s in the tissue they came from. The disadvantage, however, is that they can only be maintained for a few days, or sometimes weeks, and they tend to lose ability to function with time. This means that fresh tissue is constantly required, and the cultures cannot be used for long-term studies. Another drawback is the fact that these cultures will not permit cell/cell interactions.Another in vitro system is cell line cultures. These consist of cells that can grow indefinitely. They are often taken from human or animal tumors and some have been kept for decades. They have undergone a process called transformation that makes them able to ignore the control mechanisms that limit the number of times normal cells can divide before dying. Cell line cultures can be kept frozen in liquid nitrogen. They are used a lot because they are so easy to maintain and do not require fresh tissue. The disadvantage of this system is that it has not been possible to produce cell lines of every tissue. A further drawback is that the cells are abnormal in many ways. In some cases, the cells dont even resemble the normal cells from the tissue they came from. One advancement that has been made is the development of embryonic stem cell lines. These cells remain unaffected until the researcher manipulates them. This type of cell line culture could be used to test teratogenicity, which is the ability of a chemical to cause malformations in the fetus of an animal or human. Researchers are also interested in using embryonic stem cell lines for gene knock out studies to identify the roles of specific genes. Cell lines can be genetically engineered in many ways. For example, human genes can be inserted into an animal cell line to give it the same enzyme capabilities as human tissues (5).Tissue culture is a system that is unique in that cell/cell interactions are still possible through fragments or slices of tissue that maintains the tissues original archit...
