Polyploidy and Maturation of Rat Megakaryocytes

A commonly held concept of megakaryocyte maturation envisions a coincidence of increases in ploidy (brought about by successive replications of DNA) concomitant with the development of cytoplasmic granulation and other characteristics of mature megakaryocytes. Since certain experimental findings are difficult to reconcile with this concept, particularly labeling studies with 3HTdr, we postulated that the development of polyploidy occurs before cytoplasmic maturation begins. To test this hypothesis, three characteristics of individual megakaryocytes were examined: morphologic stage, ability to incorporate ³HTdr, and amount of DNA in individual cells. The labeling-index determinations confirmed that only young megakaryocytes incorporate ³HTdr. The youngest megakaryocytes recognized included cells of all ploidy values—4, 8, 16, and 32N—demonstrating that the highest ploidy value can be attained without concomitant cytoplasmic maturation. The more mature morphologic stages included cells with 8, 16, and 32N ploidy values, indicating that a megakaryocyte need not reach the highest ploidy value before maturing cytoplasmically. The results support the hypothesis that polyploidy develops before cytoplasmic maturation occurs, and imply that platelet production may occur in cells at several ploidy levels.