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.
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.