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.
The mitral valve apparatus consists of an annulus, cusps, chordae tendineae, and papillary muscles. The shape of the mitral valve annulus is saddle-like. The mitral valve is functionally bicuspid but embryologically composed of four cusps.