A tumour containing a miniature brain has been discovered in a 16-year-old girl’s ovary in Japan.

The tumour was found when the girl had her appendix removed. It measured 10 centimetres wide, according to the New Scientist.

The 3-centimetre brain structure was covered by a thin plate of skill bone, and was found alongside matted hair inside the tumour.

After further analysis it was discovered that the brain was a small version of the cerebellum, the part that regulates motor control in a normal human brain. It also had a mass on one side which appeared to be a brain stem, which would usually be attached to the spinal cord.

The cause of the tumour is unknown, however there is a theory that they appear when egg cells turn rogue, according to the New Scientist.

Around one fifth of ovarian tumours are found to contain tissue, such as teeth, hair, cartilage, fat and muscle. Known as teratomas, or ‘monster tumours’ in English, they are usually benign.

Masayuki Shintaku at the Shiga Medical Centre for Adults in Japan studied the tumour, and said it is extremely unusual for the teratoma cells to be organised into a brain-like structure.