Essential thrombocythaemia. Illustration of platelets, or thrombocytes, (white) in the bloodstream. Essential thrombocythaemia is a rare blood disease where the bone marrow produces too many platelets. Platelets are fragments of white blood cells. When blood clots, red blood cells (erythrocytes) are trapped within a fibrin protein mesh, which is formed in response to chemicals secreted by the platelets. Therefore essential thrombocythaemia can lead to blood clots that block blood flow in the body.