Our Skin Is Covered With Invisible Stripes

Envy the tiger and the zebra no longer. You have stripes of your own.

Human skin is overlaid with what dermatologists call Blaschko’s Lines, a pattern of stripes covering the body from head to toe. The stripes run up and down your arms and legs and hug your torso. They wrap around the back of your head like a speed skater’s aerodynamic hood and across your face. Or they would, if you could see them.

In the early 1900s, German dermatologist Alfred Blaschko reported that many of his patients’ rashes and moles seemed to follow similar formations, almost as though they were tracing invisible lines. But those lines didn’t follow nerves or blood vessels. They didn’t represent any known body system.

Here is how Blaschko depicted these lines in an early paper:

Wikimedia Commons // Public Domain

It turns out these lines are far more extensive than even Blaschko thought—closer to this:

Davide Brunelli, Med Art


And today we know what they are: cellular relics of our development from a single cell to a fully formed human. Each one of us started out as a single cell, and then a little glob of cells. As the cells divided, they differentiated. Some became muscles, others bones, still others organs. And some became skin. As those skin cells continued dividing, they expanded and stretched to cover a quickly growing body. One cell line pushed and swirled through another like steamed milk poured into an espresso to make a latte.

Blaschko’s lines are the molecular evidence of those swirls.

Most people will never see their own stripes. As Dr. Blaschko noted, there are dozens of skin conditions that follow these lines, but most of them affect patches of skin or a single body part, not the entire body. Lined and whorled nevoid hypermelanosis can create beautiful patterns.

And then there are the chimeras. Remember the single cell that turned into a glob? From time to time, two of these starter cells will merge and become a glob together. The glob eventually resolves into a chimera: an animal with two lines of DNA. As the animal’s skin develops, the two groups of cells divide and swirl just like non-chimera skin cells. The difference is that the two groups of chimerical cells are slightly different from each other. Behold the chimera cat Venus:

Sometimes, this difference is obvious. More often in humans, though, it’s too subtle to notice with the naked eye, and can only be spotted under UV light.

Why Is Pee Yellow?

Chloe Effron
Chloe Effron

WHY? is our attempt to answer all the questions every little kid asks. Do you have a question? Send it to why@mentalfloss.com.

Your body is kind of like a house. You bring things into your body by eating, drinking, and breathing. But just like the things we bring home to real houses, we don’t need every part of what we take in. So there are leftovers, or garbage. And if you let garbage sit around in your house or your body for too long, it gets gross and can make you sick. Your body takes out the garbage by peeing and pooping. These two things are part of your body’s excretory system (ECKS-krih-tore-eee SISS-tem), which is just a fancy way of saying “trash removal.” If your body is healthy, when you look in the toilet you should see brown poop and yellow pee.

Clear, light yellow pee is a sign that your excretory system and the rest of your body are working right. If your pee, or urine (YER-inn), is not see-through, that might mean you are sick. Dark yellow urine usually means that you aren’t drinking enough water. On the other hand, really pale or colorless pee can mean you might be drinking too much water! 

Your blood is filtered through two small organs called kidneys (KID-knees). Remember the garbage we talked about earlier? The chemicals called toxins (TOCK-sins) are like garbage in your blood. Your kidneys act like a net, catching the toxins and other leftovers and turning them into pee.

One part of your blood is called hemoglobin (HEE-moh-gloh-bin). This is what makes your blood red. Hemoglobin goes through a lot of changes as it passes through your body. When it reaches your kidneys, it turns yellow thanks to a chemical called urobilin (yer-ah-BY-lin). Urobilin is kind of like food coloring. The more water you add, the lighter it will be. That's why, if you see dark yellow pee in the toilet, it's time to ask your mom or dad for a cup of water. 

To learn more about pee, check out this article from Kids Health. 

12 Facts About Kidney Stones

Illustration by Mental Floss. Images: iStock
Illustration by Mental Floss. Images: iStock

Kidney stones are more common than ever. According to Harvard Medical School, every year more than 3 million people see a doctor for relief from these hard mineral and salt deposits, which form in your kidney when urine becomes too concentrated. Here's what we know about the condition formally called nephrolithiasis.

