The platelets are small non-nucleated cells circulating in our vascular system with a primary function to initially arrest bleeding from a damaged vessel by rapidly creating a platelet plug. Platelets are not replicate by cell division but originated from bone marrow where they are sh
ed off into the blood stream from the large megakaryocyte.
Platelets are the second most abundant cellular component in healthy blood, and the platelet concentration is in the range of 50-350 x 10 9/L. The average life span of circulating platelets is estimated to between 7 and 10 days . The platelet surface is packed with functional receptors to facilitate platelet activation and interactions with other platelet. The platelet also contains several granules that upon activation can be fuse with the platelet membrane and thereby release their content into the surrounding media. These granules contain a large variety of biological active substance involved in hemostasis, e.g. Platelet adhesion and activation molecules, aggregation, plasma coagulation factors and fibrinolysis proteins.
PLATELET ADHESION In Platelets Cells
Following vessel wall injury, platelets rapidly adhere to exposed elements in the sub-endothelial matrix; initiating the first step in the hemostatic response leading to the formation of platelet plug. This primary adhesion is mediated by the synergistic function of several receptors on the platelet surface. Collagen is the main component of the sub-endothelium matrix and is also considered to play the principle role in the adhesion process, out of the nine types of collagen, collagen type I and type III have been determined to constitute the main constituents.
Blood is always moving inside the vessels, but the conditions can vary from almost stagnant to extremely high flow rates. The highest shear rate, or velocity gradient, is always found closest to the vessel wall. Under high shear conditions, the initial tethering and arrest of platelets from the flowing blood to collagen is facilitated by the glycoprotein Ib-ix-v receptor, a heterotrimer composes of two disulfide linked Gpiba-Gpibβ is complex with two GPix and one GPV (Gardiner. The binding is mediated by Von Willebrand Factor (VWF), a multi-metric plasma protein that rapidly binds to exposure collagen.
The GPVI receptor binds directly to collagen via the specific Gly-pro-Hyp peptide repeat sequence and has an important role of platelet activation by outside-in signaling. The integrin2β1 adhesion receptor (also known as GP1a/11a) can bind to collagen directly. Although it has been indicated that the 2β1 integrin is not involved in the initial tethering of platelets to collagen under high shear, it is still considered important for firm adhesion and securing the platelet. 2β1 has also been proposed to be involved in platelets spreading on the collagen surface (Monnet et al, 2000.). The integrin 2β1 receptor must be activated by inside-out signdin to attain a state of high affinity for its Ligands . The two collagen receptors GPVI and 2β1 are both essential for platelet adhesion and aggregation on collagen in flow conditions, blockade of either receptor will abolish the platelet plug forming capabilities.
Platelets Cells: Platelet Adhesion