Figure 6.

Schematic interpretation of megakaryocyte development, maturation and fragmentation leading to the release of platelets. I. Megakaryoblast (2N cell); II. Megakaryocyte (MK) in endomitotic stage: (a) Anaphase A. MK with ploidy 16N has 8 mitotic spindles including centrosomes, towards which each one set of sister chromatids is pulled presumably in individual putative cell unit. (b) Interphase. 16N MK in interphase. In the cell center 8 centrosomes, although existing in individual putative cell unit, are clustered. From which 8 bundles of microtubules (MT: green line) radiate to the cell periphery; III. MK in differentiation stage: Maturation of each putative cell unit, composed of two “putative cytoplasmic compartment” (PCCs), proceeds in a clockwise fashion as shown in the illustration. (a) Migration of a single centriole from the cell center to the cell periphery, presumably along the MK original MT, in a PCC. Intrinsic organelles of platelet increase in each future platelet. Platelet demarcation vesicles (DV) are arranged at the boundary surface between neighboring PCC. (b) Fusion and fission of DV proceed at the interface of PCCs. β1-tubulin is synthesized and encapsulated in microcompartment (marked as red dot), and delivered into each platelet territory. Platelet territories in a tandem array are under elaboration inside of each PCC. (c) Demarcation membrane system (DMS) is constructed, protoplatelets are delineated by one sheet of DMS. Single centriole is located in the most peripheral platelet territory. (d) Excessively produced membrane vesicles intervene between respective protoplatelets; IV. Cytoplasmic fragmentation and platelet release: In liberated nascent platelets, MK original MTs are disassembled and β1-tubulin is liberated from encapsulated microcompartments. Respective MTOC in each nascent platelet nucleates specialized MT, leading to the formation of MT coils. In some cases, nascent platelets are released as short chain of platelets which fragment further into individual platelets in the pulmonary circulation. (Reproduced with some modification from Fig. 6 in Kosaki’s review article²³⁾ with permission from the publisher)