The scientific field of stem cell biology may not be very old, yet here and there one catches sight of its extreme possibilities. Take, for instance, a study by Japanese scientists that involved planarians, tiny flatworms. Like hydra, planarians are loaded with stem cells, so much so that a little piece of the creature—only 1/279—can regenerate the entire worm, this shown by Thomas Hunt Morgan at the end of the nineteenth century. The recent study had to do with a protein in planarians that normally ensures that only stem cells in the head region differentiate into neural tissue. When the researchers interfered with this protein, stem cells throughout the worm turned into brain tissue. Or, as one planarian scientist observed, “Brain grew all over the worm.”

Studies of meristem cells in plants, meanwhile, have biologists envisioning daisies with twice as many petals, lilies with twice as many blooms, and plants that generally might be engineered to perpetually flower. Instead of perennials—maybe diurnals? In the tree department, researchers are endeavoring to discover more about tree meristems and the on-and-off activity of the genes inside them that account for the growth habits of some of the world’s most notable specimens. Tipped off by insights into the biology underlying Methuselah, California’s 4,770-year-old bristlecone pine, or a redwood’s height, or another tree’s resistance to disease, growers

might produce trees of stunning proportions and hale-and-hardiness. Participants in the Champion Tree Project International already have cloned dozens of outstanding trees by taking cuttings from a niche rich in meristems—the tips of their branches.

For biologists studying animal stem cells, a major limitation continues to be a lack of good markers for identifying different versions of stem cells. Each time the markers improve, new observations result. Earlier this year, a Massachusetts General Hospital team unfurled its surprise finding of stem cells in the ovaries of female mice that had gone unnoticed all this time. Team leader Jonathan Tilly attests that new markers paved the way for this discovery. Should these same adult stem cells turn up in women, the data would topple the old idea that a woman is born with a finite number of egg cells, and present a range of new options, from treating low egg counts to delaying menopause.

As progress into cells and their genes continues, every day the biologist’s laboratory becomes that much more of a remarkable place. Every day the ability to isolate, merge, and manipulate Life’s little pieces expands. If you went into a Rip-van-Winkle sleep for hundreds of years, what would you find when you awoke? An overly active imagination could have a field day conjuring up a bustling, futuristic street scene, where youthful centenarians (no wrinkles, baldness, arthritis, bad teeth, or age spots) walk their tiger-retrievers and poodle-bears past buildings that are adorned with winter-resistant topiaries (twenty blooms per stem) and manned by plant-animal janitors (four-foot-high dandelion-like creatures), while across the street, in a park filled with lush flora and large tame rabbits (whose fear gene has been knocked out), people fish for giant bass and perch that reproduce so quickly, the pond is never without them.

So much for fantasy. In the here and now, the goals that medical researchers have for stem cells and their derivative cells actually seem within reason, as long as no one expects to find novel cell therapies in the clinic tomorrow. Statements are made that cell-based treatments are oversold, yet what has tended to be “over-hyped,” notes

biologist Christopher Potten, “is the time scale.” The stem cell field as a whole seems aware of this, and more cautious these days about issuing timelines than it did after the uncovering of human embryonic stem cells in 1998. Back then, it wasn’t uncommon to hear researchers say that a novel stem cell therapy would be ready in five years. These days, predictions are usually for ten, twenty, or more years, if years are mentioned. At this juncture, stem cell scientists seem genuinely sobered by the formidable chasm of biology to be crossed. Slinging a rope bridge between two Tibetan mountain peaks would be a cinch compared to the herculean task that many researchers have embarked on, which is to identify the “expression” of genes in a cell—which genes are on, which are off—that directs a cell’s stage-by-stage differentiation down a specialized path. Possibly as many as ten percent of a cell’s genes (in humans, roughly 3,000) undergo changes each time a cell differentiates another stage, Ron McKay reports.

Along with the difficult science, politics and legislation are likely to slow the emergence of stem cell therapies, and already are, in the opinion of many. Worldwide, the debate that has taken center stage is whether to ban both kinds of human cloning—therapeutic (creating blastocysts for culling stem cells) and reproductive (producing a baby)—or just the latter, which, despite persistent rumors, probably has not yet resulted in a live human birth.

