Megakaryocyte

The megakaryocyte is a bone marrow cell that creates platelets when its cytoplasm becomes fragmented. These fragments (platelets) circulate in the blood as the body's first line of cellular defense against blood loss, a process that is called hemostasis.

Megakaryocytes are large cells.(Compare to other cells) DNA synthesis is occuring 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 psuedopodia 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.

Megakaryocyte function is determined by a large number of cytokines, including thrombopoietin.

The developmental stages of the megakaryocyte is: CFU-Me (pluripotential hemopoietic stem cell or hemocytoblast) -> megakaryoblast -> promegakaryocyte -> megakaryocyte.