Tag: Physiology (e.g. Exercise Physiology)

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Gravity-defying bird beak mystery solved: Shorebirds benefit from surface tension

As Charles Darwin showed nearly 150 years ago, bird beaks are exquisitely adapted to the birds’ feeding strategy. A team of MIT mathematicians and engineers has now explained exactly how some shorebirds use their long, thin beaks to defy gravity and transport food into their mouths. The phalarope, commonly found in western North America, takes advantage of surface interactions between its beak and water droplets to propel bits of food from the tip of its long beak to its mouth, the research team reports in the May 16 issue of Science. These surface interactions depend on the chemical properties of the liquid involved, so phalaropes and about 20 other birds species that use this mechanism are extremely sensitive to anything that contaminates the water surface, especially detergents or oil.

“Some species rely exclusively on this feeding mechanism, and so are extremely vulnerable to oil spills,” said John Bush, MIT associate professor of applied mathematics and senior author of the paper. Wildlife biologists have long noted the unusual feeding behavior of phalaropes, which spin in circles on the water, creating a vortex that sweeps small crustaceans up to the surface, just like tea leaves in a swirling tea cup. The birds peck at the surface, picking up millimetric droplets of water with their prey trapped inside. Since the birds point their beaks downward during the feeding process, gravity must be overcome to get those droplets from the tip of the bird’s long beak to its mouth. Until now, scientists have been puzzled as to how that happens. Scientists speculated that the feeding strategy depended on the drop’s surface tension. Surface tension normally dominates fluid systems that are small relative to raindrops (for example, the world of insects), but it was not clear how it could benefit shorebirds. A key observation was that in order to propel the drop, the birds open and close their beaks in a tweezering motion.

To unravel the mystery, Bush teamed up with Manu Prakash, a graduate student in MIT’s Center for Bits and Atoms, and David Quere, of the Ecole Polytechnique in Paris, a visiting professor in MIT’s math department at the time of the study. They built a mechanical model of the phalarope beak that allowed them to study the process in slow motion. The process depends on a surface interaction known as contact angle hysteresis, typically an impediment to drop motion on solids. For example, raindrops stick to window panes due to contact angle hysteresis. In the case of the bird beak, the time-dependent beak geometry couples with contact angle hysteresis to propel the drop upward. “This may be the first known example where droplet motion is enabled rather than resisted by contact angle hysteresis,” Bush said. As the beak scissors open and shut, each movement propels the water droplet one step closer to the bird’s mouth. Specifically, when the beak closes, the drop’s leading edge proceeds toward the mouth, while the trailing edge stays put. When the beak opens, the leading edge stays in place while the trailing edge recedes toward the mouth.

In this stepwise ratcheting fashion, the drop travels along the beak at a speed of about 1 meter per second. The efficiency of the process, which the authors dub the “capillary ratchet,” depends on the beak shape: Long, narrow beaks are best suited to this mode of feeding. The study highlights the sensitivity of this mechanism to the opening and closing angles of the beak: “Varying these angles by a few degrees can change the drop speed by a factor of 10,” Quere said. The capillary ratchet also depends critically on the beak’s wettability–a measure of a liquid’s tendency to bead up into droplets or spread out to wet its surface. Oil is much more “wetting” than water, so if the beak is soaked in oil from a spill, this process won’t work. The researchers note a potential application of nature’s design: “We are currently exploring microfluidic devices in which this mechanism could be exploited for directed droplet transport, allowing for controlled stepwise motion of microliter droplets,” Prakash said.

Science Daily
May 27, 2008

Original web page at Science Daily

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Study links low-frequency hearing to shape of the cochlea

Specifically, it is the shape of the cochlea – the snail-shell-shaped organ in the inner ear that converts sound waves into nerve impulses that the brain deciphers – which proves to be surprisingly important. A study published online last week in the Proceedings of the National Academy of Sciences establishes a direct link between the cochlea’s curvature and the low-frequency hearing limit of more than a dozen different mammals. The relationship will be useful in conservation to estimate the impact that the noises of human activities are having on animals like Siberian tigers, polar bears and marine mammals that won’t sit still for hearing tests. It also can provide new information about the hearing of extinct mammals, like mammoths and saber-toothed tigers, and, in so doing, may contribute new insights into how the sense of hearing evolved. “It turns out that it is the curvature of the cochlea, not its size, that is highly correlated to the low-frequency hearing limit,” says Daphne Manoussaki, assistant professor of mathematics at Vanderbilt University, who headed the new study with Richard S. Chadwick, a section chief at the National Institute on Deafness and Other Communication Disorders (one of the National Institutes of Health, or NIH).

