Platelets or thrombocytes are blood cells which participate in the coagulation of blood.
Source of platelets.
Platelets are formed from their precursors in the hematopoietic tissue.
Platelets, like other blood cells, are formed from Hemocytoblast that under certain conditions, and mainly under the effects of thrombopoietin, may be converted in a megacarioblast. Megacaryoblast is transformed into Promegacariocyte, and this cell in Megakaryocyte.
During Megakaryocyte maturation process, DNA replication continues, and the nucleus suffers many divisions, but the cell continue undivided. While this process is going on, a large amount of cytoplasm accumulates.
Platelets are formed by the development of demarcation membranes in the cytoplasm, with subsequent release of the formed fragments to the venous sinusoids of the marrow bone.
A Megakaryocyte can release thousands of platelets, leaving the parenchyma cell with virtually only the nucleus and residual cytoplasma.
Platelets are very small (1 to 4 microns in diameter) and circulate between 4 and 10 days, as flattened disks without nucleus. Platelet membrane is very rich in phospholipids and contains various glycoproteins that perform a fundamental role in the reception and transduction of intracellular signals. The cytoplasm of platelets contains a microphylament system and an actin/myosin contractile structure, called thrombosthenin, which, when activated, modify the conformation of the membrane. The cytoplasm also contains microtubules, which together with the microfilaments form an internal cytoskeleton responsible for the platelets conformation, but at the same time flexible enough to allow the conformational changes that occur during the platelet activation.
Platelet cytoplasm contains also residual endoplasmic reticulum(forming the so-called dense tubular system), mitochondria, glycogen and three different types of granules: Alpha granules, dense granules and lysosomal granules, containing biologically active substances that are released during the coagulation process The energy for platelet processes (aggregation, secretion and others) derive from the aerobic metabolism in the mitochondria and anaerobic glycolysis (recall that the cytoplasm contains glycogen granules)
Platelet receptors and granules are particularly interesting from the physiological and medical point of view, so they will be discussed in more detail in future posts.
Flaumenhaft, R. et al:
The actin cytoskeleton differentially regulates platelet (alpha)granule and dense granule secretions.
The Online Metabolic and Molecular Bases of Inhereted Diseases.
The inhereted disorders of platelets