Diprosopus

Diprosopus (Greek διπρόσωπος, "two-faced", from δι-, di-, "two" and πρόσωπον, prósopon [neuter], "face", "person"; with Latin ending), also known as craniofacial duplication (cranio- from Greek κρανίον, "skull", the other parts Latin), is an extremely rare congenital disorder whereby parts (accessories) or all of the face are duplicated on the head.

Although classically considered conjoined twinning (which it resembles), this anomaly is not normally due to the fusion or incomplete separation of two embryos. It is the result of abnormal activity by the protein SHH (sonic hedgehog). (The name of this protein was inspired by the Sonic the Hedgehog video game character and is part of an idiosyncratic naming tradition in molecular biology research.)

SHH and its corresponding gene have been found to play an important role in signaling craniofacial patterning during embryonic development. Among other things, SHH governs the width of facial features. In excess it leads to widening of facial features and to duplication of facial structures. The greater the widening, the more structures are duplicated, often in a mirror image form. This has been demonstrated in the laboratory by introducing pellets of the SHH protein into chicken embryos, resulting in chickens with duplicate beaks. Inadequate amounts of that protein lead to opposite conditions such as cyclopia where facial features are insufficiently developed.

Healthy brain development is also dependent on the signaling function of SHH. During embryonic development, SHH directs embryonic cells to organize in specific areas that later become specialized neural tissues, thus controlling the size and shape of brain structures.

Diprosopus often occurs in combination with other congenital disorders, particularly anencephaly, neural tube defect and cardiac malformations. When present, the brain may show abnormalities ranging from partial to complete duplication of brain structures, and/or underdevelopment of brain tissues.

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