Reset When a blood vessel is damaged, the fibers within the connective tissue beneath the endothelial cells in the vessel wall become exposed. Platelets adhere to these fibers with the assistance of a plasma protein called . As the platelets start to stick to the vessel wall, their morphology changes dramatically. They develop that further adhere them to the blood vessel wall. As more and more platelets aggregate to the site, a(n) develops to close off the injury. This is a measure to block the flow of blood to an area where a vessel wall is damaged. Platelets undergo this morphologic change and become activated. Their cytoplasm releasing chemicals to assist with hemostasis.