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Myelodysplastic syndromes (MDS), also known as myelodysplastic neoplasms, are conditions that can occur when the blood-forming cells in the bone marrow become abnormal, resulting in the marrow not making enough healthy new blood cells. This leads to low levels of one or more types of blood cells. MDS is considered a type of cancer.

Normal bone marrow

Bone marrow is the spongy substance 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 (RBCs) 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 a condition called anemia. It can make a person feel tired and weak and look pale (which might be harder to notice in people of color). Severe anemia can cause shortness of breath.

White blood cells (WBCs, also known as leukocytes) help defend the body against infection. There are different types of WBCs:

• Granulocytes are WBCs that have granules (small particles containing proteins) that can be seen under the microscope. In 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 are another type of WBC. Some lymphocytes make proteins called antibodies that help the body fight germs. Others can directly kill invading germs. Lymphocytes are not usually abnormal in MDS.

Platelets 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 can build up in the bone marrow, crowding out the normal cells. This can lead to a person not having enough normal blood cells.

Different blood cell types can be affected in MDS, although the most common finding is a shortage of red blood cells (anemia).

Even if a person with MDS has a normal number of blood cells, their blood and bone marrow cells can still be abnormal and may not work well.

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

In about 1 in 3 people with MDS, the disease can progress to a fast-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. Now MDS is considered a form of cancer.

Related conditions of blood and bone marrow

Myelodysplastic syndromes (MDS) are part of a group of diseases known as chronic myeloid disorders that affect the blood and bone marrow.

Myelodysplastic/myeloproliferative neoplasms (ΜD?/MΡΝs)

MDS/MPNs have 2 types of features:

Myelodysplastic: The blood stem cells in these conditions don’t mature into healthy red blood cells, white blood cells, or platelets. The immature blood cells (blasts) don’t work the way they should, and they tend to die quickly.

Myeloproliferative: Too many blood stem cells become one or more type of blood cell, resulting in too many of these types of cells. The number of these blood cells goes up slowly over time.

The 3 main types of MDS/MPNs are:

• Chronic myelomonocytic leukemia (CMML)
• Juvenile myelomonocytic leukemia (JML)
• Atypical chronic myeloid leukemia (aCML)

Chronic myeloproliferative neoplasms (MΡΝs)

In myeloproliferative neoplasms (sometimes called myeloproliferative disorders or myeloproliferative diseases), too many blood stem cells become one or more type of blood cell. These cells then build up in the bone marrow and blood slowly over time. This condition usually gets worse slowly as the number of blood cells goes up.

There are 6 main types of chronic MPNs:

• Chronic myeloid leukemia (CML)
• Polycythemia vera
• Primary myelofibrosis
• Essential thrombocythemia
• Chronic neutrophilic leukemia
• Chronic eosinophilic leukemia