Site Links:
|
Descent of Man In this lecture, beginners can familiarize themselves with basic information and terms used to describe the evolution of humanity beginning with the origin of primates through the comings and goings of Genus Homo. |
New Details Of Earth's Internal Structure Emerge From Seismic Data
Posted: Thursday, November 29, 2001
Source: University Of California - Santa Cruz (http://www.ucsc.edu/)
SANTA CRUZ, CA -- About 1,800 miles beneath the surface, Earth's internal structure changes abruptly where the solid rock of the mantle meets the swirling molten iron of the outer core. But the boundary between the core and the mantle may not be as sharply defined as scientists once thought. By analyzing earthquake waves that bounce off the core-mantle boundary, researchers have found evidence of a thin zone where the outermost core is more solid than fluid.
The existence of such "core-rigidity zones"--small patches of rigid material within the fluid outer core--has been suggested before, but this report marks the first time scientists have detected one. Researchers Sebastian Rost and Justin Revenaugh of the University of California, Santa Cruz, are publishing their findings in the November 30 issue of the journal Science.
The nature of the core-mantle boundary is important because researchers now think it influences phenomena ranging from the behavior of Earth's magnetic field to the massive plumes of hot rock that rise through the mantle and erupt on the surface at volcanic hot spots such as Hawaii. The interaction of core-rigidity zones with the magnetic field, for example, may help explain the slow wobbling of Earth's rotation axis, called nutation, said Revenaugh, an associate professor of Earth sciences at UCSC.
"Studies of Earth's nutation provided one line of evidence that got people thinking there might be these little patches of rigid material in the outer core," he said. "So previous evidence was consistent with that idea, but now we have evidence that cannot be explained any other way."
The picture of the core-mantle boundary has grown increasingly complicated in recent years with advances in seismic tomography, which uses seismic waves from earthquakes to probe the internal structure of the Earth. As seismic waves radiate outward from the epicenter of an earthquake, their speed and other properties are affected by the different materials they pass through.
In the 1990s, seismic tomography showed the existence of "ultra-low velocity zones" at the base of the mantle, which some scientists interpret as evidence of partial melting of the mantle. Rost, a postdoctoral researcher, said an ultra-low velocity zone overlaps the area where he detected a core-rigidity zone, but that doesn't necessarily mean there is a connection between the two. He said the structure of the core-mantle boundary may turn out to be as complex as Earth's surface layer.
"I think what we have down there is just as complicated as the crust," Rost said. "I have a dataset that shows a very sharp core-mantle boundary just a little north of where we detected a core-rigidity zone. As we look at smaller scales, I think we will see more and more variation."
Rost and Revenaugh studied seismic shear waves, which cannot travel through a fluid and reflect off the core-mantle boundary. They looked at waves generated by earthquakes near the islands of Tonga and Fiji in the South Pacific and recorded by an array of instruments in Australia.
According to Rost, the high quality of the seismic data collected by this array was essential for detecting the rigid zone, which is only a few miles across and about 150 meters (about 500 feet) thick. "It's very thin and about the size of Santa Cruz," Revenaugh said.
There are two schools of thought about how this rigid material could occur in the molten metal of the outer core. One idea is that the core and the mantle react with one another to produce a material with intermediate density. But this process seems unlikely to produce a layer more than a few meters thick, Rost said.
The other idea relates to the growth of the solid inner core. As the Earth cools and heat flows out of the core, iron from the molten outer core solidifies onto the inner core. This increases the concentration of lighter elements in the outer core, and if those elements are near the saturation point they will also solidify out. But because they are lighter than iron, they will float to the top of the core and collect at the core-mantle boundary.
"You can think of it as an upside-down puddle formed by material rising up to the top of the core," Revenaugh said.
Whereas puddles of water form at low points on the land, "puddles" of solidified light elements from the core would form at high points in the core-mantle boundary. The seismic evidence suggests the rigid zone consists of a solid matrix with some molten iron in it, Rost said.
