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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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Animals
Why Male Hyenas Have It Worse Than Females
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A life of hunting zebras and raising young on the savanna isn’t half bad for a female hyena. Sadly, the same can’t be said for their male counterparts. As MinuteEarth explains, things take a downturn for the males of the species once they hit adolescence. No female in their pack will mate with them, a behavior scientists believe evolved to avoid inbreeding, so they head off in search of a different group to join. After dealing with vicious hazing from their new clan, they file in at the bottom of the rank and wait for other males above them to die so that they can slowly gain status.

Even after rising through the hierarchy, the most a male hyena can aspire to is being second place to the lowest-ranking female. Thanks to their bulky build and aggressive behavior, female hyenas enjoy a dominant position that’s rare in the animal kingdom.

After watching the video below, head over here for more facts about hyenas.

[h/t MinuteEarth]

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The Body
11 Facts About the Thumb
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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. Our series The Body explores human anatomy, part by part. Think of it as a mini digital encyclopedia with a dose of wow.

When it comes to the fingers on your hand, the thumb definitely does its own thing. Thumbs only have two bones, so they're obviously shorter, and they play a very important role that no other finger can claim; thanks to their unique saddle-like joint shape, and a little muscle known as the abductor pollicis brevis, you can bend and stretch your thumbs opposite your fingers to grip things. This is why they're known as "opposable thumbs." To bring you these 11 facts about the thumb, Mental Floss spoke with three experts on this unique digit: Barbara Bergin, an orthopedic surgeon in Houston; Loren Fishman, medical director of Manhattan Physical Medicine and Rehabilitation, in NYC; and Ryan Katz, attending hand surgeon at the Curtis Hand Center, located at the Medstar Union Memorial Hospital in Baltimore.

1. OPPOSABLE THUMBS MAY HAVE FREED UP OUR ANCESTORS' MOUTHS FOR LANGUAGE.

The evolution of a thumb helped our ancestors evolve to be better at defense, allowing for throwing and clubbing activities. Moreover, Fishman says, it may have even contributed to our cognitive function. "Some say this is why we have language," he says, "because we can hold things in our hands and [therefore] use our mouths for something else—such as discussing the functions of the thumb."

2. THUMBS HAVE THEIR OWN PULSE.

You might have noticed that medical professionals take a pulse with the middle and index finger. The reason is because there's a big artery in the thumb, the princeps pollicis artery, and arteries pulse, making it difficult to feel a pulse in a neck if you're using your thumb.

3. THE THUMB SEPARATES US FROM OTHER ANIMALS. MOSTLY.

"The thumb is wonderful. It evolved in such a way that we can use it to do so many amazing things, and it's one of the things that separates us from other animals," Bergin says. A handful of other animals, mostly primates, have opposable thumbs, or toes, as the case may be. These include orangutans, chimpanzees, a phylum of frogs known as phyllomedusa, some lemurs, and giant pandas—although their thumb-like apparatus is really just an extra sesamoid bone that acts like a thumb.

4. TOES CAN BECOME THUMBS.

If you should lose a thumb, fear not, says Katz. "It can be rebuilt by surgeons using your big toe." This specialized surgery uses microvascular surgery techniques to transfer your big toe to your hand, where it will function almost exactly as your thumb did. "The toe is then brought to life by sewing together small arteries and veins under a microscope," Katz says, a complicated surgery that has become vastly more sophisticated over the years. The second toe can be used too, as you can see in this medical journal, but we warn you: It's not for the faint of heart.

5. … BUT IS A THUMB WORTH LOSING A TOE OVER?

It may not seem like a big deal to lose one thumb—after all, you've got another one. But Katz cites the American Medical Association's "Guides to the Evaluation of Permanent Impairment" [PDF], which states your thumb is so important that a complete amputation "will result in a 40 percent impairment to the whole hand." In fact, they claim that it would take "a complete amputation of the middle, ring, and small fingers to equal the impairment of an amputated thumb."

6. IT'S BETTER THAN HAVING YOUR HAND SEWN TO YOUR FOOT.

Katz also points out that "there used to be a common surgical procedure for thumb reconstruction, where the patient's hand was sewn to their foot for a period of time." This procedure was called the Nicoladani procedure, after the German surgical innovator Carol Nicoladoni. "It was a precursor to transplant surgery and plastic or reconstructive surgery as we know it today," he says.

7. YOUR THUMB MAKES AN ASTONISHINGLY WIDE VARIETY OF MOTIONS.

Other than pinching and grasping, Katz points out that the thumb "translates, rotates, and flexes all at once." This coordinated set of motions provides strength and dexterity. "Thus it's the thumb that allows us to easily pen an essay, turn a nut, pick up a coin, or button a shirt."

8. THAT DEXTERITY ALSO MAKES IT FRAGILE.

The thumb may appear to only have two knuckles, but it actually has a third, right above the wrist. This is called the first carpometacarpal joint. If that starts to hurt, or gets big enough to look like a bump or a mass, you may have carpometacarpal joint disorder (CMC), a common condition that is partly genetic and partly from repetitive use, according to Bergin. "You can get arthritis in the other joints, too, but this one is the most debilitating," she says. "First it becomes painful, and then you lose the ability to use it." Surgery can help with the pain, but it won't restore full mobility.

9. PAIN IN YOUR THUMB MAY REQUIRE LIFESTYLE CHANGES.

Bergin suggests small lifestyle changes so you don't need to grip anything too hard can make a huge difference, such as buying milk jugs with handles or using an electric toothbrush. "There are a lot of things we can do [to help] on a daily basis that shouldn't affect our quality of life," she suggests.

10. SWIPING RIGHT MIGHT BE DANGEROUS.

While we generally associate thumb arthritis with older people, Bergin says she now sees it in people in their forties and even thirties. Other studies have suggested that frequent phone use can be damaging. "There must be a genetic component to premature wearing of the thumb," she says. If it runs in your family, it's a good idea to be proactive and try to avoid repetitive gripping activities.

11. WHAT IT MEANS IF YOUR THUMB IS NUMB.

If instead of pain you're experiencing numbness of the thumb that extends to your index and middle fingers, you may be showing early symptoms of carpal tunnel syndrome. Fortunately, this isn't an emergency. "The condition takes a long time to become a big problem" Bergin says. People can sometimes help the condition by wearing wrist braces and getting physical therapy. If you just can't take it, "you can get surgery at any point if you failed to improve with bracing," she says. The surgery can reduce mobility, but it should take away the numbness and pain.

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