The megakaryocyte is a large cell in the bone marrow that creates platelets by fragmenting into small, odd-shaped pieces. These fragments (platelets) of the megakaryocyte's cytoplasm circulate in the blood as the body's first line of defense against blood loss, a process that is called hemostasis.
This article will discuss how megakaryocytes give rise to platelets, as well as how megakaryocytes themselves arise from less differentiated cells, and how they mature.
The bone marrow is a compartment inside some of the bones of vertebrates, including humans, that contains important stem cells that go on to make up all the cellular elements of the circulating blood and some of the cells of the immune system that live in the solid organs of the reticuloendothelium system.
DNA synthesis is occurring in the nucleus during thrombopoiesis (stimulated by thrombopoietin) without cytokenesis, aka endoreduplication. Therefore, the nucleus of the megakaryocyte can become very large and lobulated, which, under a light microscope, can give the false impression that there are several nuclei. In some cases, the nucleus may contain up o 64N DNA.
Platelets are held within demarcation channels, internal membranes within the cytoplasm of megakaryocytes. Megakaryocytes release their platelets in one of two ways. The cell may release its platelets by rupturing and releasing its contents all at once in the marrow. Alternatively, the cell may form platelet ribbons into blood vessels. The ribbons are formed via pseudopodia and they are able to continuously emit platelets into circulation. 2/3 of these platelets will remain in circulation while 1/3 will be sequestered by the spleen.