Janus cats: The trick behind developing two faces

Janus kitten, Harvey Dent

Florida couple Nash Hand and Amanda Forsythe were glad to welcome new kitten, Harvey, into their family even though he suffers from a rare congenital disorder.  Harvey, much like his namesake, Harvey Dent, has two faces.  This phenomenon can occur in other animals, but two faced cats are referred to as 'Janus cats' because of the Roman God Janus who also had two faces.  

Though Harvey seems healthy, these animals do not usually survive longer than a few days.  Many of these deaths are due to brain abnormalities.  One exception to that rule is Frank and Louie, another Janus cat who turned 12 years old last September.  He holds the world record for longest surviving Janus cat and will be featured in the 2012 Guinness Book of World Records.  Many attribute his success to the fact that he has only one functioning esophagus and trachea.  This limits his opportunity to breathe while eating, preventing food particles from entering his lungs and causing problems such as pneumonia.  

Though Janus cats may look somewhat similar to conjoined twins, don't be fooled; this trick is more complicated than the fusion or incomplete separation of two embryos.  The disorder is more formally known as craniofacial duplication or diprosopus, meaning "two faced."  It is actually caused by an over abundance of a protein called Sonic hedgehog homolog (SHH), named after the video game character Sonic the Hedgehog.  This protein is important in coordinating the development of a growing embryo.  Embryonic cells use molecules as signals to tell them how to develop, so that the organism ends up with the correct body parts in the right places.  One of the responsibilities of SHH is to signal the cells that make up the mid and upper portions of the face.  When there is too much signal, the face widens.  The wider the face, the more likely the face will develop double features.  The opposite occurs with a lack of SHH signaling, which results in cleft lip or cleft palate.  

Little Harvey can attribute his dramatic facial appearance to some cellular miscues.