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The Uterus: A Natural History

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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.

graphic explaining the placenta and its importance
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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11 Facts About Fingernails
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Whether there's dirt beneath them or polish atop them, your fingernails serve more than just decorative purposes: They help keep your fingertips safe and have a multitude of special functions that even your doctor might not be aware of. “The nails occupy a unique space within dermatology and medicine in general, particularly because they are such a niche area about which few people have expertise,” Evan Rieder, assistant professor in the Ronald O. Perelman Department of Dermatology at NYU Langone Health, tells Mental Floss.

1. FINGERNAILS HAVE FOUR MAIN PARTS.

Along with skin and hair, nails are part of the body's integumentary system, whose main function is to protect your body from damage and infection. Fingernails have four basic structures: the matrix, the nail plate, the nail bed, and the skin around the nail (including the cuticle).

Fingernail cells grow continuously from a little pocket at the root of the nail bed called the matrix. The pale, crescent-shaped lunula—derived from Latin for "little moon"—on the nail itself is the visible portion of the matrix. If the lunula is injured, the  nail won't grow normally (a scarred lunula can result in a split nail), and changes in the lunula's appearance can also be signs of a systemic disease.

Fingernail cells are made of a protein called keratin (same as your hair). As the keratin cells push out of the matrix, they become hard, flat and compact, eventually forming the hard surface of the nail known as the nail plate. Beneath that is the nail bed, which almost never sees the light of day except when there's an injury or disease.

Surrounding the matrix is the cuticle, the semi-circle of skin that has a tendency to peel away from the nail. The skin just underneath the distal end of the fingernail is called the hyponychium, and if you've ever trimmed your nails too short, you know this skin can be slightly more sensitive than the rest of the fingertip.

2. THEY GROW AT A RATE OF 0.1 MILLIMETERS A DAY ...

That's about 3 to 4 millimeters per month. But they don't always grow at the same speed: Fingernails grow more quickly during the day and in summer (this may be related to exposure to sunlight, which produces more nail-nourishing vitamin D). Nails on your bigger fingers also grow faster, and men's grow faster than women's. The pinky fingernail grows the slowest of all the fingernails. According to the American Academy of Dermatology, if you lose a fingernail due to injury, it can take up to six months to grow back (while a toenail could take as much as a year and a half).

3. ... BUT NOT AFTER YOU'RE DEAD.

You've probably heard that your fingernails keep growing after death. The truth is, they don't, according to the medical journal BMJ. What's actually happening is that the skin around the base of the fingernails retracts because the body is no longer pumping fluids into the tissues, and that creates a kind of optical illusion that makes the nails appear longer.

4. ITS ESTIMATED THAT 20 TO 30 PERCENT OF PEOPLE BITE THEIR NAILS.

Scientists say it's still unclear why, but they suspect nail-biters do it because they're bored, frustrated, concentrating, or because it just feels comforting (and anxiety doesn't seem to play a big role). Perfectionists who don't like to be idle are very likely to have the habit. Biters expose themselves to the dangerous crud that collects underneath the nail: The hyponychium attracts bacteria, including E. coli, and ingesting that through nail-biting can lead to gastrointestinal problems down the line. Biting can also damage teeth and jaws.

5. HUMAN FINGERNAILS ARE BASICALLY FLAT CLAWS.

Our primate ancestors had claws—which, like nails, are made of keratin. As human ancestors began using tools some 2.5 million years ago (or even earlier), evolutionary researchers believe that curved claws became a nuisance. To clutch and strike stone tools, our fingertips may have broadened, causing the claws to evolve into fingernails.

6. THE NAIL ACTUALLY MAKES YOUR FINGERTIP MORE SENSITIVE.

While the fingernail may be tough enough to protect tender flesh, it also has the paradoxical effect of increasing the sensitivity of the finger. It acts as a counterforce when the fingertip touches an object. "The finger is a particularly sensitive area because of very high density of nerve fibers," Rieder says.

7. FINGERNAILS CAN REVEAL LUNG, HEART, AND LIVER DISEASES.

"One of the most interesting facts about fingernails is that they are often a marker for disease within the body," Rieder says. Nail clubbing—an overcurvature of the nail plate and thickening of the skin around the nails—is a particularly significant sign of underlying illness, such as lung or heart disease, liver disease, or inflammatory bowel disease. Two-toned nails—whitish from the cuticle to the nail's midpoint and pink, brown, or reddish in the distal half—can be a sign of kidney and liver disease. Nails that are two-thirds whitish to one-third normal can also be a sign of liver disease. However, little white marks on your nails, known as milk spots (or punctate leukonychia) are just the remnants of any kind of trauma to the nail, from slamming it in a door to chewing on it too fervently.

8. YOU CAN GET A COMMON SKIN DISEASE ON YOUR NAILS.

Psoriasis is "typically thought of as a skin disease, but is actually a skin, joint, and nail disease, and when severe, a marker of cardiovascular risk," Rieder says. Psoriatic fingernails may have orange patches called oil spots, red lines known as splinter hemorrhages, lifting of the edges of the nails, and pits, "which look like a thumb tack was repeatedly and haphazardly pushed into the nails," he says.

Doctors often prescribe topical or injected corticosteroids to treat psoriatic nails, but using lasers is an emerging and potentially more cost-effective technique. Rieder relies on a pulsed dye laser, which uses an organic dye mixed with a solvent as the medium to treat nail psoriasis, "which can be both medically and aesthetically bothersome," he says. This laser is able to penetrate through the hard nail plate with minimal discomfort and "to treat targets of interest, in the case of psoriasis, blood vessels, and hyperactive skin," Rieder says.

