More recently, there has been a major breakthrough in the animal nuclear transfer procedure. With Dolly, the famous sheep, cells were harvested from the mammary gland (adult cells) of a mature ewe for cloning. These cells were multiplied to produce a larger cell population for the cloning procedure. The production of cloned sheep was important because Dolly was the first mammal ever produced from an adult differentiated body cell (somatic cell).
The cloning of an adult sheep, reported in 1997, stimulated strong interest in nuclear transfer technology by the livestock industry. To construct cloned embryos with this new approach, a somatic cell is taken from a developing fetus or an adult animal (male or female) and microsurgically transferred into an unfertilized oocyte from which the nuclear DNA has been removed (enucleation). The enucleated oocyte with a newly introduced donor cell becomes activated, and then the reprogrammed nucleus directs embryonic development into a cloned embryo for recipient transplantation. Once the donor cell population has been prepared, hundreds of cloned embryos can be produced weekly in the laboratory, using oocytes extracted from abattoir ovaries.
The potential for the use of this new technology has left the world in amazement. Somatic cell clones have been produced in mice, rabbits, cats, sheep, goats, swine, domestic beef and dairy cattle, as well as exotic cattle and sheep. Most recently, cloned mules and a cloned horse have been produced. Cloning would provide the cattle producer an opportunity to reproduce valuable seedstock, clone animals that have suffered an injury and can no longer reproduce, or clone males that had been castrated. Cloning technology would also provide livestock producers with ready access to production-tested breeding stock, thus increasing the accuracy of selection in their breeding herds.
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