October 26 ,2011
The seating arrangements for any school trip by coach are always the same. The kids who are going to cause trouble sit at the back. Right at the front near the teacher are the more nerdy pupils. And in between, along the length of the bus, a strange gradient of cool is established, depending on how far away from the teacher each child is sitting.
The layout of the human body is established in much the same way. When an embryo first forms, concentration gradients of various signalling proteins are established – high concentrations where the protein is being secreted getting progressively lower with increasing distance from the source – that tell this amorphous blob of cells which end is the front, the back, the top, the bottom.
The interaction of these proteins lays down a basic body pattern, and this is where sonic hedgehog (SHH) comes in.
Imagine that the kids are all the same when they climb into the school bus and seat themselves at random. The “uncool” flowing from the teacher at the front of the bus sets up a concentrations gradient – from high concentration nearby to low at the back – that determines what kind of kid each becomes. So the kids at the back sprout cigarettes and spiky hair, while the kids at the front become the type to ask for extra homework. (I was one of the latter, in case you hadn’t guessed.)