Bone marrow — spongy tissue inside your bones — contains stem cells that can become red blood cells, white blood cells or platelets. Platelets travel through your blood vessels. They stick together to form clots that stop the bleeding when you damage a blood vessel, such as when you get a cut. A normal platelet count ranges from 150,000 to 450,000 platelets per microliter of blood.

If you have essential thrombocythemia, your bone marrow makes too many platelet-forming cells (megakaryocytes), which release too many platelets into your blood. The excess platelets may not function normally, leading to abnormal clotting or bleeding.

The exact cause of essential thrombocythemia and similar conditions, known as myeloproliferative neoplasms, isn't known. About half the people with the disorder have a mutation of the Janus kinase 2 (JAK2) gene. Other gene mutations also have been linked to essential thrombocythemia.

A high platelet count that's caused by an underlying condition such as an infection or iron deficiency is called reactive or secondary thrombocytosis.

Sep. 22, 2012