Essential thrombocythemia is an abnormally high production of cells called megakaryocytes in the bone marrow. The bone marrow is the soft spongy material contained in the center of the bones. These cells are responsible for producing platelets. Platelets are blood products that help the blood clot. In essential thrombocythemia, platelet levels are excessive (greater than 600,000 per microliter) and can lead to clotting problems.