Most scientists are stridently opposed to human reproductive cloning. In respect to cloning an animal, close to ninety-nine percent of the attempts purportedly fail, most clones dying soon after implantation. If they reach birth, they can have such terrible abnormalities that, as Rudolf Jaenisch at the Whitehead Institute frequently notes, those that don’t survive are “the lucky ones.” Jaenisch, who has studied cloning extensively in mice, has found that when a differentiated cell’s nucleus is transferred and asked to roll back to a totipotent state, its genes can run amuck, like a clock’s inner works when its hands are moved backwards. Scientists, not to mention nonscientists, also oppose reproductive cloning on the grounds that

the planet doesn’t need such an extreme form of reproduction when there are too many malnourished and neglected children as it is.

Presently, of the several countries that permit culling stem cells from IVF embryos—among them, Singapore, China, Japan, Finland, Greece, Sweden, the United Kingdom, and Korea—a few also permit therapeutic cloning as a way of obtaining stem cells. Other countries, several of which have large Catholic populations and which include Spain, France, Austria, Ireland, Germany, and Italy, oppose either retrieving stem cells from IVF embryos or gaining them through cloning. In the United States, a bill banning both kinds of cloning, therapeutic and reproductive, has passed in the House of Representatives, but similar legislation has been stalemated in the Senate since 2002. The word on the street is that a law that bans reproductive cloning is bound to come into existence, but that similar action against therapeutic cloning would never clear the Senate. As one knowledgeable onlooker told me, “It’s unlikely that therapeutic cloning ever will be banned, because too many people in the Senate, including conservatives, have relatives with diseases.” At the international level, a group of over sixty science academies has pledged its support for therapeutic cloning. At the national level, several high-profile Republicans, including Arlen Specter, John McCain, Orrin Hatch, and Nancy Reagan, are strongly behind stem cell research, although they don’t necessarily agree that embryos should be made for the purpose of gaining stem cells.

Current laws related to stem cell science run helter-skelter in many different directions. In the United States, scientists wanting to investigate embryonic stem cells, and who rely on government funding, have access to a restricted number of human ES cell lines, those made before August 9, 2001. Researchers supported by private funds, meanwhile, can create any number of human ES cell lines from donated surplus IVF embryos, so long as their research is in compliance with state laws. As for researchers who opt to study stem and progenitor cells from either aborted or miscarried fetuses, they face no federal funding restrictions, although they must adhere to certain

guidelines that apply to fetal tissue. That’s at the federal level. State laws are meanwhile coming into existence that support and promote stem cell research, even to the point of supporting therapeutic cloning (California and New Jersey). Certain other states have laws that prohibit one or both forms of cloning. Therefore, laws in some states pertaining to embryo or fetal tissue research or cloning can run counter to laws in other states, which can run counter to federal laws, which can run counter to legislation in other countries, which could run counter to pronouncements by the United Nations. Currently, the United Nations’ 191 members unanimously support a ban on cloning babies but remain at loggerheads over the issue of therapeutic cloning.

An interesting example of a population trying to sort out its feelings toward harvesting stem cells from IVF embryos arose in Singapore in 2001, when a government-appointed bioethics committee began a dialogue with Singapore’s lawyers, teachers, engineers, and other professional groups, as well as its numerous religious sects—among them Roman Catholics, Hindus, Bahaists, Taoists, Buddhists, Jews, and Sikhs—asking them for their views. What did each group think of using stem cells in medicine? Were IVF embryos an appropriate source for these cells? Singapore had given its stem cell researchers free rein to utilize IVF embryos for several years, with the closely monitored preconditions that these embryos should not be grown past day 13, day 14 being the start of neural tissue, or used for reproduction. This policy had enabled Ariff Bongso, working with Australian and Israeli researchers, to develop several ES cell lines, putting Singapore at the stem cell forefront. Yet Singapore, home to a sizable Catholic population, had its fair share of citizens who disagreed with the practice of using embryos in research, and before moving ahead with stem cell science any further, the government wanted a better reading of where its citizens stood on the subject.

The letters written to Singapore’s bioethics committee, which can be read on the Internet, render a moving portrayal of religious leaders searching through their ancient books, and their souls, for

guidance in making the right decision. Several responded that nowhere does it say what to do in this case. It’s a painful depiction of religion trying to keep pace with science, which, very plainly, it was never designed to do. With the exception of a few groups, including the Catholic Church and the Sikhs, the majority of respondents said that they accepted the practice of harvesting stem cells from embryos, agreeing that day 14 seemed an appropriate cutoff point.

