Myelin discovery turns neuroanatomy on its head

Myelin is the insulating material that forms around a nerve cell’s axon. It is white in color.

Harvard neuroscientists have made a discovery that turns 160 years of neuroanatomy on its head. Myelin, the white, fatty, electrical insulating material essential for fast communication between nerve cells, is not as ubiquitous as once thought, according to new work led by HSCI Nervous System Diseases Program co-leader Paola Arlotta, PhD, and collaborator Jeff Lichtman, MD, PhD, of the Department of Molecular and Cellular Biology.

“Myelin is a relatively recent invention during evolution,” said Arlotta. “It’s thought that myelin allowed the brain to communicate really fast to the far reaches of the body, and that it has endowed the brain with the capacity to compute higher level functions.” In fact, loss of myelin is a feature of a number of devastating diseases, including multiple sclerosis.

But the new research shows that despite myelin’s essential roles in the brain, “some of the most evolved, most complex neurons of the nervous system have less myelin than older, more ancestral ones,” she said.

What this means, Arlotta said, is that the higher in the cerebral cortex one looks—the closer to the top of the brain, which is the most evolved region—the less myelin one finds. Also, the “neurons in this part of the brain display a new way of positioning myelin along their axons that has not been previously seen. They have ‘intermittent myelin’ with long tracts that lack myelin interspersed among myelin-rich segments.”

She continued: “In classic neurobiology textbooks, myelin is represented on axons as a sequence of myelinated segments separated by very short nodes that lack myelin. The distribution of myelin was assumed to be always the same, on every neuron, from the beginning to the end of the axon. This new work finds this not to be the case.”

Arlotta postdoctoral fellow Giulio Srubek Tomassy, PhD, was first author on the
paper, published in Science.