Anna Marie Eleanor Ross. Scientific Thought: In Context. Editor: K Lee Lerner & Brenda Wilmoth Lerner. Volume 1. Detroit: Gale, 2009.
On February 27, 1997, the cover of the journal Nature announced the birth of Dolly, an ewe (female sheep) cloned from an adult sheep in Scotland; the publication Science called Dolly the “breakthrough of the year.” Cloned mice, calves, and cats followed, while journalists, scientists, and politicians discussed the possibility of cloned human beings.
The term cloning can describe many different processes that duplicate biological material: natural twinning, reproductive cloning (which produced Dolly), and therapeutic cloning using stem cells. This article will explore the history of cloning, ranging from natural and artificial embryo twinning to the development of more recent technologies, such as somatic nuclear transfer. The often-controversial ethical and legal ramifications of cloning and its applications will also be discussed.
Historical Background and Scientific Foundations
Reproductive cloning is reproduction without fertilization, or asexual reproduction. Identical twins, triplets, and quadruplets are clones that result from the spontaneous natural split of the embryo, producing two babies with the same genome. (Fraternal twins occur when two fertilized eggs implant in the uterus at the same time and form two embryos. Because each egg is fertilized by a different sperm cell, the two have different genomes and are not clones.) In humans, about 1 in 80 births (1.2%) are the result of a twin pregnancy, although the rate of twinning varies greatly among ethnic groups.
Early Cloning Efforts
The theories that led to embryo splitting in the laboratory were formulated in the late nineteenth century, as biologists experimented with sex cells or germ cells to determine how cells become specialized during development.
In the late 1800s, German biologist August Weismann (1834-1914) created the germ-plasm theory. This was the idea that the body contains two sorts of cells: germ cells (egg and sperm), which can transmit hereditary information, and somatic (body) cells, which cannot. He also proposed in 1885 that in the process of cell division, daughter cells received half of their genetic material from each parent. Weismann’s theories gave rise to the field of developmental biology, which studies the genetic control of embryonic development. He also believed (in a now-disproved theory) that cell differentiation (specialization of cells into different body parts and functions) resulted from the partitioning of the genome as cells divide.
In 1888 German zoologist Wilhelm Roux (1850-1924) tested Weismann’s ideas in a series of experiments. He killed one cell of a two-cell frog embryo by pricking it with a hot needle, the allowing the remaining cell to develop. This produced only a half-embryo, suggesting some crucial genetic information was lost during cell division. These results seemed to support Weismann’s partitioning theory.
This changed in 1892, however, when German experimental embryologist Hans Driesch (1867-1941) obtained some experimental results that challenged Weismann’s principles of partitioning. Driesch found that if the cells of two-cell sea urchin embryos were physically separated, entire embryos formed from each cell. This suggested that the genetic material was not partitioned between cells at this stage of embryonic development, but reproduced during cell division.
The Work of Hans Spemann
The next step in scientific understanding of genetics and cell differentiation came with the work of Hans Spemann (1869-1941) a German embryologist. He attended the University of Heidelberg and earned his doctorate at the University of Würzberg. During the winter of 1886 he read Weismann’s book The Germ Plasm: A Theory of Heredity as he recovered from a bout of tuberculosis, and his interest in embryology was sparked.
Taking Weismann’s work as his inspiration, Spemann conducted a series of experiments on salamander eggs in 1902. By using a strand of his daughter Margrette’s baby hair as a noose, he successfully split the cells of a two-celled salamander embryo and obtained a normal salamander from each individual cell. His work proved definitively that both cells contained enough genetic information to create another salamander, invalidating Weismann’s partitioning theory. Spemann’s baby-hair noose imitated nature’s own cloning methods, as when a fertilized egg divides to create identical twins, triplets, or quadruplets.
