Thursday, June 3, 2010


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Tiny variations in the isotopic composition of silver in meteorites and Earth rocks are helping scientists put together a timetable of how our planet was assembled beginning 4.568 billion years ago. The new study, published in the journal Science, indicates that water and other key volatiles may have been present in at least some of Earth’s original building blocks, rather than acquired later from comets, as some scientists have suggested.

Compared to the Solar System as a whole, Earth is depleted in volatile elements, such as hydrogen, carbon, and nitrogen, which likely never condensed on planets formed in the inner, hotter, part of the Solar System. Earth is also depleted in moderately volatile elements, such as silver.

“A big question in the formation of the Earth is when this depletion occurred,” says co-author Richard Carlson of the Carnegie Institution for Science’s Department of Terrestrial Magnetism. “That’s where silver isotopes can really help.”

Silver has two stable isotopes, one of which, silver-107 was produced in the early Solar System by the rapid radioactive decay of palladium-107. Palladium-107 is so unstable that virtually all of it decayed within the first 30 million years of the Solar System’s history.

Silver and palladium differ in their chemical properties. Silver is the more volatile of the two, whereas palladium is more likely to bond with iron. These differences allowed the Carnegie researchers, which included Carlson lead author Maria Schönbächler (a former Carnegie Institution postdoctoral scientist now at the University of Manchester) Erik Hauri, Mary Horan, and Tim Mock to use the isotopic ratios in primitive meteorites and rocks from Earth’s mantle to determine the history of Earth’s volatiles relative to the formation of Earth’s iron core. Other evidence, specifically from hafnium and tungsten isotopes, indicates that the core formed between 30 to 100 million years after the origin of the Solar System.

“We found that the silver isotope ratios in mantle rocks from the Earth exactly matched those in primitive meteorites,” says Carlson. “But these meteorites have compositions that are very volatile-rich, unlike the Earth, which is volatile-depleted.”

The silver isotopes also presented another riddle, suggesting that the Earth’s core formed about 5-10 million years after the origin of the Solar System, much earlier than the date from the hafnium-tungsten results.

The group concludes that these contradictory observations can be reconciled if Earth first accreted volatile-depleted material until it reached about 85% of its final mass and then accreted volatile-rich material in the late stages of its formation, about 26 million years after the Solar System’s origin. The addition of volatile-rich material could have occurred in a single event, perhaps the giant collision between the proto-Earth and a Mars-sized object thought to have ejected enough material into Earth orbit to form the Moon.

The results of the study support a 30-year old model of planetary growth called “heterogeneous accretion,” which proposes that the Earth’s building blocks changed in composition as the planet accreted. Carlson adds that it would have taken just a small amount of volatile-rich material similar to primitive meteorites added during the late stages of Earth’s accretion to account for all the volatiles, including water, on the Earth today.

Carnegie Institution of Washington


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On average, there are as many prime numbers for which the sum of decimal digits is even as prime numbers for which it is odd. This hypothesis, first made in 1968, has recently been proven by researchers from the Institut de Mathématiques de Luminy (CNRS/Université de la Méditerranée).

A prime number is an integer greater than or equal to 2 that has exactly two distinct natural number divisors, 1 and itself. For example, 2, 3, 5, 7, 11,..., 1789, etc. are prime numbers, whereas 9, divisible by 3, is not a prime number.

Numerous arithmetical problems concern prime numbers and most of them still remain unresolved, sometimes even after several centuries. For example, it has been known since Euclid that the sequence of prime numbers is infinite, but it is still not known if an infinity of prime numbers p exists such that p+2 is also a prime number (problem of twin prime numbers). In the same way, it is not known if there exists an infinity of prime numbers, the decimal representation of which does not use the digit 7.

Two researchers from the Institut de Mathématiques de Luminy (CNRS/Université de la Méditerranée) have recently made an important breakthrough regarding a conjecture formulated in 1968 by the Russian mathematician Alexandre Gelfond concerning the sum of digits of prime numbers. In particular, they have demonstrated that, on average, there are as many prime numbers for which the sum of decimal digits is even as prime numbers for which it is odd.

The methods employed to arrive at this result, derived from combinatorial mathematics, the analytical theory of numbers and harmonic analysis, are highly groundbreaking and should pave the way to the resolution of other difficult questions concerning the representation of certain sequences of integers.

Quite apart from their theoretical interest, these questions are directly linked to the construction of sequences of pseudo-random numbers and have important applications in digital simulation and cryptography.

