Growth factor-induced process formation of megakaryocytes derived from CFU-MK.

The extended cytoplasmic processes from megakaryocytes (MK) are believed to be structural intermediates between MK and platelets. We could observe differentiation of purified rat CFU-MK toward mature MK to form extended cytoplasmic processes. Recombinant rat interleukin-3 (IL-3), human erythropoietin (Epo), and human interleukin-6 (IL-6) each was able to stimulate process formation although they varied somewhat in their potential. Electron microscopic observations showed that these processes were very similar to those from mature MK so far reported. The combination of IL-6 with IL-3 or Epo synergistically increased the number of MK forming processes without further increase in the number of total MK formed in the presence of IL-3 or Epo alone. In addition, IL-6 significantly increased the megakaryocytic diameter and DNA content of MK induced by IL-3 or Epo and shortened the MK transit time to hasten the process formation of MK. These findings suggest that IL-6 promotes further maturity in both the cytoplasm and ploidy of MK to form extended cytoplasmic processes. The ability of these factors to generate the process formation in vitro may be related to their thrombopoietic effects in vivo.