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Extramedullary hematopoiesis refers to hematopoiesis occurring outside of the medulla of the bone (bone marrow).
In some cases, it may be physiologic. For example, during fetal development, hematopoiesis occurs at many different locations, such as the liver and spleen.
However, it is more frequently associated with pathologic processes. For example, it can be caused by myelofibrosis, after fibrotic changes within the bone marrow "crowd out" hematopoietic cells, causing them to migrate to other sites such as the liver and spleen.
Thalassemias, the deficiency of the alpha and beta chains genes, cause extramedullary hematopoiesis. symptoms include frontal bossing and malar (zygomatic bone/cheek) prominence.
It can sometimes be identified via computed tomography.