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The Uterus: A Natural History

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The human body is an amazing thing. For each one of us, it’s the most intimate object we know. And yet most of us don’t know enough about it: its features, functions, quirks, and mysteries. That’s why we’re launching a new series called The Body, which will explore human anatomy, part by part. Think of it as a mini digital encyclopedia with a dose of wow.
 

At only 3 inches long and weighing about 60 grams, the uterus isn’t a flashy, attention-grabbing organ. When it comes to human health, the heart usually comes first, followed by the brain, then perhaps the digestive system. Yet the uterus plays an outsized role. It’s the carrier of all life, the subject of scrutiny in political forums, and a source of delight and despair for sexually mature women. It causes bleeding and pain, allows 211 million women to get pregnant every year, and is partially responsible for the 10 to 20 percent of those pregnancies that end in miscarriage.

Despite its ability to create life, there are dozens of crucial things we have yet to learn about the uterus. At least we’ve abandoned the theory that it travels freely around the body, causing hysteria, and that it can be manipulated by smelling salts.

Today we know the uterus sits low in the abdomen, held in place by muscles and ligaments. It is connected to the vagina by the cervix and receives unfertilized eggs from the ovaries via the fallopian tubes, which are connected to both sides of the uterus. It expands from 3 inches to the size of a watermelon by the end of a pregnancy in order to hold the baby and placenta—and, luckily for new mothers, naturally deflates about six weeks after the child is born.

But how did we develop this organ, how does it operate—or malfunction—in the body, and what's the outlook for the future?

THE EXTRAORDINARY EVOLUTION OF THE MAMMALIAN UTERUS

Until recently, scientists didn’t even understand how mammals evolved uteruses that allowed for live birth. Soft tissue is rarely preserved in the fossil record, which means scientists can study the bone structure of past organisms but are often left guessing when it comes to organs.

Up until marsupial ancestors appeared 220 million years ago, new life came out of eggs. Before that time, even the earliest mammalian predecessors, the group called monotremes (like echidnas and platypuses) were still laying eggs. But by 105 million years ago, placental mammals had evolved elaborate uteruses that allowed for invasive placentas, maternal tolerance of the fetus, and long gestation periods. What caused this evolution? Why did mammals suddenly appear?

In 2015, a team of researchers from the University of Chicago, Yale, and several other universities found a major clue in the hunt to discover the origin of mammals: genetic parasites. Called transposons, these snippets of non-protein-coding DNA regularly changed positions in the genome, an action called “jumping genes.” The leap-frogging transposons caused genes from other tissues—like the brain and digestive system—to be activated in the uterus. As more and more genes were expressed in the uterus, organisms shifted from producing eggs to giving live birth. The shift began sometime between 325 and 220 million years ago with the appearance of monotremes, and continued for hundreds of millions of years until placental mammals appeared, sometime between 176 and 105 million years ago.

During the genetic shift, more than 1000 genes turned on in therians, common ancestors to marsupials and placental mammals (like us). Many of these genes related to maternal-fetal communication, and especially the suppression of the maternal immune system in the uterus so it didn't reject the developing fetus. Because many of the transposons had progesterone binding sites that regulated the process, the uterus evolved to be extremely sensitive to that hormone (which is produced by the ovaries during the release of a mature egg; it prepares the uterine lining to receive a fertilized egg). The study appeared in the journal Cell Reports. In a press statement, Vincent Lynch, one of the study’s authors, said the discovery shed light on how “something completely novel evolves in nature.”

“It’s easy to imagine how evolution can modify an existing thing, but how new things like pregnancy evolve has been much harder to understand,” Lynch continued. “We now have a new mechanistic explanation of this process that we’ve never had before.”

THE MYSTERIES OF MENSTRUATION

While live birth defines mammals, including everything from whales to dogs to bats, there’s one thing that sets humans apart from most other species: menstruation. We’re part of an exclusive club that’s limited to old world primates, elephant shrews, and fruit bats. All other species remodel and reabsorb the endometrium, or uterine lining. So why do humans have to deal with the hassle of a period? Scientists aren’t quite sure. One theory is that the process protects us from abnormal pregnancies. The human gestation period is so long and requires so many biological resources that it’s better to reject all but the best candidates. And the reason we have periods is far from the only thing we don’t understand about menstruation.