A question that gets asked a lot is, since a fair number of people object to using embryos for stem cell research on strong moral grounds, why don’t scientists make do with the adult stem cells that sparsely populate organs? An inescapable truth is that the adult version are the only human stem cells so far employed for therapies in humans. Stem cells from embryos, or mature cells grown in culture from them, have yet to successfully treat a human disorder. When pressed on this, investigators are apt to respond that human embryonic stem cells have only just been isolated. “Give us time to figure them out!” They also point out that twenty-five years elapsed from when embryonic stem cells were isolated from mice to when meaningful experiments began to occur with those cells. The mouse successes, while limited just a short time ago, have come on strong of late. There are early indications in mice—which should be seen as a long shot from humans—that mouse ES cells or their mature descendants can improve, to varying degrees, heart and stroke conditions, neurologic disorders including spinal cord injury, a range of blood and immunodeficiency disorders, bone irregularities, diabetes, and still other abnormalities in mice.

The ability of embryonic stem cells to give rise to a greater circle of specialized cells than the adult version is seen as their number-one drawing card. As younger cells, their immortal, self-renewing ways also avail them and their progeny of being much more long-lasting, whether in a dish or in the body, than adult stem cells, whose ability to divide and differentiate can start to diminish. On the other hand, the proliferativeness of embryonic stem cells might make them more prone to causing a malignancy than the adult kind.

The hunt meanwhile continues for a useful source of human stem cells that everyone might be behind. Over the last couple of years, several different adult stem cells have been pushed into the news by the suggestion that here’s a stem cell—whether from bone marrow, umbilical cord blood, amniotic fluid, body fat, skin, baby teeth, cadaver, or other source—that won’t cause any moral and ethical upset, and perhaps could be as effective and ambidextrous a cell as the stem cell from embryos. However, especially since adult stem cells have taken a step toward differentiating, many scientists are reluctant to put faith in the notion, before hard evidence arrives, that adult stem cells have the versatility and longevity that an embryonic stem cell can offer.

As for embryonic stem cells, there are more sources available than immediately meet the eye. They can be obtained from IVF embryos. They can also be arrived at through therapeutic cloning, whereby a donated nucleus is inserted into an egg cell, grown to blastocyst stage, and the blastocyst is harvested. In February ’04, a South Korean team reported that they had achieved the inevitable and procured a continuously dividing line of human embryonic stem cells this way.

Embryonic stem cells can also be derived from a human parthenote, an unfertilized egg cell that is stimulated, by chemical or electrical impulse, to start growing like an embryo. It, too, can be grown to roughly blastocyst stage, its stem cells then harvested. Certain species of insects, fish, and lizards reproduce parthenogenetically, with an egg launching into development without a sperm. But it’s only lately that scientists have gotten mammalian eggs to take this track. A parthenote allegedly cannot advance to fetal stage, since it lacks paternal DNA that promotes the growth of the umbilical cord. Some scientists reason that because a parthenote is not a product of egg and sperm, it can’t be considered an embryo, making it a noncontroversial source of stem cells. Whether a parthenote’s stem cells function normally still needs validation.

From China comes news of yet another way to derive embryonic

cells. A former NIH researcher reports taking human skin cells, transferring their nuclei into rabbit eggs, and, through this inventive instance of cloning, growing human embryos to five-day blastocyst stage to retrieve their stem cells. A major advantage of this approach is that it would avoid using human eggs.

All told, any method of obtaining human embryonic stem cells can be looked upon as morally objectionable, since it involves growing up human life—whether an IVF embryo, an embryo produced by cloning a cell, or even an egg cell—to roughly day 4 or 5 for its inner bounty of stem cells. A handful of groups have recently claimed, however, that their experiments have bypassed the embryo and its stem cells altogether. They maintain that they have developed culture techniques for turning one type of mature cell into another type. This represents transdifferentiation, the process so often disputed. The British company TriStem in London, for example, maintains that it can extract a person’s white blood cells, dedifferentiate them to a stem cell-like state, and then direct them into other specialized cells. Timothy McCaffrey, a biochemist at George Washington University in Washington, D.C., tested the method in his lab and calls it “striking” and “an ethicist’s dream.” “You can take blood from a person in the morning, and by the afternoon those cells have reverted to a primitive state. Then you manipulate them into cardiomyocytes or neurons,” says McCaffrey. If the method bears out in human trials, its greatest benefit, he believes, would be to allow patients to receive back their own cells as treatment for a disease instead of someone else’s, which can set off the immune system.

