Stem Cells and the Experiment That Shook the World

The development of cell lines that may produce almost every tissue of the human body is an unprecedented scientific breakthrough. It is not too unrealistic to say that this research has the potential to revolutionize the practice of medicine and improve the quality and length of life.¹

Former NIH Director and Nobel Prize Winner Harold Varmus

In the November 6, 1998, issue of the journal Science, James Thomson, a professor at the Wisconsin Regional Primate Research Center at the University of Wisconsin, reported he had developed the first line of human embryonic stem cells. Penned in the typical understatement of research writing, the abstract of the research report declares, "These cell lines should be useful in human developmental biology, drug discovery, and transplantation medicine."² Depending on one's philosophical bent, the implications of this statement were momentous or disastrous. An incredibly potent human cell was alive and well, living in an incubator in James Thomson's laboratory.

Only three pages long, the Thomson paper is packed with information and data. He describes how he obtained human embryos from a local in vitro fertilization (IVF) clinic. Couples were given the option of donating extra embryos for research purposes they were left over from the IVF procedure. They arrived packed in ice, frozen just days after fertilization in a laboratory dish. Visible only under a microscope, each embryo contained about eight cells surrounded by a very thin membrane resembling a diaphanous sac with a cluster of soap bubbles inside. Thomson placed the transparent orbs into culture dishes with carefully prepared nutrients and grew them into blastocysts, hollow spheres of about 100 cells, as shown in the figure. At this stage the embryo is between four and five days old and scarcely a tenth of a millimeter across, about twice the diameter of a human hair. Inside the cavity of the blastocyst is a mound of cells called the inner cell mass, or ICM. With a microscope, a steady hand, and a very thin, hollow glass needle, Thomson removed the clump of cells from inside the sphere and placed them in a laboratory culture dish.

Figure 1.1

Now came the difficult part: how to ensure the cells would live and thrive in the laboratory. Cell culture, the process by which biologists grow living cells in a plastic culture dish, is a tricky and time-consuming business. Cells must grow in a sterile environment, or airborne contamination will ruin the experiments. Getting the growth nutrients just right is another hurdle. The conditions in a cell culture dish (or in vitro from the Latin in glass) must essentially replicate the environment of a cell growing and dividing inside the body (or in vivo from the Latin in life). Other challenges include maintaining the right temperature, the right oxygen and carbon dioxide concentrations, and deciding when to change and refresh the growth nutrients. With some trial and error, Thomson's cells began to multiply. Fortunately, they also had the staying power to persist in this artificial environment for extended periods of time. In fact, when his paper was published, Thomson's cells were still robust and had survived and multiplied for eight months. The longevity of his cell lines was the first of three groundbreaking results reported in his Science paper.

The second result was just as crucial. The cells demonstrated no ill effects from living in their laboratory environment. Cells proliferate by dividing in two. Sometimes artificial conditions cause cells to divide incompletely, or not at all, or such conditions result in abnormal numbers of chromosomes, which are the structures that contain the cell's genes. The ingredients in the culture dish can also have ill effects on the genes themselves, causing the cell to change physical characteristics or to age prematurely and die. It was important that Thomson's cells remain consistent in type and function, even after dividing many times over many months, so they could be used reliably in future experiments.

If an apparently consistent culture of cells dividing rapidly for many months was this experiment's "lightning," the third result was its thunderclap. With careful manipulation, laboratory technicians removed the human cells from the culture dishes and injected them into experimental mice. These mice are engineered to lack an immune system so they do not reject the human cells. Once in the mice, the human cells divided rapidly and formed tumor-like structures made up of all the major human tissue types, including skin, muscle, and bone. The cells bore the unmistakable imprimatur of embryonic stem cells”next to the fertilized egg itself, the most powerful cells in the body. The thunderclap then was what Thomson showed: stem cells can be coaxed into becoming any tissue type in the human body.

