Thrombopoiesis: At a Glance!

Excerpt

Megakaryocytes are stem cell derived progenitors which give rise to platelets through successive steps of differentiation. They undergo a unique maturation process which includes nuclear polyploidization, development of internal membrane system and formation of proplatelet processes from which new platelets are shed into vascular sinusoids to reach peripheral blood [1]. The nuclear remnant is then phagocytosed by the marrow macrophages. Figure 1 shows one such megakaryocyte found in the bone marrow aspirate of a patient with immune thrombocytopenia wherein the megakaryocytic cytoplasm is extended into long serpentinous proplatelet processes (also known as platelet ribbons). The megakaryocytic nucleus is seen at one side and multiple new platelets (fragments of cytoplasm) are observed at the ends of these ribbons. As megakaryocytes progressively differentiate, they become polyploid due to endomitosis. This polyploidization facilitates massive protein production required for the formation of large number of platelets (one megakaryocyte gives rise to up to 5000 platelets). This platelet shedding into marrow sinusoids was first demonstrated in vivo using fluorescently labelled megakaryocytes [2]. Such a phenomenon is rarely visible on aspirate smears. This picture depicts a transient step of megakaryocytic differentiation with a lasting visual impression.

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