“There is so much we don’t know,” says Hilary Critchley, OB/GYN and professor of reproductive sciences at the University of Edinburgh. “Not only why do we have normal periods, but particularly why does a woman have heavier periods?” Critchley and her colleagues published a paper that compiled years’ worth of studies in Human Reproduction Update in July 2015. They found far more questions than answers. Their research confirmed what is known: that a decline in progesterone triggers menstruation, and that the endometrial coagulation system plays a part in stopping the bleeding. But plenty of questions remain about the mechanics of the process.

Doctors don’t know what regulates inflammation during menstruation, what causes the bleeding to stop, or how the uterus repairs itself so quickly without creating any scar tissue. They also don’t understand the causes of diseases associated with menstruation, like polycystic ovary syndrome and endometriosis. Neither currently has a cure, and they afflict around 1 in 10 women. In the most extreme cases of endometriosis, women have no choice but to undergo hysterectomies.

“If you’re in the workforce, period problems can be really embarrassing and really difficult to deal with. This is where I see the unmet need for new treatments,” Critchley tells mental_floss. “A woman now has 400 periods in a lifetime. A woman (100 years ago) had 40. If you’ve got more periods, you’ve got more opportunity for it to be a problem.” This increase in the number of periods by a factor of 10 in the past 100 years is due to contraception and improved nutrition. The downside is that's a lot more opportunity for menstruation to cause problems.

GROWING AN EXTRA ORGAN TO MAKE A BABY

Menstruation isn’t the only area of female reproductive health that has researchers scratching their heads. Perhaps even more confounding is the placenta, a transient organ created during pregnancy by the embryo.

“I’d say the placenta is probably the least studied and the least understood organ in the body,” says Catherine Spong, acting director of the National Institute of Child and Human Development. She oversees the Human Placenta Project (HPP), which aims to develop new tools to monitor the placenta throughout its development. “If you could understand how the placenta allows two genetically distinct entities not only to grow, but also thrive, the implications for enhancing our understanding of immunology and transplant medicine would be pretty remarkable.”

Stacy Zamudio, a recipient of a grant from the HPP and director of research at Hackensack University Medical Center, calls the placenta “the most wonderful organ ever.” Her research focuses on placenta accreta (when the placenta grows too deeply into the mother’s uterine wall and even outside organs).

“It breathes, it produces hormones, it produces immunologic factors that protect the baby against infection. It acts like a skin, a liver, a kidney, a lung—it does all the functions of the other organs in one organ,” Zamudio says.

Human Placenta Project

The placenta achieves this by hooking into arteries in the uterus, essentially hijacking the mother’s body so the embryo can have a constant stream of nutrients and oxygen as it develops. When it’s functioning normally, the placenta ensures a positive outcome: healthy baby, healthy mother. But when things go wrong with the placenta, they quickly go from bad to worse.

The placenta can be under-invasive, meaning the connection to the mother’s blood isn’t strong enough. The baby stops developing because it’s not getting nutrients, and in the worst cases the mother can suffer from preeclampsia, which causes life-threateningly high blood pressure and can only be treated by immediate delivery of the baby. Or, as with the cases Zamudio studies, the placenta can be over-invasive, infiltrating the uterus and other organs beyond it like a cancer. Finally, in a complication known as placental abruption, the placenta can peel away from the uterus before delivery, removing the baby’s source of oxygen and nutrients and causing heavy bleeding in the mother.

Pregnancy can be a dangerous balancing act, and if doctors had better ways of monitoring the placenta’s development over the course of pregnancy, they might be able to prevent or avert the worst outcomes.

FROM WOMB TRANSPLANTS TO TRICORDERS

In October 2014, a baby born to a Swedish couple became an exciting example of the possible future of maternity—he was the first child ever born of a transplanted uterus. (The first pregnancy from a womb transplant, in Turkey, was terminated in 2013 when the fetus had no heartbeat.) The 36-year-old mother, who was herself born without a uterus, received a donation from a woman in her 60s, and had a frozen embryo successfully implanted in the transplanted organ. Although the child was born prematurely, he and the mother were otherwise healthy after the pregnancy. Since then, four more women who received uterus transplants from doctors at the University of Gothenburg have gotten pregnant. 

