Hair Cells in the cochlea
Well, that's not quite true. They don't grow back in humans and other mammals, but they do in other vertebrates - birds and lizards, for example. So why is that? What are these creatures doing that we can't?
First, lets ask another question. All our cells contain the same DNA code, so why do they act differently? How does an eye cell develop differently to skin cell?
It's mostly because of Transcription Factors. These are chemicals that stick to the outside of your DNA without affecting the DNA code itself. By doing this they can switch on or off parts of your genetic code.
When you are conceived, you split off into a lump of cells called stem cells. As different transcription factors start working in these cells, they fulfill different roles, and your body starts to form. The Transcription Factor that leads to inner hair cells forming seems to be Atoh1. But after you are born, Atoh1 stops being created in your ear, so no new hair cells.
In birds it is different. When their hair cells die, the supporting cells surrounding them react. They start producing Atoh1 again, triggering DNA activity that changes these cells into hair cells, and the birds hearing gets repaired.
Scientists are looking at ways to replicate this. As with gene therapy (see previous post), the techniques are not yet being tested on humans. But some hopeful results have been seen in mice, where supporting cells have been turned into hair cells. This has been done by combining stem cells with Atoh1, and switching on some other chemicals.
I've skipped a lot of detail here - in reality there are many different factors interacting, and not all of these are fully understood. But it's good to know progress is being made!
