Number 688, June 11, 2004
by Phil Schewe and Ben Stein
A New Chemotaxis Assay
A new chemotaxis assay reveals nerve cells' surprising sensitivity.
A new method for studying the guidance (change in direction) of neurons
amid a sea of protein molecules shows how sensitive this process is
to the surrounding protein gradient.
Chemotaxis is the process by which living cells sniff out their local
environment and act accordingly, which usually means moving or growing
toward higher concentrations of beneficial molecules.
In the case of neurons removed from their natural setting and put down
on a bed of collagen gel in a dish, growth will follow the increasing
gradient of proteins in their vicinity, such as the nerve growth factor
(NGF) protein. Neuronal growth, the way in which the long axon bodies
of a nerve cells wire themselves into a network, is of great interest
since this aids in knowing how brains form.
Now a team of scientists at Georgetown University has developed a new
method for measuring the gradient of local proteins (which have been
fluorescently tagged) and the axon's response. In this case the neural
cells come originally from a rat's brain.
The Georgetown team of neuroscientists and physicists find that axon
growth is sensitive to gradients so small (0.1%) that they correspond
to about one additional molecule across the spatial extent of the axon's
"growth cone," the sensing device at the tip of the growing axon.
This is a remarkable feat considering that, at any one instant, there
are large statistical fluctuations in the 1000 or so NGF molecules in
the vicinity of the growth cone. The researchers suggest that axons
may thus be "nature's most-sensitive gradient detectors." (Rosoff et
al., Nature Neuroscicence,
June 2004; contact Jeffrey Urbach, urbach@physics.georgetown.edu, 202-687-6594;
or Geoffrey Goodhill, geoff@georgetown.edu.)
Performing Boolean Surgery to Unlock Biosonar's Secrets
Over the last approximately 60 million years of evolutionary history,
bats have developed highly optimized biosonar systems in which they
broadcast ultrasound at various frequencies and then detect the echoes
to sense their surroundings.
At last month's meeting of the Acoustical Society of America in New
York, researchers (Rolf Mueller, University of Southern Denmark, +45-6550-3655,
rolfm@mip.sdu.dk) presented the first high-resolution, three-dimensional
maps to depict spatial regions in which the ears are sensitive to low-
mid-, and high-frequency ultrasound. These biologically based ultrasound-sensitivity
maps vary considerably over the studied sample of bat species and are
likely to vary even more over the approximately 1000 species which exist
in total. They may help inspire much better designs for artificial antennas
of any type, from the acoustic ones in ship sonar systems and medical
devices to the electromagnetic antennas in cell phones.
In their approach the researchers perform CT scans of bat ears to obtain
highly detailed images and 3D shapes which are then rendered on a computer.
Next they model the interaction between each ear shape and ultrasound
waves from the bat's surroundings. The researchers can understand how
the anatomical features of an ear shape bring about the spatial sensitivity
patterns by performing painless "Boolean surgery," in which they can
modify an ear's shape on a computer (often by removing some features
and--as part of their future plans--mixing features from different species)
and see how the modifications change the ear's detection of ultrasound.
(Paper 4aAB6 at meeting; also see lay-language
paper.)
Microwave Tissue Welding
A conventional microwave oven uses an antenna to squirt microwaves
into a reflective box where they preferentially excite and heat anything
rich in water molecules. A new experiment performed in the group of
Michael Golosovsky and Dan Davidov at the Racah Institute of Physics,
the Hebrew University of Jerusalem reduces the antenna size and dispenses
with the resonant box and, by getting really close to the sample of
soft matter, can heat a tiny spot, one half by one quarter of a millimeter
in size, up to temperatures of 120 C (or 250 F).
One possible application would be "tissue welding," the process of
binding together edges of cut tissue using "biological solder" such
as albumin. Infrared lasers can do such welding, but Golosovsky (golos@vms.huji.ac.il,
972-2658-6551) says that the microwave approach uses much lower power,
can do the job faster, can deposit radiation at deeper levels in the
wound, and bandages are transparent to the microwaves. Also collateral
tissue damage would be better controlled. (Copty
et al., Applied Physics Letters, 21 June 2004.)
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