An agent that is smaller than a virus causes scrapie. It is either a protein called a prion or it is another substance closely associated with the prion protein. Scientists still have a lot to learn about the scrapie agent and exactly how it works.
Tissue valves are made with tissues from porcine (pig) heart valves or bovine (cow) cardiac tissue because they function like human heart valves. Once the tissue is removed from the animal, it is chemically treated to preserve the tissue and prevent immulogic reactions once it is placed in a patient.
Summary Information: Valve Cusps Valve Cusps: Valve cusps are located in the valves of the heart and are flaps of tissue that open in one direction as blood is pumped through them. The valve cusps are made of thin fibrous tissue, covered with a smooth membrane (endocardium) and reinforced by connective tissue.