This Week in Science

Science  26 Apr 1996:
Vol. 272, Issue 5261, pp. 461
  1. An edge-on view of Saturn's rings

    About every 15 years,Earth, the sun, and Saturn are positioned such that Saturn's rings appear edge-on. The Hubble Space Telescope has been used to take advantage of this rare set of reference frames in 1995 (see cover and the Perspective by Murray, p. 507). Bosh and Rivkin (p. 518) identified the F ring inner satellite, Prometheus, lagging behind its predicted position by about 20 degrees and the A ring outer satellite, Atlas, ahead of its predicted orbit by about 25 degrees, during the first Earth ring-plane crossing in May. During the second Earth ring-plane crossing in August and the solar crossing in November, Nicholson et al. (p. 509) confirmed Prometheus' lag, which suggests that this satellite may not have collided with the F ring as predicted but may have encountered a co-orbital satellite instead. They also studied the structure of the fainter F, E, and G rings. Hall et al. (p. 516) used the faint object spectrograph on Hubble just before the August crossing to more clearly define a tenuous OH gas cloud enveloping the rings that may be derived from meteoritic impacts.

  2. Proper connections

    Carbon nanotubes, like graphite,are electrically conducting, but accurately measuring their conductivity can be difficult—very small probes are necessary, and the resistance of the contacts to the tube itself can dominate the measurement.Dai et al. (p. 523)used lithographic methods to expose parts of nanotubes while holding the rest of the nanotube under a conducting layer of gold. A conducting probe tip was then used to contact the exposed nanotubes and also map out theirstructure. Conductivity measurements were made at several points, and thus contact resistance could be factored out. Kinks in the nanotubes greatly increase their resistivity.

  3. Moving molten iron

    Formation of iron cores in Earth and other planets would have involved early segregation of iron-rich melts from silicate-rich mantles. Minarik et al. (p. 530) provide experimental evidence suggesting that large degrees of meltingof the planets would have been required for the iron melts to segregate. These authors found large wetting angles for iron-rich melts in olivine up to pressures of 11 gigapascals, which suggests that small volumes of melt could not form an interconnected networkthrough solid olivine.

  4. Homeobox connection

    Homeobox-containing (Hox) genes regulate pattern formation during development. No human mutations, however, have been linked to a Hox gene to date. Muragaki et al. (p. 548) found that synpolydactyly, an inherited abnormality of the hands and feet, is caused by a mutation in HOXD13. The phenotype is the result of an insertion of a polyalanine stretch in the protein in a region outside of the homeodomain.

  5. Sense in the cerebellum

    Because the cerebellum is activated by body movements and damage to it causes a loss of motor control, it has been regarded as a motor organ. Gao et al. (p. 545; see t he news story by Barinaga, p. 482) obtained magnetic resonance images of the lateral cerebellum (the dentate nucleus, whichsends out signals) of human subjects as they performed motor tasks that did or did not require sensory input, as well assensory comparison tasks. Only the tasks that required sensory input activated the cerebellum. The cerebellum may not bespecialized for motor control, as commonly thought, but only for certain motor ta sks that are used to make sensory discriminations.

  6. Visual cortex development

    Covering one eye in a mammal's early life can permanently impair the response of the visual cortex in the brain to that eye—the brain will learn to respond only to the open eye (ocular dominance plasticity). How this phenomenon occurs has been debated, and a role for one of the neurotransmitter receptors—the metabotropic glutamate receptor—has been postulated. Hensch and Stryker (p. 554) show that in vivo activation of the receptor is not required for the ocular dominance plasticity. Howeve, they also demonstrate that in vitro long-term depression, which had been thought to occurthrough a similar mechanism, does require receptor activation.

  7. Damage control

    DNA damage induced by ultraviolet light or exposure to carcinogens can be repaired by nucleotide excision. Transcription-coupled repair (TCR) couples nucleotide excision repair to transcription; in Escherichia coli, mutations in genes for mismatch repair also lead to defects in TCR. Mellon et al. (p557) have identified mutations in human homologsof genes required for mismatch repair in E. coli in cell lines from patients with hereditary nonpolyposis colorectal cancer, a cancer predispositionsyndrome. These cell lines are also defective for TCR, suggesting that in addition to mismatch defects, environmentally induced defects also avoid repair in these cells and may lead to increases in sporadic tumors.

  8. Fashioning an image

    Two brain imaging techniques,positron emission tomography and functional magnetic resonance imaging, rely onchanges in blood flow and oxygenation that result from neuronal activity. The temporal and spatial connection between neuronal activity and these changes has been an unresolved issue. Malonek and Grinvald (p. 551) optically imaged the cat visual cortex during presentation of oriented gratings that are known to yield well-defined cortical responses. They suggest that aerobic metabolism in localized neurons first creates a highly localized increase in deoxyhemoglobin. This effect triggers an increase in the blood flow that maintains oxyhemoglobin in the immediate vicinity and is then followed by a large regional increase inblood flow that only slowly returns to prestimulus levels.

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