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Fetus in fetu

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Fetus in fetu (or ftus in ftu) is a developmental abnormality: a mass of tissue inside the body that more or less resembles a fetus. There are two theories of origin concerning fetus in fetu. One theory is that the mass begins as a normal fetus but becomes enveloped inside its twin.^[1] The other theory is that the mass is a highly developed teratoma. Fetus in fetu is estimated to occur in 1 in 500,000 live births.^[2]

Contents [hide] 1 Is it alive? 2 Theories of development 2.1 Teratoma theory 2.2 Parasitic twin theory 3 Examples in the media 4 Examples in fiction 5 See also 6 References

[edit] Is it alive?

A fetus in fetu can be considered alive, but only in the sense that its component tissues have not yet died or been eliminated. Thus, the life of a fetus in fetu is akin to that of a tumor in that its cells remain viable by way of normal metabolic activity. However, without the gestational conditions in utero with the amnion and placenta, a fetus in fetu can develop into, at best, an especially well differentiated teratoma; or, at worst, a high-grade metastatic teratocarcinoma. In terms of physical maturation, its organs have a working *lo** supply from the host, but all cases of fetus in fetu present critical defects, such as no functional brain, heart, lungs, gastrointestinal tract, or urinary tract. Accordingly, while a fetus in fetu can share select morphological features with a normal fetus, it has no prospect of any life outside of the host twin. Moreover, it poses clear threats to the life of the host twin on whom its own life depends.^[3]

[edit] Theories of development

There are two main theories about the development of fetus in fetu; one simple, the other complex.

[edit] Teratoma theory

Main article: Teratoma

Fetus in fetu may be a very highly differentiated form of dermoid cyst, itself a highly differentiated form of mature teratoma.^[4]

[edit] Parasitic twin theory

Main article: Parasitic twin

Fetus in fetu may be a parasitic twin fetus growing within its host twin. Very early in a monozygotic twin pregnancy, in which both fetuses share a common placenta, one fetus wraps around and envelops the other. The enveloped twin becomes a parasite, in that its survival depends on the survival of its host twin, by drawing on the host twin's *lo** supply. The parasitic twin is anencephalic (without a brain) and lacks some internal organs, and as such is almost always unable to survive on its own. As the normal twin has to "feed" the enveloped twin from the nutrients received over a single umbilical cord, it usually dies before birth.^{[citation needed]}

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