Wilmut’s LambWilmut then set out to attempt the key breakthrough, the experiment that had eluded researchers since Spemann proposed it 59 years before: to transfer the nucleus from an adult differentiated cell into an enucleated egg, and allow the resulting embryo to grow and develop in a surrogate mother, hopefully producing a healthy animal.
Wilmut removed mammary cells from the udder of a 6-year-old sheep (figure 23). The origin of these cells, gave the clone its name, “Dolly” after the country singer Dolly Parton. The cells were grown in tissue culture, and some frozen so that in the future it would be possible with genetic fingerprinting to prove that a clone was indeed genetically identical with the 6-year-old sheep.
In preparation for cloning, Wilmut’s team reduced for 5 days the concentration of serum on which the sheep mammary cells were subsisting. In parallel preparation, eggs obtained from a ewe were enucleated, the nucleus of each egg carefully removed with a micropipette.
Mammary cells and egg cells were then surgically combined in January of 1996, the mammary cells inserted inside the covering around the egg cell. Wilmut then applied a brief electrical shock. A neat trick, this causes the plasma membranes surrounding the two cells to become leaky, so that the contents of the mammary cell passes into the egg cell. The shock also kick-starts the cell cycle, causing the cell to begin to divide.
After 6 days, in 30 of 277 tries, the dividing embryo reached the hollow-ball “blastula” stage, and 29 of these were transplanted into surrogate mother sheep. A little over 5 months later, on July 5, 1997, one sheep gave birth to a lamb. This lamb, “Dolly,” was the first successful clone generated from a differentiated animal cell. The Future of http://biologywriter.com/backgrounder//cloning-2
Wilmut’s successful cloning of fully differentiated sheep cells is a milestone event in gene technology. Even though his procedure proved inefficient (only one of 277 trials succeeded), it established the point beyond all doubt that cloning of adult animal cells can be done. In 1998 researchers succeeded in greatly improving the efficiency of cloning. Seizing upon the key idea in Wilmut’s experiment, to clone a resting-stage cell, they returned to the nuclear transplant procedure pioneered by Briggs and King, but used 800 resting-stage cumulus cells from a female mouse. From them they produced 10 healthy mouse clones, all identical to the female source, and, soon after, 7 clones of the clones.
Transgenic cloning can be expected to have a major impact on medicine as well as agriculture. Animals with human genes can be used to produce rare hormones. For example, sheep that have recently been genetically engineered to secrete a protein called alpha-1 antitrypsin (helpful in relieving the symptoms of cystic fibrosis) into their milk may be cloned, greatly cheapening the production of this expensive drug.
It is impossible not to speculate on the possibility of cloning a human. There is no reason to believe such an experiment would not work, but many reasons to question whether it should be done. Because much of Western thought is based on the concept of human individuality, we can expect the possibility of human cloning to engender considerable controversy.
Recent experiments have demonstrated the possibility of cloning differentiated mammalian tissue, opening the door for the first time to practical transgenic cloning of farm animals.
Figure 23. Wilmut’s animal cloning experiment. Wilmut combined a nucleus from a mammary cell and an egg cell (with its nucleus removed) to successfully clone a sheep.
