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.
This allows the leaves to obtain water and nutrients that are necessary for the manufacture of food from light energy (photosynthesis). Food made in the leaves is then transported down to the roots and to other parts of the tree for growth. The pipes in the trunk are known as vascular tissue.It is this vascular tissue which we exploit to make paper and other forest products. Wood is primarily made of xylem (tracheids in conifers, vessel elements in hardwoods) that behave very much like a bundle of straws. Try crushing a bundle of straws from the side.his allows the leaves to obtain water and nutrients that are necessary for the manufacture of food from light energy (photosynthesis). Food made in the leaves is then transported down to the roots and to other parts of the tree for growth. The pipes in the trunk are known as vascular tissue.