In a healthy adult, bone marrow is factory of blood cell production.
However, under certain pathological conditions, body reverts to an embryonic state where organs outside bone marrow take over production of red blood cells, white blood cells, and platelets.
This process is known as Extramedullary Hematopoiesis (EMH). It signals significant underlying hematologic stress.
Why Does It Happen?
EMH typically occurs when bone marrow can no longer meet body’s demand for blood cells. It usually falls into two categories:
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Marrow Replacement: The marrow space is fill with fibrosis (scarring), tumors, or lipid storage diseases.
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Increased Demand: The marrow is functioning, but destruction of blood cells is so rapid (as in severe hemolytic anemias) that body must utilize auxiliary sites to keep up.
Sites
During fetal development, liver and spleen are primary hematopoietic organs. When EMH occurs in adults, body usually reactivates these sites first:
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Spleen: The most common site, often leading to splenomegaly (enlarged spleen).
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Liver: Leading to hepatomegaly (enlarged liver).
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Lymph Nodes: Frequently involved in various myeloproliferative disorders.
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Paravertebral Areas: In rare cases, hematopoietic “tumors” can grow along the spine, which may lead to spinal cord compression..