Megakaryopoiesis and thrombopoiesis

Abstract

Platelets are formed from the cytoplasm of megakaryocytes (MKs) that reside in the bone marrow (BM). MKs arise from hematopoietic stem cells in the osteoblastic niche in BM. The primary regulator of megakaryopoiesis is thrombopoietin. It is generally accepted that MKs migrate into the vascular niche and produce platelets; however, the mechanism by which platelets are formed and released from MKs remains controversial. The most prevalent proposed mechanism regarding platelet formation is the proplatelet model; in this model, MKs form long extensions (proplatelets) by remodeling their cytoplasm and release cell fragments into sinusoid vessels. Detached cell fragments in the circulating blood are heterogeneous population of cells in regard to size, shape, or structure. Large fragments (platelet progenitors) are believed to have the ability to convert into mature platelets. After leaving BM sinusoids, these platelet progenitors may convert into individual mature platelets in the bloodstream.