1. KIDNEY STONES TYPICALLY CAUSE REALLY PAINFUL SYMPTOMS.

At first you may notice your urine is cloudy, bloody, and foul smelling. Your back may begin to ache, and nausea may come over you. Then, as the stone moves from your kidney into your urinary tract or bladder, sometimes becoming trapped, there’s often an intense, stabbing pain that many people say they wouldn’t wish on their worst enemy.

2. MOST PEOPLE DEVELOP ONE TYPE OF STONE …

What kind of kidney stone you get depends on your diet, fluid intake, genetics, hereditary disorders, and even whether you take certain medications, but the vast majority of people get calcium oxalate stones. They're formed from a mix of calcium in urine and the compound oxalate, which is found naturally in food like nuts, chocolate, and some vegetables, including beets and spinach; oxalate is also produced by your liver. There's some evidence that people who take the seizure medicine topiramate can develop these stones in the form of calcium phosphate.

3. … BUT THERE ARE THREE OTHER KINDS TOO.

Struvite stones are fast-growing mineral deposits that typically develop in response to a urinary tract infection, and can grow large enough to block the kidney, ureter, or bladder before you notice any symptoms; they affect women more than men. Uric acid stones turn up in people who eat a lot of red meat, shellfish, and organ meats, which contain hefty doses of an organic compound called purine that can lead to more uric acid than the kidneys can excrete. Cystine stones are caused by a rare hereditary disorder called cistinuria in which your kidneys excrete excessive amounts of the amino acid cystine.

4. THEY'RE EXTREMELY COMMON—ESPECIALLY IN MEN.

There's a solid chance you could end up with a kidney stone. The National Kidney Foundation notes that one in 10 people will develop one during the course of their life. And if you’re male, take note: Your gender alone is considered a kidney stone risk factor. Men are twice as likely as women to develop them. Another factor is age: Although stones are most common from ages 20 through 50, they tend to peak around age 30.

5. IF YOU’VE HAD A KIDNEY STONE, YOU’LL PROBABLY DEVELOP ANOTHER ONE …

Sorry to say, but simply having a kidney stone puts you at risk for a recurrence. If you’ve had one, the U.S. National Library of Medicine notes that there’s a 30 to 50 percent chance more stones will form within five years.

6. … BUT YOU CAN TAKE STEPS TO PREVENT THEM.

Cutting back on sodium (i.e. deli meats, packaged soups, and processed foods) can help, because a stone can form from excessive salt consumption. You should also avoid too much animal protein—it produces urine containing more acid, which is known to increase your risk for kidney stones—and increase your intake of fruits, vegetables, whole grains, and low-fat dairy. And be sure to drink plenty of fluids, especially water—at least 12 glasses a day. (That's good advice for everyone, not just those prone to kidney stones.)

Don't drink much apple or cranberry juice as both contain oxalates and are linked to an increased risk of developing calcium oxalate stones. High doses of Vitamin C may boost the concentration of oxalate in urine; the Cleveland Clinic recommends a daily maximum of 500 milligrams.

7. IT'S A MYTH THAT CALCIUM CREATES SOME KIDNEY STONES.

Despite the fact that the word calcium is part of the most common kind of kidney stone, you don’t need to treat calcium as the enemy. In fact, having too little calcium can actually increase the odds you’ll get these types of stones. According to the Cleveland Clinic, eating about two or three servings of calcium-rich foods daily reduces oxalate absorption, helping to keep calcium oxalate stones away. So get out the cheese.

8. IF YOU PASS A STONE, CONGRATULATIONS! NOW TAKE IT TO A DOCTOR.

Ninety percent of kidney stones are passed through urination. Getting one out this way may hurt a lot, but once the stone has finished causing you agony, it could provide clues that could help you avoid developing another one. If you’re able to retrieve the stone, bring it to your doctor, who can order an analysis. Identifying its components can reveal the kind of stone it is and potentially point to a treatment or prevention plan.