Spemann then did a series of experiments in which he constricted the egg with the hair noose without separating the 2-cell embryo completely. This produced salamanders with two heads and one trunk and tail. He stated, “Such animals came to the stage of feeding and it was now most remarkable to see how once the one head and at another time the other caught a small crustacean, how then the food moved through the separate foreguts to the joint posterior intestine.” Though Spemann was understandably intrigued by the results of his experiments, his work was compared to that of the fictional Dr. Frankenstein in the German press.
In the late 1920s Spemann continued his work with salamanders, constricting a zygote with another baby-hair noose, causing the cell to assume the shape of a dumbbell. One side had all the nuclei, and the other side only cytoplasm. When the nucleated part reached the 16-cell stage, he loosened the constriction and allowed a nucleus from the embryo to slip into the clear part of the egg. The cell took up the nucleus and developed into a normal salamander. With this process, Spemann completed one of the first cloning experiments using the nuclear transfer method. This experiment also confirmed the idea that the complete genome is replicated during cell division, at least during the early stages.
Spemann then wanted to see at which stage older embryonic cells lost their ability to have their genome replicated or cloned. In other words, at what stage did embryonic cells lose their totipotency—their ability to make a complete embryo. His attempts at dividing zygotes beyond the 16-cell stage did not result in normal embryonic development, so he concluded that beyond this stage, cells had their fates “determined.” In other words, beyond the 16-cell stage, cells were programmed to produce different types of cells.
In his 1936 publication Experimentelle Beiträge zu einer Theorie der Entwicklung (Embryonic development and induction), Spemann proposed a “fantastical experiment” of cloning an organism from differentiated or even adult cells using his nuclear transfer method. Spemann, however, did not have the technology to transfer the nuclei from older embryonic cells into those whose nuclei had been removed (enucleated). It was not until the work of Thomas King (1921-2000) and Robert Briggs (1911-1983) at the Fox Chase Cancer Center in Philadelphia in the 1940s and 1950s that it was possible to perform the “fantastical experiment” Spemann had envisioned.
The Work of Briggs and King
Briggs conducted a cloning experiment to study the activation and deactivation of genes during cell development using the North American leopard frog, Rana pipiens, whose large eggs were easily manipulated. He intended to transfer the nucleus of a cell from a blastula (an early embryo that consists of around 8,000 to 16,000 cells), into a fertilized, enucleated egg. King had removed the nucleus of a frog egg in 1949 using microsurgical techniques. Both researchers realized that an egg from which the nucleus was removed lacked a genome and could only develop partially. This meant that further development of the embryo could be started again by the introducing another cell nucleus.
In the next stage of their experiments, Briggs and King pricked a frog egg cell with a needle, imitating the changes that occur naturally in the cell when a sperm pierces the egg surface. After 15 minutes, a black dot appeared on the egg’s surface; these were the egg’s chromosomes, which were known at the time to contain genetic material. The black dot was removed with a glass needle, essentially enucleating the egg. A donor blastula cell was then isolated, and Briggs and King gently broke its cell membrane. The broken cell was transferred to the enucleated egg, and the two cells combined, completing the nuclear transfer.
Briggs and King showed that tadpoles developed from enucleated eggs injected with blastula nuclei, a result that was confirmed by other investigators. In other words, blastula nuclei were totipotent. By the project’s completion Briggs and King had successfully cloned 35 complete embryos and 27 tadpoles from 104 successful nuclear transfers.
Continuing their research, Briggs and King attempted to clone tadpoles using the nuclei of older embryo cells. But they found as the embryo cells developed it became much more difficult to produce clones from them. The few tadpoles cloned from differentiated cells that survived to become tadpoles grew abnormally. Though neither investigator knew it at the time, this amphibian produced by nuclear transplantation became the prototype for cloning animals. The abnormal growth of the clones had also demonstrated some of the potential problems of cloning technology.
