Researchers have found evidence that the placenta plays an important role in fetal brain development during the early stages of pregnancy.
Experiments in mice show that during a key period, the placenta becomes a source of the chemical serotonin, which helps determine the wiring of key circuits in the brain.
The finding, published in the journal Nature,could help explain what leads to brain disorders such as autism and schizophrenia. And it shows that the placenta does a lot more than simply transport nutrients from a mother to her unborn baby.
"The placenta is not just a passive bag of cells sitting there just allowing things to flow freely between the mom and the fetus," says Pat Levitt, director of the Zilkha Neurogenetic Institute at the University of Southern California's Keck School of Medicine.
"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," Levitt says.
Levitt's research team discovered this while studying the role of serotonin in early brain development.
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.
They were looking at the chemical's effect on the frontmost part of the brain, called the forebrain.
"The forebrain has the circuits that we know are disrupted in autism and schizophrenia, and bipolar disorder, and in anxiety and depression," Levitt says.
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: the mother.
"For about 50 or 60 years there's been this idea that serotonin can be supplied by the mom to the developing fetus," Levitt says.
That makes sense. After all, mothers do make serotonin. But experiments by Levitt's team showed there was no way for a mother'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.
"I was completely taken aback," Levitt says
His 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.
Other research suggests that serotonin probably isn't the only important brain-signaling chemical being made by the placenta, Levitt says. He says there are hints that it can make dopamine, which is also involved in wiring up the brain — and in mental illness.
Levitt's research offers a whole new view of the placenta, says Ron McKay, a researcher at the Lieber Institute for Brain Development in Baltimore.
"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," he says. "And that influence is likely to be long lasting."
For example, he says, it might determine the risk that a child is going to someday develop autism or mental illness.
The new finding, though unexpected, is likely to lead to a much better understanding of what happens to the brain during pregnancy, McKay says.
"It's nice because we're very interested now in pinning down the sources of important signals in brain development," McKay says. "This is a good detective story with a very happy ending."
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