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.
Calvin SH Ng, Song Wan, Tak Wai Lee, Alan DL Sihoe, Innes YP Wan, Ahmed A Arifi, Anthony PC Yim. Video assisted thoracoscopic surgery for mediastinal extramedullary haematopoiesis. Ann R Coll Surg Engl 2002;84:161-163