Growth Factors / Platelets
Growth Factors
Growth factors are important for cell growth and are found all over the body. They are concentrated in platelets and this provides a convenient source that is easy to harvest and prepare. If the target organ or structure is small the platelets can be concentrated in the plasma before breaking the platelets to release the growth factors. Hence the term Platelet Rich Plasma (PRP). When used to stimulate stem cells prior to injection the platelets are concentrated and the clotting factors activated to release the growth factors.
Platelets
Are small regularly-shaped clear cell fragments (i.e. cells that do not have a nucleus containing DNA), 2–3 µm in diameter. The average lifespan of a platelet is normally just 5 to 9 days. The physiological range for platelets is 150–400 x 109 per liter.
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.
The aggregated platelets help this process by secreting chemicals that promote the invasion of fibroblasts from surrounding connective tissue into the wounded area to completely heal the wound or form a scar. This fibroblast stimulation can be used to grow new collagen in the skin so reducing wrinkles.
Activation
The inner surface of blood vessels is lined with a thin layer of endothelial cells that, in normal hemostasis, acts to inhibit platelet activation by producing nitric oxide, endothelial-ADPase, and PGI2. Endothelial-ADPase clears away the platelet activator, ADP.
Endothelial cells produce a protein called von Willebrand factor (vWF), a cell adhesion ligand, which helps endothelial cells adhere to collagen in the basement membrane. Under physiological conditions, collagen is not exposed to the bloodstream. vWF is secreted constitutively into the plasma by the endothelial cells, and is stored in granules within the endothelial cell and in platelets.
When the endothelial layer is injured, collagen, vWF and tissue factor from the subendothelium is exposed to the bloodstream. When the platelets contact collagen or vWF, they are activated (e.g. to clump together). They are also activated by thrombin (formed with the help of tissue factor). They can also be activated by a negatively-charged surface, such as glass.
Platelet activation further results in the scramblase-mediated transport of negatively-charged phospholipids to the platelet surface. These phospholipids provide a catalytic surface (with the charge provided by phosphatidylserine and phosphatidylethanolamine) for the tenase and prothrombinase complexes. Calcium ions are essential for binding of these coagulation factors.
