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.
One type of this tissue is known as fibrous connective tissue. This type of tissue is formed by elastic and collagen fibers and is used to bind other types of body tissues. Ligaments and tendons are made of fibrous connective tissue. There are no living cells in most of this connective tissue. Instead, it is made mostly of water, proteins, and polysaccharides.