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Sixty-three bone marrow (BM) biopsy paraffin sections from patients with platelet counts of 1000 x 10(9)/1 or greater were examined to determine the incidence of megakaryocytic emperipolesis for the various myeloproliferative disorders (MPDs) and for reactive thrombocytosis. Of those cases classified as specific MPDs, 77% of primary thrombocythemia (PT) specimens, 100% of the polycythemia vera (PV) specimens, a single idiopathic myelofibrosis (IMF) specimen, and 17% of the chronic granulocytic leukemia (CGL) specimens demonstrated emperipolesis within megakaryocytes. Two of three cases grouped as MPDs but not further classified also demonstrated emperipolesis. Of the cases of reactive thrombocytosis (RT), 75% showed the presence of emperipolesis. Our results indicate that, with the exception of CGL, emperipolesis can be found in the BM megakaryocytes of the great majority of patients who have extreme thrombocytosis. The underlying cause, whether myeloproliferative or reactive, does not apparently influence the incidence of the phenomenon.

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