use of cell, tissue, and organ cultures are used usually only as relative replacements, because they require freshly obtained animal cells and tissue. However, the animals are used more economically in this way, because one animal can provide tissue for a number of cultures. Human tissue can sometimes be used, but it is difficult to obtain, store, and distribute. Some human tissue is available after surgeries. Human placenta has been suggested as a source of tissue for various types of research. Because it contains mast cells that share certain structures with nerve cells, it can be used in some neurological studies. One drawback is that when human tissue is used, there is a greater risk that it will contain dangerous viruses, and so greater precautions must be taken. One idea is to establish tissue banks, much like blood banks, where the tissue can be screened and then be used to supply researchers. Because in vitro procedures isolate the system under study from the rest of the organism, they are ideal when trying to avoid the effects of influences such as hormones. This can also be a disadvantage because these external factors may have a crucial effect on the question being studied. There are several different kinds of in vitro systems, and each one has its own advantages and disadvantages.The first method is to use subcellular fractions of one cell component. An example of this type of in vitro would be to study the role of liver microsomes and their importance to drug metabolism. One drawback to this type is the fact that this system will not provide information about the influence of factors in the cell, let alone in the organ or the entire organism (5).Primary cell cultures are produced from fresh tissue that has been disrupted to obtain individual cells. These cultures are fairly easy to set up, and the advantage is that they contain normal cells with all the characteristics that determine their specialized function...
