Megakaryocytes are large polyploid cells present of the bone marrow, which produce the blood platelets or thrombocytes, small disk-shaped cytoplasmic fragments that circulate in the blood and are essential components of the haemostasis system. Formation and release into the blood is under the primary regulation of thrombopoietin, a glycoprotein produced in the kidney and liver. During differentiation of megakaryocytes, continued DNA replication in the presence of abortive mitosis leads to the formation of polyploid nuclei, a process called endomitosis. It has been shown that polyploidisation is connected with the suppression of stathmin, a microtubule-regulatory protein that has a major role in the formation of the mitotic spindle. Platelets are pre-formed in the cytoplasm of megakaryocytes at sites of protrusions, called proplatelets. A megakaryocyte may form 10–20 proplatelets from the tips of which the platelets are released. Maturing platelets become filled with organelles and granules, which move from the megakaryocyte cell body to the tips of the proplatelets.