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.
Aortic valve leaflets. The three leaflets form the aortic valve and provide its main sealing mechanism. Anatomically the valve leaflets can be divided into three parts: The free margin, with a thickened circular node (nodule of Arantius), which provides the coaptation area to the corresponding neighboring valve leaflets.