Push to understand the mysterious placenta

Minutes after a baby girl was born on a recent morning at UCSF Medical Centre at San Francisco, her placenta — a pulpy blob of an organ that is usually thrown away — was packed up and carried off like treasure through a maze of corridors to the laboratory of Susan Fisher, a professor of obstetrics, gynecology and reproductive sciences.

There, scientists set upon the tissue with scalpels, forceps and an array of chemicals to extract its weirdly powerful cells, which storm the uterus like an invading army and commandeer a woman’s body for nine months to keep her fetus alive.

The placenta is the life support system for the fetus. A disk of tissue attached to the uterine lining on one side and to the umbilical cord on the other, it grows from the embryo’s cells, not the mother’s. It is sometimes called the afterbirth: It comes out after the baby is born, usually weighing about a pound, or a sixth of the baby’s weight.

It provides oxygen, nourishment and waste disposal, doing the job of the lungs, liver, kidneys and other organs until the fetal ones kick in.

If something goes wrong with the placenta, devastating problems can result, including miscarriage, stillbirth, prematurity, low birth weight and pre-eclampsia, a condition that drives up the mother’s blood pressure and can kill her and the fetus. A placenta much smaller or larger than average is often a sign of trouble. Increasingly, researchers think placental disorders can permanently alter the health of mother and child.

Given its vital role, shockingly little is known about the placenta. Only recently, for instance, did scientists start to suspect that the placenta may not be sterile, as once thought, but may have a microbiome of its own — a population of micro-organisms — that may help shape the immune system of the fetus and affect its health much later in life.

Dr Fisher and other researchers have studied the placenta for decades, but she said: “Compared to what we should know, we know almost nothing. It’s a place where I think we could make real medical breakthroughs that I think would be of enormous importance to women and children and families.”

The National Institute of Child Health and Human Development calls the placenta “the least understood human organ and arguably one of the more important, not only for the health of a woman and her fetus during pregnancy but also for the lifelong health of both.”

In May, the institute gathered about 70 scientists at its first conference devoted to the placenta, in hopes of starting a Human Placenta Project, with the ultimate goal of finding ways to detect abnormalities in the organ earlier, and treat or prevent them. 

Seen shortly after a birth, the placenta is bloody and formidable looking. Fathers in the delivery room sometimes faint at the sight of it, doctors say.

 It is bluish or dark red, eight or nine inches across and about an inch thick in the middle. The side that faced the fetus is covered by a network of branching blood vessels, the umbilical cord emerging like a fat stalk. The side that faced the mother, glommed onto the uterine wall, looks raw and meaty.

In some cultures, the organ has spiritual meaning and must be buried or dealt with according to rituals. In recent years in the United States, some women have become captivated by the idea of eating it — cooking it, blending it into smoothies, or having it dried and packed into capsules. Not much is known about whether this is a good idea.

Silent invader

When scientists describe the human placenta, one unsettling word comes up repeatedly: “invasive.” The organ begins forming in the lining of the uterus as soon as a fertilized egg lands there, embedding itself deeply in the mother’s tissue and tapping into her arteries so aggressively that researchers liken it to cancer. In most other mammals, the placental attachment is much more superficial.

The placenta establishes a blood supply at 10 to 12 weeks of pregnancy. Ultimately, it invades 80 to 100 uterine vessels called spiral arteries and grows 32 miles of capillaries.

 The placental cells form minute fingerlike projections called villi, which contain fetal capillaries and come in contact with maternal blood, to pick up oxygen and nutrients and get rid of wastes.Spread out, the tissue formed to exchange oxygen and nutrients would cover 120 to 150 square feet.

 Every minute, about 20 percent of the mother’s blood supply flows through the placenta. The front line of the invasion is a cell called a trophoblast, from the outer layer of the embryo.

 Early in pregnancy, these cells multiply explosively and stream out like a column of soldiers.“The trophoblast cells are so invasive from the get-go,” Dr. Fisher said. “They just blast through the uterine lining to get themselves buried in there.”

They shove other cells out of the way and destroy them with digestive enzymes or secrete substances that induce the cells to kill themselves. Michael McMaster, a professor of cell and tissue biology at the University of California, San Francisco, said that failures of this early process probably happened fairly often.

People often assume that miscarriages and other problems arise from the fetus itself, but he said, “it’s probably true that at this early stage, a lot of trophoblast malfunction can underlie pregnancy loss or future disease.”

Trophoblasts are so invasive that they will form a placenta almost anywhere, even if they land on tissue other than the uterus. Occasionally, pregnancies begin outside the uterus, in fallopian tubes or elsewhere in the abdomen, and the rapid, penetrating growth of the placenta can rupture organs.

Placentas that form over a scar on the uterus, where the lining is thin or absent — say, from a previous cesarean section — can invade so deeply that they cannot be safely removed at birth, and the only way to prevent the mother from bleeding to death is to take out the uterus.

Dr Fisher’s lab discovered that as trophoblasts invade, they alter certain proteins on their surfaces, called adhesion molecules, to become more motile.

Researchers later found that cancer cells do the same thing as they spread from a tumor to invade other parts of the body. “When I first read this anatomy, I couldn’t believe the whole world wasn’t studying this,” Dr.Fisher said.

“The placenta gives the answer in many term stillbirths,” Dr Roberts said. Half of those deaths are never explained, but many of them involve abnormalities in the placenta, including infections or unusual conditions in which the mother’s immune system appears to have rejected the placenta.

I can’t tell you how important it is to the family just to have an answer,” she said. Knowing can help ease the guilt that many parents feel when a child is stillborn. The information can also tell doctors what to watch for in future pregnancies. 

In one case, Dr Roberts said, examining the placenta helped diagnose an immune incompatibility between the parents that had caused multiple stillbirths and miscarriages. The mother was treated and went on to have a healthy child.