Platelet

Definition
noun, plural: platelets
A discoid cellular fragment (2–3 µm diameter) important for blood coagulation and for haemostasis by repairing breaches (small breaks) in the walls of blood vessels
Supplement
Hematopoiesis is the process of forming new blood cellular elements in vertebrates. In this regard, thrombopoiesis is a hematopoiesis leading to the formation of thrombocytes, which are also called platelets. The process begins with a multipotent stem cell, a hemocytoblast, which gives rise to a common myeloid progenitor cell. The progenitor cell, in turn gives rise to colony forming unit (particularly, CFU-Me, which is also referred to as CFU-Meg or CFU-Mega) fated to become megakaryocytes. The CFU-Me gives rise to megakaryoblasts. A megakaryoblast is the precursor for a promegakaryocyte. The promegakaryocyte, in turn, is the precursor cell for a megakaryocyte. The megakaryocyte is the cell from where the platelets are derived.
A platelet is a discoid cellular fragment of a megakaryocyte. It has a diameter of about 2 to 3 µm. It pinches off from the megakaryocyte and move to the peripheral blood where it plays a role in blood clotting and haemostasis. This fragment has granules at the center portion (called granulomere). Its periphery, which is clear, is referred to as hyalomere. Other components of a platelet are plasma membrane, sub-membrane area, cytoskeleton (particularly in the sol-gel zone), lysosomes, mitochondria, dense bodies (platelet-activating mediators such as ADP, serotonin, and calcium), peroxisomes, and alpha granules (e.g. coagulation factor V, coagulation factor XI, fibrinogen, growth factors, etc.).
Platelets binds to fibrinogen at the site of a wound to begin the blood clotting process. They also release platelet-derived growth factor which presumably contributes to later repair processes by stimulating fibroblast proliferation. Reserved platelets are stored in the spleen for later use. Worn-out platelets are destroyed in spleen and liver.
Word origin: Greek platús (“broad”, “flat”, “wide”) + –let (“small”)
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