Spiral-shaped cochleae are exclusive to mammals. Birds and reptiles generally have plate-like or slightly curved versions of this critical organ, limiting the span of octaves that they can hear. Animals with tightly coiled cochleae tend to have greater hearing ranges, but previous attempts to associate these auditory effects with the physical characteristics of the cochlea have proven unsatisfactory because they did not take a critical acoustic effect into account. In 2006 Manoussaki and her NIH collaborators published a paper proposing that the helical shape of the cochlea enhances low-frequency sounds through an effect analogous to the well-known “whispering gallery effect” in which soft sounds that travel along curved walls in a large chamber remain loud enough that they can be heard clearly on the opposite side of the room. When sound waves enter the ear, they strike the eardrum and cause it to vibrate. Tiny bones in the ear amplify and transmit these vibrations to the fluid in the cochlea, creating pressure waves that travel along a narrowing canal in the coiled tube-like organ. The canal is one of two main chambers that are created by an elastic membrane that runs the length of the cochlea. The mechanical properties of this “basilar” membrane vary from very stiff at the broad, outer end to increasingly flexible toward the inner end as the chambers narrow. The basilar membrane’s graded properties cause the waves to grow and then die away. Different frequencies peak at different positions along the membrane.

Sensory cells are attached to the basilar membrane and have tufts of tiny hairs called stereocilia that stick up into adjacent structures in the canal. As the basilar membrane moves it tilts the sensory cells, causing the stereocilia to bend. The motion generates electric signals that travel along the auditory nerve to the brain. As a result, the sensory cells near the outer end of the cochlea detect high-pitched sounds, like the notes of a piccolo, while those at the inner end of the spiral detect lower-frequency sounds, like the booming of a bass drum. This mechanical ordering of response from high to low frequencies works in the same fashion whether the cochlear tube is laid out straight or coiled in a spiral. But Manoussaki’s calculations predicted that the spiral shape causes the energy in the low-frequency waves to accumulate against the outside edge of the chamber. This uneven energy distribution, in turn, causes the membrane to move more toward the outer wall of the chamber, enhancing the bending of the stereocilia. The enhancement is strongest at the apex of the spiral, where the lowest frequencies are detected. Manoussaki and her collaborators calculated that the increase in the sound pressure level can be as much as 20 decibels, equivalent to the difference between the aural ambience of a quiet restaurant and a busy street.

“The idea that the cochlea’s curvature has a significant effect on hearing has been quite controversial for many years,” says Darlene R. Ketten, a senior scientist at Woods Hole Oceanographic Institution and assistant professor at the Harvard Medical School, who participated in the current study. “Curvature was often dismissed or, when examined, the theories were not entirely satisfactory. Now we have a theory that we have confirmed with a number of concrete examples using real ear shapes and hearing abilities.” Ketten provided Manoussaki and her collaborators with high-resolution CT scans of the cochleae of a number of different species of land and marine mammals. Together with her biophysicist colleagues, Manoussaki analyzed these shapes and found that low- frequency hearing limits of species ranging from mice to cats to cows to whales varied in step with the ratio of the radii of curvatures at their cochlea’s base to that of its apex. This ratio varies from about two to nine: The larger it is the lower the frequencies that the animal can hear.

“This makes sense because the bigger the ratio, the tighter the spiral is wound and more of the sound wave energy in the low-frequency waves is forced against the cochlea’s walls,” Manoussaki says. Animals like mice, which have a radii ratio of about two, can’t hear much below 1000 hertz (Hz). Species like cows and elephants, which have a ratio of about nine, hear sounds as low as 20 Hz. The power of this approach is illustrated by the cat, guinea pig and sea lion. The cochlea of the cat is longer than that of the guinea pig, but the guinea pig has a ratio of 7.2 and can hear down to 47 Hz, while the cat, with a smaller ratio of 6.2, has a higher threshold of 55 Hz. Similarly, the sea lion has a basilar membrane three times as long as that of the guinea pig. But its radii ratio is 5.2, lower than either the cat or the guinea pig, and it cannot make out sounds below 180 Hz. (This limit is for the sea lion’s hearing in air; under water it can hear down to 200 Hz.) “What I like about this is that a macroscopic feature of the ear has such a major effect on our hearing,” says Manoussaki. “As colleagues have pointed out, so much research today is done at the genetic and cellular level that you don’t often see cases like this where simple geometry proves to be so important.”

