Multiple In Vivo Effects of Interleukin- 3 and Interleukin-6 on Murine Mega karyocyt o poiesi s

HE COMPLEX regulation of megakaryocytopoiesis T has been extensively re~iewed.'.~ Although mega- karyocyte development is a continuous process, it has been useful to consider early and late stages. The early stage comprises the transition of stem cells to committed mega- karyocyte precursors and is measured by megakaryocyte colony number and size in culture. In vitro, this stage is stimulated by one or more megakaryocyte colony-stimulat- ing factors (Meg-CSF) that have been identified in the urine and plasma of patients with aplastic anemia,536 amega- karyocytic thrombocytopenia,' and thrombocytopenia fol- lowing chemotherapy.8 The late stage comprises the matura- tion of megakaryocyte precursors into platelet producing cells and may be assessed by megakaryocyte number, size, ploidy or acetylcholinesterase activity (in certain species) or by measuring isotope incorporation into newly forming platelets.' Megakaryocyte maturation and platelet produc-