FACTORS REGULATING MEGAKARYOCYTE PROGENITOR COMMITMENT TO POLYPLOIDIZATION

Recognizable megakaryocytes are polyploid cells generated by a clonoge nic, diploid progenitor, termed CFU-MKC (colony forming unit megakaryo cyte). In order to quantify polyploidization, ploidy histograms of meg akaryocytes determined by microphotometric or flow cytometric measurem ents of megakaryocyte DNA have generally been used. However there tech niques provide no information on the rate of commitment of CFU-MKC to polyploidy. Using a technique of clonal analysis deter mining the dist ributions of the number of doublings (NbD) undergone by CFU-MKC before committing to polyploidization, the polyploidization probability of C FU-MKC could be derived. This probability was found to be a constant i ndependent from CFU-MKC mitotic history, since NbD distributions are e xponential functions characterized by a constant rate of decay per dou bling By studying the effects of growth factors on polylploidization p robability, it was also shown that: (1) this parameter is negatively r egulated by growth factors contained in poke-weed or WENI conditioned media, as well as by erythropoietin; (2) commitment to polyploidizatio n does not require prior CFU-MKC division; (3) bipotent erythroid-mega karyocyte progenitors have a lower polyploidisation probability than C FU-MKC; (4) determination of polyploidization probability reflects the activity of growth factors with greater accuracy than megakaryocyte c olony count.