EurekAlert! Medicine
May 13, 2008

Original web page at EurekAlert! Medicine

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Biological link between pain and fatigue discovered

A recent University of Iowa study reveals a biological link between pain and fatigue and may help explain why more women than men are diagnosed with chronic pain and fatigue conditions like fibromyalgia and chronic fatigue syndrome. Working with mice, the researchers, led by Kathleen Sluka, Ph.D., professor in the Graduate Program in Physical Therapy and Rehabilitation Science in the UI Roy J. and Lucille A. Carver College of Medicine, found that a protein involved in muscle pain works in conjunction with the male hormone testosterone to protect against muscle fatigue. Chronic pain and fatigue often occur together — as many as three in four people with chronic, widespread musculoskeletal pain report having fatigue; and as many as 94 percent of people with chronic fatigue syndromes report muscle pain. Women make up the majority of patients with these conditions.

To probe the link between pain and fatigue, and the influence of sex, the UI team compared exercise-induced muscle fatigue in male and female mice with and without ASIC3 — an acid-activated ion channel protein that the team has shown to be involved in musculoskeletal pain. A task involving three one-hour runs produced different levels of fatigue in the different groups of mice as measured by the temporary loss of muscle strength caused by the exercise. Male mice with ASIC3 were less fatigued by the task than female mice. However, male mice without the ASIC3 protein showed levels of fatigue that were similar to the female mice and were greater than for the normal males. In addition, when female mice with ASIC3 were given testosterone, their muscles became as resistant to fatigue as the normal male mice. In contrast, the muscle strength of female mice without the protein was not boosted by testosterone.

“The differences in fatigue between males and females depends on both the presence of testosterone and the activation of ASIC3 channels, which suggests that they are interacting somehow to protect against fatigue,” Sluka said. “These differences may help explain some of the underlying differences we see in chronic pain conditions that include fatigue with respect to the predominance of women over men.” The study, which was published in the Feb. 28 issue of the American Journal of Physiology — Regulatory, Integrative and Comparative Physiology, indicates that muscle pain and fatigue are not independent conditions and may share a common pathway that is disrupted in chronic muscle pain conditions. The team plans to continue their studies and investigate whether pain enhances fatigue more in females than males. “Our long-term goal is to come up with better treatments for chronic musculoskeletal pain,” Sluka said. “But the fatigue that is typically associated with chronic, widespread pain is also a big clinical problem — it leaves people unable to work or engage in social activities. If we could find a way to reduce fatigue, we could really improve quality of life for these patients.”

Science Daily
April 29, 2008

Original web page at Science Daily

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Alligators’ muscles move lungs around for sneaky maneuvers in water

Without a ripple in the water, alligators dive, surface or roll sideways, even though they lack flippers or fins. University of Utah biologists discovered gators maneuver silently by using their diaphragm, pelvic, abdominal and rib muscles to shift their lungs like internal floatation devices: toward the tail when they dive, toward the head when they surface and sideways when they roll. It allows them to navigate a watery environment without creating a lot of disturbance,” says doctoral student T.J. Uriona. “This is probably really important while they are trying to sneak up on an animal but don’t want to create ripples.” The discovery in American alligators suggests “special muscles that manipulate the position of the lungs — and thus the center of buoyancy — may be an underappreciated but important means for other aquatic animals to maneuver in water without actively swimming,” says C.G. Farmer, an assistant professor of biology. Those animals include crocodiles, African clawed frogs, some salamanders, turtles and manatees, she adds, noting that the use of muscles to move the lungs may be “incredibly important or you would not see it evolve repeatedly.”

