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It may be true that humans are born with most of the neurons we’ll ever have, but they’re not always where one might expect them to be.

Two papers from the lab of Shawn Sorrells, assistant professor of neuroscience in the Kenneth P. Dietrich School of Arts and Sciences, show the movement of neurons into the brain after birth may be the key to pivotal times in a young person’s life.

Neuroscientists have long known of a cache of unusual neurons in a region near the amygdala that appear much younger than the rest of the 86 billion neurons of the brain and nervous system.

In , Sorrells and team used these cells to help answer the longstanding question: Do new neurons form after birth?

Immature neurons in the amygdala paralaminar nucleus, called PL neurons for short, seem to be created later than others. But there are also clues suggesting PL neurons, which make up about 20% of all human amygdala neurons, are not newly formed, just slow to mature.

Pia Alderman, a research assistant in Sorrells’ lab, was keen on the idea that it might be possible to find answers by doing analogous tests in mice, but first she had to find out if mice had PL neurons. When Alderman compared human brains and mouse brains, she found a small collection of immature neurons in the mouse amygdala that, like those in humans, seemed to delay their development.

Using the mouse model, the researchers were able to interrogate questions they couldn’t answer with post-mortem human brain tissue.

“All these things we were hypothesizing to be true in humans, we were able to confirm experimentally in the mouse,” Sorrells said. “We confirmed that mouse PL neurons are not born postnatally.” Despite their immature appearance, they are created before birth, just like normal neurons.

“They delay their growth. We’ve confirmed that a subset is migrating. We’ve also confirmed that some PL neurons remain immature in adult mice, though a good portion of them mature by adolescence while others continue migrating into their final locations,” he added.

In humans, those mature migrating neurons appear to move into the amygdala, an area of the brain linked to emotional maturity.

That, Sorrells said, makes perfect sense. “Anyone who knows a teenager knows — there’s a lot going on in those years.”��

The next question for Sorrells in this line of research is to determine the true role of these neurons in adolescent behavioral development. Answering this question could help explain why humans have so many amygdala neurons that follow this uniquely delayed developmental timeline.

In , published in Nature, Sorrells and collaborators found a different subset of neurons continues through toddlerhood to migrate to an area of the brain essential for making memories. The finding suggests the neural migration is related to periods of neuroplasticity when the brain is especially receptive to changes and adaptation.

Sorrells' research team was first to show that, unlike what was previously thought, neuronal migration of such scale and duration is extensive within regions that control thoughts and emotions.

“It is generally thought that most parts of the brain are done recruiting neurons by the time an individual is born,” said Sorrells. “We were incredibly excited to learn that not only does large-scale neuronal migration continue into specific brain regions, but that this process also continues into ages when children are crawling and beginning to walk.”

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In the figure, courtesy of Shawn Sorrells, large chains of migrating cells break down into a shower of young migrating neurons that continue to integrate into the entorhinal cortex and neighboring regions until 2-3 years of age.