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A baby's brain starts to develop early in pregnancy, and it needs to get just the right amount of certain chemicals at just the right time. If that doesn't happen, scientists suspect the result can be problems like autism or schizophrenia. Scientists used to think that those important brain chemicals came from the mother's body or from the fetus itself.
But NPR's Jon Hamilton reports that new research has uncovered another surprising source: the placenta.
JON HAMILTON: The conventional wisdom is that the placenta is there to transport nutrients from a mother to her unborn baby. But a team led by Pat Levitt from the University of Southern California has shown that it can do a lot more than that.
Dr. PAT LEVITT (Director, Zilkha Neurogenetic Institute, University of Southern California's Keck School of Medicine): The placenta is not just a passive bag of cells sitting there just allowing things to flow freely between the mom and the fetus, but rather we can think of it as a machine that can produce its own hormones, its own chemicals that can have an effect on the developing fetus itself.
HAMILTON: Levitt's team discovered this while looking at how a baby's brain develops during pregnancy. They were concentrating on the role of the chemical serotonin. Once we're all grown-up, serotonin affects our mood, but during pregnancy, it helps wire up key circuits.
Levitt's team was studying serotonin's effect on the frontmost part of the brain, called the forebrain.
Dr. LEVITT: The forebrain has the circuits that we know are disrupted in autism and schizophrenia, and bipolar disorder, in anxiety and depression.
HAMILTON: So Levitt and his team wanted to know how serotonin is supplied to the forebrain of a developing fetus. He says experiments in mice showed that, at least early on, it wasn't coming from the fetus itself. So they figured it must come from the other obvious source.
Dr. LEVITT: You know, in the field, for about 50 or 60 years, there's been this idea that serotonin can be supplied by the mom to the developing fetus.
HAMILTON: That made sense. After all, moms do make serotonin. But experiments by Levitt's team showed that there was no way for a mom's serotonin to get to the fetus. It couldn't cross the placenta. So the team members were forced to consider another possibility: Maybe the source of serotonin was the placenta itself. Levitt says he thought that was pretty unlikely. Then he saw the results of some experiments done by his team.
Dr. LEVITT: I was not kind of taken aback. I was completely taken aback.
HAMILTON: Levitt's team showed that, at least in mice, the placenta was making serotonin. What's more, the placenta only produced serotonin during a specific period of early fetal development. Then it stopped. And when the team looked at the human placenta, they found it also had the potential to make serotonin.
Levitt says other research suggests that serotonin probably isn't the only important brain-signaling chemical being made by the placenta. He says there are hints that it can make dopamine, which is also involved in wiring up the brain, and in mental illness.
Ron McKay is a researcher at the Lieber Institute for Brain Development in Baltimore. He says Levitt's research offers a whole new view of the placenta.
Dr. RON McKAY (Researcher, Lieber Institute for Brain Development): What this study shows is that the placenta itself is the source of a specific signal at a very particular period in development, which is influencing the brain of the new child. And that influence is likely to be long-lasting.
HAMILTON: For example, it could determine the risk that a child will go on to develop autism or mental illness. McKay says the new finding, though unexpected, is likely to lead to a much better understanding of what happens to the brain during pregnancy.
Dr. McKAY: It's nice because we're very interested now in pinning down the sources of important signals in brain development. Many of us know that brain development is creating risk for later disease or later well-being. And so my response is this is a good detective story with a very happy ending.
HAMILTON: The new research appears in the journal Nature.
Jon Hamilton, NPR News.