Extramedullary hematopoiesis

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.

[4] Hematopoiesis also takes place in many other tissues or organs such as the yolk sac, the aorta-gonad mesonephros (AGM) region, and lymph nodes.

The main objective of the production of these cells will be the facilitation of tissue oxygenation to support rapid embryonic growth.

[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.

When red blood cell (RBC) numbers are low, the body induces a homeostatic mechanism aimed to increase the synthesis of RBCs, typically via the production of erythropoietin.

Other manifestations occur in the thymus, heart, breast, prostate, broad ligaments, kidneys, adrenal glands, pleura, retroperitoneal tissue, skin, peripheral and cranial nerves, and the spinal canal.

Despite the hypoxic/acidic conditions of the splenic microenvironment, supplied with a legion of macrophages making it inhospitable for HSCs, EMH usually occurs within the red pulp.

Among the various organs associated with EMH, the spleen offers a unique site for evaluation of hematopoietic stem cell (HSC)/niche interactions.

The most common sites of EMH associated with neoplastic disorder are the spleen, lymph nodes, skin, bone, small intestine, orbit, breast, cervix, nasal sinus, mediastinum and brain.

[20] The fact that EMH frequently occurs in the red pulp, is supported by current data that suggests that splenic sinus endothelial cells expressing CXCL12 may contribute to the attachment and recruitment of circulating hematopoietic precursor cells, forming bone marrow niche-like regions of EMH in the human spleen.