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If you've gone through puberty—and especially if you've ever looked back at your seventh-grade yearbook photo—you might remember that time of your life with a little bit of a cringe. But no matter how you felt about it, from a scientific standpoint, puberty is pretty impressive.

So much goes on in bodies as they transform from a child into an adult. But as complex as it is, just one hormone is the key to controlling it all —and it isn't testosterone or estrogen. It's a modest-sized protein that you have probably never heard of.

For a long time, researchers didn't know what kickstarted puberty. They knew a lot about what happens in the body during it, and that somehow, it gets going around the time kids are between 10 and 12, but they didn't know what flipped the switch. In fact, it was 2003 before researchers found the first clue.

Two separate teams—one in Massachusetts and one in France —actually stumbled across that first clue separately, right around the same time. Each team had been studying adults with a disorder called hypogonadotropic hypogonadism, or HH. People with HH never produce significant amount of sex hormones, so they don't naturally go through puberty.

And the researchers were trying to understand what gene defect was causing that. To do that, each team independently analyzed the genomes of related adults who had HH. They figured that if they could just find a mutation that these family members had in common, that was probably the source of the disorder.

And they found it: The family members with HH all had a certain receptor protein that didn't work. Receptor proteins are like little landing pads on or inside cells that intercept certain substances, like other proteins. Once a molecule latches on to the receptor, the cell responds in some way.

And the faulty receptor on the patients with HH was the one for a protein called kisspeptin. Kisspeptin had just been discovered seven years earlier, but no one was really thinking about it in the context of puberty. Instead, the main thing we knew was that the protein can stop cancer cells from spreading.

Researchers at a lab in Hershey, Pennsylvania figured that one out by identifying the gene that tells the body to make kisspeptin, and they called that gene kiss-1. So, if you're wondering why this protein is called kisspeptin, it's because kiss-1 was named in honor of Hershey's Kisses. So, for the first few years after its discovery, kisspeptin was super interesting in cancer research, but that was about it.

Except, now, the new research suggested that this protein played a much bigger role in the body than anyone knew. After all, the whole reason the people in those studies had HH —and didn't develop into sexually mature adults without treatment— seemed to be because their cells weren't picking up kisspeptin. So this little-known protein named after a chocolate kiss seemed to be key to kicking off the biological transition from child to adult.

Researchers worked out that, just as kisspeptin tells cancer cells to stop growing, it can also trigger responses in other cells—including the release of hormones that kicks off puberty. It all starts in the brain. At the beginning of puberty, kisspeptin binds to receptors sitting on a group of neurons at the base of the brain, and that basically sets off a biological Rube Goldberg machine.

First, the kisspeptin makes those neurons release some hormones. Then those hormones travel into tiny blood vessels that wrap around the pituitary gland, right under the brain, and they tell it to release two sex hormones, called gonadotropins. These gonadotropins head over to the sex organs to signal that it's time to start maturing.

In response, bodies with ovaries start producing estrogen and developing mature eggs, and bodies with testes start producing sperm and testosterone. But these sex hormones don't just go to the reproductive system. They also land on receptors all over the body and tell cells to do a number of new things, like start growing hair in new places.

And that's the beginning of puberty. So, if kisspeptin never successfully binds to its receptors in the brain, the whole process never gets started. As for how kisspeptin knows when it's time to go bind to those receptors in the first place—that's still a bit of a mystery.

It might be responding to metabolic changes, which could signal that the body has reached the right size to begin transitioning to adulthood, but scientists are still trying to understand that piece of the puzzle. What we do know is that the complex process of going from child to an adult has everything to do with a very little-known protein, named for a very little chocolate treat. Thanks for watching this episode of SciShow!

And to find out more about what happens to the body during puberty, you might like our episode about what goes on inside a teenage brain. You can watch it right after this. [♪ OUTRO].