Platelets, also called “thrombocytes”, are blood cells whose function (along with the coagulation factors) is to stop bleeding. Thrombocytes have no nucleus: they are fragments of cytoplasm which are derived from the megakaryocytes of the bone marrow, and then enter the circulation. These unactivated platelets are biconvex discoid structures shaped like a lens, 2–3 µm in greatest diameter. Platelets are found only in mammals, an adaptation that may have evolved to offset the risk of death from hemorrhage at childbirth – a risk unique to mammals.
On a stained blood smear, thrombocytes appear as dark purple spots, about 20% the diameter of red blood cells. The smear is used to examine platelets for size, shape, qualitative number, and clumping. The ratio of platelets to red blood cells in a healthy adult is 1:10 to 1:20.
The main function of thrombocytes is to contribute to hemostasis: the process of stopping bleeding at the site of interrupted endothelium. They gather at the site and unless the interruption is physically too large, they plug the hole. First, platelets attach to substances outside the interrupted endothelium: adhesion. Second, they change shape, turn on receptors and secrete chemical messengers: activation. Third, they connect to each other through receptor bridges: aggregation. Formation of this platelet plug (primary hemostasis) is associated with activation of the coagulation cascade with resultant fibrin deposition and linking (secondary hemostasis). These processes may overlap: the spectrum is from a predominantly platelet plug, or “white clot” to a predominantly fibrin clot, or “red clot” or the more typical mixture. The final result is theclot. Some would add the subsequent clot retraction and thrombocyte inhibition as fourth and fifth steps to the completion of the process and still others a sixth step wound repair.
Low thrombocyte concentration is thrombocytopenia and is due to either decreased production or increased destruction. Elevated thrombocyte concentration is thrombocytosis and is either congenital, reactive (to cytokines), or due to unregulated production: one of the myeloprolerative neoplasms or certain other myeloid neoplasms. A disorder of platelet function is a thrombocytopathy.
Normal thrombocytes can respond to an abnormality on the vessel wall rather than to hemorrhage, resulting in inappropriate thrombocyte adhesion/activation and thrombosis: the formation of a clot within an intact vessel. These arise by different mechanisms than a normal clot. Examples are: extending the fibrin clot of venous thrombosis; extending an unstable or ruptured arterial plaque, causing arterial thrombosis; and microcirculatory thrombosis. An arterial thrombus may partially obstruct blood flow, causing downstream ischemia; or completely obstruct it, causing downstream infarction.