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.
Characteristics and Location. Serosa are made up of two layers of tissue with a thin layer of serous fluid in the middle. One layer is made up of mesothelium, which produces the fluid, while the other is made up of connective tissue, which attaches the membrane to other parts of the body and provides blood and nerves for the epithelial layer.