The pioneering surgery is now spreading across the world. Doctors at Cleveland Clinic performed the first successful uterus transplant in the U.S. just last week. The 9-hour surgery was performed on a 26-year-old patient with uterine factor infertility (an irreversible condition affecting 3 to 5 percent of women that prevents pregnancy). If the patient heals and can become pregnant, the surgery could offer new hope to women who previously thought they were doomed to infertility.

Despite the enormous advances made in the last decades concerning women's health, many questions about the uterus remain unanswered. Scientists don’t know why the placenta sometimes grows too little or too much, or how it communicates with the rest of the organs in the mother’s body. They don’t know why some women have debilitating cramps during their periods that have been likened to the pain of having a heart attack. But with scientists around the globe investing time and resources into such questions, it might not be long before we have real answers and solutions to these problems.

"We're not that far away from the tricorder in Star Trek," Zamudio says, referring to developing technologies like nanomagnetics. "I'm hoping that I'll be alive long enough to see a doctor be able to wave the instrument over the woman's abdomen and tell me what the glucose level is in that body."

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The Prehistoric Bacteria That Helped Create Our Cells Billions of Years Ago
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We owe the existence of our cells—the very building blocks of life—to a chance relationship between bacteria that occurred more than 2 billion years ago. Flash back to Bio 101, and you might remember that humans, plants, and animals have complex eukaryotic cells, with nucleus-bound DNA, instead of single-celled prokaryotic cells. These contain specialized organelles such as the mitochondria—the cell’s powerhouse—and the chloroplast, which converts sunlight into sugar in plants.

Mitochondria and chloroplasts both look and behave a lot like bacteria, and they also share similar genes. This isn’t a coincidence: Scientists believe these specialized cell subunits are descendants of free-living prehistoric bacteria that somehow merged together to form one. Over time, they became part of our basic biological units—and you can learn how by watching PBS Eons’s latest video below.

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11 Primal Facts About Dian Fossey
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Born in San Francisco on January 16, 1932, Dian Fossey came from a world far removed from the dense jungles of East Africa. She discovered that environment in her thirties and spent the final decades of her life studying the gorillas that lived there. From her groundbreaking primatology work to her mysterious death, here are 11 facts about the scientist behind Gorillas in the Mist.

1. HER LOVE OF ANIMALS BEGAN WITH A PET GOLDFISH.

Though she went on to become one of history's most famous animal-lovers, Fossey didn't grow up in a pet-friendly household. The only animal she was allowed to keep as a child was a single goldfish. She loved her fish, but when it died, her parents barred her from getting another animal to replace it. Even a pet hamster offered to her by a classmate was forbidden from entering the house.

2. SHE WAS A PRIZE-WINNING EQUESTRIAN.

Not permitted to keep pets in the home, Fossey nurtured her passion for animals through equestrianism. She received her first horseback-riding lesson at age 6. By the time she reached her teen years, she was advanced enough to merit an invitation to join the riding team at Lowell High School in San Francisco. Her hobby earned her several awards and pushed her to pursue an education in animal husbandry at the University of California, Davis. Even after she'd shifted career aspirations to occupational therapy, Fossey chose to move to Kentucky to be closer to farm life.

3. SHE SPENT HER LIFE SAVINGS ON HER FIRST TRIP TO AFRICA.

Dian Fossey was 31 when she first stepped foot on the continent where she'd complete her most important work. Inspired by a friend's trip to Africa, she collected her life savings (about $8000), took out a three-year bank loan, and planned a seven-week trip through the wilderness of Kenya, Tanzania, Congo, and Zimbabwe. On her adventures there she met Louis Leakey, the anthropologist famous for sponsoring the all-woman trio of primatology pioneers (the "trimates") that included Jane Goodall, Biruté Galdikas, and eventually Dian Fossey herself. It was also during this period when Fossey saw gorillas in the wild for the first time. She met wildlife photographers Joan and Alan Root and joined them on an expedition to photograph the animals in the Congolese mountains. The vacation wasn't scientific in nature, but as Fossey later wrote, "The seed was planted in my head, even if unconsciously, that I would someday return to Africa to study the gorillas of the mountains."

