Immune cells in the human brain may be critical to orchestrating the organ’s development in the womb because they trigger a dramatic increase in an important type of nerve cell, new research suggests.
Estimates suggest that these key cells, known as inhibitory interneurons, make up some 25% to 50% of the neurons in the adult cortex, the wrinkled tissue that covers the surface of the brain. In fact, the human cortex carries more than double the number of interneurons as the mouse cortex does.
These interneurons relay signals between other brain cells and help keep that signaling in check with a chemical messenger called GABA. As the brain’s main “inhibitory” messenger, GABA helps turn down brain activity by making neurons less likely to fire, thus balancing out the “excitatory” signals that amplify brain activity. Various disorders have been tied to problems with interneurons, including epilepsy, autism and schizophrenia.
Now, in a study published Aug. 6 in the journal Nature, researchers have uncovered a force that drives interneurons to multiply in the developing human brain — and they say it may be unique to our species.
“That’s why we cannot use traditional animal models,” study co-author Diankun Yu, an assistant researcher in pediatrics at the University of California, San Francisco (UCSF), told Live Science. To uncover this mechanism that may unfold only in the human brain, the researchers developed an organoid — a miniature 3D structure, grown from stem cells, that mimics a full-size structure found in the human body.
Prior to the organoid study, research in lab animals suggested a link between the activation of the maternal immune system during pregnancy and a lower number of interneurons in the cortices of their offspring, compared to offspring that didn’t experience an immune upset. That kind of activation might occur in response to a viral or bacterial infection, for example. The study authors explored this in previous research with lab mice, in which they pinpointed a key player behind the link: microglia, the brain’s resident immune cells.
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