platelet

Tiny disc-shaped structure found in the blood, which helps it to clot. Platelets are not true cells, but membrane-bound cell fragments without nuclei that bud off from large cells in the bone marrow.

They play a vital role in blood clotting as they release blood clotting factors at the site of a cut. Over twelve clotting factors have been discovered and they produce a complex series of reactions which ultimately leads to fibrinogen, the inactive blood sealant always found in the plasma, being converted into fibrin. Fibrin aggregates into threads which form the fabric of a blood clot.