The researchers found that alligators are somewhat like pilots using controls to adjust an aircraft’s pitch and roll, except the reptiles’ controls are muscles that help them shift their lungs backward to dive, forward to surface or sideways to roll. Farmer says the new study asked how gators “manage to maneuver so gracefully without the fins and flippers used by fish, seals and other adept swimmers.” “The secret to their aquatic agility lies in the use of several muscles, such as the diaphragmatic muscle, to shift the position of their lungs. The gases in the lungs buoy up the animal, but if shifted forward and backward cause the animal to pivot in a seesaw motion. When the animals displace gases to the right or left side of the body, they roll.” Uriona says that during the Triassic Period, which began 250 million years ago, the crocodilian ancestors of alligators were cat-sized animals that lived only on land. “Until now, it was believed the diaphragmatic muscle evolved to help them breathe and run at the same time,” he says. “Showing they are actually using it to move around in water gives an alternative explanation for why the muscle evolved.” It also suggests the muscle didn’t evolve until after crocodilians took to the water during the Cretaceous Period, which began 145 million years ago. During that time alligators’ ancestors also evolved a flattened skull, shorter limbs and a big tail.

Science Daily
April 1, 2008

Original web page at Science Daily

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New hearing mechanism discovered

MIT researchers have discovered a hearing mechanism that fundamentally changes the current understanding of inner ear function. This new mechanism could help explain the ear’s remarkable ability to sense and discriminate sounds. Its discovery could eventually lead to improved systems for restoring hearing. MIT Professor Dennis M. Freeman, working with graduate student Roozbeh Ghaffari and research scientist Alexander J. Aranyosi, found that the tectorial membrane, a gelatinous structure inside the cochlea of the ear, is much more important to hearing than previously thought. It can selectively pick up and transmit energy to different parts of the cochlea via a kind of wave that is different from that commonly associated with hearing. Ghaffari, the lead author of the paper, is in the Harvard-MIT Division of Health Sciences and Technology, as is Freeman. All three researchers are in MIT’s Research Laboratory of Electronics. Freeman is also in MIT’s Department of Electrical Engineering and Computer Science and the Massachusetts Eye and Ear Infirmary.

It has been known for over half a century that inside the cochlea sound waves are translated into up-and-down waves that travel along a structure called the basilar membrane. But the team has now found that a different kind of wave, a traveling wave that moves from side to side, can also carry sound energy. This wave moves along the tectorial membrane, which is situated directly above the sensory hair cells that transmit sounds to the brain. This second wave mechanism is poised to play a crucial role in delivering sound signals to these hair cells. n short, the ear can mechanically translate sounds into two different kinds of wave motion at once. These waves can interact to excite the hair cells and enhance their sensitivity, “which may help explain how we hear sounds as quiet as whispers,” says Aranyosi. The interactions between these two wave mechanisms may be a key part of how we are able to hear with such fidelity – for example, knowing when a single instrument in an orchestra is out of tune.

“We know the ear is enormously sensitive” in its ability to discriminate between different kinds of sound, Freeman says. “We don’t know the mechanism that lets it do that.” The new work has revealed “a whole new mechanism that nobody had thought of. It’s really a very different way of looking at things.” The tectorial membrane is difficult to study because it is small (the entire length could fit inside a one-inch piece of human hair), fragile (it is 97 percent water, with a consistency similar to that of a jellyfish), and nearly transparent. In addition, sound vibrations cause nanometer-scale displacements of cochlear structures at audio frequencies. “We had to develop an entirely new class of measurement tools for the nano-scale regime,” Ghaffari says. The team learned about the new wave mechanism by suspending an isolated piece of tectorial membrane between two supports, one fixed and one moveable. They launched waves at audio frequencies along the membrane and watched how it responded by using a stroboscopic imaging system developed in Freeman’s lab. That system can measure nanometer-scale displacements at frequencies up to a million cycles per second.

The team’s discovery has implications for how we model cochlear mechanisms. “In the long run, this could affect the design of hearing aids and cochlear implants,” says Ghaffari. The research also has implications for inherited forms of hearing loss that affect the tectorial membrane. Previous measurements of cochlear function in mouse models of these diseases “are consistent with disruptions of this second wave,” Aranyosi adds. Because the tectorial membrane is so tiny and so fragile, people “tend to think of it as something that’s wimpy and not important,” Freeman says. “Well, it’s not wimpy at all.” The new discovery “that it can transport energy throughout the cochlea is very significant, and it’s not something that’s intuitive.”

