The Amazing Placenta

A placenta is less an organ contained and owned by a mother than it is one temporarily leasing space in her body. An object lesson.
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I had not given much thought to placentas until the one inside me refused to come out. I’d never thought of the placenta as an object that had agency, let alone its own personality. But there I was, wondering why it didn’t obediently expel itself during what had always been described to me as a mundane third stage of labor.

By not gracefully exiting when it was expected to, the placenta inside me was given an adjective to indicate its obstinate, misbehaving status. It was no longer a plain-old “placenta.” Instead, it was deemed a “retained placenta,” one that could potentially cause bleeding and threaten my life. Now it was a fugitive holing up in my uterus, resisting arrest . The placenta had done a good job during the pregnancy, but apparently even that wasn’t enough to redeem it.

After a standoff involving two shots of oxytocin and a nurse tugging unsuccessfully on the umbilical cord, the placenta and I were wheeled into the operating room. A doctor dug her hands and tools inside me and retrieved it. The placenta was quickly whisked away before I could even catch a glimpse. No one mentioned its fate to me. I’d guess that it was disposed of as biohazardous waste, maybe after a poke or two from the pathologist. It seemed like an unceremonious end, even for an organ that is unflatteringly nicknamed “the afterbirth.”

The same year I had an encounter with a retained placenta I never saw, I also made the acquaintance of a placenta from a Hollywood movie. In the comedy “Wanderlust” (Universal Pictures, 2012), George Gergenblatt and his wife stumble upon a commune and become indefinite guests.

In one scene, one of the commune’s resident couples enters the breakfast room with their newborn. The placenta and umbilical cord are in tow, still attached after the “lotus birth” the couple had selected. George is the only one who is alarmed by the placenta. It, and the couple’s plan to make soup with it, drive him over the edge. While the placenta rests quietly in a bowl on the kitchen table, George flies into an abusive rant: “Cut and discard that shit off your infant immediately. It’s gross!”

For the George Gergenblatts of the world, the placenta seems like an inactive, amorphous lump of bloody flesh. But it is far from inactive. Even after its delivery, it lives for minutes longer, as the umbilical cord continues to pump nutrients from it to the newborn. The placenta is the dedicated, underpaid employee that continues to work overtime. Some doctors and midwives delay clamping the umbilical cord for a few minutes after delivery, to reap the benefits of the placenta’s dedication.

Nor is the placenta a simple blob of anonymous tissue. Peel away the outer membranes that give it its shiny appearance, and one discovers a disc-shaped organ with distinct features. On the side connected to the uterus is a dark and lobed “maternal” side. On the other side is a shiny “fetal” side from which the umbilical cord dangles. On the inside, fetal blood vessels branch off in an intricate tree-like pattern from a trunk of three blood vessels in the umbilical cord.

Whether cast as a villain in a real-life medical drama or as a prop in a Hollywood comedy, the placenta is a highly sought-after actor. In folklore, it has been attributed various important roles, such as sibling or twin, second soul, or part of the child. In such roles, the placenta would be given an appropriate disposal. According to some traditions, the placenta is planted underneath a tree. A sea burial is not unusual in Indonesia, and some fathers even dive to bring the placenta to its final resting place. Other traditions prescribe burning it, while still others caution against doing so, lest harm could come to the child.

As old as these beliefs might be, they are consistent with our current understanding of the placenta. Understanding the placenta as a “sibling” isn’t entirely inaccurate, genetically speaking. The placenta is comprised of a mix of fetal and maternal tissues. Therefore, like most siblings, it shares some but not all of its genetic composition with the child it cradles in the mother’s womb.

The view that the placenta is a part of the child is consistent with its developmental origins. Most of the placenta comes from the embryo that later also becomes the child. A developmental biologist peering through a microscope at a human embryo as young as four days old can quickly point out which cells are destined to become placenta. These cells, the trophobloasts, are the first type of cell in the embryo to choose their future vocation. They sit at the outer edge of the spherical embryo, forming a thin layer of cells around an inner cavity and a lopsided mass of cells that later form the fetus.

Trophoblasts literally invade the tissue in the uterus, burrowing into the endometrium and setting up a semi-permanent campsite for the nascent placenta. The maternal blood supply is rerouted to the developing fetus, and the elaborate architecture of branched fetal vessels form. Evasive maneuvers are undertaken to avoid detection and attack by the mother’s immune system. This process of placentation is sometimes likened to tumor formation, as many of the mechanisms and molecular players are the same.

