Surgeons have discovered a tumor that contains a miniature brain, bones and hairs inside a 16-year-old's ovary.

A paper, recently published in Neuropathology, described the strange incident. As per the authors of the paper, the tumor measured 10 centimeters. It was identified as a mature cystic teratoma, derived from the Greek word teratos/teras, which means monster.

Describing the "monster," which was accidentally discovered by the surgeons when removing the girl's appendix, New Scientist reported that the tumor has a skull covering the 1.18-inch brain, which included a tiny version of a cerebellum and a mass that resembled a brain stem. Amazingly, the brain could transmit electric impulses like a normal brain.

Cancer Research UK said ovarian teratoma is a type of germ cell tumor. The mature teratoma is the most common type of ovarian germ cell tumor.

However, what was found in the teenager's ovaries was rare as it already contained foreign tissues. In addition, it is puzzling how and unlikely that the tissues found in the tumor had assembled themselves to form proper brain-like structures.

Angelique Riepsamen at the University of New South Wales in Australia told New Scientist, "Neural elements similar to that of the central nervous system are frequently reported in ovarian teratomas, but structures resembling the adult brain are rare."

Teratomas are made from our own errant cells. They are most likely stem cells that went rogue for yet to be identified reasons.

Fox News reported that neurological symptoms can occur when the immune system labels the brain cells in the ovary as foreign and attacks and this could result to foggy brain. Fortunately, this was not experienced by the patient. Doctors removed the tumor and the patient is recovering.

Meanwhile, Science Alert said that this is not the first case in Japan. In 2002, a tumor with developing organs, head, body, extremities, and even the beginnings of a penis was removed from a 25-year-old Japanese woman.