Science Daily
October 30, 2007

Original web page at Science Daily

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Gazelles shrink liver and heart to reduce oxygen consumption during drought

How do gazelles and other large desert mammals adjust their physiology to survive when food and water are in short supply? A fascinating new study from the July/August issue of Physiological and Biochemical Zoology reveals that gazelles in the deserts of Saudi Arabia have evolved the ability to shrink oxygen-demanding organs such as the liver and heart, allowing them to breathe less. Fewer breaths reduce the amount of water lost to respiratory evaporation during prolonged periods of drought.

“We found that gazelles had the lowest total evaporative water loss ever measured in an arid zone ungulate [hoofed animal],” write Stéphane Otrowski (National Wildlife Research Center, Saudi Arabia), Pascal Mésochina (National Wildlife Research Center, Saudi Arabia), and Joseph B. Williams (Ohio State University). Sand gazelles’ livers and hearts — which are important determinants of metabolic rate — decrease significantly in mass during four months of food and water restriction. Conversely, the gut walls, which are responsible in ruminants for 28-46% of whole-body protein synthesis, an energy demanding process, did not decrease significantly in mass. There are few sources of drinking water in the desert, so sand gazelles must rely on vegetation for both food and water requirements.

“The deserts of the Arabian Peninsula are among the most austere of terrestrial environments, with low, unpredictable rainfall, and high ambient temperature,” explain the authors. “The sand gazelle has evolved a remarkable capacity to reduce its evaporative water losses, which is likely a component of their success.” Unexpectedly, the researchers also found that deprived sand gazelles had a higher fat content in the brain, revealing that gazelles may store fats in the brain to secure brain metabolism during prolonged food and water deprivation.

Science Daily
July 3, 2006

Original web page at Science Daily

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Hearts and minds

What do giraffes and fighter pilots have in common? They both experience extreme rushes of blood from the head: pilots (from the force created by rapid acceleration) and giraffes (merely from lifting their necks). Pilots wear special flight suits to avoid fainting. Now, a new study suggests that a powerful heart is what keeps the giraffes from swooning. Thanks to its long neck, a giraffe’s head can rise up to 5 meters in mere seconds after the creature takes a drink. One would expect this dramatic motion to trigger a massive drain of blood from the brain, but giraffes obviously aren’t fainting all over the place. As long ago as 1955, researchers speculated that giraffes keep their head full using a sort of siphon system, whereby the pull created by blood flowing from the brain via the jugular vein draws extra blood from the heart via the carotid artery. Others hypothesized that the heart alone did the job, pumping blood at sufficiently high pressure to keep the brain running smoothly.

To solve the dizzying conundrum, zoologist Graham Mitchell and his team at the University of Wyoming in Laramie built a machine that replicated the length, rigidity, and pressure of the blood-flow system in a giraffe’s neck. Rubber and PVC tubes substituted for the jugular vein and carotid artery, and an electric pump became the heart. When the researchers tested the siphon model, they found that the pressures within the tubing were different from those known to exist in real giraffes. In contrast, when the team had the “heart” do all the work, the blood-vessel pressures nicely matched up with those seen in the living creature.

If the model holds, a giraffe’s blood pressure is about twice as high as that seen in people, says Mitchell. He suspects that a muscular cuff at the base of the jugular vein constricts as the giraffe stands to maintain that pressure in the brain. The team reports its results in the current issue of the Journal of Experimental Biology. “It’s a very good paper from a modeling standpoint,” says Alan Hargens, a physiologist at the University of California, San Diego. The next step, Hargens says, is more hands-on work with giraffes to put the model to the test.

ScienceNow Magazine
July 3, 2006

Original web page at ScienceNow Magazine

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Reactive oxygen species shown essential for development of inner ear’s balance machinery

Reactive oxygen species (ROS) are normally produced as a product of metabolism, and, as their name implies, they are highly reactive with surrounding biological components. The ability of ROS to damage DNA and other critical molecules underlies their reputation for causing deleterious cellular effects and their association with aging, carcinogenesis, and atherosclerosis. However, in an unanticipated discovery suggesting that ROS may play important positive roles in development, researchers have found that the production of ROS by a particular enzyme is essential for inner ear development and for the ability to properly maintain balance.

