The Girl with Two Faces (Medical VIDEO )

 

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 a protein called sonic hedgehog homolog (SHH). (The unlikely sounding name of this protein was inspired by the Sonic the Hedgehog videogame character and is part of an idiosyncratic naming tradition in molecular biology research that some have criticized as frivolous.)

The SHH protein and its corresponding gene have been found to play an important role in signaling craniofacial patterning during embryonic development. Among other things, the SHH protein 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 of the 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. Insufficient 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 the SHH protein. During embryonic development, the SHH protein directs embryonic cells to organize in specific areas that later become specialized neural tissues, thus controlling the size and shape of brain structures