''Fetus in fetu'' (or ''foetus in foetu'') is a rare

developmental abnormality A birth defect, also known as a congenital disorder, is an abnormal condition that is present at birth regardless of its cause. Birth defects may result in disabilities that may be physical, intellectual, or developmental. The disabilities can r ...

in which a mass of tissue resembling a

fetus A fetus or foetus (; plural fetuses, feti, foetuses, or foeti) is the unborn offspring that develops from an animal embryo. Following embryonic development the fetal stage of development takes place. In human prenatal development, fetal develo ...

forms inside the body of its

twin Twins are two offspring produced by the same pregnancy Pregnancy is the time during which one or more offspring develops ( gestates) inside a woman's uterus (womb). A multiple pregnancy involves more than one offspring, such as with t ...

. An early example of the phenomenon was described in 1808 by George William Young. There are two hypotheses for the origin of a "fetus in fetu". One hypothesis is that the mass begins as a normal fetus but becomes enveloped inside its twin. The other hypothesis is that the mass is a highly developed

teratoma A teratoma is a tumor made up of several different types of tissue, such as hair, muscle, teeth, or bone. Teratomata typically form in the ovary, testicle, or coccyx. Symptoms Symptoms may be minimal if the tumor is small. A testicular tera ...

. "Fetus in fetu" is estimated to occur in 1 in 500,000 live births.

Classification as life

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 A neoplasm () is a type of abnormal and excessive growth of tissue. The process that occurs to form or produce a neoplasm is called neoplasia. The growth of a neoplasm is uncoordinated with that of the normal surrounding tissue, and persists ...

in that its cells remain viable by way of normal metabolic activity. However, without the gestational conditions ''in utero'' with the

amnion The amnion is a membrane that closely covers the human and various other embryos when first formed. It fills with amniotic fluid, which causes the amnion to expand and become the amniotic sac that provides a protective environment for the devel ...

and

placenta The placenta is a temporary embryonic and later fetal organ (anatomy), organ that begins embryonic development, developing from the blastocyst shortly after implantation (embryology), implantation. It plays critical roles in facilitating nutrien ...

, a ''fetus in fetu'' can develop into, at best, an especially well differentiated teratoma; or, at worst, a high-grade

metastatic Metastasis is a pathogenic agent's spread from an initial or primary site to a different or secondary site within the host's body; the term is typically used when referring to metastasis by a cancerous tumor. The newly pathological sites, then, ...

teratocarcinoma. In terms of physical maturation, its organs have a working blood 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.

Hypotheses of development

There are two main hypotheses about the development of fetus in fetu.

Teratoma hypothesis

Fetus in fetu may be a very highly differentiated form of

dermoid cyst A dermoid cyst is a teratoma of a cystic nature that contains an array of developmentally mature, solid tissues. It frequently consists of skin, hair follicles, and sweat glands, while other commonly found components include clumps of long ...

, itself a highly differentiated form of

mature teratoma A teratoma is a neoplasia, tumor made up of several different types of biological tissue, tissue, such as hair, muscle, teeth, or bone. Teratomata typically form in the ovary, testicle, or sacrococcygeal teratoma, coccyx. Symptoms Symptoms may ...

Parasitic twin hypothesis

Fetus in fetu may be a

parasitic twin A parasitic twin, also known as an asymmetrical or unequal conjoined twin, is the result of the processes that also produce vanishing twins and conjoined twins, and may represent a continuum between the two. Parasitic twins occur when a twin emb ...

growing within its

host A host is a person responsible for guests at an event or for providing hospitality during it. Host may also refer to: Places *Host, Pennsylvania, a village in Berks County People * Jim Host (born 1937), American businessman * Michel Host ...

. Very early in a

monozygotic Twins are two offspring produced by the same pregnancy.MedicineNet > Definition of TwinLast Editorial Review: 19 June 2000 Twins can be either ''monozygotic'' ('identical'), meaning that they develop from one zygote, which splits and forms two em ...

twin pregnancy, in which both fetuses share a common placenta, one fetus wraps around and envelops the other. The enveloped twin becomes a

parasite Parasitism is a Symbiosis, close relationship between species, where one organism, the parasite, lives on or inside another organism, the Host (biology), host, causing it some harm, and is Adaptation, adapted structurally to this way of lif ...

, in that its survival depends on the survival of the host twin, by drawing on the host twin's blood supply. The parasitic twin is

anencephalic Anencephaly is the absence of a major portion of the brain, skull, and scalp that occurs during embryonic development. It is a cephalic disorder that results from a neural tube defect that occurs when the rostral (head) end of the neural tube fail ...

(without a

brain The brain is an organ that serves as the center of the nervous system in all vertebrate and most invertebrate animals. It consists of nervous tissue and is typically located in the head ( cephalization), usually near organs for special ...

) and lacks some

internal organ In biology, an organ is a collection of tissues joined in a structural unit to serve a common function. In the hierarchy of life, an organ lies between tissue and an organ system. Tissues are formed from same type cells to act together in a ...

s, and as such is unable to survive on its own. As the host twin has to "feed" the enveloped twin from the nutrients received over a single umbilical cord, they usually die before birth.

References

External links

Xray of fetus in fetu
{{Twin conditions Congenital disorders Germ cell neoplasia Pediatrics Supernumerary body parts Parasitic twin