by Neil Bauman, Ph.D.
A man asked:
When the microscopic hair cells are damaged and break off in the cochlea, what happens to them? Are they harmlessly absorbed, or do they float around and impair the action of the remaining hair cells?
Great question. Most people have the misconception that the tiny hairs “break off” from being exposed to loud sounds, or other causes—much like tree branches breaking in a hurricane. This is not the right analogy.
In actual fact, as I understand it, the tiny hairs don’t “break off”. Rather the whole hair cell itself dies—taking with it the bundle of “hairs” numbering between 30 and 300 tiny stereocilia per bundle (what we colloquially call “hairs”) that stick up from each hair cell. These dead cells are then absorbed by the body.
What causes these hair cells to die? One mechanism is being “zapped” by “free radicals” released as a result of loud noise or ototoxic drugs for example. The hair cell either dies from a “direct hit”, or if mortally wounded, programs itself to die through a process called apoptosis.
In addition to dying hair cells, another mechanism is that as we age, the stereocilia seem to slowly disappear—becoming shorter and shorter and finally the whole hair cell is “overrun” by adjacent supporting cells and “disappears”.
In any case, the dead cells are not left to float around and cause havoc with the remaining hearing mechanism.