4 minute read

Human Cloning

Possible Role Of Cloning In Stem Cell Therapy, Aging And ReproductionBenefits of animal cloning

An oocyte is an unfertilized egg. Oocytes and spermatozoa are called gametes, and represent different cells that fuse their genes to form a new cell, the fertilized egg. The fertilized cell is called a zygote, and it rapidly divides into several totipotent cells (cells capable of developing into any cell type) called blastomers. Totipotent cells can be considered the opposite of differentiated cells (cells that are biochemically and morphologically specialized to perform a specific function), and it is worth noting how differentiated cells (gametes) can produce totipotent cells. As the fertilized egg continues to divide, totipotent cells become more differentiated and specialize into nerve cells, blood cells, muscular cells, and the many other cells that are required in order to produce a complete new individual. In the laboratory, this biological process can be modified. If an unfertilised egg is enucleated (the nucleus is removed) and fused with a somatic cell (any cell other than germ cells that produce gametes) from an adult individual, the resulting cell will have inside the nucleus only the genes from the adult individual that donated the somatic cell with his relative nucleus. Thus, a new "twin" individual is theoretically generated. Once an embryo is generated, it can be implanted in the uterus of a surrogate mother. This method defines cloning, i.e., the creation of a new being by nuclear transfer from a somatic (differentiated) cell. The first successfully cloned mammal was the sheep Dolly. Dolly was created in this manner using a mammary cell. Dolly, however, was not as identical as a naturally occurring twin because some of the mitochondrial DNA from the oocyte was present in the resulting zygote. The mitochondria provide energy needed by the cell and may play other roles as well, possibly even storing information in neurons and thus, playing a role in memory. Cloning is a process with a low rate of success; hundreds of experiments are needed to clone a single animal. Furthermore, in cloned animals, a higher rate of malformations and genetic disease, as well as signs of early aging have been observed.

Pharmaceutical proteins and nutraceuticals

The possibility of deriving live animals from cultured cells provides an efficient way of producing transgenic farm animals. Furthermore, in normal transgenic breeding, successive generations often loose the incorporated gene. Once a transgenic animal is made, cloning makes sure that its genetic variation remains through successive generations. In this way, human proteins can be produced avoiding purifications from blood, an expensive process associated with risk of contamination of viruses such as AIDS and hepatitis C. Target proteins can be purified from milk of transgenic animals as well as sheep, goats, and cattle with relatively low costs. For example alpha-1-antitrypsin and factor IX can be produced and used to treat cystic fibrosis and haemophilia, respectively. Again, human serum albumin, which is in high demand for treatment of burns and other trauma, can be produced in transgenic cows by substitution with the human albumin gene for its bovine equivalent. By altering the nutritional content of cows' milk it is possible to insert genes for human proteins in order make high-nutrition milk for premature infants, for example, or to create milk without the specific proteins responsible for allergic immune responses or lactose intolerance.

Xenotransplantation organ source animals

Xenotransplantation is the use of animal organs for human organ transplantation. Recent advances in understanding of organ rejection and in animal genetic modification and cloning have made it possible to consider animals as a viable source of organs for transplant into humans. This need stems form the worldwide shortage of donated human organs for transplant. It is estimated that in the United States, about 1,200 patients die each year on heart and lung transplant waiting lists. Research into xenotransplantation has concentrated on the use of pigs. The prospect of xenotransplantation presents a whole new set of risks for consideration as well as the so-called xenozoonoses. This neologism refers to animal diseases that may be transmitted to the recipient of a xenotrans-plant. Some zoonotic pathogens are known to scientists and screening protocols to detect them have been developed, but it is likely that others exist that have not been identified. The use of pigs as a source for donor organs seems to reduce the risk of unusual infections, as pigs and humans, for the most part, share the same environment.

Animal models of human disease

Cloning can produce genetically identical laboratory animals that can be used as models for the study of human disease. The most commonly used laboratory animal, the mouse, reproduces rapidly and its genetics have been well studied for the discovery of new treatments for disease. Several other mammals have served as scientific models. Cats, for example, aided research on human AIDS using the feline AIDS (FIV) as a model. Rabbits have proved valuable for studying human cardiovascular diseases, and primates have been models for studying human diseases such as viral hepatitis.

Additional topics

Science EncyclopediaScience & Philosophy: Heterodyne to Hydrazoic acid