The scientific and medical implications contained in this short paper are profound and unambiguous. Embryonic stem cells could be used to generate new tissue and organs for transplantations. Defective and dying tissues caused by diseases such as Parkinson's or diabetes could be replaced with an unlimited supply of specially grown stem cells. Cultures of human stem cells could be used as laboratory tools to help identify new drugs and therapies. For pure scientists like Thomson, observing stem cells in the laboratory could provide insights into how all animals embark upon the magnificent developmental process that begins with a single cell.

A Recipe for Success

When the 1998 Science article rolled off the press, James Thomson was just 38. Originally trained as a biophysicist, he had a doctorate in veterinary medicine and a Ph.D. in molecular biology. For any research biologist, a Science publication is a significant achievement. But it was far from Thomson's first paper. He already had twenty others listed on his resume.

Thomson started his graduate career at Pennsylvania's Wistar Institute in the mid-1980s under the wing of an early pioneer of developmental biology, Davor Solter. Known for his work on mouse embryology, Solter took a blastocyst from the animal's uterus, teased it apart, and placed the cells of the ICM in special culture conditions that allowed them to survive and multiply. What Thomson learned from his training with Solter was pivotal to his later years of research. The recipe for a specially designed laboratory "soup" (or in the parlance of biologists, the medium) into which embryonic cells are placed to grow and divide means the difference between cells that thrive and cells that wither. Through trial and error, Solter perfected the ingredients in his mouse embryonic cell medium it worked so well that the basic recipe is still widely used today.

During the interim period between his graduate thesis defense and the next stage of his training (called a postdoctoral fellowship), Thomson continued his work with mouse embryonic stem cells. At the Roche Institute of Molecular Biology, he teamed up with stem cell expert Collin Stewart. At that time, embryologists were trying unsuccessfully” to grow human embryonic cells using what they had learned from mouse embryonic cells. During lunch with Stewart in 1988, Thomson had an epiphany. "Collin told me about people in Britain who had attempted to derive human embryonic stem cells but had failed," he recalls.³ "The problem became obvious. If you compare a mouse embryo to a human embryo, they are as different as night and day. Even some of the molecules that control the embryo's development in the mouse are different or missing entirely in humans."⁴ He reasoned that if he could work out the cell culture recipe on a species closely related to humans, he would be one step closer to solving the scientific hurdles blocking human embryonic stem cell research.

At the tender age of 30, Thomson did just that. He went off to do his fellowship at the Oregon Regional Primate Center, which was at that time the best training ground for primate biologists. While there, he perfected his cell culture techniques and, in 1991, he was recruited to the University of Wisconsin to work on monkey embryonic stem cells. Four years later, he derived the first primate embryonic stem cell line and, in 1995, published his research in the Proceedings of the National Academy of Sciences of the USA.⁵ As the paper went to press, Thomson held his breath. "After the monkey research was published, I fully expected other labs to use our methods to do the same thing with human cells," he said. "But no one did." Two years later, Thomson figured he would try it himself. "It was surprisingly easy," he recalls. "We had worked out most of the techniques already." He paused for a moment. "You know, there is a certain amount of finesse to growing the cells of this type, and most of our failures came with the monkey stem cells. It was worth the time: the very first human stem cell we isolated gave us a cell line!"

Although the human health implications of a line of human stem cells were not lost on him, Thomson focused on the mechanics of an animal's development: how genes orchestrate the process, what chemical signals are involved, and how the combination leads to organized structures such as skin and bone. He knew that a system to grow embryonic stem cells would be used as a standard tool for other biologists, and as a result, the entire field would benefit. Ted Golos, a fellow faculty member at the University of Wisconsin and collaborator on the monkey research, describes Thomson as a "how things work" kind of scientist. Golos says, "It can be dangerous if your interests don't have immediate benefit to solving a human disease because the government sometimes doesn't fund how things work kind of projects."⁶

Advances from the reproductive biology field aided Thomson's success with human cells. After fertilizing a human egg in a test tube, an IVF clinician incubated it for a brief amount of time before placing it back into the mother. Early procedures met with limited success. The cell culture medium was unable to mature the fertilized egg to an age where it could "take on" the environment of the uterus and survive. As a result, doctors transferred embryos too early, resulting in what Thomson calls "developmental mismatches." By the mid 1990s, the culture medium had improved markedly, along with the rate of successful IVF pregnancies. Coincidentally, when Thomson switched to human embryonic stem cell research, the new media became available. Then Thomson recruited a postdoctoral fellow who had trained with the inventor of the new medium and adapted it to his own methods.

