Clone (cell biology)

The process of generating identical individuals from a single organism is known as cloning. The resulting identical individual, called a clone, is presumed to carry exactly the same type of cells, tissues, and even genetic material. This biotechnological approach ensures that a specific livestock or agricultural crop is generated with the same features and quality, which in turn facilitates in continuous economic income for farmers and other agricultural producers.

In 1996, the first animal that was derived from an adult cell was presented to the scientific world and the general public. Despite the excitement associated with this biotechnological breakthrough, there was also extensive scrutiny and debate regarding this controversial news. First, scientists were doubtful whether this animal clone would address the need to increase the yield of this particular livestock. Second, there was also concern whether this animal would develop into an organism identical to that of its origin.

Background

A clone pertains to an organism that originated and is 100% similar to a single common ancestor. This biotechnological approach entails the generation of similar copies of an organism via asexual reproduction. Various fruits and vegetables that are commonly sold in the market, including apples, bananas, and potatoes, are clones. Furthermore, cloned livestock has served as a major component of animal agriculture in the past two decades. Two distinct methods have been employed in cloning animals: mechanical embryo splitting and nuclear transfer.

The cloning technique of embryo splitting pertains to the separation of a multicellular embryo early on during its development to produce clones, or simply called twins. For example, a 32-cell embryo could be split into two 16-cell clones or twins. Embryo splitting may occur naturally, such as that observed in human identical twins. However, it is important to understand that fraternal twins do not emerge via embryo splitting, but result from two separate eggs that were simultaneously fertilized by two independent sperms. Fraternal twins are the result of a single pregnancy yet may be different genders.

Embryo splitting may also be performed in the laboratory using various types of animal species. The first attempt to generate a clone through embryo splitting was conducted in the 1980s with dairy animals. To date, more than two thousand split clones have been registered in the United States alone. However, the application of this technique to cattle and sheep has been limited to a maximum of four clones from a single embryo.

Nuclear transfer involves the relocation of DNA from one nucleus that was derived from a single cell into an unfertilized egg that had its own nucleus removed by using a procedure known as enucleation. The introduced nucleus is thus designated as the donor, whereas the nucleus-free unfertilized egg in considered the recipient. In order to initiate the process of development, the donor nucleus is stimulated to fuse with the recipient egg through the application of a short electrical pulse or by adding a specific chemical that results in the fusion process. This stimulation then results in the division of the embryo, similar to that occurring in a fertilized egg. In the case of nuclear transfer involving mammals, the division-induced embryo is received by a surrogate mother and continues to develop until birth. Upon completion of embryonic development in the surrogate mother, the developed organism is then delivered as with any newborn.

The first mammalian clone created using nuclear transfer was generated in the early 1980s. This experiment was preceded approximately thirty years earlier by nuclear transfers using frogs. Upon the success of the first mammalian clone, several other mammalian species were cloned, which included mice, pigs, sheep, and monkeys. By 1990, around fifteen hundred cattle were generated via nuclear transfer alone. These clones were produced by transferring the nuclei of early-stage embryos, namely those consisting of eight to thirty-two cells. Numerous other animals were cloned throughout the late twentieth and early twenty-first century. These included rats, cats, horses, dogs, and wolves. Though scientists continued to hope to use cloning to revive extinct species, no successful attempts were carried out.

Impact

The first mammal that was cloned using the nuclear transfer technique was the sheep named Dolly. This embryo was then allowed to develop until adulthood. One unique characteristic of the nuclear transfer experiment that generated Dolly was that it involved the use of a somatic cell that originated from an adult sheep. This form of nuclear transfer using a somatic cell allows the production of an unlimited number of cells that derived from an adult mammal that possessed specific agriculturally important and superior traits. This approach facilitated the establishment of elite animals that were used in breeding based on their recorded performance, as well as on the characteristics of its offspring.

One of the major obstacles that has been associated with nuclear transfer using somatic cells is that its success rate in generating live offspring is relatively very low. The majority of these failed cases were due to pregnancy losses that occurred after the transfer of an egg that contained an adult cell nuclei into the enucleated recipient cell. An average of 9% of these nuclear transfer assays have actually resulted in live calves, and the efficiency of this cloning technique varied between 0% and 45%, as it largely depended on the type of somatic cell that was used as donor. Similar problems of very low success rates have also been observed in other cloning techniques, such as embryo transfer as well as in vitro fertilization. Interestingly, these pregnancy losses are also observed in natural matings; therefore, there is currently a need to further examine the factors that influence the occurrence of these negative events.

Another issue that has arisen in relation to cloning via nuclear transfer involves the development of various abnormalities in the resulting offspring. These physical abnormalities include a condition wherein the cloned animals are significantly large at birth, as well as placental disorders, edema, and fetal death. These abnormalities have also been related to the type of somatic cells used in the nuclear transfer experiment. Reports have also shown that only around 40% of cloned calves survive the first 150 days after delivery. However, those offspring that survive the first 150 days show normal patterns and rates of breeding, thus suggesting that there is a need to further investigate the mechanisms responsible for this cloning-related mortality.

Bibliography

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