Reproductive Cloning Using the Nuclei of Differentiated Cells
In 1962 British developmental biologist John Gurdon (1933-) at Oxford University announced that he had cloned South African clawed frogs (Xenopus laevis). Exposing a frog egg to ultraviolet (UV) light destroyed its nucleus. Gurdon then used microsurgical techniques to remove the nucleus from a tadpole intestinal cell and put it into the enucleated egg. The egg grew into a tadpole that was identical to the tadpole that donated its nucleus and DNA.
Gurdon knew this was the first time that the nucleus of a determined or differentiated cell had been proved totipotent, and his work helped establish techniques for future nuclear transplant experiments. But researcher Dennis Smith was skeptical of Gurdon’s results, even though they had electrified the scientific community. Smith believed there were some undifferentiated sex cells in the tadpole intestines, and the tadpoles may have been cloned from these cells.
In the face of Smith’s criticism, Gurdon produced still more frogs from differentiated cells. These later experiments silenced dissenters, and the techniques he used for nuclear transfer are standard today. In fact, in a 1963 speech entitled “Biological Possibilities for the Human Species of the Next Ten-Thousand Years,” British biologist J.B.S. Haldane (1892-1964) first used the word “clone” (from the Greek word meaning “twig”—as in horticulture, when a twig or branch is used to create a new plant) to describe Gurdon’s work. Gurdon had firmly established that a cell’s genetic potential did not diminish as the cell became differentiated.
Continuing Controversy: The Work of Karl Illmensee
Though amphibians could be cloned with nuclear transfer, mammals were a different matter. Mammalian eggs are very small—less than 0.1% the volume of amphibian eggs, and microsurgical techniques that could isolate, enucleate, and fuse a mammalian egg with a single somatic cell were not developed until the late 1960s and early 1970s. The second difficulty was the nature of cell differentiation in mammals. Though Gurdon had firmly established that a frog cell’s genetic potential did not diminish as the cell became differentiated, was this true for mammalian cells as well?
In 1979 Austrian biologist Karl Illmensee (1939-) claimed to have cloned three mice via nuclear transfer at the Jackson Laboratory in Maine. His work at first was hailed as a breakthrough, since Illmensee was known for his skill in micromanipulation. By utilizing a four-day-old mouse embryo cell, he claimed he crushed the outer parts of the cell by sucking it into a pipette, but left the nucleus intact. Then he injected the nucleus into a fertilized mouse egg. In this way, the original genetic material of the mouse egg was removed and the cell was grown into an adult mouse.
Though his microsurgical skills were well known, the next year Illmensee’s work was questioned when he refused to demonstrate this nuclear transfer technique. His results also could not be verified by other scientists working in his lab. Because peer review and verification are essential to the scientific method, a commission in 1983 decided that Illmensee’s work was “scientifically worthless.” To this day, it is unclear if Illmenssee did successfully clone three mice in 1979, but no one thus far has been able to reproduce his results.
Modern Cultural Connections
Despite Illmensee’s failure to demonstrate somatic cell nuclear transfer, there was much public interest in cloning technology. In 1978 writer David Rorviks’s book In His Image: The Cloning of a Man became a best seller—it told the story of an American millionaire, known to readers only as Max, who secretly had himself cloned. Although the publisher admitted in 1982 that the book was a hoax, Congressional hearings on cloning were held as a result of the ethical and legal implications that the book raised, and one scientist whose work was referenced sued the publisher, who was forced to pay damages to him. The hoax, combined with the questionable experimental results of Illmensee, led many scientists to claim that mammalian cloning in the laboratory was impossible.
Fiction writers’ fascination with cloning continued unabated. The movie The Boys from Brazil, adapted from American novelist Ira Levin’s (1929-2007) 1976 novel, tells the story of a generation of boys cloned from Hitler’s DNA. American author Michael Crichton’s (1942-) novel (1990) and movie (1993) Jurassic Park imagines the ultimately disastrous recreation of dinosaurs for a theme park.
