Extramedullary hematopoiesis (EMH or sometimes EH[1]) refers to hematopoiesis occurring outside of the medulla of the bone (bone marrow).[2] It can be physiologic or pathologic.
Physiologic EMH occurs during embryonic and fetal development; during this time the main site of fetal hematopoiesis are liver and the spleen.
Pathologic EMH can occur during adulthood when physiologic hematopoiesis can't work properly in the bone marrow and the hematopoietic stem cells (HSC) have to migrate to other tissues in order to continue with the formation of blood cellular components. Pathologic EMH can be caused by myelofibrosis,[3] thalassemias or disorders caused in the hematopoietic system.
Physiologic EMH[edit]
During fetal development, hematopoiesis occurs mainly in the fetal liver and in the spleen followed by localization to the bone marrow.[4] Hematopoiesis also takes place in many other tissues or organs such as the yolk sac, the aorta-gonad mesonephros (AGM) region, the spleen, and lymph nodes. During development, vertebrates go through a primitive and a definitive phase of hematopoiesis. The lungs also play a role in platelet production in adults.[5]
Primitive hematopoiesis[edit]
Primitive hematopoiesis occurs in the yolk sac during early embryonic development. It is characterized by the production of erythroid progenitors or nucleated erythrocytes, also known as erythroblasts or megaloblasts. The main objective of the production of these cells will be the facilitation of tissue oxygenation to support rapid embryonic growth. This primitive phase is transitory and the cells that are produced express embryonic globins, aren't pluripotent, and aren't capable of self-renewal.
Definitive hematopoiesis[edit]
Definitive hematopoiesis differs from the primitive phase through the production of hematopoietic stem cells. The formation of these cells occurs in the AGM later in development. Later, they migrate to the fetal liver where the majority of physiologic EMH takes place. Finally, once the bone marrow has developed, they migrate there. They can also migrate to the spleen and lymph nodes where hematopoiesis can occur, but to a lesser degree.
Pulmonary hematopoiesis[edit]
Pulmonary hematopoiesis also appears to play an important role in adults.[5] In comparison to the bone marrow, where trilineage hematopoiesis occurs, the lungs preferentially contribute to the production of platelets through a resident population of megakaryocytes. This is supported by studies showing that blood leaving the lungs has more platelets and fewer progenitor cells than blood entering the lungs. It has been seen that in cases of severe thrombocytopenia, pulmonary megakaryocytes migrate out of lungs into the bone marrow, where they help to replenish the depleted bone marrow population.
https://en.wikipedia.org/wiki/Extramedullary_hematopoiesis
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