Thrombocytogenesis by megakaryocyte; Interpretation by protoplatelet hypothesis.

Serial transmission electron microscopy of human megakaryocytes (MKs) revealed their polyploidization and gradual maturation through consecutive transition in characteristics of various organelles and others. At the beginning of differentiation, MK with ploidy 32N, e.g., has 16 centrosomes in the cell center surrounded by 32N nucleus. Each bundle of microtubules (MTs) emanated from the respective centrosome supports and organizes 16 equally volumed cytoplasmic compartments which together compose one single 32N MK. During the differentiation, single centriole separated from the centriole pair, i.e., centrosome, migrates to the most periphery of the cell through MT bundle, corresponding to a half of the interphase array originated from one centrosome, supporting one "putative cytoplasmic compartment" (PCC). Platelet demarcation membrane (DM) is constructed on the boundary surface between neighbouring PCCs. Matured PCC, composing of a tandem array of platelet territories covered by a sheet of DM is designated as protoplatelet. Eventually, the rupture of MK results in release of platelets from protoplatelets. (Communicated by Tadamitsu Kishimoto, M.J.A.).

Platelet production by megakaryocytes: protoplatelet theory justifies cytoplasmic fragmentation model.

The maturation of megakaryocytes (MKs) leading to platelet production is carefully reviewed. For instance, when MK with ploidy 16N enters the maturation stage, eight centrosomes are clustered in the cell center surrounded by 16N nucleus. Each bundle of microtubules (MTs) emanated from the respective centrosome supports and organize eight equally volumed cytoplasmic compartments which together compose one single 16N MK. Then, the following three processes take place in parallel until the complete maturation of MKs Virchow's Arch. 1906;186:55-63. Two centrioles, composing centrosome, are separated and each one with pericentriolar material migrates to just beneath the plasma membrane through the MT bundle [corresponding to a half of the interphase array, originated from one centrosome, supporting one "putative cytoplasmic compartment "(PCC)] (Blood. 2005;106:4066-75). Platelet specific granules and other cellular components, newly formed in the central field of the cell, are transported along the MTs and many platelet territories, future platelets, are elaborated as a tandem array from the center to periphery in each PCC [3]. All the important membranes including plasma membrane and platelet demarcation membrane (DM) are synthesized de novo and those are transported as membrane vesicles (MVs) from Golgi body along the MTs. MVs arranged on the boundary surface between neighboring PCCs undergo fusion and fission to yield a paired membrane. Further connection with the external milieu results in the completion of DM system. The PCC covered by a sheet of DM is designated as protoplatelet. Excessive production of the MVs, most probably intervenes between the respective protoplatelets. Eventually, the matured MK ruptures as a whole, resulting in release of platelets from protoplatelets and many of MVs, though the mechanism is not fully elucidated yet.

In vivo platelet production from mature megakaryocytes: does platelet release occur via proplatelets?

Although platelets are universally accepted to be born from megakaryocytes (MKs), the mechanism by which platelets are formed and released from MKs in vivo remains controversial. One theory, known as the proplatelet theory, postulates that platelets are released from proplatelet processes protruding from MKs into sinusoids located in the bone marrow hematopoietic compartment. Proplatelet formation (PPF) has been observed in in vitro experiments involving detailed analyses of related molecular events. PPF has also been used as a marker of MK maturation. However, PPF is suggested to be a nonphysiological phenomenon. On the other hand, transmission electron microscopy (TEM) analyses have revealed platelet formation via explosive fragmentation of MK cytoplasm in bone marrow and lung capillaries prepared by immersion fixation. Moreover, TEM and scanning electron microscopy studies of liquid-cultured MKs kept in suspension show that platelet formation occurs without PPF. Rather, an explosive and global fragmentation of the MK cytoplasm composed of platelet territories has been reported as the mechanism of platelet formation. In addition, in vivo and ex vivo observations of platelet release from MKs with phase-contrast microscopy strongly support the explosive-fragmentation theory. With all observations taken into account, PPF may not be a prerequisite for platelet release from MKs under real-life conditions. In this review, a new "protoplatelet" concept is proposed to support the explosive-fragmentation theory. Additionally, the role of the lungs in platelet production is reviewed and discussed.