Although mammalian cloning seemed impossible, in 1984 Steen Willadsen (1944-), a Danish scientist working in England, cloned a sheep from embryo cells, and American geneticist Neal First (1930-) cloned a cow embryo. Working independently from each other, both researchers invented new procedures that led to higher survival rates among cultured embryo cells.
Willadsen also used a technique called electrofusion, in which an electric current fused cells of 8- or 16-cell embryos into enucleated eggs, producing healthy cloned animals. Previous studies had found that in mammals, different genes are repressed during egg and sperm development and division, and that normal embryo development requires a genetic contribution from both the male and female genomes. Thus, by using nuclear transfer with embryonic donor cell nuclei, more healthy adults were produced. In the mid 1980s through the mid 1990s, rabbits, pigs, mice, cows, and monkeys were cloned with embryonic cell nuclei. A profitable business developed in duplicating embryos of prize cattle, as the process was more cost-effective than artificial insemination.
At this point, successful cloning in mammals involved the use of totipotent cells. Attempts to clone an adult mammal by nuclear transfer from differentiated cells had failed—until the work of two English scientists: embryologist Ian Wilmut (1944-) and biologist Keith Campbell (1954-) at the Roslin Institute in Scotland in 1996 and 1997.
In 1996 Wilmut and Campell cloned a sheep called “Dolly” from a single cell taken from the mammary cells of a six-year-old ewe’s udder. (She was named after the entertainer and country-western singer Dolly Parton, who said that she was honored and that there is no such thing as “baaaaad publicity.”) Dolly was the first mammal ever created from the nonreproductive tissue of an adult animal. First, Wilmut and Campbell extracted unfertilized egg cells (oocytes) from the ovaries of ewes and ennucleated them, leaving empty, DNA-free oocytes. Next, using electro-fusion, they fused the empty oocyte with the mammary cell. The current triggered the egg to start dividing, and the donor nucleus directed the development of the egg cell. After growing and dividing for a week or so in a laboratory culture dish, the fused cell formed an early embryo, which Wilmut’s team implanted into a surrogate mother. Five months later, Dolly was born.
What made Wilmut and Campbell successful? Primarily their research on cell cycles. Each cell goes through a series of stages in its life and development, including times when it reproduces and divides called the cell cycle. Egg cells in particular undergo a state of suspended animation after fertilization while they coordinate their DNA with those of the sperm. This stage, called gap zero or G0, can be induced artificially by depriving cells of nutrients, forcing them into a quiescent state. Scientists theorize that when starving cells interrupt their cell cycle and fall into a resting state, the genes in their nuclei are more responsive to the instructions of the egg into which they were introduced. In other words, the donor cells’ nuclei are reprogrammed.
Wilmut and Campbell found that synchronizing the cell cycle between the donor cell and the empty oocyte led to a greater chance of normal development, although they do not yet know why coordinating cell cycles has this effect. By depriving the donor mammary cell of nutrients and enucleating the egg right after it was fertilized with sperm, they ensured both donor and recipient cell were at G0.
Though Wilmut and Campbell were ultimately successful, Dolly was only produced after 277 unsuccessful attempts. The high failure rate of somatic cell nuclear transfer (SCNT) was and is a concern to other researchers. Although the success rate has improved since Dolly (improving in cattle, for example, from 1 to 20%) cloned animals often possess developmental abnormalities called “cloning syndrome.” Lower survival rates at birth are common, as is prolonged gestation (delayed birth) and higher birth weight.
Dolly gave birth to normal offspring in 1998, a lamb called Bonnie, but she also had a shorter-than-typical life span and problems with arthritis. One characteristic of mammalian aging is the steady loss of sections of DNA, called telomeres, from the ends of chromosomes. At each cell division, a bit of telomere is lost. Because Dolly was derived from an older cell, some researchers have suggested that she might have shorter telomeres that led to her premature death. As it stands, the reasons behind “cloning syndrome” are not yet well understood, but genetic error or cellular aging are likely explanations.
