Extramedullary hematopoiesis (EMH) implies the production of erythroid and myeloid progenitor cells outside of the bone marrow. EMH in adults is typically seen in patients with myeloproliferative neoplasms (MPNs) but its association also with other conditions, including thalassemia, has long been recognized 1. In both MPN and non-MPN settings, the liver and spleen are the two most frequent sites of EMH and it has been hypothesized that circulating hematopoietic cell filtration (entrapment), possibly via endothelial cell expressed ligands, such as chemokine ligand 12, rather than splenic stroma account for the particular phenomenon 2–4; the concordant detection of MPN-specific mutations, such as JAK2V617F, and specific cytogenetic abnormalities in both bone marrow and splenic tissue of affected patients, supports this contention 5, 6.

Circulating hematopoietic progenitors mobilized as a result of otherwise nonspecific hematopoietic stressors have also been implicated in seeding non-hepatosplenic EMH (NHS-EMH) 7. The latter has been reported in a variety of organs, including the central nervous system 8, ovaries and tubes 9, the skin 10, the lungs and pleura 11, the pericardium 12, 13, lymph nodes 14, and other sites. In a previously published report of 27 Mayo Clinic cases of NHS-EMH diagnosed antemortem between 1975 and 2002 15, the most common associated condition was myelofibrosis and the most frequent involved site the thoracic vertebral column. The current study focuses on the Mayo Clinic experience with EMH cited in the absence of MPN. The objectives were to systematically