9. ANCIENT CULTURES DISPLAYED SOCIAL STATUS WITH NAIL ART.

Painting and other forms of decorating nails have a history of offering social and aesthetic cues through variations in nail color, shape, and length, Rieder says. In fact, he adds, in some cultures ornate and well-decorated fingernails "serve as a proxy for social status."

Five thousand years ago in China, men and women of the Ming Dynasty aristocracy grew their nails long and covered them with golden nail guards or bright home-made polishes. The long nails allegedly announced to the world their social rank and their freedom from performing menial labor.

10. A FORMER BEAUTICIAN HELD THE WORLD RECORD FOR THE LONGEST NAILS.

Lee Redmond of Utah started growing her nails in 1979 and kept at it until she held the world record for "longest fingernails on a pair of hands ever (female)" in 2008. Her right thumbnail was 2 feet, 11 inches and the collective length of all her nails was 28 feet, 4 inches. She also applied nail hardener daily and painted them a reflective gold. Unfortunately, she broke her nails in a 2009 car accident and has no plans to regrow them.

11. THE FIRST NAIL CLIPPERS WERE PATENTED IN 1875.

Today, biters don't have to use their teeth to trim their nails. While the earliest tools for cutting nails were most likely sharp rocks, sand, and knives, the purpose-built nail clipper—though it might be more accurately called a circular nail file—was designed by a Boston, Massachusetts inventor named Valentine Fogerty and patented in 1875. The nail clippers we know today were the design of inventors Eugene Heim and Oelestin Matz, who were granted their patent for a clamp-style fingernail clipper in 1881.

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What's Really Happening When We See 'Stars' After Rubbing Our Eyes?
Photo illustration by Mental Floss. Images: iStock.
Photo illustration by Mental Floss. Images: iStock.

It's likely happened to you before: You start rubbing your eyes and almost immediately begin seeing colors, specks, and swirls from behind your closed lids. So what's happening when you see these 2001-esque "stars"? Do they only occur upon rubbing? Does everyone experience them?

Before we can get to what causes the lights, we need to understand a bit about how the eyes work. Angie Wen, a cornea surgeon at New York Eye and Ear Infirmary of Mount Sinai, tells Mental Floss that the retina—the innermost layer of the eye—consists of millions of cells, or photoreceptors. These cells, she says, "are responsible for receiving information from the outside world and converting them to electrical impulses that are transmitted to the brain by the optic nerve. Then, the brain interprets them as images representing the world around us."

However, what we see doesn't just stop there. Sometimes "we see light that actually comes from inside our eyes or from electric stimulation of the brain rather than from the outside world," Wen says. "These bursts of seemingly random intense and colorful lights are called phosphenes, and appear due to electrical discharges from the cells inside our eyes that are a normal part of cellular function."

People have been writing and theorizing about phosphenes for thousands of years. Greek philosophers thought the bursts of light were the result of fire inside our heads: "The eye obviously has fire within it, for when the eye is struck fire flashes out," wrote Alcmaeon of Croton (6th–5th century BCE), a philosopher and early neuroscientist, of the swirls and specks someone sees after getting a blow to the head. A century later, Plato—who believed that a "visual current" [PDF] streamed out of the eye—wrote that "Such fire as has the property, not of burning, but of yielding a gentle light they [the Gods] contrived should become the proper body of each day."

Plato's take was still the dominant one through the Middle Ages. Eventually, Newton (1642–1727) theorized a concept that's more in line with what's believed today about these strange sparkly visions: The phenomenon is due to light that's produced and observed when pressure and motion is placed on the eyes.

Eleonora Lad, an associate professor of ophthalmology at Duke University Medical Center who has a background in neuroscience, explains exactly why eye rubbing generates these visions: "Most vision researchers believe that phosphenes result from the normal activity of the visual system after stimulation of one of its parts from some stimulus other than light," including putting external pressure on the eyes. (Interestingly, due to retinal damage, blind people can't see phosphenes caused by pressure, but they can see them when their visual cortex is electrically stimulated. In hopes of turning this phenomenon into improved vision for the blind, scientists have developed a cortical visual prosthesis, implanted in the visual cortex, that generates patterns of phosphenes. The device has been approved by the FDA for clinical trial.)

As Alcmaeon rightly pointed out, there are causes for the bursts of light beyond just rubbing your eyes: Getting hit in the eye can produce this phenomenon—as can a sneeze, a surprisingly powerful event that tends to clamp our eyes shut, Wen says.

Receiving an MRI or EEG may also trigger it. MRIs, for example, produce a changing magnetic field which can stimulate the visual cortex, making a person see these flashing lights. When it comes to an EEG, depending on the brain stimulation frequency band (Hz) used, some patients experience the phenomenon when closing their eyes, which is believed to come from retinal stimulation during the process.

And the activity doesn't only happen on Earth; astronauts in space have also been known to experience them. As reported in 2006 in the journal Vision Research, "over 80 percent of astronauts serving in today's NASA or ESA (European Space Agency) programs have perceived phosphenes at least in some missions and often over several orbits." They're mainly attributed to interactions between the eye and cosmic ray particles in space, outside the Earth's protective magnetic field.

No matter the cause, the bursts of light are perfectly normal—but that doesn't mean you should engage in excessive eye rubbing. Wen says ophthalmologists advise against rubbing your eyes or applying vigorous pressure; according to Lad, too much rubbing may be damaging to the cornea and lens or "result in a loss of fatty tissue around the eyes, causing the eyes to look deep-set."

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