Teratoma: from classical greek, meaning ‘monstrous tumour‘.
Simply put, a teratoma is an encapsulated tumour which contains tissue or organ components. these tissues may be quite different from the surrounding tissues, and have been known to contain hair, teeth, bone, and on rare occasions more complex organs such as eyes or limbs.
There are 2 rare forms of this condition:
fetus in fetu and fetiform teratoma.
In these cases, the cyst contains tissue components which resemble a malformed foetus. Both may contain partial/complete organ systems, even major body parts such as torso or limbs. However, fetus in fetu differs in that it has an apparent spine and bilateral symmetry.
The popular medical interpretation of fetus in fetu is as a congenital complication, whereupon one foetus begins growing within it’s twin. However, without the appropriate in utero conditions, a fetus in fetu cannot develop to physical maturity. There are reported cases of mature teratome which contain partially developed organ systems, cranial bones, and a rudimentary beating heart.
A fetus in fetu can be considered alive only in a very limited sense, as it’s blood supply for tissue and organs are provided directly by it’s host. additionally, all cases of fetus in fetu present critical defects, such as no functional brain, heart, lungs, gastrointestinal tract, or urinary tract.