Things Heat Up

Thomson and his colleagues, like most research biologists, are part of an international network of scientists working in universities, research institutes, and corporations. Since 1945 American universities with biological and medical sciences programs have benefited from the bounty of the Department of Health and Human Services (HHS) and its biggest agency, the National Institutes of Health (NIH). It is a foregone conclusion that without the NIH, the National Science Foundation, the Department of Energy, and other government agency funding, Americans would not enjoy some of the best medical care in the world. Although the brute force of government spending on biological science hasn't always yielded immediate results, by most measures, America has benefited greatly from the investment. Americans have access to a powerful repository of drugs, therapies, and medical device a dizzying array of technology designed to propel us into a happy and healthy old age.

But not all has been congenial between biomedical scientists and their funding agencies. Presidents and Congresses, both liberals and conservatives, have used their authority to guide, redirect, and limit funds. In this aspect, the fate of science funding is no different than funding for interstate highway systems, municipal police departments, or the National Endowment for the Arts. As it turned out, James Thomson and other human embryologists did their work not with government resources but with private funds. Why? Because government support of research using human embryos has been banned by Congress for decades. The controversy began in the late 1970s with the advent of IVF and the spare embryos generated by the procedure. Most proponents of biomedical research hold that it is morally permissible, even morally required, to use the extra embryos for potentially life-saving biomedical research. Opponents object, saying that the destruction of any embryo is the moral equivalent of killing a human life.

Soon after the Thomson paper was published, the NIH, recognizing the potential of human embryonic stem cell research, sought to lift the congressional ban, and NIH director Harold Varmus said he would draft guidelines regulating the use of embryonic cells. In 1999, President Clinton asked for a review of the matter by his ethics experts, and they concluded that the federal government should fund research provided that only embryos left over from fertility treatments be used. The recommendations clearly stated that the parents must have donated the embryos expressly for the research and that the IVF clinics must not profit from the exchange.

That year, Science proclaimed the development of human stem cell lines as the most important advance of the year. Cn its annual top ten list, it said, "In just one short year, stem cells have shown promise for treating a dizzying variety of human diseases." Similar reports followed from the major media outlets. CNN breathlessly reported, "Researchers isolate human stem cells in the lab; breakthrough could lead to treatments for paralysis, diabetes."⁷ Amidst the commotion, however, were growing criticisms and warnings from religious and moral leaders. The National Council of Catholic Bishops protested, calling the White House policy to allow the use of otherwise-discarded early embryos "guidelines on how to ethically destroy human life."⁸ Pope John Paul II weighed in, calling stem cell research an "accommodation and acquiescence in the face of other related evils, such as euthanasia [and] infanticide." He went on to say, "A free and virtuous society, which America aspires to be, must reject practices that devalue and violate human life at any stage from conception until natural death."⁹ The fact that cloned animals were now part of the scientific scene muddied waters further the procedure used to make Dolly the sheep shares its scientific history with embryo research. Scientists and journalists used words such as embryo and cloning so cavalierly that the lay public wasn't sure what distinguished animal cloning from babies conceived through IVF and embryonic stem cell research. As the millennium drew to a close, many people felt that a knock on the door from their human clones seemed a distinct possibility.

Clinton signed the NIH guidelines in August 2000, opening the door to scientists who needed funding for embryonic stem cell research. But few were willing to risk the precious time to write grant proposals that could be rescinded with sudden shifts in political winds. During a campaign speech, George W. Bush made clear his intentions regarding the issue, saying, "I oppose federal funding for stem cell research that involves destroying living human embryos."¹⁰ In an election year riddled with controversy, the stage was set for a raging battle in which scientists, politicians, religious leaders, doctors, and patients would find themselves unwilling soldiers.