Potential Applications of Cloning
Despite these concerns, Wilmut and Campbell’s cloning breakthrough was an exhilarating scientific success with tremendous commercial value, particularly in animal agriculture and human medicine. Somatic cell nuclear transfer technology has been used to clone cattle, pigs, and goats as well as sheep, all of which are important food-producing animals. ABS Global of De-Forest, Wisconsin, announced in August 1997 that it had developed a cattle-cloning technique that prevents cloning syndrome, showing off its first success, a calf appropriately named “Gene.” ABS claimed a success rate of 80%, much higher than the 1:277 ratio that produced Dolly.
SCNT has been proposed as a way to restore endangered and extinct species, clone dearly loved dead pets, and to investigate the role of the cell nucleus and cytoplasm in embryonic development. Genetic Savings and Clone, a company in Sausalito, California, produced the first clone of a domestic cat in December 2001 and delivered the first of two cloned-to-order cats in 2005. The company went out of business a year later, however, because almost no one was willing to pay the $50,000 cost of cloning an animal.
Cloning for Reproduction
In theory SCNT could be used for reproduction, with the cloned embryo implanted into a womb and brought to term. This would enable infertile couples or single parents to have genetically related children. Reproductive cloning would also enable people who carry x-linked and autosomal recessive disorders (such as forms of muscular dystrophy) to have children without passing on the disease.
Wilmut’s original goal was a transgenic clone—an animal that contains a human gene. By linking a gene for human clotting factor or growth hormone with one that controls lactation, for example, the desired substance would be secreted in the cloned animal’s milk, from which it could be extracted.
The first transgenic clones were sheep, Polly and Molly, born in 1997. They carried the genetic sequences for human clotting factor IX (used to treat hemophilia), which was secreted in their milk. To clone these sheep, Schnieke’s research team grew sheep fetal cells in culture and inserted into them the human gene for clotting factor IX, as well as the sequence to direct the gene to function only in the mammary gland. The nucleus from the fetal cells was transferred to enucleated eggs, the egg was put into a surrogate mother, and Polly and Molly were born.
Therapeutic Cloning and Stem-Cell Research
Another potential application of SCNT is therapeutic cloning using stem cells. Embryonic stem cells are totipotent, able to differentiate in all human tissues. Scientists hope that embryonic stem cells may eventually be used to repair or rebuild damaged organs or systems, and that treatment could be tailored to each recipient via cloning. Because the cloned tissue would be genetically identical to the patient, the problem of transplant rejection would be eliminated.
In this process, the nucleus of a cell from the patient would be transferred into a human embryo that had been ennucleated. The embryo would be grown to the 8- or 16-cell stage, after which its stem cells would be harvested and cultured for transplantation. Researchers hope that embryonic stem cells might be used some day to treat neurodegenerative conditions like Parkinson’s disease; therapies for some types of cancer, diabetes, and osteoporosis may also be possible. Scientists are also exploring the use of embryonic stem cells to create replacement human organs, including skin and cartilage.
Stem-cell research continues to be a controversial issue, however, because the process destroys viable human embryos. Despite these concerns, on January 22, 2001, the United Kingdom became the first country to approve government-funded human embryonic stem cell research with “adequate safeguards,” limiting research to infertility and other specific situations.
Interesting as the prospects of cloning may be, its ethical implications were and have been of great concern to many. If a sheep could be cloned, were humans next? Were we playing God? What would our kinship relationships be to our clone? Would “cloning syndrome” make human cloning too dangerous to contemplate?
Shortly after Dolly’s birth was announced, in March 1997 President Clinton (1946-) issued a moratorium that halted federal funding for any research involving human cloning, stating “There is virtually unanimous consensus in the scientific and medical communities that attempting to use these cloning techniques to actually clone a human being is untested and unsafe and morally unacceptable.” In June 1997 the National Bioethics Advisory Commission, appointed by President Clinton in 1995, proposed legislation to prohibit human cloning. Like other such bills before and after it, however, this one was defeated in the legislature.
