The level of differentiation of megakaryocyte progenitors and the subsequent modulation of megakaryocyte antigens on developing colony cells.

Human megakaryocyte development was studied in cell culture by first determining the differentiation stage of the clonable progenitor cell population and then analyzing the kinetics of expression of two megakaryocyte antigens using an immunoalkaline phosphatase system. Studies on the expression of two antigens present on megakaryocytes but not detectable on platelets (Mk-A, Mk-B) revealed that Mk-A antigen was found on megakaryocytes in colonies of less than six to eight cells at early times of cell culture. By contrast, Mk-B antigen was detected only in large megakaryocytes starting day 7 of culture and observed primarily in colonies of greater than eight cells. These studies show that human megakaryocyte progenitors are among the more mature of the precursor classes and link the antigen A bearing megakaryocytes to the developmental sequence as the immediate progeny of the precursor population with limited mitotic capacity. Amplification of antigen B expression is late in the developmental process, being detected by this method on only large megakaryocytes in well-formed colonies after 7-9 days of culture.