Highly wired

New brain study finds more synapses in men than in women

4:04pm, September 8, 2008
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Men are dense — in the temporal neocortex anyway.

An investigation of brain tissue recovered from epilepsy patients during surgery showed men had a higher density of brain cell connectors, called synapses, than their female counterparts, researchers report September 8 online in the Proceedings of the National Academy of Sciences.

The find might explain why men have better spatial perception, while women better remember what they hear and can talk faster, the researchers suggest.

“Or, it could mean men’s brains are just more redundant,” says Edward Jones, director of the Center for Neuroscience at the University of California, Davis, who was not involved in the study. Right now, it’s hard to know exactly what the difference means, he says.

For many years, scientists have searched for structural variations between men’s and women’s brains to explain psychological studies showing that, overall, the sexes think and act differently. Past studies found differences in brain mass and neuron density, but “they were hyped and untrustworthy,” Jones says.

This study is meticulously detailed, he notes. It is the first to show gender differences on such a fine scale — at the synapse, which is the juncture where an electrical signal passes from one brain cell to another. “The level of detail and meticulousness are why I have confidence in the results,” he says.

To measure the difference in synapse density, four Spanish scientists studied brain tissue taken from eight patients, four men and four women. The patients were having surgery on the hippocampus regions of their brains to treat epileptic seizures. As part of the procedure, tissue from the temporal neocortex was extracted, along with the culprit hippocampus tissue. The temporal neocortex is related to speech, memory and hearing. Tests showed that the temporal tissue was not affected by the patients’ epilepsy, the researchers report.

The team then analyzed the temporal tissue with an electron microscope. All the samples had similar numbers and densities of neurons, as well as similar thicknesses throughout the six layers of tissue. The only difference by gender was synapse density. The four men had, on average, 33 percent more synapses per cubic millimeter of tissue, says study coauthor Javier DeFelipe of the Cajal Institute in Madrid, Spain.

“But, the sample size is small,” comments Karl Zilles of the Institute of Neurosciences and Biophysics in Jülich, Germany. And, he adds, epilepsy leads to synapse changes even outside the epileptic focus. So, undetected changes could have occurred in the synapses of the temporal neocortex.

DeFelipe admits that this study is a first step and only focuses on one area of the brain. Women’s brains could have a higher synapse density in other regions, he explains.

“Given the challenges, like getting fresh tissue, it is great work,” but more research is needed, Zilles says.

Jones notes, though, that the epilepsy treatment that produced the samples for this study is becoming more common. “I just hope the results encourage researchers to start taking a look at that available tissue,” he says.

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