A biologically constructive function for ROS in development was unanticipated. Even a previously known beneficial role for ROS seems to be intimately linked to toxicity: White blood cells generate ROS by an NADPH oxidase enzyme to kill invading bacteria. In recent years, evidence has been accumulating that other NADPH oxidases, similar to that of white blood cells, are widespread in the body, but their function remains largely obscure.

In their study, the researchers show that a spontaneously discovered line of mutant mice, named “head slant” because of the odd head and body posture of these animals, carries a mutation in the Noxo1 gene, which encodes an NADPH oxidase enzyme. This error in Noxo1 inactivates the enzyme in the inner ear, leading to very specific consequences: Mutant mice lack the tiny calcium carbonate crystals of the inner ear, which, because of their large inert mass, normally enable animals to determine the direction of gravitational pull. The Noxo1 mutant mice are unable to sense gravity and therefore often fall, rest in a slanted posture, and are unable to remain on the surface of water. By inserting an intact Noxo1 gene into the genome of “head slant” mutants, researchers enabled the mutant mice to maintain balance. These findings indicate that reactive oxygen species produced at the right place at the right time can have a constructive developmental role, in contrast to their previously known toxic effects.

Science Daily
February 14, 2006

Original web page at Science Daily

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Trials of the heart

Adult human stem cells may offer the opportunity to use one of biomedical science’s most promising technologies without the ethical dilemmas of embryonic cells. But whether the cells’ plasticity-or ability to ignore germ-line heritage and differentiate into therapeutically useful tissues-warrants clinical application at this stage remains controversial. “We’re still debating it,” says Amy Wagers, Harvard Medical School investigator and plasticity critic. “It’s too early to tell which way things will fall.” Biologists generally agree that even the most potent adult stem cells can’t approach the therapeutic power of embryonic stem cells. Nevertheless, at least a dozen clinical trials based on adult-cell plasticity have commenced in patients with serious heart disease – prematurely, some contend.

In embryonic development, cells form three germ layers: ectoderm, mesoderm, or endoderm. Generally, biologists considered cell differentiation overwhelmingly unidirectional and progressively restrictive. A cell fated to make neurons could not make blood cells; a stem cell fated to make white and red blood cells could not make a heart or a liver. Somatic cells could not transdifferentiate, switching from one lineage to the next; nor could they dedifferentiate, reverting to less specialized versions. Research since the late 1990s argued that some stem cells are significantly more flexible than originally thought. A flurry of animal research hinted that hematopoietic stem cells (HSCs) could become neural, muscular, skeletal, liver, kidney, lung, and skin cells. HSCs went to the site of the injury and apparently changed into a mélange of tissues. In one influential paper, Donald Orlic at the US National Institutes of Health reported that massive numbers of HSCs had changed into cardiomyocytes in heart-injured mice.

The relative safety of injecting heart patients with their own mobilized blood prompted a clinical trial in Brazil, and by 2003, ten international trials had enrolled human subjects with end-stage heart disease. Clinicians reported anecdotal cases of formerly bedridden patients jogging after the procedure. In early 2004, the US Food and Drug Administration reviewed the European data and approved similar trials in Boston, Texas, and most recently, at Johns Hopkins University in Baltimore. The Baltimore studies use powerful mesenchymal cells, a multipotent stem cell found in bone marrow. Nevertheless, the European trials showed modest improvement in heart function after myocardial infarction, along with some evidence of angiogenesis. Left ventricular ejection fraction, the percentage of blood pumped into the aorta, improved as much as 6% more for treated patients than for controls. Others showed no significant differences in improvement.

Douglas Losordo, a Tufts University clinician who has treated his patients with HSCs, says the therapy isn’t designed as a cure but is akin to “giving a booster dose of the natural mechanism for tissue repair.” Robert Lanza, vice president of medical and scientific development at Advanced Cell Technology in Worcester, Mass., says he’s not surprised by the early clinical data. He reports that in his mouse research, blood progenitor cells replaced nearly 40% of damaged heart tissue. Lanza maintains, “As long as the HSC-heart trials are done safely, the results give us good information.” He adds, “If I had a heart infarct, I’d be happy with a 10% improvement.” Critics point out that the initial trials were conducted with few patients and note potential flaws in the conclusion that hematopoietic cells changed into heart muscle cells. Stanford Nobel laureate and biochemist Paul Berg says, “The mouse studies used unpurified populations of stem cells. So did the heart trials. It could be some growth factor carried along with the supernatant or a resident heart stem cell that’s responsible for the effect, not plasticity.”

