LONDON, England (CNN) -- Teratomas of the brain are very rare and comprise less than 1 per cent of all brain tumors.
Teratomas have been the subject of intense fascination among scientists because of their sometimes strange qualities -- some have been reported to contain hair, teeth, bone and, very rarely, more complex organs such as eyeballs.
Thoughts about the origin of these tumors have attracted much debate.
CNN spoke with Dr. Thomas L. Ellis, senior neurosurgeon at Wake Forest University School of Medicine, in North Carolina, United States, about the tumors sometimes known as "the monsters".
He explained some quick facts:
• "Teratoma" comes from the Greek word "teraton," meaning "monster".
• Teratomas are seen most commonly in children and young adults.
• Teratomas are made up of tissue from all three germ layers -- mesoderm, ectoderm, and endoderm -- that occur during the formation of an embryo.
• Although teratomas can occur during embryonic development, most arise much later in life.
• Teratomas occur most often in the midline of the brain, therefore often obstructing and putting pressure on critical areas of the brain. This can lead to loss of basic functions, but this loss can be temporary -- until the tumor is removed or reduced in size.
• Teratomas are commonly very solid and rubbery making them resistant to dissection with standard instruments.