It sounds like the plot of a horror movie: a patient goes in for the removal of a benign mass, and instead the surgeon finds a tumor with teeth, hair, bone and something resembling an eyeball. While often the subject of exaggeration and urban legend, it's a documented phenomenon known as a teratoma, a tumor containing developed tissue or organ components arising from more than one germ layer. When an embryo is in the very early stages of development, it has three primary layers of cells called germ layers⎯the ectoderm, the mesoderm and the endoderm⎯which further differentiate into the rest of the tissues of the organism.

Germ Layer Develops Into: Ectoderm Epidermis, teeth, nervous system and eyes Mesoderm Bones, many organs, muscles, and blood vessels Endoderm Gastrointestinal lining, pancreas, lungs, thyroid

Teratomas are fairly rare, but can occur in anyone, and have been observed across several species including felines, canines and horses. So how does such an unusual tumor arise? People who develop teratomas are actually born with the precursor cells at the location of the tumor, which then grow over the course of the person's life and differentiate into the malformed tissues. These can include cartilage, bone, teeth, hair, alveoli, and neural-like tissues, usually contained in a membrane to form a cyst. Most teratomas are fairly small and only contain weakly-differentiated tissues like cartilage, hair and epithelial tissue, but in rare cases can exhibit more developed characteristics that resemble limbs, eyes or organs.

This type of teratoma is also referred to as fetus in fetu, and may contain highly developed or even parafunctional organs. How this condition arises is the subject of debate in the scientific community. One theory is that during a monozygotic (identical) twin pregnancy, one twin becomes dominant and absorbs the other, halting its development such that it only possesses a few tissue types. While parasitic twins are a documented phenomenon, with smaller growths it is not always clear which classification the abnormality falls under. The other theory is that fetus in fetu is a highly developed teratoma giving the appearance of a second entity inside the body. It can be unclear which is correct in these cases.

The good news is that teratomas are seldom cancerous, and usually well-encapsulated, making them straightforward to remove with minimal risk to surrounding structures. While certainly a frightening experience, and one requiring surgery, well over 90 percent of patients make a full recovery and experience improved quality of life once the resource-draining structure is removed. Who needs teeth in their abdomen, anyway?

Source:

Hamilton, C., MD. (2016, April 15). Cystic Teratoma. Retrieved October 21, 2016, from http://emedicine.medscape.com/article/281850-overview