Scientists once thought the adult brain was set in its ways. Now they’re discovering that adult neurons have a remarkable ability to grow and change – and researchers at MIT have shown that some neurons can sprout new branches and retract old ones.
These findings, published in last week’s issue of PLoS Biology, an online journal from the Public Library of Science, add to a growing body of evidence that older brains are, in fact, still agile. The same method that researchers applied in the current study could be used to assess the best ways for encouraging brain cells to grow, which would help people with spinal cord injury, stroke, and other disorders. “This could be a powerful diagnostic or a way to test therapies,” says Elly Nedivi, the professor of neurobiology at MIT who led the research team.
Nedivi and her collaborators used mice that had a few neurons labeled with fluorescent dye, so the cells could be seen under a microscope. They shaved off a small piece of a mouse’s skull and covered the opening with glass. Using that “window,” they then took pictures of the fluorescent neurons with a two-photon microscope, a technique that provides very high-resolution images of living brain tissue. And, finally, to record how the neurons changed over time, they captured images of the same neurons over several weeks.
Their laborious work paid off. What they found was that dendrites – those treelike extensions on neurons which receive information from other brain cells – grew, shrank, and changed over time. “You see the full range of types of growth you see during development, such as growth spurts, new processes, or new branching from [the main neuronal process],” says Nedivi. “This is what the brain is doing on a day-to-day basis.”
In previous studies, scientists had observed structural changes in tiny spikes on the surface of dendrites, called spines. But, by painstakingly reconstructing larger portions of neurons, Nedivi and her colleagues Wei-Chung Lee, Peter So, and Hayden Huang revealed larger-scale changes in the dendrites, which may have gone unnoticed with other methods.
“We know that throughout life we learn things, so synapses must be changing in some way. But people didn’t know if the same synapses were getting stronger or weaker, or if whole new ones were forming or old ones were disconnected,” says Harvard neuroscientist Joshua Sanes, who generated the mice used in the MIT study. “This raises those last possibilities: that some changes may involve wholesale formation of new synapses or loss of old ones.”