Platelets or thrombocytes are small, irregularly shaped anuclear cell fragments, 2-3µm in diameter, which are derived from fragmentation of precursor megakaryocytes.
The average lifespan of a platelet is normally just 5 to 9 days.
Platelets play a fundamental role in haemostasis and are a natural source of growth factors. They circulate in the blood of mammals and are involved in haemostasis, leading to the formation of blood clots.
If the number of platelets is too low, excessive bleeding can occur. However, if the number of platelets is too high, blood clots can form (thrombosis), which may obstruct blood vessels and result in such events as a stroke, myocardial infarction, pulmonary embolism or the blockage of blood vessels to other parts of the body, such as the extremities of the arms or legs.
More on Platelets
An abnormality or disease of the platelets is called a thrombocytopathy,which could be either a low number of platelets (thrombocytopenia), a decrease in function of platelets (thrombasthenia), or an increase in the number of platelets (thrombocytosis). There are disorders that reduce the number of platelets, such as Heparin-Induced Thrombocytopenia (HIT) or thrombotic thrombocytopenic purpura (TTP) that typically causes thrombosis, or clots, instead of bleeding.
Platelets release a multitude of growth factors including platelet-derived growth factor (PDGF), a potent chemotactic agent, and TGF beta, which stimulates the deposition of extracellular matrix. Both of these growth factors have been shown to play a significant role in the repair and regeneration of connective tissues.
Other healing-associated growth factors produced by platelets include basic fibroblast growth factor, insulin-like growth factor 1, platelet-derived epidermal growth factor, and vascular endothelial growth factor. Local application of these factors in increased concentrations through Platelet-Rich Plasma (PRP) has been used as an adjunct to wound healing for several decades. Platelets are produced in bone marrow by fragmentation of the cytoplasm of megakaryocytes, one of the largest cells in the body.
The precursor of the megakaryocyte, the megakaryoblast arises by a process of differentiation from the haemopoitic stem cell. The megakaryocyte matures by endomitosis synchronous replication (that is DNA replication in absence of nuclear or cytoplasmic division) enlarging the cytoplasmic volume as the number of nuclear lobes increase in the multiples of two.
Very early on invaginations of plasma membrane are called the demarcation membrane, which evolves through the development of the megakaryocyte into the branched network. At a various stage in development, most commonly at the eight nucleus stage the cytology becomes granular. Mature megakaryocytes are extremely large, with an eccentric placed single lobulated nucleus and a low nuclear to cytoplasmic ratio.
Platelets form by fragmentation of megakaryocyte cytoplasm, approximately each mega cytoplasm, giving rise to 1000-5000 platelets. The time interval from differentiation of the human stem cell to the production of platelets averages approximately 10 days.
Thrombopoietin is the major regular of platelets production and is constitutively produced in the liver and kidneys. Thrombopoietin increase the number and the rate of maturation of platelets.