Centre National de la Recherche Scientifique


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Protestants who are reminded of their religion do more good deeds and trust that other people will do so, as well. But when Catholics are reminded of their religion, they do less public service and are less trusting that others will do so.

These are among the findings of a new Cornell study that explored the connections between religious affiliation and economic behavior.

Previous work on the effect of religion was based on correlations. For example, "People find [that] a sample of Protestants is more hard working than Catholics, and they want to conclude from that that it's some sort of effect from Protestantism," said Daniel Benjamin, assistant professor of economics and co-author of the study, which is published as a National Bureau of Economic Research working paper.

"But Protestants, Catholics and Jews differ in all sorts of ways besides their religion, and those other factors -- some of which might be very hard to measure -- could in fact be what's responsible for the difference in behavior," he said.

To isolate the effect of religion, Benjamin and co-authors James Choi of Yale and Cornell undergraduate Geoffrey Fisher '10 used a method developed by psychologists called priming in which 827 Cornell students unscrambled sentences. One group's sentences contained words associated with religion -- sacred, God, prophet -- and the second group's did not.

"Each of us has multiple identities. I'm white, I'm Jewish, I'm an economist," said Benjamin. "But the extent to which my behavior is affected by any of those identities depends on how salient that aspect of my identity is at a particular moment."

Belief-related words made the subjects' religious identity salient by "activating neural connections related to whatever sort of norms of behavior the subjects associate with their religion, in ways they might not be conscious of," Benjamin said.

The researchers then observed the behavior of the primed and unprimed subjects in a series of economic games, which allowed them to infer that whatever differences they uncovered were the causal effect of the religious identity norms. They then analyzed the data separately by stated religion of the participants, and tested various hypotheses.

"We found support for the hypothesis, for example, that Protestants who were exposed to the religious prime would give more money than Protestants [whose] religion was not made salient," said Benjamin, and "that the primed Catholics contributed less to the public good. That was exactly consistent with the existing hypothesis literature."

However, the researchers "found no effect of priming religion on any measure of work ethic [or] for the idea that religion makes people more forward looking, more willing to delay gratification," Benjamin said.

The study produced mixed evidence about risk taking: Primed Protestants were less inclined to take risks, while primed Catholics took more risks. Interestingly -- and unpredicted by any pre-existing hypothesis -- Jews who were primed were more reciprocal, repaying kindness with kindness, than Jews who were not primed.

"Another interesting thing we found is that Protestants who believe more in divine punishment give more to the public good when primed," Benjamin said.

As his role in the study grew, Geoffrey Fisher '10 -- who helped program the experiment, analyze data and edit the paper -- was promoted from research assistant to co-author. "Having the opportunity to co-author a paper with Dan and James has been a wonderful experience to have at such an early point in my career," said Fisher, who will be a graduate student at Caltech in the fall studying how the brain makes economic decisions and is affected by environmental stimuli, such as primes. "They gave me a glimpse into what a real research project is like."

(Photo: Cornell U.)

Cornell University


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The age-old question of why men store fat in their bellies and women store it in their hips may have finally been answered: Genetically speaking, the fat tissue is almost completely different.

“We found that out of about 40,000 mouse genes, only 138 are commonly found in both male and female fat cells,” said Dr. Deborah Clegg, assistant professor of internal medicine at UT Southwestern Medical Center and senior author of the study appearing in the International Journal of Obesity. “This was completely unexpected. We expected the exact opposite – that 138 would be different and the rest would be the same between the sexes.”

The study involved mice, which distribute their fat in a sexually dimorphic pattern similar to humans.

“Given the difference in gene expression profiles, a female fat tissue won’t behave anything like a male fat tissue and vice versa,” Dr. Clegg said. “The notion that fat cells between males and females are alike is inconsistent with our findings.”

In humans, men are more likely to carry extra weight around their guts while pre-menopausal women store it in their butts, thighs and hips. The bad news for men is that belly, or visceral, fat has been associated with numerous obesity-related diseases including diabetes and heart disease. Women, on the other hand, are generally protected from these obesity-related disorders until menopause, when their ovarian hormone levels drop and fat storage tends to shift from their rear ends to their waists.

“Although our new findings don’t explain why women begin storing fat in their bellies after menopause, the results do bring us a step closer to understanding the mechanisms behind the unwanted shift,” Dr. Clegg said.

For this study, researchers used a microarray analysis to determine whether male fat cells and female fat cells were different between the waist and hips and if they were different based on gender at a genetic level.

