Platelets, or thrombocytes, are produced in the bone marrow by giant cells called megakaryocytes. When megakaryocytes reach their full size, they start to fall apart and release hundreds of small platelets. The hormone thrombopoeitin (TPO) controls the growth of megakaryocytes and platelet production.
Mature megakaryocytes produce proto-platelet bodies within their cytoplasm, then they release them into the bloodstream via processes filled with platelet ribbons or by releasing the platelets into the bone marrow cavity by fracturing. TPO triggers the growth and fracturing of megakaryocytes, and therefore it controls platelet production. High levels of TPO hormones stimulate the growth of more megakaryocytes, and as these cells mature, they fracture to create more platelets.
The production of platelets is controlled via a feedback loop. Increased platelet numbers cause a decrease in TPO as the circulating platelets bind to the TPO produced in the liver, kidneys and bone marrow, causing TPO levels to fall and platelet production to decrease. Errors in this feedback cause abnormalities in the platelet count, either known as thrombocytopenia (low platelet counts) or thrombocytosis (high platelet counts).
The total mass of platelets remains constant in a person, although the total number of platelets varies. As the size of platelets increases, the number of platelets decreases. The size of platelets is determined by the number of pieces the megakaryocyte fractures into, which may be either many small pieces or fewer large pieces.