Thrombocytes are cellular derivatives from megakaryocytes which contain factors responsible for the intrinsic clotting mechanism. They represent fragmented cells (See Figure 17.12) which contain residual organelles including rough endoplasmic reticulum and Golgi apparati. They are only 2-microns in diameter, are seen in peripheral blood either singly or, often, in clusters, and have a lifespan of 7 to 10 days. Partitioning of the granular cytoplasm by invagination of the plasma membrane produces platelets. Inside the platelets, the granulomere, an intensely stained core, contains granules which release serotonin and protease enzymes.