Because the fat distribution patterns of male and female mice are similar to those of humans, the researchers used the animals to compare genes from the belly and hip fat pads of male mice, female mice and female mice whose ovaries had been removed – a condition that closely mimics human menopause. Waist and hip fat (subcutaneous fat) generally accumulates outside the muscle wall, whereas belly fat (visceral fat), a major health concern in men and postmenopausal women, develops around the internal organs.

In addition to the genetic differences among fat tissues, the researchers found that male mice that consumed a high-fat diet for 12 weeks gained more weight than female mice on the same diet. The males’ fat tissue, particularly their belly fat, became highly inflamed, while the females had lower levels of genes associated with inflammation. The female mice whose ovaries had been removed, however, gained weight on the high-fat diet more like the males and deposited this fat in their bellies, also like the males.

“The fat of the female mice whose ovaries had been removed was inflamed and was starting to look like the unhealthy male fat,” Dr. Clegg said. “However, estrogen replacement therapy in the mice reduced the inflammation and returned their fat distribution to that of mice with their ovaries intact.”

Dr. Clegg said the results suggest that hormones made by the ovaries may be critical in determining where fat is deposited. Her overall goal is to determine how fat tissue is affected by sex hormones and whether it would be possible to develop a “designer” hormone replacement therapy that protected postmenopausal women from belly fat and related diseases such as metabolic syndrome.

(Photo: UTSMC)

UT Southwestern Medical Center


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Paleontologists have discovered a rich array of exceptionally preserved fossils of marine animals that lived between 480 million and 472 million years ago, during the early part of a period known as the Ordovician. The specimens are the oldest yet discovered soft-bodied fossils from the Ordovician, a period marked by intense biodiversification. The findings, which appear in the May 13 issue of the journal Nature, greatly expand our understanding of the sea creatures and ecosystems that existed at a crucial point in evolutionary history, when most of the animal life on the planet was found in the oceans.

The team—led by Peter Van Roy, a Yale postdoctoral associate, and Derek Briggs, the Frederick William Beinecke Professor of Geology & Geophysics and director of the Yale Peabody Museum of Natural History—uncovered more than 1,500 fossils of soft-bodied marine animals in newly discovered sites in southeastern Morocco during a field expedition last year. Many are complete fossils, and include sponges, annelid worms, mollusks and horseshoe crabs—in particular, a species similar to today’s horseshoe crab, which appeared some 30 million years earlier than previously known.

The Cambrian period, known for the “Cambrian Explosion” that saw the sudden appearance of all the major animal groups and the establishment of complex ecosystems, was followed by the “Great Ordovician Biodiversification Event,” when the number of marine animal genera increased exponentially over a period of 25 million years.

Because hard shells fossilize and are preserved more readily than soft tissue, scientists had an incomplete and biased view of the marine life that existed during the Ordovician period until now.

“The early Ordovician was a critical moment when massive diversification takes off, but we were only seeing a small piece of the picture that was based almost exclusively on the shelly fossil record,” Briggs said. “Normal faunas are dominated by the soft-bodied organisms we knew were missing, so these exceptionally well-preserved fossils have filled in much of the missing picture.”

The site in Morocco where the fossils were discovered was conducive to preserving even the soft tissues of the creatures that lived in its waters so long ago, thanks to generally calm waters, occasional rapid burial that protected the animals from scavengers, and favorable chemical conditions within the sediment that allowed for the rapid mineralization of soft tissue as it decayed.

In addition to providing a more complete understanding of marine life at that time, the team’s discovery upends a long-held belief that so-called Burgess Shale-type faunas, which are typical for the Early to Middle Cambrian, disappeared at the end of the Middle Cambrian epoch, some 499 million years ago.

“There was an anomaly in the fossil record. Most of these animals just seemed to disappear at the end of the Middle Cambrian,” said Van Roy, first author of the paper.

The team found that these Burgess Shale-type species survived well into the Ordovician period, which would have had a major impact on those ecosystems and their evolution, Van Roy said.

The team expects to find even more fossils representing other species during future planned expeditions in Morocco. “We’re only scratching the surface,” Van Roy said. “I’m certain there will be more spectacular fossils coming out of this site in the near future.”

