Extra-medullary haematopoiesis is haematopoiesis occurring outside the typical sites in the bone marrow (axial skeleton, ribs and long bones). It typically occurs when in increased demand or where there is failure of normal bone marrow compartments due to destruction or replacement by abnormal bone marrow precursors.
Sites that commonly take over include the liver, spleen, lymph nodes and the paraspinal areas of the thorax. Involvement of other organs occurs, but is less common.
Causes may include: