Boston’s Schepens Eye Research Institute—sandwiched between a nursery school and a methadone clinic—is a Harvard-affiliated research establishment named for the renowned retinal surgeon Charles Schepens. There, Michael Young, a neurobiologist, has carved out a difficult goal for his lab that connects back to the struggles of eighteenth-century naturalist Charles Bonnet.

To briefly revisit that story, Bonnet, his eyesight failing, had attempted to find out if there was any truth in Lazzaro Spallanzani’s claim that a salamander could regenerate its eye with the same panache it did its tail. When he tried to remove the eye, Bonnet only succeeded in knocking out its lens and doing other serious optical damage. In a matter of months, however, the salamander’s eye would completely heal, new lens and all, while Bonnet’s poor vision would steadily worsen.

By now, no one questions Spallanzani’s contention. Researchers would just like to know by what means salamanders, newts, frogs, goldfish, and various other lower vertebrates can regrow a lens, iris, or other eye part, so that they might prod the human eye to regenerate in a similar fashion. While this might seem like wishful thinking, evidence already exists that the mammalian eye is responsive to such ploys.

Young and his postdocs have taken on the challenge of regenerating the retina, the thin tissue that lines the back of the eye and converts light to image. Observes Young, research groups around the world have singled out the re-tin-a—he pronounces the word with a swift patter of three equally accented syllables—because it is the only major tissue in the global part of the eye for which ophthalmologists lack effective repair methods. By comparison, two other important eye tissues—the lens and cornea—are much more easily treatable. “If the lens is damaged, an artificial lens can replace it,” he notes. “And corneal surgeons restore eyesight all the time”—by replacing a defective cornea with a healthy one obtained from a fresh cadaver. Since corneal transplantation’s modern beginnings in the ’50s, it has become one of the most successful types of transplants available.

But in the case of a diseased or injured retina, few treatment options exist, and consequently the large majority of vision disorders in the Western world are retina related. Among the most common of these are diabetic retinopathy, which affects the eye’s blood vessels in diabetics; glaucoma, which affects pressure in the inner eye; retinitis pigmentosa, which affects the retina’s epithelial tissue; and macular degeneration, which affects vast numbers of photoreceptors in the retina’s center (the macula), causing vision loss in the eye’s center.

An affable, low-key fellow in his mid-thirties, Young is a walking—and motorcycle-riding—testament to the extent to which eye afflictions have been successfully dealt with in our modern era. The glasses he wears to correct astigmatism and nearsightedness leave him free for his favorite pastime, a memento of which hangs in his office: a large poster of a bright yellow Ducati motorcycle. If ever there were a time to push further to curb retinal diseases, it is now, stresses Young. A report issued by Prevent Blindness America estimates that as baby boomers age, the number of Americans with impaired vision will double over the next three decades. Macular degeneration alone, which is already the leading cause of blindness in older Americans, is expected to affect one in six individuals over the age of sixty.

Only recently have scientists dared to think that the human

retina might be made to regenerate. It’s long been known that retinal cells of certain lower vertebrates are capable of regrowth, but the phenomenon seemed restricted to cold-blooded creatures, whose eyes continue growing in adulthood by adding cells. In contrast, the eyes of warm-blooded mammals grow after birth when cells stretch, not multiply, which made it seem as if there was no cell birth or regeneration happening in the eye, just as there wasn’t in the brain. Indeed the retina, which is central nervous system tissue, is an extension of the brain. It was only with Joseph Altman’s findings, and then Fernando Nottebohm’s, that people began gaining a new perspective on the brains of higher vertebrates, notably bird and mammal. These brains did harbor immature cells that constantly divide and differentiate into specialized cells, and suddenly scientists had a reason to think that the mammalian brain—and the eye—might be candidates for regenerative medicine. Then University of Toronto researchers created a terrific stir when, in 2000, they found stem cells in the eyes of mice, cows, and humans, stem cells that generated retinal cells. The notion came alive that perhaps a mammal’s retina did have regenerative power, just like that seen in the eye of Bonnet’s salamander, but that some sort of inhibitory process was keeping it under wraps.

At Schepens, Michael Young’s game plan is relatively straightforward. After experimenting with and learning from other animals, he hopes eventually to be able to isolate progenitor cells from the human retina, multiply them, and then “transplant them back into the eye and have them become the type of cell we want them to be, which in most cases are photoreceptors cells,” says Young.

The human retina contains three main classes of cells—photoreceptor cells, retinal ganglion cells, and inner nuclear layer cells—but of these three, it is mainly the photoreceptors, which consist of rods (for black-and-white and night vision) and cones (for color and daylight vision), that are lost in blinding diseases. (In two notable exceptions—glaucoma and optic neuropathy—the retinal ganglion cells suffer.) Even when photoreceptors are destroyed, for a period of

time the rest of the retinal system and its connection to the brain can remain pretty much intact, according to Young, which is all the more reason to think that the strategy of replacing photoreceptors can help people gain back their vision.

Photoreceptors are one of Nature’s supreme gifts to animals, and if you have any doubt about this, look at a micrograph of a rod, with its fancy quiver of cilia at one end of an elongated body. Equipped with as many special features as the Mars Rover, a photoreceptor turns light energy entering the eye into chemical energy that gets routed to the brain and is transformed into everything we see. Here’s a cell that makes one appreciate the notion that as humans have evolved, our cells may have become too specialized to undergo the changes that a salamander’s cells undergo when a salamander needs to replace a part.

When I first met Young in ’01, he and his labmates were in the process of extracting progenitor cells from the retinas of healthy baby mice, then injecting these cells into the eyes of mice that had no working rod cells and thus were blind by six months of age. Once transplanted into the space behind the retina, the cells were seen to migrate into the retina photoreceptor region and form what appeared to be working rods. “I think we’re pretty confident that they are integrated and have the capacity to function,” Young had told me at that point. “But we don’t know if they are passing visual information to the mouse.” Ascertaining whether a mouse’s vision has improved isn’t all that easy—the animal can’t exactly tell researchers, “Hey, I can see!”—and for this reason, a mouse behaviorist from England was about to visit Young’s lab to spend two weeks conducting the necessary tests.

By the time I returned to Schepens nearly two years later, Young and his crew had determined that the mice that had received transplants indeed were seeing a little better, but not because of the implanted cells, exactly. Rather, it was because the new cells that they differentiated into seemed to stop cone cells from degenerating, Young explained. It was a perfect illustration of the maxim, “Science

is never what it seems.” Young, in the meantime, had formed collaborations with groups in Denmark, Sweden, and California, and was pursuing the same goal as before—retrieving progenitor cells from healthy eyes and transferring them into eyes lacking photoreceptors—this time in pigs. A pig’s eye is much closer in size and structure to a human eye than is a mouse eye, and a lot easier to work with, he told me. What’s more, “There’s an RP pig in North Carolina,” Young said with boyish excitement, RP standing for retinitis pigmentosa, a disorder that desecrates rod cells. This pig model was expected to serve as a human-like proving ground for cell therapy.

Throughout the stem cell research community, I was finding, research groups were taking the same course, and still are. In short, they are using animal models that mimic a particular disease as a way of testing the effectiveness of a particular cell therapy approach. Mice with blood deficiencies; rats with spinal cord injury; monkeys with motor atrophy similar to Parkinson’s or ALS; zebrafish with heart defects. Learning the biology as they go, these modern-day Bonnets and Trembleys are going for broke. Yet whether cell-based therapies can sufficiently materialize without a more complete understanding of the intricate biology that underlies stem and progenitor cells is one of the questions that researchers are currently asking themselves. Some researchers, fearing that the cart is going before the horse, predict that only when the biology is more fully worked out will effective and safe treatments follow.

During a lecture on stem cells at Schepens in May of 2001, Ruben Adler, an eminent ophthalmologist from Johns Hopkins School of Medicine, said something that stood out both for its refreshing candor and also its reminder that human biology is still at an early hour. Noted Adler, “Everything is so much more complex than one pretends it to be.” In his talk—which was titled “Is the potential of stem cells going to be realized?”—he brought home how many things must tie perfectly together if transplanted cells are to integrate with other cells and restore normal function in the central

nervous system. Ostensibly, he was talking about cells emptied into the eye. But I would hear other scientists make the same point about cells transplanted into other parts of the central nervous system, whether the substantia nigra, the small midbrain region vandalized by Parkinson’s; or the spinal column, the span of, in humans, thirty-three vertebrae that is so vulnerable to injury. Of all the systems that researchers would like to mend with cells, the nervous system is easily the most complicated, and if adding cells can regenerate any part of it, it would be a triumphant endorsement of cell-based medicine in general. In his talk Adler referred to the promising observations centered around using stem cells to restore nerve function, yet warned that the complexity of chemical exchanges that take place between nerve cells and dozens upon dozens of growth factors, which are the proteins that direct and nourish cells, is beyond staggering. “If you’re transplanting stem cells into a retina expecting that something within the retina is going to induce their differentiation into the cell types you desire,” he told the gathered scientists, “you better pray real hard, because very likely the signaling molecules aren’t going to be in the right place at the right time for the stem cells to see.”

At a conference on neural stem cells later in 2001, which was held at a Hilton resort next to San Diego’s Mission Bay, a neurobiologist I met at lunch one day, Don Gash, had gone further than Adler when he conveyed to me his deep concern about the attempts being made to reverse neurodegenerative diseases through cell replacement measures. Gash, the chairman of anatomy and neurobiology at the University of Kentucky Chandler Medical Center, said that his involvement with early fetal transplants geared to repairing Parkinson’s had left him skeptical that a course of action as “simplistic” as putting one type of neuron into a problem area was “going to fix the problem.”

Using Parkinson’s as an example, Gash cited two major problems. The first had to do with the “monumental” undertaking of trying to reconstruct the complex neural circuitry that Parkinson’s pervades. Much of the cellular damage might be concentrated in the

substantia nigra, yet neurons in that small midbrain region are part of a loop of processes that include several other parts of the brain. The transplanted neurons “need not only make the right connections, but also receive the right inputs” from neurons in other areas, Gash stressed. I found myself imagining a mechanic trying to fix a car engine by randomly dropping spark plugs under its hood. Those who believe cell transplantation can work to diminish Parkinson’s—and that includes many reputable scientists—acknowledge that one of the many details that need clarification is precisely where cells should be introduced in this loop for the greatest, safest effect.

The second problem Gash cited was “control over the neurons you’re putting in. Control is a very, very important issue,” he warned. If the transplanted cells “do the wrong thing, how are you going to stop them?”

I’d only just been on the phone with a Brigham and Women’s neuropathologist, Rebecca Folkerth, who had talked at some length about a disconcerting experience she had had in 1991 that possibly illustrated what Gash was getting at, although no one would ever know for sure. Folkerth, then on the staff of the New England Medical Center in downtown Boston, had been called in one evening to do a brain-only autopsy. The deceased was a middle-aged Parkinson’s patient who had traveled to China in 1989, where an American surgeon had implanted fragments of fetal tissues into the putamen region of the man’s brain, as well as infused individual cells derived from a six-week embryo into the brain’s ventricular system, hoping this surgery would help the patient’s severe symptoms. At the time, funding for fetal work of this sort was prohibited in the United States. For many following months, the man’s gait, swallowing, speech, and other abilities had notably improved, he and his family had reported. Yet one morning, after telling his wife he felt tired, the man had suddenly died.

That night, when Folkerth dissected the ventricles, she was amazed to find that they were filled with tissue that didn’t belong in the brain. “It was bizarre. I could see hair shafts, cartilage, and con-

nective tissue,” she recalls. “I was there by myself in the autopsy room, and thought no one would believe me, so I took lots of photographs.” The surgeon who had performed the transplant had asked her to check whether the transplanted fetal cells had proliferated. “He really hoped that the surgery had worked to some degree. But I didn’t see any nerve cells,” she recounts. “My own feeling is that this overgrowth” of nonneural tissue “was compressing his brain stem and that’s what caused his death.” Where had this extraneous tissue come from? Folkerth imagines that in the course of cutting out and implanting the neural material, the surgeon had unknowingly included progenitor cells from other tissues.

To be sure, this particular case represents a worse-case scenario and emerges from a time, although only a decade ago, when less was understood about brain cell transplantation. It still can serve as a valuable reminder that the very aspect that makes immature cells potentially so useful in medicine—their ability to rapidly multiply and differentiate—can also be their most dangerous.

There isn’t a scientist around who would tell you that replacing brain cells is a piece of cake; yet more than a few researchers view the avenue of retinal regeneration as one of the most promising attempts at stem cell therapy currently under investigation. It fits a presumed criterion, which is that stem cells, and the cells derived from them, will be most effective for disorders that are confined to one area of the body and involve one type of cell, and less effective for disorders that are more diffuse and affect many cell types. Juvenile diabetes, the type 1 variety that arises in children, is viewed as a particularly good candidate for cell therapy for that very reason. It involves just one of the pancreas’s ten or so cells: the beta cell, which makes insulin. Similarly, Parkinson’s attacks dopamine-making neurons. One doesn’t hear much discussion about a cell-based therapy for Alzheimer’s, on the other hand, largely because the disease creates such a broad swath of destruction in the brain’s limbic system. “The problem with Alzheimer’s is that whole neural networks consisting of many different types of neurons in various brain regions succumb

to the disease,” cites Rudolph Tanzi, a Harvard Medical School neuroscientist. “Replacing pockets of cells here and there will not mean that the patient will regain lost connections between nerve cells.”

One particularly encouraging achievement for scientists who are working on retinal regeneration is that a stem cell-based therapy for the cornea has already been terrifically successful, and for as long as twenty years. Different from a corneal transplant but often done in conjunction with one, the technique is performed when the surface of the cornea has been lacerated by chemical burns, intense heat, or some other injury. The hero in this case is an adult stem cell that lives in the border, or limbus, of the cornea in the front of the eye, as compared to the stem cell that resides near the retina in the back of the eye. A “limbal stem cell,” it’s called. Surgeons retrieve limbal stem cells from a patient’s good eye, preferably, or from a donor, and then transplant them into the limbus of the damaged eye, where, nearly always engrafting, they replace the dysfunctional cells that had been clouding the cornea’s surface and preventing vision.

“The corneal surface almost invariably recovers in ten to fourteen days. The patient can regain total eyesight,” notes Kenneth Kenyon, who developed limbal cell transplants in the early ’80s during his tenure as the director of the cornea transplant program at the Massachusetts Eye and Ear Infirmary. Kenyon relates that in the ’70s one of his mentors at the Eye and Ear, Richard Thoft, had shown that the eye’s conjunctiva—the transparent tissue that covers the outer eye—could be transplanted, one eye to another. Scientists are only now realizing that the conjunctiva likely has its own stem cells that permit this grafting procedure. But when Thoft began these transplants, Kenyon points out, “it was before the dawn of stem cells, and no one was thinking stem cells.” Thoft’s success “planted a seed.” With the assistance of Scheffer Tseng, then a research fellow, Kenyon tried his first limbal cell transplant in 1984. It was spectacularly successful, yet initially viewed with suspicion. “Like many a new idea, it took a decade to come into parlance and be accepted.”

Patients who had this procedure twenty years ago still show no

sign of limbal cell depletion, says Kenyon. “That’s the essence of stem cells; they’re the gift that keeps on giving.”

Another encouraging aspect of research into retinal regeneration is noticeable elsewhere in the stem cell field: For every organ and disease being studied, a range of treatment approaches are under investigation, a situation spawned by scientists’ different orientations and strengthened by science’s competitive edge. So if one team’s approach doesn’t pan out, there’s the chance that another team’s will, or maybe another’s. On the Seattle campus of the University of Washington, for example, biologist Thomas Reh is approaching the challenge of replacing cells in the retina in a distinctly different way than Michael Young at Schepens. Instead of transplanting progenitor cells from one animal’s eye to another’s, Reh would like to goad mature glial cells that are already in the eye, in the vicinity of the retina, to dedifferentiate and then redifferentiate into ganglia cells that would migrate into the retina and replace those killed by glaucoma. This endogenous approach of using in-body cells relies heavily on a finding that Reh and teammate Andrew Fischer made a few years back, which is that glial cells in the chick retina have the ability to dedifferentiate and revert back to immature status, then re-mature into a different eye cell. “Since glial cells are spread across the retina, it seems an interesting, sensible strategy—to stimulate glia [in the eye] to undergo this process,” justifies Reh.

Compared to the amphibian eye’s one-hundred percent ability to regenerate, the chick eye has ten percent regenerative power, and the human eye, zero, says Reh. Why don’t birds and mammals regenerate? What’s changed in the course of evolution? Those are the questions that presently consume his lab, where the mission is to try to identify factors that will bolster the chick eye’s regenerative ability to one-hundred percent. Reh expects it will be many years before researchers are able to tap the human eye’s full potential, yet he has implicit faith this will happen some day. Look at the celebrated example of transdifferentiation in the eye of Bonnet’s salamander, he

exclaims. A salamander’s eye lens, iris, and retina can all grow back because adjacent cells, reacting to injury, dedifferentiate and redifferentiate into the appropriate tissues. Most likely, the same molecular components are “still” in place in the human eye, says Reh. What he’s indirectly referring to by “still” is that humans were once amphibian, too.

Despite being friends, neither Michael Young nor Tom Reh holds back when touting their respective plans of attack—to transplant cells or to activate in-body cells. These represent the two fun-damental approaches throughout the stem cell community. Young at one point said to me, “I hear people say that the job of the endogenous stem cell people is to put transplanters out of business, which makes perfect sense. If you can get the body to fix itself, then you don’t have to do a transplant, which is invasive and difficult and fraught with rejection problems. But can it fix itself?” Reh meanwhile thinks that the transplantation approach is equally suspect. “I’m skeptical whether it will work in a highly organized tissue like the retina. I’m skeptical that in transplanting these cells and letting them crawl around, they’ll organize so well as to give back vision.” He feels, in general, that the history of transplanting cells is “littered with failure.” “But hey,” he laughs, “I’ve been surprised every year for the last ten years!”

Whatever research nook one looks into, there is a very real sense that cell-based medicine isn’t too good to be true, rather that it’s too true from a biological standpoint not to be good medically, and that given time, supportive laws, funding, knowledge, patience, and more time, healing the body with its own cells will be the norm—some day. As researchers make headway in applying stem, progenitor, and even mature cells to diverse disorders, it’s expected that society’s reli-

ance on chemical-based drugs would diminish, but it’s also likely that synthesized chemicals and drugs would work in tandem with cell therapies to alleviate different illnesses.

To date, very few cell-based products exist in the United States. The FDA’s Center for Biologics Evaluation and Research [CBER] reports that it has licensed only one cell therapy product—Carticel, which is used to repair cartilage. The Center for Devices and Radiological Health (CDRH) meanwhile has cleared several tissue-engineered products, primarily in the area of skin. (Tissue-engineered products are apt to combine a biological product with a medical device.) FDA approval cannot be taken as a true indicator of the number of cell treatments on the market, because approval is not required for “human cells, tissues, and cellular and tissue-based products that are minimally manipulated” and not combined with a drug or device, according to a CBER spokesman. This noted, the case remains that not many cell products are yet available, and when I made inquiries into why this is, the primary explanation given was that cells haven’t been understood well enough to be effectively and safely put back into the body for therapeutic purposes. Also, given that these products contain living cells, the manufacturing and safety challenges are formidable, to say the least.

Despite their optimism, stem cell scientists acknowledge that some of the obstacles facing them are positively gargantuan. Consider a few of the difficulties associated with transplanting cells, a procedure that would first require growing embryonic stem cells, then converting them to a desired cell type, and finally delivering them to the right site in the body. Currently, the reported doubling time of human ES cells is twenty-four to forty-eight hours, depending on the cells; at that rate, say scientists, it could take a very long time to get the therapeutic amounts needed for a procedure. As for coaxing ES cells to become a specific cell type, this remains a difficult art, and so far it’s hard to get quantities of almost any desired cell, according to scientists who are trying. Moreover, to what stage of differentiation should cells be grown? Which stage of maturation

will be effective for therapy, which will be detrimental? Then there’s the question of, when cells are injected, where exactly should they go in an organ? In respect to Parkinson’s, for example, some researchers suggest one brain region, while others suggest another. Will enough of the delivered cells live? Will they stay in the right region or will they wander invasively into an area where they don’t belong? Finally, if the cells are from a donor, can they be kept from causing a catastrophic immune reaction?

One of the greatest fears is that transplanted cells can create a cancer. Embryonic stem cells are seen as riskier in this respect than older cells, because they are so proliferative. If put in the wrong place, they can lead to teratomas, and perhaps other types of tumors as well, warn scientists. If you’re pouring millions of mature cells into the body, can you be certain there’s no immature cell among them that will divide uncontrollably and form a malignancy? The endogenous approach—stimulating healthy, in-body cells to replace diseased cells—could be a cause for even greater concern, advises Michael Young. “If you’re trying to induce a cell in the body to proliferate that shouldn’t be proliferating, the potential to give rise to tumors might be a big problem,” he speculates.

A question that is especially hard for researchers to answer at this early stage, yet is so crucial, is, if you go to the trouble of substituting healthy cells for diseased cells, what guarantee is there that the new cells won’t suffer the same fate as the old cells? “It probably depends on the context of the disease,” says Melissa Carpenter, a stem cell biologist at the Robarts Research Institute in Ontario. “One of the standard answers is that in the case of, say, Parkinson’s disease, it might have taken fifty or sixty years for those cells to die.” Therefore, the replacement cells might be under no immediate danger.

Scientists are just beginning to understand stem cells; and yet what can come as a surprise is how many therapeutic practices already hinge on the regenerative properties of stem and progenitor cells. Better-known examples include transplants for blood disorders that are reliant on bone marrow, bloodstream, or umbilical cord stem

cells; the fusing and healing of bone done by mesenchymal stem cells in bone marrow; and the time-honored skin graft carried out by epidermal stem cells. Lesser known are therapies that utilize blood stem cells to cure immunodeficiency diseases in fetuses; and, as just seen, limbal cell and conjunctival transplants for the eye.

Regenerative medicine is already here, say researchers, due to a very different sector—the pharmaceuticals. Not to be overlooked are the increasing lineup of manufactured drugs that cause stem or progenitor cells in the body to proliferate, resulting in flourishing populations of specialized cells. Examples include erythropoietin, which stimulates blood cell precursors to produce red blood cells and thrombopoietin, which similarly builds up the blood’s platelets. Synthesized interleukins and colony-stimulating factors boost the generation of immune system cells. A lab-made parathyroid hormone meanwhile represents the first FDA-approved drug to stimulate bone growth, which it does by causing progenitor cells to make more osteocytes. In respect to the brain, Prozac and other antidepressants reportedly make neurons multiply. Some drugs create the opposite effect and inhibit cell production.

Streaming from the labyrinthian network of biological research in this country and beyond comes an even bigger surprise, which is the impressive number and diversity of cell-dependent treatments that are undergoing rigorous investigation. In the United States, the government’s current funding restrictions are said to be curtailing work at the human level and making junior scientists think twice about entering the field. This new science in our midst is such a drawing card to so many researchers, however, it would appear that there’s no way of keeping the genie in the bottle. Researchers are finding outlets for studying stem cells at the animal level; or at the human level through private funding; or with federal dollars available for fetal-tissue research or the approved “Presidential” cell lines.

Brief descriptions of some of these research endeavors follow, enough to demonstrate the hungry ingenuity that the notion of treating disease with cells—cells at all stages of maturation—is inspiring.

Be aware that much of this research is in its infancy and must satisfy many more inquiries into safety and efficacy before gaining acceptance.

A few years ago, researchers discovered that when bone marrow was transplanted into mice with a skeletal-muscle disorder corresponding to Duchenne muscular dystrophy (DMD), stem cells in the bone marrow traveled into skeletal muscle and actually restored, at least to a small extent, the protein that is deficient in this illness. Observes Emanuela Gussoni at Children’s Hospital in Boston, one of the researchers who made this finding, it was an encouraging sign that stem or progenitor cells might be made to reverse the tide of this incurable disease, whose male victims are almost always wheelchairbound by their mid-teens and often die by their early thirties. For now, researchers face the snag that stem cells in bone marrow don’t repair muscle “with high enough efficiency,” says Gussoni. Two other cell sources for DMD therapies are under investigation: stem cells derived from muscle, and stem cells derived from the vascular endothelium of the aorta of embryonic mice. An Italian team recently showed that the latter version, when injected into a leg artery of dystrophic mice, effectively reconstituted skeletal muscles downstream.

Although different autoimmune disorders affect different parts of the body, their shared feature is that lymphocyte cells in the immune system exert a sustained attack on the body’s tissues. In the case of multiple sclerosis, tissue in the central nervous system suffers. In rheumatoid arthritis, connective tissue suffers. In diabetes, pancreatic tissue suffers. In lupus, tissue in the brain, lungs, kidney, and virtually any other organ can suffer.

Peripheral blood and bone marrow transplantation, it’s turning out, and the stem cells at their core, just might be the treatment of the future for some autoimmune disorders, if animal and human studies continue to be as encouraging as they have been. For a bone marrow transplant to work, lymphocytes circulating in the blood first are eliminated by chemotherapy and antilymphocyte globulin. Then, when bone marrow or peripheral blood cells are reinfused in a person, it’s like “rebooting the computer,” likens Ann Traynor, who directs stem cell transplantation for autoimmune diseases at the University of Massachusetts Medical Center in Worcester. Transplanted stem and progenitor cells are counted on to reprogram the immune system, leading to new, naive lymphocytes that “allow tissues to repair themselves.” Autologous transplants that accommodate a person’s own tissues can be used, thus avoiding the high chance of an immune rejection raised by a donor’s tissue. While a person’s own stem cells may carry a genetic proclivity for the disease, they are not skewed to enact the disease as are mature lymphocytes, says Traynor.

In a clinical trial currently in progress at Northwestern University in Chicago and at UMass, of the forty lupus patients who have so far received peripheral blood stem cell transplants, seventy-five percent appear in remission from active lupus, some for up to seven years, while twenty-five percent have lapsed back into serious disease, according to Traynor. The NIH, meanwhile, is funding a multicenter clinical trial that will evaluate the effectiveness of bone marrow and peripheral blood transplantation for eighty patients with advanced lupus. If safety and efficacy are confirmed, a larger study will follow.

Transplanting stem cells from bone marrow or the circulating blood may hold promise as well for multiple sclerosis patients, particularly those whose disability is less advanced. “With MS, transplantation appears to be much more effective earlier in the disease than later, after actual nerve loss starts,” says Traynor.

Altogether, she believes that better days are coming for autoimmune patients: “Soon we’ll see more and more stem cell transplants

for lupus, Crohn’s disease, and earlier MS. With time, I expect we’ll see transplants used for rheumatoid arthritis and diabetes as well. We feel we’re approaching a place where people should not die of lupus anymore, should not die of Crohn’s disease anymore, should not experience disability that is sustained from these diseases anymore.”

Levels of glucose in our blood are maintained by insulin, and insulin is made by beta cells, one of four types of islet cells in the pancreas. In type 1 diabetes, an autoimmune disorder that appears mostly in childhood, beta cells are largely destroyed, and if the body doesn’t get insulin by other means, soaring blood sugar can severely damage the body’s organs. This is not a rare disease. Approximately one in every 400 to 500 children and adolescents in the United States has this form of diabetes, estimates the NIH’s National Institute of Diabetes and Digestive and Kidney Disease.

Seen as one of the most exciting advances toward liberating patients from insulin injections, a Canadian team at the University of Alberta in Edmonton has devised a procedure—the “Edmonton Protocol”—whereby beta islet cells from fresh cadavers are transplanted, via the portal vein, into the liver. Once engrafted, the cells send insulin to the rest of the body via the bloodstream. Since the first transplant in 1999, the Edmonton group has treated more than sixty patients. “Eighty percent of patients with completed transplants remain insulin-free at one year,” according to James Shapiro, the protocol’s developer. It’s widely observed that the Edmonton Protocol has pretty much proven to the world that if there were enough cadavers, insulin injections might be a thing of the past. Unfortunately, there are only enough cadavers to provide islet cells for a tiny fraction of patients.

Researchers, therefore, are pursuing a better source of beta cells. Since an adult stem cell has not been found in the pancreas, using ES cells to produce beta cells might be the most promising route.

“We know that embryonic stem cells can lead to islets, because if you put an ES cell into a mouse blastocyst, among the cells it contributes to are islets. But we don’t know how to do that outside a mouse,” says Douglas Melton, a Harvard biologist whose laboratory is working away at this challenge. In 2001, Ronald McKay’s NIH group reported that it had actually coaxed a mouse ES cell to differentiate into an insulin-producing cell. A subsequent study, however, has raised some doubt about McKay’s findings. So the field goes one step forward, one step back.

Meanwhile, doctors at the University of Cincinnati Pancreatic Disease Center have designed a new cell treatment for pancreatitis that involves removing a patient’s diseased pancreas and salvaging as many healthy islets cells as possible; then reinserting the islets into the patient’s liver, where, just as in the Edmonton Protocol, they can produce sufficient insulin for the body. Unlike the Edmonton Protocol, patients get back their own islets and therefore aren’t beholden to antirejection drugs. The procedure being fairly new, there’s no way of yet knowing if the liver-implanted islets will keep working long-term. To date, over forty patients have been treated, all have recovered, and over fifty percent are producing enough of their own insulin. They no longer require injections, according to one of the program’s coordinators.

Nearly twenty years ago, Douglas Cotanche, a Boston neuroscientist, accidently discovered that chickens can regenerate hair cells located in their cochleas, the coiled inner-ear tissue so important for hearing. Stem cells in the cochlea appear to account for this ability. According to Cotanche, most hearing loss in humans can be attributed to the loss of sensory hair cells, not to nerve loss, which has spurred his Children’s Hospital team to try to induce cochlea stem cells into regenerating sensory hairs in a mammal’s ear.

Convincing embryonic stem cells to differentiate into a specific kind of cell remains a tall order. Yet if the art can be perfected and made reproducible from lab to lab, it could be tremendously useful, and not just for the purpose of transplanting mature cells into ailing organs, but for testing drugs as well. Take the example of a Geron team’s having turned human ES cells into what it believes are hepatocytes, the liver’s primary specialized cell. If hepatocytes could be mass produced from ES cells, drugmakers, who must show that a new drug has no adverse effects on the liver, would have vast quantities of liver cells. Currently, a chief source of liver cells for drug tests are cadavers.

As patients who receive treatment for cancer of the head and neck know all too well, radiation can cause wicked dry mouth. Explains Simon Tran, a dental clinician at the NIH’s National Institute of Dental and Craniofacial Research, “The good news is that radiation can get rid of the cancer, but the bad news is that it can destroy one of two types of cells that make up the salivary glands.” To make life a bit more pleasant for patients, Tran would like to replace the cells lost to radiation, even engineer artificial salivary glands from scratch. He therefore has joined up with Éva Mezey at the NIH to see if the bone marrow stem cells that she has proposed can morph into cells of other tissues might serve to replace lost cells in the salivary gland that are responsible for secreting saliva. While there are currently two FDA-approved medications for dry mouth, if a patient lacks saliva-making cells, they aren’t of much use, says Tran.

Every tooth, it appears, is the appointed outcropping of a few stem cells. Stem cell populations within a tooth give rise to its inner pulp, the surrounding dentin, the outer enamel, and periodontal ligaments that anchor teeth to bone. At the University of Texas Health Science Center at San Antonio, among other places, biologists are studying both the idea of developing teeth directly from human embryonic and adult stem cells, or else triggering an interaction between epithelial and mesenchymal stem cells that might produce a tooth on site. “Our ultimate goal is to be able to regenerate teeth at the site where they are lost,” shares Mary MacDougall, a molecular biologist. “I don’t think the idea of growing teeth is science fiction; I think it will be science fact.” Meanwhile, the dental pulp of baby teeth has been found to be a rich source of stem cells that give rise not only to dentine, but also bone and cartilage.

Up until recently, it was unimaginable that a mammal’s eggs and sperm could be made anywhere but in the body. But in 2003, that quickly changed when a University of Pennsylvania team took mouse embryonic stem cells and differentiated them into eggs in a dish. Not much later, the other shoe fell when investigators elsewhere, including George Daley’s Harvard Medical School lab, managed to direct ES cells into sperm cells. Should the derived eggs and sperm check out and prove normal, they could be used to treat infertility in women and men, and refresh the levels of egg and sperm of cancer patients whose own gametes have been clobbered by chemotherapy.

Stem cells are found in fairly sizable numbers in the bulge of hair follicles in mammals. Their role in this niche is to generate mature

cells of the epidermis, sebaceous glands, and follicles. In some cases, researchers are trying to utilize these stem cells for skin replacement; others view them as the answer to hair replacement. Dr. George Cotsarelis’s University of Pennsylvania team is pursuing the latter goal and recently took a large step forward. Removing these stem cells from the hair follicle bulge of mice, the Cotsarelis crew transplanted them to furless mice, and in four weeks the so-called nude mice began to grow fur. It’s apt to be many years before this same approach can be used to restore human hair. But furrier times may be ahead.

Recent studies by Yale researchers suggest that Prozac (fluoxetine) and other antidepressants stimulate the brain’s hippocampus to make more new neurons, and that this heightened state of neurogenesis may be why these drugs alleviate depression. Previous studies had shown that brain cell depletion can accompany depression, so it would follow that halting cell loss, as antidepressants appear to do, can reverse it. Some scientists put these findings in a different light, however, arguing that any brain cell changes resulting from antidepressants or other drugs may be harmful to the brain.

In ’99, Henriette van Praag at the Salk Institute made a discovery that should have every able-bodied person, young and old, running around the block every day. She showed that when mice were given running wheels and ran to their heart’s content, the exercisers, in comparison to mice that had no running wheels and were much more sedentary, made fifty percent more new neurons in the hippocampus, and their ability to learn was significantly greater. In short, exercise may advantageously activate stem cells in the hippocampus. Other studies have demonstrated that exercise influences the brain’s

metabolic pathways, causing an increase in serotonin and other chemicals. Van Praag notes that to combat depression, “Exercise is probably better than taking antidepressants. If you exercise, you are activating your own system under normal circumstances.”

An oft-repeated phrase in the stem cell community is “the Orlic mouse.” It refers to mouse studies conducted by Donald Orlic and colleagues at the NIH and New York Medical College that left dramatic evidence in 2001 that stem cell therapy can fortify damaged heart muscle in mice. The researchers took bone marrow cells that had been partially purified for stem cells, injected these mouse cells into heart tissue, and witnessed a major increase in new myocytes, or muscle cells. The new cells appeared to go to work in the heart, helping the mice to recover and live longer than mice with poor cardiac muscle that hadn’t received bone marrow. There was a sixty-eight percent increase in the survival rate, according to Orlic.

Small, intermittent studies with humans leave the impression that stem cells from bone marrow or blood can revitalize human heart tissue as well, although “the qualifier is that these studies must be done in a more rigorous manner,” says Orlic. In recent investigations—including two in Germany and one in Hong Kong—upon arriving at the hospital, victims of severe heart attacks received traditional measures of balloon angioplasty and stent implantation or coronary bypass surgery. One week later they also had their own bone marrow withdrawn and injected into the coronary artery responsible for the infarct. Although the benefits varied depending on the patient, on the whole the investigators reported positive results. In some cases, patients showed striking improvement in blood flow and a significant reduction in the zone of damaged tissue. Most significant, all three teams noted that implanting bone marrow stem cells directly into the heart seemed safe.

Stem cells in these situations can’t do the impossible; they cannot revive dead heart tissue. Yet it could be that they can generate enough new tissue to give the injured heart a better fighting chance than it would otherwise have. The concept of using stem cells to bolster heart muscle “is brand new in only the last five years,” says Orlic. “Before, cardiologists had never been able to form new heart tissue.”

Organ transplantation has come a long way since the first organ—a kidney—was successfully implanted in 1954. Because of Cyclosporine, Prednisone, and other immunosuppressive drugs, as many as ninety percent of transplanted organs survive for at least one year. In dulling the immune system, these drugs can take a toll, however, leaving patients vulnerable to infections, cancers, heart disease, and other conditions. And they often fail to fend off organ rejection in the long run.

Enter a novel procedure that is the talk of the transplant community and one that might mean that organ recipients won’t have to rely on these drugs in the future. The procedure’s mainstay are immature cells in bone marrow—be they adult stem cells or progenitors. Right before an organ is transplanted, the patient first receives a bone marrow transplant, the marrow donated by the same person who is donating the organ. Immature cells in the donated marrow basically prime the patient’s immune system T-cells for the organ that follows, teaching the cells to recognize the donor’s cells “as self,” says David Sachs, director of the Transplantation Biology Research Center at Massachusetts General Hospital, the first hospital to perform this technique on humans. The first patient to receive the procedure—a kidney recipient—hasn’t had to take antirejection drugs since 1998, according to Sachs. If these good results continue, along

with freeing patients from the many complications of immunosuppressive drugs, “it could mean longer survival times for organs,” predicts Sachs.

Since there have been bones to break, bones have healed themselves due to the stromal stem cells found within bone marrow. (These stem cells are also called mesenchymal.) These cells produce osteoprogenitor cells, which generate osteoblasts, the mature cells that form bone. Although bone is naturally regenerative, repairing it isn’t necessarily simple, as when a fracture doesn’t heal, or bone is lost to cancer, infection, a mineral deficiency, a deformity, or an accident. Orthopedic researchers are studying how to control the differentiation of mesenchymal stem cells, so as to use their derivative cells almost as patching ingredients for the skeleton. One approach under investigation is to harvest marrow cells from a large bone with a fair amount of marrow (such as the hip bone), identify the mesenchymal stem cells, expand them in culture, and transplant them beside the fracture or other bone malady. Researchers are also applying stromal stem cells to injured cartilage, muscle, and tendon, which can be harder to treat than bone.

Motivated by the many women who lose a breast to cancer and opt for reconstructive surgery, Hava Avraham, a cell biologist at the Beth Israel Deaconess Medical Center in Boston, is in the preliminary stages of trying to direct stem cells to recreate the mammary fat pad, the tissue that lends structure to the breast. She and two postdocs are looking into whether transplanting either human or mouse bone marrow stem cells or embryonic stem cells into emptied fat pads can lead to tissue replacement. Meanwhile, other groups have reported that there does appear to be a single, self-renewing adult stem cell

that gives rise to the human mammary gland’s different types of specialized cells.

Despite the inherent difficulties of replacing brain cells, advances in growing and managing cells that might be used to treat and alleviate Parkinson’s disease invite guarded optimism. For a number of years, Ron McKay has been determining how to bring ES cells through a series of well-defined steps so that they will yield dopamine neurons. “Being able to control transitions—that’s what the game is all about,” says McKay, who is one of the field’s masters at maneuvering stem cells down a specific pathway. In 2000, McKay and his NIH group reported that they had induced mouse ES cells to differentiate into neural precursors, then dopamine neurons. Two years later, they went a step further. After growing dopamine neurons from ES cells, they injected them into the brains of rats with Parkinson’s-like symptoms, the rats “clearly benefitting,” states McKay.

Apart from the animal work, since the 1980s other researchers have applied experimental treatments directly to patients in the throes of advanced Parkinson’s. Neural tissue from aborted human fetuses has been transplanted into disease-affected brain regions of hundreds of patients worldwide, dozens of whom have experienced “up to a fifty percent reduction in their symptoms,” according to a report in Science in 2000. A few patients have even remained off medication for several years. Researchers see this as an encouraging sign that neural transplants not only can slow Parkinson’s but have the potential to reverse its symptoms and sidestep the disease’s root cause, which remains unknown. Less encouraging, most Parkinson’s patients reportedly have not benefitted long-term from fetal neuronal transplants, and more than a few have developed side effects—notably movement disorders in addition to those they already experienced. While apparently fetal tissue transplantation can sometimes work for Parkinson’s, because of its uneven track record, the

controversy that surrounds aborted fetuses, and the fact that tissue from as many as six fetuses is needed for every transplant, interest has largely swung to ES cells, and directing them into neurons and transplanting them, as McKay is doing in mice.

Adult stem cells are another possibility. In 2002, Michel Levesque, a neurosurgeon at Cedars-Sinai Medical Center, treated a former fighter jet pilot who has Parkinson’s with “remnants of adult neural stem cells” removed from the man’s prefrontal cortex, recounts Levesque. The cells were put into the man’s left putamen, a disease-affected brain region. The patient has maintained an eighty percent reduction in symptoms on the treated side, according to Levesque, but has developed symptoms on the untreated side. Levesque says he is waiting for the FDA to approve the same surgery for fifteen more patients.

Although using stem, progenitor, or mature cells to fashion organs and other three-dimensional body parts may seem over the top, there’s been gradual progress in this area for many years. Tissue engineers follow a basic design. First they create a degradable scaffold—a structure made from a synthetic or natural material meant to support the cells. Then they seed it with cells. While these 3-D creations are largely in the experimental phase, some clinical successes have resulted. One such instance arose in 1998, when a man arrived in the ER unit at the University of Mass Medical School having lost the top of his thumb to a machine. Charles Vacanti, then on the staff at UMass, and his lab crew configured a piece of coral into the same shape as the lost bone and seeded it with the man’s own osteoprogenitor cells. They derived these regenerative progenitor cells “from a biopsy of the periosteum (lining) of the man’s wrist bone,” Vacanti recounts. Once this scaffold was attached to the remaining part of the man’s thumb, “the bone cells matured and laid down new bone matrix, filling in the pores of the coral.” The man, further reports

Vacanti, “recovered more than ninety percent function in his thumb, as determined by standard hand surgery function tests.”

Major organs aren’t likely to be produced anytime soon; but there are definite steps in that direction. For example, researchers are employing mesenchymal stem cells from sheep bone marrow to develop heart valves; mesenchymal stem cells from humans to grow bone and cartilage; human vaginal progenitor cells to grow urethras and bladders; ES cells as well as differentiated muscle cells for crafting blood vessels; and liver progenitors to grow liver tissue.

Holding back tissue engineering for many years has been “the inability to expand cells in vitro,” according to Anthony Atala, the director of Wake Forest University’s new institute for regenerative medicine. It took his lab (previously at Children’s Hospital in Boston) as long as five years to learn how to grow specialized urothelial cells, which he went on to use for fashioning entire bladders in dogs. Tissue engineers say that the challenge will be to convert stem cells into large numbers of specialized cells for building specific structures.

As unthinkable as the notion of repairing the spinal cord once was, it isn’t any longer. Researchers believe that in the future the paralysis that presently incapacitates the approximately 2.5 million people living with spinal cord injury worldwide may respond to treatment with regenerative cells. The natural ability of damaged spinal cords of certain adult fish and amphibians to regenerate is taken as an optimistic sign.

Spinal cord regeneration likely will depend on a multifaceted approach. That’s because when the spinal cord is severed, “it isn’t just that nerves are cut,” observes Marie Filbin, a biologist at Hunter College of the City University of New York. Along with neurons, other cells—oligodendrocytes—that myelinate and insulate the axons of nerve cells can die, which also interferes with impulses be-

tween the brain and body. Antecedents for spinal cord regeneration possibly exist in humans, and yet degraded myelin will work against these molecular signals. Researchers are therefore headed in many directions. Using rodents, some are directing ES cells in a dish into oligoprogenitors and then injecting those cells into the injury site. Reports cite improved myelination and better locomotion in rats. Other groups are transplanting undifferentiated ES cells directly into rodents to see if they respond to the injury and replace missing tissue. “If you want to repair the [axons] that are cut, instead of getting them to grow a great distance, you might want to transplant in neurons so that they bridge the gap. It seems far-fetched, but it’s a possibility,” says Filbin.

Filbin’s group, in collaboration with Thomas Jessell’s at Columbia, is attempting to switch off genes in neural stem cells that inhibit regeneration, hoping that when transplanted, these cells will react to signals to regrow spinal cord tissue. Jessell lately gave transplanters much to cheer about. His lab demonstrated that mouse ES cells could be coaxed to become motor neurons in culture. A Johns Hopkins team then transplanted these cells into the spinal cord, where they survived and even sent out axons toward muscle.

Still other teams are trying to incite cells within the animal to migrate to the injured area of the spine to make new tissue.

Could aging be closely tied to a slowdown in stem cell production? That’s what a small but growing circle of investigators are looking into. A Duke team, for instance, has reported a link between aging in mice and a decline in bone marrow stem cells. With fewer numbers of these regenerators spinning out differentiated cells that repair arteries and keep them healthy and supple, the report related, atherosclerosis developed, a condition in which artery walls lose their elasticity.

Michael Young’s lab at the Schepens Eye Research Institute, with its expanding posse of postdocs, has recently moved from a cluster of small, ill-shaped rooms into spacious refurbished quarters down the hall. It’s a sign that Young’s funding is going well and that the powers that be are warming to the idea of restoring the retina through cell therapy, a notion that just a few years ago was looked upon as overly avant-garde. Throughout the vision field, a contagious interest in cell therapy is inviting a greater variety of studies into how cells might be directed to mend vision woes. Researchers are exploring the use of immature cells from the lacrimal gland to treat dry-eye syndrome; glial cells from the back of the eye to assist in retinal reattachment; immature cells from umbilical cord blood to reconstruct capillaries damaged by retinopathy. One occasionally hears of even more ambitious goals, such as attempts to regenerate a damaged optic nerve or grow an entire cornea, the latter work motivated by a shortage of donor corneas.

What about the ultimate challenge, that of replacing an entire eye? It seems an outrageous expectation. Then again, researchers have identified a patch of cells in the early embryo that becomes the eye, and they can remove these cells from frog embryos and sustain them in culture, whereupon they grow into an eye-like structure. Ask about the future, and it’s very much in the eye of the beholder.

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