Thousands of scientists around the world, however, have voluntarily agreed not to pursue human cloning. The European Parliament and the Council of Europe have also banned human cloning to protect the genetic identity and dignity of human beings. The Council of Europe, which represents governments of 40 European nations, stated “Any intervention seeking to create a human being genetically identical to another human being, living or dead, is prohibited.”
In May 1997, an international religious cult called the Raelians announced it was creating a company called Clonaid to sponsor “research” in human cloning. By 2001 the company claimed to have cloned a human embryo, a baby called Eve, who was supposedly born in 2002, but Clonaid refused to allow genetic tests to prove its case. The company’s CEO promised to deliver proof of the successful clone in 2003, but produced only a photograph. Scientists dismissed the cult’s claims to have produced several other living clones as ludicrous, and the federal government began to investigate the group for fraud, because they had purportedly taken $200,000 from an American couple who wanted to clone their son, an infant who had died from congenital heart defects.
In 1998, Richard Seed, a physicist from Chicago, announced his plans to perform human cloning experiments before Congress enacted a ban on cloning. Although he held Ph.D. from Harvard University in nuclear physics, Seed was no stranger to reproductive technology. He founded a company in the 1960s to transfer embryos from prize cows to surrogate mothers, and in the 1980s created the firm Fertility and Genetics to transfer fertilized eggs from healthy women to those with fertility problems. His work attracted professional interest, and was published in the Journal of the American Medical Association as well as the Lancet.
Societal preference for in-vitro fertilization led to the failure of his business venture, so Seed established a human cloning clinic, remarking that if Congress banned human cloning in the United States he would move his venture to the Caribbean or Mexico. Indeed, shortly after Seed’s announcement, in late January 1998, the Food and Drug Administration announced its authority to regulate human cloning, making it a violation of federal law to attempt somatic cell transfer without approval.
Seed’s attitude reflects the combination of scientific prestige and financial reward that awaits a researcher who successfully clones humans. Jon Gordon, a cloning expert from Mount Sinai Hospital in New York City, stated “You can make cloning against the law but I think people will try anyway. They’ll do it because if they do, they’ll never be forgotten.”
After the initial reaction to the birth of Dolly and the ban on human cloning, the continued success and potential of therapeutic cloning continued to make the issue more ethically complex. Wilmut himself plans to attempt human cloning, arguing that “stem cells taken from cloned human embryos could be of enormous benefit for medical research, as well as providing a means of treating disease.” Most controversially, he argues that cloning techniques could be combined with genetic engineering to cure hereditary diseases. The potential benefits are so great, Wilmut says, that “it would be immoral not to do it.”
The lure of scientific fame drew hucksters, too. In February 2004, research teams at Seoul National University in South Korea, led by Woo Suk Hwang (1953-) claimed in the journal Science to have derived stem cells from a cloned human embryo. (South Korea permits cloning research while banning it for reproductive purposes.) A year later Hwang claimed to have created the world’s first cloned dog, Snuppy, an Afghan hound born to a yellow Labrador surrogate mother. (Snuppy’s name was a blend of “Seoul National University” and “puppy.”) Unfortunately, Dr. Hwang’s work was quickly declared fraudulent, he was expelled from the university, and indicted for embezzlement and bioethics law violations.
Therapeutic cloning’s potential for new medical therapies and lobbying by the scientific community has frustrated efforts to impose legal bans on human cloning. The United Nations adopted a declaration condemning human cloning, passed by 84 votes to 34, with 37 abstentions. The statement prohibits “all forms of human cloning inasmuch as they are incompatible with human dignity and the protection of human life.” Despite the significant differences between therapeutic and reproductive cloning, the overall concept is still very much a political and ethical controversy.