Light micrograph of a bone marrow film from a person suffering from thrombocytopenia of unknown origin. Thrombocytopenia is a reduction in numbers of blood platelets, which results in spontaneous bruising and prolonged bleeding after injury. Dominating the image is a purple-stained megakaryocyte, the site of production of blood platelets (thrombocytes), which bud from the cell's large cytoplasm. Platelets function to plug small defects in the walls of small blood vessels, and contribute to the clotting process by releasing serotonin, which constricts damaged blood vessel walls. Magnification: X 100 at 35mm size.