(Photo: Peter Van Roy/Yale University)

Yale University


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Deep inside the ear, specialized cells called hair cells detect vibrations in the air and translate them into sound. Ten years ago, Stefan Heller, PhD, professor of otolaryngology at the Stanford University School of Medicine, came up with the idea that if you could create these cells in the laboratory from stem cells, it would go a long way toward helping scientists understand the molecular basis of hearing in order to develop better treatments for deafness.

After years of lab work, researchers in Heller’s lab report in the May 14 issue of Cell that they have found a way to develop mouse cells that look and act just like the animal’s inner-ear hair cells — the linchpin to our sense of hearing and balance — in a petri dish.

If they can further perfect the recipe to generate hair cells in the millions, it could lead to significant scientific and clinical advances along the path to curing deafness in the future, they said.

“This gives us real hope that there might be some kind of therapy for regenerating hair cells,” said David Corey, PhD, professor of neurobiology at Harvard University who was not involved in the study. “It could take a decade or more, but it’s a possibility.”

Using both embryonic stem cells from mice as well as reprogrammed mouse fibroblasts (a type of relatively undifferentiated cell found in many parts of the body), the researchers present a step-by-step guide on how to coax these cells into the sensory cells that normally reside in the inner ear.

“We knew it was really working when we saw them in the electron microscope,” Heller said. “They really looked like they were more or less taken out of the ear.”

Humans are born with 30,000 cochlear and vestibular hair cells per ear. (By contrast, one retina harbors about 120 million photoreceptors.) When a significant number of these cells are lost or damaged, hearing loss occurs. The major reason for hearing loss and certain balance disorders is that — unlike other species such as birds — humans and other mammals are unable to spontaneously regenerate these hearing cells.

As the population has aged and noise pollution has grown more severe, health experts now estimate that one in three adults over the age of 65 has developed a handicapping hearing loss due to the destruction of these limited number of hair cells.

One of the roadblocks to understanding the molecular basis of hearing is the paucity of hair cells available for study, Heller said. While researchers will ultimately need human hair cells, the mouse version is a good model for the initial phases of experimentation, he said. In addition to using mouse embryonic stem cells, the researchers used fibroblasts that had been reprogrammed to behave like stem cells: These are known as induced pluripotent stem cells, or iPS cells.

“Our study offers a protocol to generate millions of functional hair cells from a renewable source,” Heller said. “We can now generate these cells and don’t have to go through dozens of mice for a single experiment. This allows us to do molecular studies with much higher efficiency.”

The study details how the researchers succeeded in coaxing the mouse embryonic stem cells and the iPS cells through different phases of development that occur in the womb. According to lead author Kazuo Oshima, MD, PhD, a research instructor at Stanford who works in Heller’s lab, they started by turning the stem and iPS cells into the type of cells that form a young embryo’s ectoderm — the embryo’s outer layer of cells that eventually differentiate into many tissues and structures, such as skin and nerve cells. Next they used specific growth factors to transform them into “otic-progenitor” cells (otic means ear). And after that, they varied the chemical soup in the dish, so that the cells clustered in a manner similar to hair cells and developed stereociliary bundles, which are also characteristic of hair cells.

“We looked at how the ear develops in an embryo, at the developmental steps, and mimicked these steps in a culture dish,” Heller said.

Hair cells in the inner ear contain tiny clumps of hair-like projections, known as stereocilia. Sound vibrations cause the stereocilia to bend slightly, causing mechanical vibrations that are then converted into an electrochemical signal that the brain interprets as sound.

The cells in the petri dish, under close examination, had this same structure.

“These cells have a very intriguing structure,” Heller said. “They look like they have hair tufts of stereocilia.”

More importantly, further study showed that the cells also responded to mechanical stimulation by producing currents just like hair cells. Using a probe, researchers stimulated the bundles and recorded the currents that were evoked. Co-author Anthony Ricci, PhD, professor of otolaryngology, was responsible for this step of the work.

Heller, a leader in stem-cell based research on the inner ear, has recently been focused on two paths for possible cures for deafness: drug therapy — which could be as simple as an application of ear drops — and stem cell transplantation into the inner ear.

Both paths could be further advanced by the ability to develop hair-cell-like cells, he said. “We could now test thousands of drugs in a culture dish,” he explained. “It is impossible to achieve such a scale in animals. Within a decade or so we could reap the benefits of this type of screening.”

The lab’s research into the regeneration of hair cells for transplantation into the inner ear to cure deafness will also continue.

“We made hair-cell-like cells in a petri dish,” said Oshima. “This is an important step toward development of future therapies.”

(Photo: Stanford U.)

Stanford University




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