When the author Nancy Redd laments eating her placenta, or when the model Holly Willoughby announces that she won’t rule out eating hers, we probably don’t bat an eye at their use of the possessive pronoun. But the placenta is really an unequal mix of maternal and fetal tissues—the latter forming the majority of the organ. Perhaps the term “mother cake” that Swedes and Germans use to refer to the placenta should also be called into question. The placenta is less an organ contained and owned by the mother than it is one temporarily leasing space in her body for a purpose orthogonal to the mother’s immediate concerns.

Perhaps its uncanny status helps explains the placenta’s many oracular uses, ancient and modern, ritual and medical. Traditional midwives in the Ukraine use the placenta to predict the number of future children. West African wise men examine the placenta to predict the child’s future.  Pathologists also inspect the placenta, often looking at cross-sections of the tissue for clues after an unusual pregnancy or labor. Following a study published earlier this year, clinicians might begin examining the placenta for abnormal folds and creases, which may indicate increased risk of developing autism. Molecular biologists may also examine the placenta, looking at levels of certain molecules to predict an infant’s future behavior.

The placenta is a transient organ that doesn’t even appear in all of us. Yet it forms a lasting part of our evolutionary identity, our being. We classify ourselves as “placental mammals,” to distinguish from other mammals that either lay eggs (“monotremes”) or give birth to young fetuses and carry them in pouches until they’re well developed (“marsupials”). Strictly speaking, we’re not the only kinds of animals to have placentas. Marsupials and some reptilian species have rudimentary versions of placentas. But our "eutherian" placentas are far more sophisticated, capable of bringing nutrients and oxygen to the fetus until it completes its gestation.

Indeed, our placenta is a remarkable biological innovation. Gray’s Anatomy describes the placenta as “the organ by which the connection between the foetus and the mother is established, and which subserves the purposes of nutrition, respiration, and excretion.” This simple-seeming definition belies the complexity of its engineering. Fetal blood vessels, shaped into branch-like structures called chorionic villi, immerse themselves in a pool of maternal blood. The intimate proximity with maternal blood allows transport of gasses, nutrients, waste products, and other molecules like antibodies between the two blood systems. But even with an estimated 12 square meters of area available for diffusion between fetal and maternal blood, never or rarely do the two bloods meet. A four-membrane placental barrier sees to that.

Placentas are often imitated but never duplicated. Artificial placentas are simply not up to par with the real thing. In his 1931 novel “Brave New World,” Aldous Huxley envisioned the existence of artificial wombs and placentas that would support fetal development to term. But after sixty years of efforts, completely functional placental replacements are still the work of fiction. Some devices generously named “artificial placentas” are limited to respiration support. They do not tackle the delivery of nutrients that the placenta handles so deftly. The most advanced devices to date can only maintain animal fetuses for up to a few weeks.

As though its tedious work sustaining life in the uterus were not enough, the placenta has also served as raw material for medicines. One disease for which the placenta has offered relief is Gaucher’s disease, a rare genetic disorder in which patients are unable to process lipids (such as fats). Glucocerebrosidase, the enzyme deficient in Gaucher’s disease, can be extracted from the placenta. The drug manufacturer Genzyme did just that, preparing its enzyme replacement drug Ceredase from pooled placentas. After the gene for glucocerebrosidase was isolated, Ceredase was eventually replaced by Cerezyme, a version of the enzyme made by cells grown in the laboratory.

The placenta has also starred in a more controversial drug story. Merieux UK began collecting placentas in 1976 to extract the protein albumin, which it marketed for use in medical emergencies such as severe burns. But Merieux was ordered to stop in 1993, as the placentas had been collected without patient consent.

More recently, the placenta was discovered to be a rich source of stem cells. Individually tailored, genetically compatible therapies would theoretically become possible by harvesting and saving stem cells from a child’s placenta. For prenatally diagnosed birth defects, there are even visions of obtaining stem cells before birth, seeding them into tissue scaffolds to grow desired tissue grafts, and then performing corrective surgeries in the uterus or immediately after birth.

But it’s not only drug and biotech companies that make use of the placenta as raw materials. So does the beauty industry. Proteins obtained from the placenta are featured as an ingredient in some cosmetics, including anti-aging products. Placenta facials are also being offered at up-scale beauty clinics.

The placenta boasts an unusual relationship to us humans, for each of us has spent the better part of a year intimately connected to one. Whatever it can or cannot tell us about our future, we cannot deny the placenta’s importance in our personal histories.  Our common early dependence on it binds all of us, whether we sympathize with George Gergenblatt’s disgust in the face of the placenta, or revere it, or never think of it at all.

 


An ongoing series about the hidden lives of ordinary things
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Suzanne Nguyen is a biologist living near Gothenburg, Sweden.

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