"It fits with the idea of an area where solid material has collected within the liquid outer core," he said.
Send page by E-Mail
Posted: Thursday, November 29, 2001
Source: University Of California, San Diego (http://www.ucsd.edu)
Scientists at the University of California, San Diego have produced dramatic images of brain cells forming temporary and permanent connections in response to various stimuli, illustrating for the first time the structural changes between neurons in the brain that, many scientists have long believed, take place when we store short-term and long-term memories.
In a paper published in the November 30 issue of the journal Cell, researchers from UCSD's Divisions of Biology and Physical Sciences describe their achievement, a "Holy Grail" for neuroscientists who have long sought concrete evidence for how nerve connections in the brain are changed temporarily and permanently by our experiences.
"The long-term memories stored in our brain last our entire lives, so everybody had assumed that there must be lasting structural changes between neurons in the brain," says Michael A. Colicos, a postdoctoral fellow at UCSD and the lead author of the paper. "Although there's been a lot of suggestive evidence to indicate that this is the case, it's never before been directly observed."
"While most people assumed that some sort of rearrangement of nerve cell connections took place in the brain, this was extremely difficult to demonstrate experimentally," says Yukiko Goda, a professor of biology at UCSD who headed the research team, which included Michael J. Sailor, a professor of chemistry and biochemistry at UCSD, and Boyce E. Collins, a postdoctoral fellow in Sailor's lab. "Some investigators saw increases in the number of synapses in the brain in response to stimuli, while others saw no changes. There are a billion synapses in a cubic centimeter of brain tissue, so no one could tell for certain whether the statistical comparisons of synapse density between one sample and another showed a real increase."
To resolve this problem, the UCSD researchers focused their attention on individual nerve cells, specifically neurons from the hippocampus -- the portion of the human brain crucial to forming particular types of memory -- and filmed them as their synapses made new connections to other nerve cells in response to electrical impulses.
The ability of the scientists to do this without impairing the normal physiological functions of the cells depended on two new techniques implemented in Goda's lab to study synaptic connections. One was a method of visualizing the rods and filaments of actin -- the girders that make up the cytoskeleton, the internal skeleton of the cell. Using molecular biology techniques, fluorescent versions of actin were constructed and visualized as the neurons grew and changed shape to establish new connections.
The second development, which resulted from a collaboration between Goda and Sailor, was a method of stimulating nerve cells in a manner that mimicked their stimulation in the brain. This involved using the "photoconductive" properties of silicon in a way that allowed the researchers to deliver a short, high frequency burst of electricity to a specific area of a neuron on a silicon chip by simply shining light on that area. Light excitation in that area of the silicon created a narrow pathway through which Colicos and his colleagues could apply a tiny voltage below the chip to target the neuron.
"We stimulate these cells with a short, high-frequency burst," says Colicos, working in Goda's lab. "That type of stimulation is what other researchers believed for many years was the type that formed these connections between neurons."
A key advantage of this method is that it doesn't damage the cell. "Part of the reason people haven't been able to demonstrate this before is that the technology hasn't been available to do this before," says Colicos. "The standard way of stimulating a neuron is to use an electrode. But as soon as you stab the cell with an electrode, it begins to die. So the advantage of this new technique is that we can keep the cells in their physiologically normal state. And when we stimulate the cells of our choice by shining light, we can induce the actual structural changes that occur in the brain -- the formation of these new synapses."
In their experiments, the UCSD researchers discovered that when they stimulated a cell once, the actin inside the cell was activated and temporarily moved toward neurons to which they were connected. The activity in the first cell also stimulated the movement of actin in neighboring neurons, which moved away from the activated cell. Those changes in the cells were temporary, however, lasting for about three to five minutes and disappearing within five to 10 minutes.
