) syndromes are a group of conditions caused by abnormal blood-forming cells of the bone marrow. In myelodysplastic syndromes (MDS), the bone marrow cannot produce blood cells effectively. Many of the blood cells formed are defective. These abnormal blood cells are usually destroyed before they leave the bone marrow or shortly after entering the bloodstream. As a result, patients have shortages of blood cells, which are reflected in their low blood counts.

Although MDS has not been considered cancer in the past, most hematologists (specialists in diseases of the blood) now consider it 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 are exactly alike – just like identical twins – and they share abnormal growth properties. Clonal growth is typically seen in cancer where all the cells appear to have started from an original abnormal cell. Although MDS is a clonal disorder, there are many different forms, as described below.

A second reason MDS is considered a form of cancer is that in about 30% of MDS cases, the abnormal bone marrow cells eventually progress into acute myeloid leukemia, a rapidly growing cancer of bone marrow cells. Some doctors think MDS is an early form of leukemia although it often doesn’t progress into leukemia. In the past, myelodysplastic syndromes were called preleukemia or smoldering leukemia. (For more information on leukemia, see the American Cancer Society document, “Acute Myeloid Leukemia.”)

What Are Myelodysplastic/Myeloproliferative Diseases?

Experts have realized that certain blood diseases that were once called myelodysplastic syndromes didn’t fit in that category. This is because they produced too many white blood cells. They actually resembled myeloproliferative diseases (myelo – bone marrow, proliferative – excessive growth) such as leukemia, where the problem is an overproduction of blood cells. As a result, the experts created this new category. The most common disease in this group is chronic myelomonocytic leukemia. Much less common are atypical chronic myeloid leukemia and juvenile myelomonocytic leukemia. All of these diseases have a high production of abnormal blood cells.

Normal Bone Marrow

Bone marrow is the soft, inner part of some bones such as the skull, shoulder blades, ribs, pelvis, and backbones. Bone marrow is made of blood-forming cells, fat cells, and supporting tissues that help blood-forming cells grow. A small fraction of the blood-forming cells are stem cells that continually reproduce to form new cells. Some of these new cells remain as stem cells, while others go through a series of changes to develop into one of 3 types of blood cells: red blood cells, white blood cells, or platelets.

Red blood cells contain hemoglobin, which carries oxygen from the lungs to all other tissues in the body while returning carbon dioxide. Having too few red blood cells (anemia) typically causes weakness, fatigue, and, when it is severe, shortness of breath.

Platelets are usually classified as a type of blood cell, but they are actually small fragments from a bone marrow cell called the megakaryocyte. Platelets are important in plugging damaged areas of blood vessels caused by cuts or bruises. A shortage of platelets, called thrombocytopenia, can result in too much bleeding or bruising when blood vessels are damaged.

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.

Granulocytes are a group of white blood cells that destroy bacteria. The granulocytes (neutrophils, basophils, and eosinophils) are distinguished by the size and color of their granules (spots seen inside the cells under the microscope). These granules contain enzymes and other substances that can destroy germs that cause infections. In the bone marrow, granulocytes dev