By early 2004, many of the original claims of stem cell switching had been refuted. Early results couldn’t be repeated and experimental design was called into question. Several papers indicated that adult stem cells had not changed into, but rather fused with organ cells. Two labs attempted to repeat experiments showing blood-heart plasticity in mice and primates, but failed. In a follow-up study, Wagers, along with Stanford’s Leora Balsam, Robert Robbins, and HSC pioneer Irving Weissman, injected highly purified populations of genetically tagged HSCs into the heart muscle of 23 mice. The transplanted cells did not increase the survival rate in mice, did not typically persist in the heart muscle more than 30 days, and did not produce the signature proteins of heart cells. They did notice slightly improved pumping efficiency, however.

It’s important to note that the bone marrow-heart trials have been conducted safely, and the clinical effect, while small, is significant. Despite this, many basic scientists say the clinicians moved too swiftly. Transplanting mixed populations of cells, they say, leaves the mechanisms of the therapy locked in a black box. “It is important to do as much basic research as we can before going to clinical trials,” says Johns Hopkins University professor Saul Sharkis. “We still don’t completely understand the mechanisms of stem cell repair.”

Wagers worries about declaring victory too early. “If we consider a 6% improvement in cardiac function a success, then we’ve left behind an opportunity to understand why this is happening and aim for a 60% improvement.”
Clinicians bristle when told that they should slow down. Joshua Hare, principal investigator for the Johns Hopkins trial says, “It’s unethical to wait. We won’t fully understand the mechanism until we do the clinical research. That’s what evidence-based medicine is all about.”

Basic scientists continue to dispute whether truly plastic cells can be found in adult cell populations. Yale University professor Diane Krause says she believes there is a potent stem cell in the bone marrow that can change into lung, liver, and skin. She rejects that hematopoietic stem cells (HSCs) transdifferentiate, however, and instead says she has found a yet-uncharacterized “marrow-derived cell.” According to Krause, “There is no question that a marrow-derived cell can make epithelial cells. The question is how: fusion or differentiation. I feel it could be both.” Johns Hopkins University oncologist Saul Sharkis, who collaborated with Krause in 2001, rules out fusion. “The most important thing is to know what kind of cell you transplant. A primitive cell has more potential.”

In 2002, the University of Minnesota’s Catherine Verfaillie provided evidence for multipotent adult progenitor cells (MAPCs).2 MAPCs are extremely rare – numbering less than 2,000 in a single mouse. They live indefinitely in culture and can be coaxed to generate different germ layers and liver, neural, and endothelial cells. The difficulty with MAPCs is that they haven’t been identified in vivo and have a finicky nature, making Verfaillie’s experiments hard to repeat. Scott Dylla, a scientist with OncoMed Pharmaceuticals, spent five years working with Verfaillie. Dylla says though MAPCs were thought to be HSC “changelings” early on, Verfaillie was more careful than most about genetically marking clonal populations before observing their differentiation pathways.

Better characterization of the starting material could help. Before transplanting stem cells, Harvard Medical School’s Amy Wagers uses fluorescence-activated cell sorting to pick out pure HSCs on the basis of 12 surface antigens, including the important CD45 markers. For her part, Krause purifies cells taken from bone marrow, though she admits some heterogeneity. Sharkis first elutes cells based on size and density, and then removes nonstem cells with an antibody (a step called lineage-depletion). He labels the remaining cells with a dye and puts them back into an irradiated mouse. The damaged marrow swings into a repair mode, enriching the cells 1000-fold. The marrow is harvested and transplanted, repairing damaged organs.

Sharkis and Krause may indeed have their hands on a rare MAPC-like cell. But Wagers sees no differentiation of her cells into heart or lung. Krause says Wagers’ purification scheme could leave behind a new “marrow-derived” cell. And Dylla posits that the claims of new potent adult stem cells found in blood could be “rediscovered” MAPCs. The round robin of claims and counter claims are evidence of a subtle – but important – shift in the debate about stem cell plasticity. At least among this group of hematopoietic stem cell experts, the discussion has turned to whether novel varieties of multipotent stem cells exist in the bone marrow.

The Scientist
July 19, 2005

Original web page at The Scientist