One year later from his ranch in Crawford, Texas, President Bush made a sweeping announcement: funding from the NIH would be used for research only with preexisting embryonic cell lines (which numbered only in the dozens), and no federal funds would be used for the creation or use of new stem cell lines or to clone human embryos for any purpose. Later that same year, the House of Representatives followed the administration's lead and, by a wide majority, banned cloning of humans and voted to criminalize so-called therapeutic cloning, a laboratory method used to generate embryonic stem cells. The penalty was set at a $1 million fine and up to ten years in jail. In January 2002, the Senate swung into action, and Sam Brownback (R, Kansas) introduced a proposal that mirrored the House's bill. The Senate failed to act on the legislation in 2003 and 2004. In 2005, momentum in favor of stem cell research began to swing slowly the other direction. In a challenge to President Bush, the House of Representatives approved legislation to lift the ban on embryonic stem cell research. The vote was 238 to 194, 47 votes short of the two-thirds majority needed to override a presidential veto. "I made it very clear to the Congress that the use of federal money, taxpayers money, to promote science which destroys life in order to save life, I'm against that," Bush said before the vote. "And therefore, if that bill does that, I will veto it."¹¹

Adult versus Embryonic Stem Cells

As the debate escalated, opponents of embryonic stem cell research mined emerging scientific evidence suggesting that adult stem cells could be used in therapy. Although they show up prior to birth, adult stem cells are developmentally older, specialized cells that exist in many places in the body, biding their time before they replace old and damaged cells and the diseased tissues in which they reside. The principal difference between adult and embryonic stem cells lies in their potential to become different types of cells and tissue. Embryonic stem cells have enormous potential, they can become any cell or organ in the body. Adult stem cells, by contrast, are restricted in what they can become. As they mature, their ability to change becomes increasingly limited until they are a fully matured cell, such as a skin cell or a neuron.

Preliminary results from some adult stem cell research laboratories in late 1999 and early 2000 hinted that adult stem cells were every bit as powerful as their embryonic counterparts. Political and religious groups used this data to make the case that embryonic stem cell research was unnecessary. But other laboratories were unable to repeat the experiments, and a burst of new data refuted the original claims. As a result, few stem cell researchers today will say that adult stem cells are the sole answer for curing disease and physical dysfunction. Indeed, experts say the opposite embryonic stem cells have tremendous therapeutic potential.

The final answers won't come any time soon. Cures may come from other disciplines of biology and some diseases may prove too stubborn to treat. Human stem cell research is in its infancy and is extremely fluid: results published in last year's scientific journals are quickly refuted this year. Biology yields its secrets grudgingly, and it is quite possible that a decade or more will pass before anything resembling a general theory of stem cell biology is articulated, and then only if research is allowed to proceed under open conditions. Our knowledge of human development relies on investigating both types of cells: prohibiting one line of research and not another is like asking Einstein to understand relativity without gazing at the stars or asking da Vinci to understand flight without watching birds.

A Glimpse of What Lies Ahead

What are stem cells? Can they cure diabetes or make a new heart? What is the scientific controversy that swirls around embryonic and adult stem cells? Which stem cell discoveries will develop into actual therapies? Which diseases will benefit, and how long will it take? If we aggressively pursue embryonic stem cell research, will human clones be far behind? Do we destroy a human being when we use an embryo for research?

The pages ahead provide the biological and scientific basics needed to explore the most recent advances and therapeutic applications of human stem cells. But a glimpse into biology and medicine provides only part of the answers. No area of science is so deeply interwoven with ethical concern. The final chapters cover the moral and political dimensions that, along with their medical promise, make stem cells front-page news. It is difficult to find a biologist who will say that stem cells alone hold the key to solving our most intractable diseases. But it is safe to say that no single area of biomedicine holds such great promise for improving human health.