"The short-term changes are just part of the normal way the nerve cells talk to each other," says Colicos. "The long-term changes in the neurons occur only after the neurons are stimulated four times over the course of an hour. The synapse will actually split and new synapses will form, producing a permanent change that will presumably last for the rest of your life."
"The analogy to human memory is that when you see or hear something once, it might stick in your mind for a few minutes. If it's not important, it fades away and you forget it 10 minutes later. But if you see or hear it again and this keeps happening over the next hour, you are going to remember it for a much longer time. And things that are repeated many times can be remembered for an entire lifetime."
"It's like a piano lesson," says Goda. "If you play a musical score over and over again, it becomes ingrained in your memory."
In their experiments, which were financed in part by grants from the National Science Foundation and the National Institutes of Health, the researchers observed no changes in these newly formed nerve connections once they were established, indicating that they were permanent.
"Once you take an axon and form two new connections, those connections are very stable and there's no reason to believe that they'll go away," says Colicos. "That's the kind of change one would envision lasting a whole lifetime."
Editor's Note: The original news release can be found at http://ucsdnews.ucsd.edu/newsrel/science/mccell.htm
Send page by E-Mail
Same Flower Chemicals Tell Some Insects "This Bud's For You," Posted: Thursday, November 22, 2001
Source: Cornell University (http://www.cornell.edu)
ITHACA, N.Y. -- When some insects zero in on a flower for nectar, their ultraviolet vision is guided by a bull's-eye "painted" on the plant by chemical compounds. Now, chemical ecologists at Cornell University have discovered a second job for these compounds: warding off herbivores.
Even before a flower bud -- such as the creeping St. John's wort -- opens for business, the same chemicals, called DIPs (for dearomatized isoprenylated phloroglucinols), are both coloring the flower in patterns unrecognizable to the human eye and protecting the plant's reproductive apparatus by killing or deterring caterpillars, the scientists report in the upcoming Proceedings of the National Academy of Sciences (Vol. 98, No. 24).
"Now that we know where to look, anti-feedant chemicals like the DIPs undoubtedly will be found in other plant species, and they offer clues to more natural insect control agents," says Thomas Eisner, Cornell's Schurman Professor of Chemical Ecology and one of six authors of the report. An anti-feedant chemical discourages herbivorous insects and can harm those that don't get the message.
One place DIPs are found is in hops, the female flowers of the commercial hop, which give beer its bitter flavor and also protect against pathogenic microorganisms, Eisner says. "If your beer is safe and enjoyable to drink, you ought to thank a flower."
Also participating in the Cornell study, which was supported by grants from the National Institutes of Health, were Jerrold Meinwald, the Goldwin Smith Professor of Chemistry; Athula Attygalle, director of the Mass Spectrometry Facility in the Department of Chemistry and Chemical Biology; Mathew Gronquist, graduate student in that department; Alexander Bezzerides, graduate student in the Department of Neurobiology and Behavior; and Maria Eisner, senior research associate in that department, who is Thomas Eisner's wife and research partner. The DIP finding follows 30-year-old studies by the Eisners of floral "nectar guide" patterns that only creatures with vision in the ultraviolet part of the spectrum can see. Using combinations of special camera lenses and filters, photographic films and video imaging, the Eisners revealed a bug's-eye world where flowers display patterns that are visible only to insects. Besides making a target on the part of the flower where nectar and pollen occur, the distinctive patterns also are believed to help insects recognize a familiar flower among a field of competing images.
"But we had a nagging suspicion that the ultraviolet-absorbing pigments had other functions for the plant," recalls graduate student Bezzerides, who subsequently helped to demonstrate toxicity and a deterrent effect of the chemicals. "We wondered if the chemicals originally served the plants as a sunscreen against ultraviolet radiation."
