Read more: Instant Expert: Cloning
Artificial cloning of animals is one of the leading achievements of 20th-century biology, providing the means to create an organism that is an exact genetic copy of another 鈥� an identical twin. It is a far more complex business than the long-established techniques of cloning plants by taking a cutting such as a twig or stem and letting it take root 鈥� the origin of the term 鈥渃lone鈥�, from the Greek for twig.
There are now ways to clone all kinds of creatures so that the original animal and its clone share every scrap of DNA. Cloning has become a fundamental tool, with widespread uses in biology, from generating stem cells for medicine to breeding elite, genetically modified and endangered animals.
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Nature鈥檚 clones
It鈥檚 not always appreciated that cloning is commonplace in nature, taking place in creatures as diverse as bacteria, vertebrates, and sometimes even in human beings too.
A few days after a mammalian egg has been fertilised by a sperm, the resulting embryo has to 鈥渉atch鈥� from its rubbery covering, called the zona, so that it can implant in the wall of the uterus. Occasionally, the embryo will split and go on to develop into a pair of identical twins 鈥� each one a clone of the other.
This process of embryo fission was exploited in the earliest attempts to create an artificial clone.
Pioneers of twinning
The German philosopher and biologist Hans Driesch (1867-1941) was the first to create twins, back in 1891. He took a two-cell sea urchin embryo, shook it apart, and showed that each cell developed into a complete individual. This finding refuted the then prevalent idea of another eminent German biologist, August Weismann (1834-1914), who believed that if the cells from a two-cell embryo were separated, each could create only half a creature.
Driesch鈥檚 embryo-splitting experiments showed that for several generations after fertilisation, embryonic cells retain the potential to turn into any type of cell 鈥� from heart cells to egg or sperm cells, and even the huge variety that make up a whole individual. These cells, which are now called totipotent or omnipotent stem cells, have become the focus of attention in laboratories worldwide because they offer extraordinary medical opportunities for growing replacements for defective cells and tissues in the human body.
鈥淒riesch鈥檚 experiments showed that embryonic cells retain the potential to turn into any cell type鈥�
However, a key technical hurdle to the artificial twinning of animals remained for almost a century. It was only overcome in 1984 by Steen Willadsen, a Danish scientist working at the UK Agricultural Research Council鈥檚 Unit on Reproductive Physiology and Biochemistry in Cambridge.
The problem Willadsen faced was that when the protective zona of a mammalian embryo is damaged, the mother鈥檚 immune system destroys it. Willadsen prevented this when splitting sheep and cow embryos by protecting each half in a jelly-like casing made from seaweed. With Carole Fehilly, he went on to mix cells from embryos of different species to create a 鈥済eep鈥� (a goat-sheep chimera), with the aim of studying embryonic development.
The technique could also be used to help save endangered species. If a subspecies that is threatened with extinction has a related subspecies that is common, chimeras could be created that develop in the uterus of the common subspecies and yet produce the sperm and eggs of the endangered species and so boost the animal鈥檚 dwindling gene pool. But there are limits to the degree to which an early embryo can be split and thus the possible number of clones that can be produced this way. A powerful alternative that would overcome this limit was waiting in the wings.
Pioneers of nuclear transfer
A powerful cloning method called nuclear transfer was pioneered by the German embryologist Hans Spemann (1869-1941), a Nobel laureate and director of the Kaiser Wilhelm Institute of Biology in Berlin. After his death, a lock of hair from his baby daughter Margarete was found in an envelope, tucked in among his files for 1899, and baby hairs were key to his astonishing technique.
Peering down a microscope at hundreds of slippery, newly fertilised salamander eggs, Spemann used a loop of hair as a noose to constrict the egg into a dumb-bell shape, with the nucleus in one half and only cytoplasm and other cellular material in the other.
After the nucleated side had divided four times, creating a 16-cell embryo, Spemann loosened the hair and allowed one of the 16 nuclei to slip back into the empty half of the dumb-bell. Cell division now began on this side as well.
By tightening the loop again, Spemann broke apart the two embryos. The astonishing pay-off was twin salamanders, one moments younger than the other 鈥� the first in vitro animal clone produced by nuclear manipulation.
In the late 1950s, University of Oxford developmental biologist John Gurdon鈥檚 pioneering work on nuclear transfer in Xenopus frogs showed how the genome remains intact as an embryo develops and its cells become specialised 鈥� the process known as differentiation. His research made a splash when he took a spectacular photograph of 30 little albino frog clones, all developed from the cells of a single albino tadpole introduced into the eggs of normally pigmented frogs.
In 1986, Steen Willadsen stunned his peers with a paper in Nature reporting that he had used nuclear transfer to clone sheep. For this he used cells from early embryos.
The next year he produced live calves from 128-cell embryos, which already contain two cell types. This finding ran counter to the dogma of the day, which said cloning would not work with DNA from more specialised cells, but he did not present his work in public for several years.
Willadsen鈥檚 experiments provided some of the impetus for Dolly, the clone that finally overturned the idea that it was impossible to make adult cells embryonic again.