4. SHE PROVED HER DEDICATION WITH AN APPENDECTOMY.

Leakey reconnected with Fossey back in the States in 1966. The anthropologist had spent the last several years supporting his former secretary Jane Goodall in her chimpanzee research, and now he was in search of a candidate to do for gorillas what Goodall had done for chimps. After getting to know Fossey better, he decided she was the right woman for the job. He offered to gather the funding for her trip back to Africa, but before she left she would need to remove her appendix as a precaution. This didn't scare her off. When Leakey wrote six weeks later to say the surgery wouldn't be necessary and he had just wanted to make sure she was committed, she was already appendix-less.

5. HER FIRST RESEARCH EXPEDITION ENDED ABRUPTLY.

Fossey returned to the Congo toward the end of 1966—just months before a civil war erupted in the already volatile region. Rebel soldiers captured her at her base camp in July 1967. After spending two weeks in military detainment, she was able to bribe her way out with promises of cash and her Land Rover. The guards agreed to drive her to Uganda, and shortly after they arrived, she had them arrested. After the scare, Fossey was ready to resume her research almost immediately: This time she set up camp in Rwanda, ignoring warnings from the U.S. Embassy.

6. SHE UNCOVERED THE GORILLAS' TRUE NATURE.

Prior to Fossey's research, the public viewed gorillas as beasts similar in temperament to King Kong. She quickly disproved the notion that gorillas were bloodthirsty animals that would attack humans when given the chance.

To infiltrate their society, she adopted their habits. Walking on her knuckles and chewing on celery stalks allowed her to gain the apes' trust. As long as she maintained a nonthreatening profile and made her presence known at all times, she was safe around the gentle behemoths. Today we know that despite their intimidating size, gorillas are some of the least violent members of the great ape family.

7. SHE EARNED A UNIQUE NICKNAME FROM LOCALS.

Dian Fossey spent enough time at her research center in Rwanda to garner a reputation. To the locals she was Nyiramachabelli, a Swahili name that when roughly translated means "the woman who lives alone on the mountain."

8. SHE USED THE GORILLAS' NOSES TO TELL THEM APART.

Many of the gorillas Fossey studied were given names, such as Peanut, Rafiki, and Uncle Bert. Fossey used another method to tell her subjects apart: She drew sketches of their noses. Each gorilla has a unique pattern of wrinkles around its nose that makes it easy to identify. These nose prints are the equivalent of fingerprints in humans, but instead of getting up close to study them, Fossey was able to document them from far away using binoculars and a sketchpad.

9. ONE OF HER GORILLAS IS ALIVE TODAY.

Hundreds of gorillas made it into Dian Fossey's body of research. In 2017, only one specimen from that original pool is still alive. Poppy was born into a group of gorillas on Fossey's radar in 1976. The researcher documented the animal's birth and childhood in her journals. Today, at 41, Poppy is the oldest gorilla currently monitored by the Dian Fossey Fund.

10. HER WORK IS THE SUBJECT OF A BOOK, A MOVIE, AND AN OPERA.

In 1983, Fossey published the book that helped make her famous. Gorillas in the Mist is the autobiographical account of her first 13 years in the African jungle and the scientific discoveries she made about the gorillas living there. The title went on to become a bestseller. Five years later, Sigourney Weaver starred as Fossey in a film of the same name. The biopic snagged five Oscar nominations and converted Weaver into a gorilla conservationist.

There's another dramatization of Fossey's life that's not so widely known: In 2006, the Kentucky Opera VISIONS! program staged an opera called Nyiramachabelli—a nod to the researcher's nickname.

11. HER DEATH REMAINS A MYSTERY.

Next to her groundbreaking gorilla research, Fossey is perhaps best known for her mysterious and tragic murder. On December 27, 1985, she was found dead in her cabin at her Rwandan research camp. The cause of death was a machete blow to the head, but the identity of her assailant remains unknown to this day. (A Rwandan court convicted in absentia her American research assistant, Wayne McGuire, for her murder and sentenced him to death. McGuire, who fled Rwanda before the conviction, has always maintained his innocence.) Fossey was buried in the nearby mountains beside the grave of her favorite gorilla Digit, who had been slaughtered by poachers years earlier. Before she was killed, Fossey wrote one final entry in her diary. It reads:

“When you realize the value of all life, you dwell less on what is past and concentrate on the preservation of the future.”

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