Surgeons find miniature brain and skull growing in a teenager's ovary

A team of surgeons recently found something wildly unexpected when they performed a routine appendectomy on a 16 year old girl. The girl's body was harboring a four inch-wide tumor in her ovary which contained a wad of matted hair, small bone plates, and a very tiny brain.

The girl was suffering from a teratoma, which is a kind of tumor which causes cells to go haywire and start specializing into something they aren't. Perfectly normal muscle cells might, for example, start growing into teeth or bones.

Teratomas often occur in ovarian cells, possibly in immature egg cells. However, they rarely result in structures as intricate as the miniature brain, which had a recognizable cerebellum and brain stem. 

Teratomas are a separate case from parasitic twins, which is what happens when one twin's body becomes absorbed into the other twin's during the pregnancy. The absorbed twin 

A teratoma is typically present from birth, at a rate of approximately 1 teratoma per 500,000 live births. The teratoma continues to develop (albeit usually very slowly) throughout the patient's life.