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Here’s a horrifying little story to kick off your weekend: A 16-year-old girl in Japan recently had a tumor surgically removed from her ovary — and when her doctors split it open, they found a tiny brain growing inside.

To be more specific, they found “clumps of greasy, matted hair inside, and a 3-centimetre-wide brain-like structure covered by a thin plate of skull bone,” New Scientist reported. In a case report published last week in the journal Neuropathology, the girl’s doctors explained that the structure turned out to be a cerebellum, the part of the brain that coordinates movement, and part of a brain stem, which connects the brain to the spine.

This particular type of tumor — the kind containing tissues normally found elsewhere in the body — is called a teratoma, Greek for “monstrous tumor,” and it’s a fairly common thing to find in the ovaries: According to New Scientist, “about one-fifth of ovarian tumours contain foreign tissue, including hair, teeth, cartilage, fat and muscle,” and, yes, brain cells. But as Japanese researcher Masayuki Shintaku, one of the report’s co-authors, told the magazine, it’s rare for those cells to create something actually resembling a human brain — and especially a brain so fully formed that it actually had functioning neurons, as this one did.

The patient, who’s since made a full recovery, didn’t show any symptoms from the tumor (doctors just happened to notice it during an otherwise normal appendectomy). But if you thought, “Hey, I bet things can get a little weird when you have a tiny second brain inside your body,” oh boy, would you be correct:

[T]here have been a handful of reports worldwide of women with ovarian teratomas who have developed personality changes, paranoid thoughts, confusion, agitation, seizures or memory loss.



Some of these neurological symptoms can arise when the immune system recognises brain cells in the ovary as foreign and launches an attack, because cells in the woman’s real brain can end up being attacked as well, leading to inflammation.



This, unfortunately, is not even the creepiest example of a teratoma in recent history. In 2003, according to the Guardian, doctors operating on a 25-year-old patient discovered a tumor that contained “a small, doll-like body,” complete with a brain, four limbs, bones, several baby teeth, and even eyelashes. Maybe you should cancel your weekend plans and just spend that time trying to get that image out of your head. Good luck.