CFU - MEG CULTURES FROM PATIENTS WITH ET 1545 Table 1 . Clinical Chara cteristics of P atients With Essential Thrombocythemia Patient Platelet Mean Platelet Hemoglobin Treatment

Peripheral blood mononuclear cells from five patients with essential thrombocythemia (ET) were cultured in vitro to evaluate restricted megakaryocytic (CFU-Meg). myeloid (CFU-GM). and erythroid (BFU-E) progenitor cell development. Varying concentrations of aplastic canine serum served as the source of megakaryocyte colony-stimulating activity. and cultured megakaryocyte ploidy distributions were determined by Feulgen staining and microfluorometry. Megakaryocyte colony growth was strikingly abnormal in all five patients evaluated. Four of the 5 had a marked expansion in the concentration of circulating CFU-Meg and 3 of 4 manifested abnormalities in cultured megakaryocyte colony size (2 unusually large and 1 small). Unstimulated megakaryocyte colony growth was substantially increased in three patients. However, the fraction of megakaryocyte progt riitOrs in cell cycle was near or below normal in all instances. Endomitotic megakaryocyte development was disordered in each of the four ET patients in whom it was evaluable. In normal subjects. mean megakaryocyte ploidy values vary biphasically with aplastic canine serum concen-