When you hear the word “tumor,” you probably think “not good.” And they can be pretty scary.

In their mildest form, tumors are just a lump of cells hanging out in your body. At their worst, those cells are dividing and spreading like wildfire — they’re cancerous.

But there’s one kind of tumor that’s basically straight out of a horror movie. If you peek inside a teratoma, you might find hair, teeth, eyes, or even brain cells. As strange as these tumors are, the jumbles of body parts aren’t actually turning into people.

But they have taught us a lot about stem cells and how early development works. Most tumors start when a random mutation in one cell’s DNA makes it go rogue and divide too much. So, for instance, a lung tumor might be a cluster of the cells that would normally line your lungs.

If that tumor keeps growing, the cells can develop more random mutations, which can create more small differences between them. But — on the whole — it’s still lung-like tissue. Teratomas, on the other hand, look like you blended up a human and started growing something from the goop.

And we think that’s because of the cells that spawn these tumors. Most teratomas develop from germ cells in the ovaries and testes. Those are the cells that divide to produce mature sperm and eggs.

As you probably know, sperm and eggs fuse together to make embryos. And the cells in early embryos have an ability biologists call pluripotency, which means that with the right physical and chemical signals, they can become pretty much anything, from skin cells to liver cells. If enough random mutations happen in germ cells, they can unlock pluripotency at the wrong time.

Combined with wildly dividing cells, that leads to teratomas. As teratomas form, they have at least a tiny bit of all three kinds of developmental tissue that embryos have. There’s the endoderm, which goes on to make your gut lining, the mesoderm, which makes your muscles, blood vessels, and skeleton, and the ectoderm, which makes your skin and brain.

To be clear, they are not embryos — their growth isn’t nearly controlled enough. But that developmental tissue is how hair and eyeballs can start sprouting in these tumors. Scientists really started digging into teratomas in the 1950s and 60s.

A team using mice to study the health hazards of cigarettes found teratomas in a mouse’s testes, then switched over to tumor research soon after. Through careful experimentation with these tumors, biologists started to realize that embryonic development happened in a similar way. In embryos it was just … less broken.

Plus, they discovered that mouse embryonic stem cells implanted into the testes of healthy adult mice could grow into teratomas too. So really, you can pretty much trace the field of stem cell research back to teratomas. And it’s a field that could help a lot of people someday.

Normally, it’s hard to donate tissues to patients that need them, because our bodies reject foreign stuff. That’s where pluripotent stem cells come in, like embryonic stem cells. We might be able to use them to grow tissues that can be transplanted without triggering those defenses, which could be really useful for medicine.

But, obviously, scientists don’t want to accidentally transplant tissues that create teratomas. Even if it turns out to be benign, finding a tooth-filled lump inside you would not be a good surprise. So there’s a lot we have to learn before these potential therapies get close to becoming a reality.

And on that list is why seemingly normal embryonic stem cells grow into tumors with a bunch of body parts. Thanks for watching this episode of SciShow! If you want to learn about more weird things hiding in human bodies, check out our video about the natural painkiller in your spit!

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