Developmental staging of in vitro–generated MKs. Schematic representation of the stages as immature human MKs mature and then undergo injury. (A) The immature LG MK population has low ploidy and few internal granules and show limited response to agonist stimulation. These LG MKs mature to become the HG/CD42b⁺ MK population. (B) The mature HG/CD42b⁺ MKs have increased ploidy, RNA, and α-granular content as compared with the immature LG MKs. They are also maximally responsive to stimulation by various agonists. (C) Mature HG/CD42b⁺ MKs take up fluorescently labeled FV into their α granules and release FV-labeled platelets in vitro and in the circulation of mice when infused. The FV-labeled platelets are similar in size or larger than human donor platelets. FV-labeled platelets are incorporated into clots and activate in response to agonist stimulation, suggesting that they are functional. (D) GM6001 inhibits CD42b shedding from MKs, but does not prevent apoptosis or improve MK functionality. (E) On the other hand, blocking apoptosis also prevents CD42b shedding, and redirects MKs to increase the number of CD42b⁺ MKs that take up FV and release functional platelets.