Biology textbooks may need a revision, say Johns Hopkins Medicine scientists, who present new evidence that an armlike structure of mammalian brain cells may be a different shape than scientists have assumed for more than a century.

Their study on mouse brain cells shows that the cells’ axons — the armlike structures that reach out and exchange information with other brain cells — are not the cylindrical tubes often pictured in books and on websites but are more like pearls on a string.

A report on the findings is published online in Nature Neuroscience.

“Understanding the structure of axons is important for understanding brain cell signaling,” says Shigeki Watanabe, associate professor of cell biology and neuroscience at the Johns Hopkins University School of Medicine. “Axons are the cables that connect our brain tissue, enabling learning, memory and other functions.”

To be able to see axons on brain cells (neurons), which are 100 times smaller than the width of a human hair, the scientists used high-pressure freezing electron microscopy. Like standard electron microscopy, which shoots beams of electrons at a cell to outline its structure, Watanabe and his team froze mouse neurons to preserve the structures’ shape.

The researchers studied three types of mouse neurons: ones grown in the lab, those taken from adult mice and those taken from mouse embryos. The neurons were nonmyelinated (they were without the myelin insulating cover that surrounds the axon).

The researchers found the bubbly, pear shape of axons among all of the tens of thousands of images taken of the tissue samples. They named the pearl-like structures in which the axon swells “nonsynaptic varicosities.”

“These findings challenge a century of understanding about axon structure,” says Watanabe.