More than a few scientists voice skepticism over a technique that so easily changes one cell to another. “If cells can run around and become any other cell, why aren’t I a pile of goo?” observes one stem cell scientist. Other researchers mention that even if a mature cell could be demoted to immature and made pluripotent, it would contain mutations that typically accumulate in the DNA of older cells, making it and its progeny less than ideal for cell therapy.

Still, cloning and its ability to revert the nucleus of a cell to a

pluripotent state make even veteran biologists believe that, in theory at least, it should be possible to take any cell, sit it in the right culture, and prompt its genes to revert back to a zygotic state capable of beginning an embryo, leading to a person. Then the question comes down to, if every cell has the ability to become a human being, does that make every cell sacred? Several years ago, Harold Varmus, who currently presides over the Memorial Sloan-Kettering Cancer Center, was quoted in The New Republic as saying, “If we say any cell has the potential to be a human being, then every time you cut your finger, do you have to wear black?” Most stem cell scientists as well are pretty clear about where things start crossing over the line into absurdity. “The Dolly experiment says that any cell in your body is totipotent, but that doesn’t make every cell in your body equivalent to a baby,” maintains James Thomson.

Investigators estimate that for decent progress to be made in developing cell therapies, hundreds, if not thousands, of human ES cell lines are needed worldwide. In the United States, of the seventy-eight “presidential lines” that qualify for federal grants, as of May ’04 less then twenty are usable. They include three batches resulting from James Thomson’s work at the University of Wisconsin and five from Ariff Bongo and colleagues in Singapore. Even if all seventy-eight were up for grabs, none could go toward patient treatment, since they were made with mouse feeder cells, which can carry viruses. In 2002, Doug Melton, then chairman of Harvard’s Department of Molecular and Cellular Biology, told me that “it would be surprising if the first lines were any good” in the first place. “If you look at the mouse embryonic stem cell work, it took years before robust cell lines were established,” many of the early ones having been “duds.” Melton estimated that at the time at least ninety percent of stem cell investigators were doing basic research with animal stem cells while they waited for the political scene surrounding human ES cells “to sort itself out.”

Melton didn’t want to wait, not after having learned first that his son had juvenile diabetes, then that his daughter had the same disor-

der. Teaming up with Doug Powers at Boston IVF and Andrew McMahon, another Harvard biologist, he put into motion the gutsy plan of creating human ES cell lines using frozen IVF embryos contributed by couples. Since he had generous private funds obtained through the Howard Hughes Foundation and the Juvenile Diabetes Research Foundation, he was not beholden to federal restrictions and could proceed. As for state restrictions, Massachusetts had passed a law in 1974 that banned research on live human fetuses, which, by extension, could be construed to include IVF embryos. Yet a researcher could be exempted from this ruling by getting an institutional review board’s approval for a research undertaking, which Melton got from Harvard. And so, by intelligently dodging bullets, Melton was in the clear.

Boston IVF began thawing surplus embryos for the project in January ’02. They were transported to Melton’s lab in Cambridge, where Melton and a team of assistants grew them to blastocyst stage, removed the blastocysts’ outer cells, set loose their inner pearls, and cultured them. When I visited later in ’02, Melton said he had at least two, maybe four, “non-presidential lines” already flourishing. As soon as he made a few more and verified their pluripotency, he planned to start giving them away to research groups, free for the asking, in comparison to the cell lines that were eligible for federal grants, which cost up to $5,000 per line. “I can’t wait to give them out!” he exclaimed. He saw each cell batch as an experimental treasure for medical researchers. The unfortunate hitch was that, in the U.S., only researchers with private funding would be able to profit scientifically from these new lines. Researchers outside the country might benefit to a greater extent.

Now, in the spring of 2004, Melton has as many as seventeen human ES cell lines in the fridge and is just beginning to send them off to other benches. Meanwhile, at Boston IVF Doug Powers reports that as many as thirty to forty percent of couples who have finished their infertility treatment are choosing to donate their extra embryos to the stem cell project.

From across the country, and from other continents as well, including the UK, Singapore, Australia, and Europe, comes news that a growing number of laboratories are bringing fresh new ES cell lines into existence. The freshness factor might be an important criterion for their use, for it’s been discovered that ES cells that grow in culture for long periods can acquire mutations in their chromosomes, which could possibly mar their clinical value. “There may be an ongoing need to generate new ES cells,” says Andy McMahon. “But we won’t know the answer until we work on more lines.” Melton’s stem cell lines, like so many others, have been grown in the company of mouse cells. McMahon doesn’t see that as a hindrance at this early stage, as there’s much more to learn about ES cells before they take the form of new medicines.

And so, ingenuity by ingenuity, the stem cell field presses onward—not quickly, but with an eagerness that neither difficult science nor unsettled legislation can necessarily squelch. As young as the stem cell field is, already there are signs of certain obstacles giving way. For example, Ariff Bongso’s Singapore lab and other groups are developing ways of culturing ES cells without animal feeder cells, which otherwise would keep ES cells from clinical use. Also, the technology of rendering pure batches of stem cells continues to improve. Stanford’s Irv Weissman believes that if you can produce a completely pure population of hematopoietic stem cells, one that excludes the mature lymphocyte cells that trigger a person’s immune system, it would be possible to transfer bone marrow stem cells from one person to another without having to worry that the transplant would cause an immune system rejection. “Because there would be no T-cells there to trigger one!” exclaims Weissman. This sort of creative problem solving is everywhere to be seen in the field.

An area that is turning into a research hot spot is the study of stem cells, not as a treatment for disease, but as the origin of disease. Since stem cells are biological building blocks, presumably if they falter, they carry the recipe for disaster. NIH bone specialist Pam Robey refers to her lab’s cells, which are bone marrow stromal (or

mesenchymal) stem cells that give rise to bone, cartilage, and fat, as “the cause and the eventual cure” of the rare genetic skeletal disorders that she and her labmates investigate. “Any mutation, any environmental factor that causes a change in stem cell metabolism can lead to one of these diseases,” she says. “The cure, meanwhile, is skeletal regeneration, and figuring out how to incorporate our stem cells into the skeleton.”

In the area of cancer, to what extent different cancers are the result of stem cells that go awry, no one can yet say. However, it’s easy enough to see how stem cells can launch a rapidly growing malignancy. Notes Christopher Potten, “If stem cells are normally dividing in a 50-50 way,” where fifty percent are self-renewing and fifty percent are differentiating, “to get cancer, there would only need to be a change to 49-51,” or a slight shift to too many self-renewing cells. Whether cancer really can be called a stem cell disease, which implies that the stem cell is wholly responsible, needs further examination. This said, various researchers have recently presented evidence that tumor-inducing stem cells may play a role in breast cancer, gliomas (the most common kind of brain tumor in adults), and a variety of leukemias.

In Harvard’s Countway library hangs Robert Hinckley’s oil painting, First Operation Under Ether, featuring doctors in somber dress and a patient out cold with his head thrown back. The occasion, which took place under Massachusetts General Hospital’s Ether Dome in 1846, was the first public demonstration of ether’s effects. It’s a large, heroic painting, capturing as it does the challenge of curing people in a humane way. To this day anesthesia remains a godsend. Hinckley’s portrait, though, seems set back in the Dark Ages. Not even ten years had passed since cells had been recognized

as the little worlds that Life is made of. One hundred and fifty years later, are we finally at a turning point of knowing enough about cells to begin to make them our medicines? Is it just an illusion that medicine can be better, and at the same time kinder, than what patients in the past have experienced? The new presence of stem cells in our midst opens a new chapter and makes one a believer.

Some biologists believe that the greatest legacy of stem cells will lie in their therapeutic value. Others say their best gift will be what they can tell us about the miracle of Life, and how a tiny cell has the wherewithal to grow into a thriving form, of whatever nature. Other biologists instead look at a stem cell as the immortal cell that will extend human life indefinitely and reveal aging to be a mere aberration. That may sound far-fetched, but it isn’t altogether. One of Abraham Trembley’s successors, Howard Lenhoff, tells of how, when he was just starting his career, he fed the same hydra for three years, until he got “bored with feeding it” and stopped. Had he kept up the routine, the animal would still be living, he maintains. “A hydra’s cells have a life span of about six weeks; they constantly die and are sloughed off.” And yet at the same time, “they are constantly being renewed.”

“You see,” he says, “a hydra is essentially an immortal animal. It doesn’t die.”

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