9. IF YOU CAN’T PASS A STONE, TREATMENTS ARE AVAILABLE …

In an attempt to exit the body, a stone travels from the kidney to the bladder through a narrow tube called the ureter. If the stone is larger than a quarter-inch, it's simply too big to pass through the ureter, and will get trapped there. (If it can make it through to your bladder, it's small enough to pass out out of your body through the urethra.) This causes intense pain, blocked urine flow, and possible bleeding from urinary tract walls. That's when it's time for treatment.

There are several methods for getting rid of a kidney stone, all of which aim to break the stone into smaller pieces so they can leave the body. In an extracorporeal shock wave lithotripsy (from the Greek for "crushed stone"), high-frequency sound waves are applied externally to break stones up, allowing them to pass when you pee. Laser lithotripsy takes a similar approach: Stones in the ureter are broken up with a laser and also leave the body naturally. More invasive is percutaneous ultrasonic lithotripsy, which involves passing narrow instruments (including a fiberoptic camera) through your back to your kidney; ultrasound breaks the stones up, and then fragments are removed by an instrument. Finally, a ureteroscopy is a treatment option in which a small scope is inserted in the ureter towards the bladder to determine the stone's location. Then it's broken up for natural passage or removed altogether. Luckily, you're unconscious under general anesthesia during the last procedure.

10. … AND THEY'RE FAR SUPERIOR TO THOSE USED IN THE PAST.

Kidney stones are nothing new—mentions of the painful formations go back more than 5000 years, to Mesopotamian medical texts—and medical interventions have occurred for just as long. Stones made it into the Hippocratic Oath, in which physicians swore they would "not use the knife, not even on sufferers from stone," leaving the procedure to "such men as are engaged in this work" [PDF]. Surgeons in ancient Greece and India were attempting stone removal as far back as the 7th century BCE.

The 16th to 18th centuries were a heyday for stone surgeons, who were largely self-taught. The most notorious of them was Frere Jacques Beaulieu. He pioneered the lateral perineal lithotomy—which involved making an incision in the perineum, inserting a terrifying cutting instrument into the bladder, cutting up the stone, and then extracting the pieces with the instrument or his fingers—in the late 17th century. Unfortunately for his patients, he had no technical training, and his method was often deadly; in 1698, after 25 of his 60 patients died, he was banned from doing the procedure—but he didn't stop. He's thought to have performed more than 5000 lithotomies. (And no, the song doesn't seem to be about him.)

11. IF ALL ELSE FAILS, TRY RIDING A ROLLER COASTER.

If you’re a thrill seeker who happens to have kidney stones (and some vacation time), you may be in luck. After a "notable number" of patients reported that riding the Big Thunder Mountain roller coaster at Walt Disney World in Orlando helped them to pass their kidney stones, Michigan State University urologist David Wartinger decided to investigate. He created a kidney replica—complete with kidney stones—put it in a backpack, and let it ride the roller coaster 60 times. It worked—but passing the stones depended on where the backpack was placed in the coaster. Rides in the last car were the most effective, with the stones passing 64 percent of the time, while the front few cars yielded only a 16 percent success rate.

Big Thunder Mountain was the only ride in the theme park that was effective. Neither Space Mountain nor Aerosmith's Rock 'n' Roller Coaster did the trick, likely because they were too fast, with a G-force that pinned the stones in place. Of course, while this is an interesting finding, if you suspect you have kidney stones, speak to your doctor before you high-tail it to Walt Disney World.

12. A KIDNEY STONE THE SIZE OF A MOUSE WAS REMOVED FROM A MAN IN 2004.

The stone measured 5.11 inches at its widest point—a world record. Five years later, a whopping 2.5-pound stone was surgically removed from a man in Hungary in 2009. Perhaps seeing a bunch of kidney stones in one place other than originating from your own body will put you at ease. If that’s the case, check out the International Museum of Surgical Science in Chicago, where a collection of stones is on display in glass jars.

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