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Acts as much more than a passive conduit // Shapes the development of the fetus // Provides information science can’t get anywhere else.

Placenta: Organ of Change

By Meera Lee Sethi // Illustrations by Paul Blow // Summer 2011
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placenta opener

Paul Blow

It wasn’t clear why blood began hemorrhaging into the ventricles of the newborn’s brain, causing seizures. The baby, at Stanford University’s Lucile Packard Children’s Hospital, was a first child, but the delivery wasn’t premature, there was no sign of infection, and there had been no obvious risk factors.

The biggest clue lay in the placenta. A microscopic examination revealed that the villi (fingerlike projections, covering one side of the placenta, that embed themselves in the uterine wall) were chock-full of lymphocytes. Those white blood cells, says Stanford University neonatologist Anna Penn, belonged to the mother and showed that her immune system had been attacking the placenta.

This complication isn’t hard to understand. Half the genetic makeup of the placenta (which forms from embryonic cells during the earliest days of pregnancy) comes from the father, and because of those paternal genes, placental cells are covered in proteins that the mother’s immune system registers as foreign. Normally the placenta can counteract that response, but sometimes things go awry. “What happened in this pregnancy was essentially a graft vs. host reaction,” Penn says. It was as if an organ recipient’s body had rejected a transplant.

Knowing what went wrong couldn’t reverse the bleeding, and only time will tell about any permanent damage to the baby. But because unwanted immune responses often recur, the diagnosis should help this mother during future pregnancies. She’ll be closely monitored by an obstetrician who specializes in high-risk cases. “If we hadn’t looked at the placenta, we would have had no explanation of the problem and no way to prevent it from happening again,” says Penn.

In the vast majority of U.S. hospitals, no one would have looked; most placentas are dumped into a biohazard bucket. This lack of attention is partly a problem of timing: By the time anyone sees the placenta, its job is over and it seems irrelevant. More important, placentas are overlooked because, until recently, we didn’t understand much of what their work entails.

Scientists have long grasped the core function of this organ—to act as an intermediary between maternal and fetal blood circulation systems by transporting oxygen and nutrients to the fetus and carrying away waste. Research has also linked the health of the placenta to the health of the baby. A placenta of abnormally low volume, for example, may signal such complications as restricted fetal growth. Yet science has generally thought of the placenta as a passive conduit with no capacity to act independently.

Now, however, Penn and other scientists have helped usher in a very different view of the spongy, veiny placenta. It has turned out to be an active, even aggressive orchestrator of the events of pregnancy and fetal development. Interrupting the placenta in that job has serious consequences for mother and baby alike. And though the organ (like no other in the body) has an ephemeral existence, the molecules it produces, its tissues and other features provide essential information about the pregnancy and the fetus that is available from no other source. While researchers are still learning how to interpret and use much of that information, studies have already yielded compelling conclusions that argue for a deeper appreciation of the placenta’s role.

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Placenta: The Shape of Things to Come

Could unusual configurations forecast health problems?


1. “The Placenta: The Lost Neuroendocrine Organ,” by Anca M. Pasca and Anna A. Penn, NeoReviews, February 2010. A review of current understanding of the placenta’s role in fetal development, with a focus on how placental hormone production affects the fetal brain.

2. “Cancer and Pregnancy: Parallels in Growth, Invasion, and Immune Modulation and Implications for Cancer Therapeutic Agents,” by Shernan G. Holtan, Douglas J. Creedon, Paul Haluska and Svetomir N. Markovic, Mayo Clinic Proceedings, November 2009. Many cellular mechanisms that support healthy pregnancies are also employed by cancers as they evade immune-system attack; this review summarizes similarities between the two otherwise very different phenomena.

3. The Placenta and Human Developmental Programming, edited by Graham J. Burton, David J.P. Barker, Ashley Moffett and Kent Thornburg (New York: Cambridge University Press, 2011). This text covers the latest research into the placenta’s role in the resetting of gene expression during gestation and its effect on adult diseases.

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