Body processes

Extramedullary Hematopoiesis – Function, Role & Diseases

Extramedullary hematopoiesis

Extramedullary hematopoiesis is a special form of blood formation. Basically, the term “hematopoiesis” stands for blood formation or the production of blood cells that takes place outside of the bone marrow. During the fetal period, blood formation outside the bone marrow is physiological. However, this form of blood formation occurs postnatally only in a pathological context.

What is extramedullary hematopoiesis?

Basically, the term “hematopoiesis” refers to the formation of blood cells or blood cells. These cells are produced by special stem cells that are able to form blood cells. Many blood cells have a limited lifespan.

The erythrocytes live between 40 and 120 days, while the thrombocytes have a lifespan of three to ten days. For this reason, the constant reproduction of new blood cells is necessary. In adults, billions of new blood cells are formed every day.

Basically, the term hematopoiesis comes from the Greek and stands for the production of blood. In humans or fetuses, blood cells are initially formed in the blood islands within the yolk sac. Only later are those organs formed that are responsible for the production of blood cells after birth. A special form of blood formation is extramedullary hematopoiesis .

These primarily include the spleen, thymus and bone marrow. In the unborn human, the liver is primarily responsible for the formation of blood cells. This is where the first mature erythrocytes without a nucleus are produced. The fetal liver also makes other important types of blood cells.

After birth, blood formation in healthy people takes place in the bone marrow. This is the so-called myelotic system. In addition, the lymphatic system is also able to form blood cells. Basically, the cells of the blood are formed from the stem cells.

In the first step, these mature in the bone marrow and finally pass into the blood. Some stem cells are pluripotent and produce myeloid and lymphoid blood cells. Other stem cells only make one type of blood cell.


There are a number of possible causes for extramedullary hematopoiesis, i.e. the formation of blood cells outside of the bone marrow. These are primarily certain diseases associated with extramedullary hematopoiesis. On the one hand, various myeloproliferative disorders are capable of causing extramedullary hematopoiesis. 

Examples of myeloproliferative diseases are chronic myeloid leukemia , which is also known by the abbreviation CML, or osteomyelofibrosis . These diseases also include various bone marrow metastases , such as breast cancer , small cell bronchial carcinoma and prostate carcinoma .

In addition to myeloproliferative diseases, a so-called Rhesus incompatibility is also a potential cause for the development of extramedullary hematopoiesis. Finally, various types of toxins are also able to trigger extramedullary hematopoiesis. This includes, for example, the toxic substance pentachlorophenol.

Symptoms, Ailments & Signs

Extramedullary hematopoiesis is associated with various typical symptoms that are characteristic of the disease. Basically, the disease can occur in several organs. These include, for example, the spleen, the liver and the skin.

In the majority of cases, the extramedullary hematopoiesis is triggered by chronic myeloproliferative diseases. A direct injury to the bone marrow, which is responsible for the formation of blood cells, can also result in extramedullary hematopoiesis. In principle, extramedullary hematopoiesis represents a displacement of blood formation from its usual range.

Diagnosis & course of disease

If a person experiences characteristic signs and symptoms of extramedullary hematopoiesis, seek medical advice promptly. The attending doctor first discusses the patient’s personal medical history with the respective patient and carries out the anamnesis. Various clinical examinations then take place.

The most important element in the diagnosis of extramedullary hematopoiesis is a peripheral blood smear . If there is extramedullary hematopoiesis, this shows that an unusually large number of immature blood cells leak into the peripheral blood. These are, for example, drop-shaped erythrocytes with a nucleus or so-called dacryocytes.

In some cases, the granulocytes are not fully mature. If extramedullary hematopoiesis is suspected, a bone marrow biopsy is indicated. If the typical fibrous or malignant processes can be seen here, the diagnosis is considered relatively secure. In the context of the differential diagnosis, primarily an aplastic anemia has to be clarified.

Treatment & Therapy

Since extramedullary hematopoiesis is usually the result of or an accompanying symptom of another underlying disease, treating the underlying disease is the drug of choice. For this reason, it is essential to correctly diagnose the underlying disease and then initiate appropriate therapy . If treatment is successful, regression of the extramedullary hematopoiesis is possible.


Effective measures for the direct prevention of extramedullary hematopoiesis are currently not known. In principle, preventive measures should start with the possible underlying diseases, which may result in extramedullary hematopoiesis. 

Since extramedullary hematopoiesis is a threatening condition, appropriate therapy should be instituted as soon as possible. For this reason, a doctor should be consulted if there are typical signs of extramedullary hematopoiesis.

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Hello! I am Lisa Newlon, and I am a medical writer and researcher with over 10 years of experience in the healthcare industry. I have a Master’s degree in Medicine, and my deep understanding of medical terminology, practices, and procedures has made me a trusted source of information in the medical world.