<P> Fetus in fetu and fetiform teratoma are rare forms of mature teratoma that include one or more components resembling a malformed fetus . Both forms may contain or appear to contain complete organ systems, even major body parts such as torso or limbs . Fetus in fetu differs from fetiform teratoma in having an apparent spine and bilateral symmetry . </P> <P> Most authorities agree that fetiform teratomas are highly developed mature teratomas; the natural history of fetus in fetu is controversial . There also may be a cultural difference, with fetiform teratoma being reported more often in ovarian teratomas (by gynecologists) and fetus in fetu being reported more often in retroperitoneal teratomas (by general surgeons). Fetus in fetu has often been interpreted as a fetus growing within its twin . As such, this interpretation assumes a special complication of twinning, one of several grouped under the term parasitic twin . In this regard, it is noteworthy that in many cases the fetus in fetu is reported to occupy a fluid - filled cyst within a mature teratoma . Cysts within mature teratoma may have partially developed organ systems; reports include cases of partial cranial bones, long bones and a rudimentary beating heart . </P> <P> Regardless of whether fetus in fetu and fetiform teratoma are one entity or two, they are distinct from and not to be confused with ectopic pregnancy . </P> <P> A struma ovarii (literally: goitre of the ovary) is a rare form of mature teratoma that contains mostly thyroid tissue . </P>

How does a tumor grow teeth and hair