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 the bone marrow. During the fetal period, blood formation outside the bone marrow is physiological. Postnatally, however, this form of hematopoiesis occurs exclusively in a pathological context.
What is extramedullary hematopoiesis?
Extramedullary hematopoiesis is a specific form of blood formation. Basically, the term “hematopoiesis” refers to the formation of blood cells or blood corpuscles. These cells are produced by special stem cells that are capable of forming blood cells. A large number of blood cells have a limited life span. For example, erythrocytes live between 40 and 120 days, while platelets have a life span of three to ten days. For this reason, the constant replenishment of new blood cells is necessary. In adult individuals, several billion new blood cells are formed per day. Basically, the term hematopoiesis comes from the Greek and stands for the production of blood. In humans or fetuses, the formation of blood cells initially takes place 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. This primarily includes the spleen, thymus and bone marrow. In unborn humans, the liver is initially responsible for the formation of blood cells. This is where the first mature erythrocytes without nuclei are produced. In addition, the liver of the fetus also forms other important types of blood cells. After birth, blood formation in healthy individuals takes place in the bone marrow. This is the so-called myelotic system. In addition, the lymphatic system is also capable of forming blood cells. Basically, the cells of the blood are formed from stem cells. These mature in the bone marrow in the first step and finally pass into the blood. Some stem cells are pluripotent and produce myeloid and lymphoid blood cells. Other stem cells form only one type of blood cell.
Causes
There are a variety of possible causes for extramedullary hematopoiesis, which is the formation of blood cells outside the bone marrow. Primarily, these are certain diseases associated with extramedullary hematopoiesis. First, various myeloproliferative diseases are capable of causing extramedullary hematopoiesis. Myeloproliferative diseases include chronic myeloid leukemia, also referred to by the abbreviation CML, or osteomyelofibrosis. In addition, various bone marrow metastases belong to these diseases, as well as, for example, breast cancer, small cell bronchial carcinoma, and prostate carcinoma. In addition to myeloproliferative disorders, so-called rhesus incompatibility is also a potential cause of extramedullary hematopoiesis. Finally, various types of toxins are also capable of triggering extramedullary hematopoiesis. These include, for example, the toxic substance pentachlorophenol.
Symptoms, complaints, and signs
Extramedullary hematopoiesis is accompanied by various typical symptoms that are characteristic of the disease. Basically, the disease can occur in several organs. These include, for example, the spleen, liver and skin. In the majority of cases, extramedullary hematopoiesis is triggered by chronic myeloproliferative diseases. Direct injury to the bone marrow, which is responsible for the formation of blood cells, may also result in extramedullary hematopoiesis. In principle, extramedullary hematopoiesis represents a displacement of hematopoiesis from its usual area.
Diagnosis and course of the disease
If a person suffers from characteristic signs and symptoms of extramedullary hematopoiesis, a specialist should be consulted immediately. The attending physician first discusses with the respective patient his or her personal medical history and takes a medical history. This is followed by various clinical examinations.The most important element for the diagnosis of extramedullary hematopoiesis is a peripheral blood smear. If extramedullary hematopoiesis is present, this shows that an unusually large number of immature blood cells leak into the peripheral blood. These are, for example, drop-shaped erythrocytes with nuclei or so-called dacryocytes. In some cases, they are also not fully mature granulocytes. If extramedullary hematopoiesis is suspected, a biopsy of the bone marrow is indicated. If the typical fibrous or malignant processes can be seen here, the diagnosis is considered relatively certain. In the differential diagnosis, aplastic anemia should be clarified in the first instance.
Treatment and therapy
Because extramedullary hematopoiesis is usually the consequence or an accompanying symptom of another underlying disease, therapy of the underlying disease is the treatment of choice. For this reason, it is essential to correctly diagnose the underlying disease and then initiate adequate therapy. If the treatment is successful, regression of extramedullary hematopoiesis is possible.
Prevention
Effective measures for direct prevention of extramedullary hematopoiesis are currently unknown. In principle, preventive measures would have to address the possible underlying diseases that may result in extramedullary hematopoiesis. Since extramedullary hematopoiesis is a threatening condition, appropriate therapy should be initiated as soon as possible. For this reason, a physician should be consulted if typical signs of extramedullary hematopoiesis are observed.
