Scientists have found a way to track tiny features of individual brain cells in living mice, providing a glimpse at how brains change over time. In one case, they watched the animals' brains rewire after their whiskers were clipped.
The technique will help scientists explore how the brain forms memories and reacts physically to its owner's experiences.
The advance ``will have far-reaching implications'' for studying the brain, according to Ole P. Otterson and P. Johannes Helm of the University of Oslo in Norway. They wrote a commentary that accompanies two reports on the technique in Thursday's issue of the journal Nature.
Brain cells called neurons signal each other across tiny gaps, called synapses. The signals are received by tiny spines. Researchers in both studies followed the growth and destruction of the spines over time.
The researchers tracked the spines, which measure less than one twentieth the width of a human hair, using specially bred mice that carry a gene which makes some of their neurons glow. They used laser and electron microscopes to peek through windows implanted in the mouse skulls, or through skull bones thinned with a drill.
In one study, the researchers clipped the highly sensitive whiskers of mice and watched changes in the part of the brain that receives signals from the whiskers. Two to four days after clipping, the number of spines created or lost in that area increased significantly, indicating new synapses were being created and others destroyed, the researchers reported.
Graham Knott, a researcher at the University of Lausanne in Switzerland and an author of the study, said the group had for the first time tracked a brain as it adapted to change.
``By changing the input to the brain, you've got a situation where you're changing the connections,'' Knott said.
``You knew you would get change... the way in which cells receive the information changes. But to actually look at and see physical changes, that's actually what's happening, you're actually watching the brain connect.''
Michael Merzenich, a University of California at San Francisco researcher who studies the brain but was not involved in the new research, said the results were expected, but ``no one has ever actually watched it happen.''
In a separate study, researchers at the New York University School of Medicine found that such spines can be long-lived, much longer than the first group found.
Spines in the visual cortex, the part of the brain that responds to information from the eyes, lived longer in adult mice than young mice. The NYU group tracked young and adult mice over periods ranging from several hours to several months.
The findings suggest some spines can last an entire lifetime, but the NYU researchers said they also found evidence of changes in the shape of individual spines, which could be a means of improving the efficiency of synapses.
Larry R. Squire, a neurobiologist at the University of California at San Diego School of Medicine, said the findings of the two studies are not necessarily contradictory because the researchers looked at different parts of the brain using differing techniques.
The findings of the two studies also are ``some of the best evidence'' about how synapses behave in adult mammalian brains and could help explain on how the human brain works, Squire said.
``There can be no doubt that similar phenomena occur in the human brain,'' Squire said.