So the Cornell biologists and chemical biologists joined forces to see what would make a caterpillar sick. Adding to their suspicion that DIPs and similar compounds might have an anti-feedant function was the finding that the compounds were particularly prevalent in plant ovary walls -- making up as much as one fifth by dry weight -- as well as in other reproductive structures such as the anthers. "Just as important as attracting pollinators to a plant is producing viable seed, so there is an evolutionary incentive to protect the reproductive apparatus from herbivores," says graduate student Gronquist, who characterized the chemicals
The flowering plant chosen for the study was Hypericum calycinum, a native of southeastern Europe that is popular with gardeners worldwide as an ornamental. When H. calycinum flowers are fully open, they appear to humans as a uniform yellow disk. But to insects with ultraviolet-sensitive eyes, the disk is highlighted by a dark, ultraviolet-absorbing center, giving the flower a bull's-eye.
While Gronquist performed analyses that led to isolation of the chemical compound, the biologists devised feeding studies. They offered to larvae of the Utheisa ornatrix moth (also called the rattlebox moth) filter-paper discs soaked with chemicals from plants the insects normally relish.
Then the caterpillars were offered paper disks also soaked with DIP chemicals. The ultraviolet-absorbing chemicals deterred most of the caterpillars. But the DIPs were lethal to those that sampled the chemical-laced paper.
The experiments showed, according to the Cornell chemical ecologists, that DIPs both contribute to the ultraviolet pattern in flowers and serve as an anti-feedant, with potentially lethal consequences. Says Eisner, "With the same chemical, the plant is saying to pollinating insects that it needs to attract, 'this bud's for you,' and to herbivores that pose a threat, 'bug off.' "
And speaking of beer, Cornell chemistry professor Meinwald notes that similar chemicals from hops, which have been used in brewing for centuries, are not in the form or quantity to harm human drinkers -- or even to deter fans of the bitter beverage. "What we use to flavor and to
preserve beer is also used by plants both to entice the pollinators and to deter the enemy," according to Meinwald. "Nature quite often has a way of using the same chemical idea to solve diverse problems."
Related Web sites:
o Proceedings of the National Academy of Sciences: http://www.pnas.org/current.shtml
o Cornell Dept. of Chemistry and Chemical Biology: http://www.chem.cornell.edu/
o Cornell Dept. of Neurobiology and Behavior:
http://www.nbb.cornell.edu/neurobio/sofneurobio.html
o CIRCE (Cornell Institute for Research in Chemical Ecology):
http://www.cfe.cornell.edu/circe/aboutcirce.html
Editor's Note: The original news release can be found at http://www.news.cornell.edu/releases/Nov01/DIP.flowers.hrs.html
Send page by E-Mail
Gene Controls Neural Stem Cell Growth Posted: Friday, November 9, 2001
Source: Howard Hughes Medical Institute (http://www.hhmi.org/)
Researchers have discovered that a gene previously implicated in a variety of forms of cancer is also a key regulator of neural stem cell proliferation. Understanding how the gene, called PTEN, promotes the proliferation of neural stem cells could aid efforts to use stem cells in treating neurological disorders.
Howard Hughes Medical Institute investigator Hong Wu and colleagues at the UCLA School of Medicine reported on the regulatory role of Pten in the November 1, 2001, Science Express, the online counterpart of the journal Science.
According to Wu, PTEN is the second most frequently deleted tumor suppressor gene, giving rise to human cancers including brain, breast, prostate, and endometrial cancers.
There was also evidence, said Wu, that the PTEN protein produced by the gene played a normal role in neural development. “It was known that humans who have inherited deletions or mutations of the PTEN gene often showed macrocephaly, or abnormally large brains,” she said.
The gene is expressed in the central nervous systems of developing human and mouse embryos, but no one had ever done a detailed study to understand the precise role of PTEN in the nervous system.
Knocking out the gene in mice caused early death in embryos, before significant brain development. So, Wu and her colleagues used the Cre-loxP system to genetically manipulate the mice so that the gene would be knocked out later in gestation. The researchers discovered that knocking out Pten in the mouse embryos appeared to hyper-activate a signaling pathway that regulates cell proliferation and cell death in the brain.
