These cells are the only white blood cells that return to the blood stream after migrating to the tissues. These agranulocytes vary in diameter from 12-20 microns and have an oval, horseshoe or kidney shaped nucleus with more cytoplasm than lymphocytes. The nucleus does not stain as darkly as that of lymphocytes in blood smears. After leaving the ciculatory system, monocytes differentiate into macrophages in connective tissues . Platelets These cell fragments originate from large cells in the bone marrow called megakaryocytes, have a lifespan of about 10 days and are nonnucleated.
Platelets promote blood clotting and repair gaps in the walls of blood vessels.
Upon injury to the vessel endothelium the platelets aggregate and form a platelet plug. They then release substances that induce further platelet aggregation and blood clotting. After the vessel is repaired by the formation of new tissue the clot is removed by plasma and platelet derived enzymes. Normal platelet count ranges from 200,000 to 400,000 per microliter of blood.