Anatomical studies revealed a significant increase in brain size in the mutant animals. The researchers also noted an increase in the size of the brain cells themselves – the first evidence that the PTEN protein regulates cell size in mammals, said Wu. The scientists next used antibodies to mark specific types of brain cells.
Those experiments revealed that the neural stem cells in the mutant mice developed into the normal lineages of cells in the embryonic brain.
Additional cell-labeling studies indicated that the increase in brain cells likely resulted both from increased proliferation of cells and reduced programmed cell death.
Wu and her colleagues also used “neurosphere” cell cultures of stem cells from the brains of both normal and mutant mice to explore in detail how the stem cells developed. Neurospheres are tiny aggregates of brain cells that include stem cells and their progeny at different stages of development. By applying growth factors, the neurospheres of cells from different brain areas can be induced to proliferate and differentiate.
“We found that mutant neurospheres proliferated more readily than normal neurospheres,” said Wu. “Also as in the in vivo studies, we found that the mutant neurospheres, like the normal neurospheres, produced the normal range of neural cells – neurons, astrocytes and oligodendrocytes. We conclude that these experiments suggest that the PTEN protein is a major modulator in neural stem cells of the proliferative cell cycle and of programmed cell death,” she said.
While Wu emphasized that it is still relatively early, it may be that “the signaling pathway elucidated by this study will have an impact on future clinical studies aimed at manipulating stem cell populations,” she said.
Also, she said, the findings of PTEN’s role in regulating neuronal stem cell development will lead to better understanding of how mutations that abolish PTEN function allow unchecked cell proliferation in cancers.
Wu and her colleagues plan further studies to explore whether PTEN serves as a switch that triggers normally quiescent stem cells to enter the cell cycle and proliferate. They also plan to analyze in detail how knocking out PTEN in adult animals triggers cancers. Such understanding could lead to drug therapies that would prevent hyperactivation of the PTEN-controlled pathway, to treat such cancers.
Send page by E-Mail
Link To Our Ancient Past Is Confirmed In Potassium Channel Research Posted: Friday, November 9, 2001
Source: University Of Pennsylvania Medical Center (http://www.med.upenn.edu/)
Research on components of the brain's electrical signaling system has answered a basic question about our human evolution, confirming scientific belief that we two-legged, computer-using creatures are descended from prokaryotes -- cellular organisms so primitive and simple that they exist without nuclei or cell walls.
The study, led by Zhe Lu, MD, PhD, an Associate Professor in the Department of Physiology at the University of Pennsylvania School of Medicine have been recently published in the journal Nature.
The research by Lu and his colleagues focused on the structure and function of molecules called potassium channels, which are essential to how the brain works. When potassium channels open and close, they control the flow of potassium ions across cell membranes. The current contributes to the electrical signals in nerve, muscle and endocrine cells.
Scientists who study the brain's electrical signals have relied on a blue-print developed from functional studies of eukaryotic (neuronal) potassium channels and structural studies of prokaryotic (bacterial) potassium channels, based on the assumption that the two channels are essentially the same. However this assumption has recently been challenged.
Lu and his collaborators devised a project in which the pore of a prokaryote's potassium channel (the interior core of the channel) was substituted for the pore of a potassium channel in a euokaryote. The scientists found that the eukarotic channel continued to function essentially as it had previous to the substitution.
"This has very profound implications for evolution," Lu said. "It appears the potassium channels in advanced brains and hearts of mammals have evolved from something like this bacterial channel. So what we learn from the more easily studied bacterial channels can be directly applied to our understanding of potassium channels such as those in our brains."
In the study, Lu worked with Penn colleagues, Angela Klem, research specialist, and Yajamana Ramu, PhD.
The work was funded by the National Institutes of Health.
Send page by E-Mail
Designed and maintained by: Renee, Hendricks, Vashti, Meri & Amon. S.E.L.F.