When it comes to recovering lost hearing ability, birds and amphibians have more advantages than mammals. In humans, the cells present in the inner ear endowed with the responsibility of detecting sound and transmitting those signals to brain develop during very early stage of development. If due to any reason say injury, illness or aging, the hearing ability is lost it cannot be replaced.
Scientists at the Washington University School of Medicine in St. Louis have identified two signaling molecules that are important for the right development of cochlea a part of the inner ear. If these signaling molecules are not present, the embryo doesn't produce enough cells which eventually form the adult cochlea. That can result in shortened cochlear duct and impaired hearing. This study is a step towards understanding inner ear development, which will take us near towards the goal of being able to recover lost hearing. One can check the study online in the journal eLife.
David M. Ornitz, MD, PhD, the Alumni Endowed Professor of Developmental Biology and senior author of the study said that in order to eventually be able to restore hearing, regeneration of the sensory hair cells of the cochlea is important. In case of birds and fishes, if the cells in the inner ear are damaged, those cells are naturally turned back into progenitor cells that are capable of replacing the sensory cells. But, mammals are more complex organisms. They boast of a better sense of hearing over a wider range of sounds, but they do not have the ability to regenerate sensory hair cells.
In this new study, Ornitz and his colleagues shave found that proper inner ear development in mice depends on the presence of two signaling molecules called FGF9 and FGF20. In mice, the normal signaling of these molecules in the inner ear turns on at about day 11 of the mouse embryo’s typical 20-day development. It takes another two to three days for these two molecules to direct the progenitor cells to multiply. By embryonic day 14, the progenitor cells stop multiplying and begin to differentiate to become functional adult sensory cells. At this point, the cellular population that comprises the adult ear is largely complete. It has been deduced that if the FGF signals are not present during inner ear development, shortened cochlea and impaired hearing are likely.
Study's first author Sung-Ho Huh, PhD, instructor in developmental biology added that in mammals, including mice and people, the number of sensory progenitor cells is fixed. This number is determined by cell division or cell death in early stages of development. Between embryonic days 11 and 14 is the period when it happens in mice. When that developmental window closes, the number of cells that are formed is all you get. There is no compensation if the number of these cells is low.
This new study reveals that FGF9 and FGF20 send signals to their receptors, located in nearby cells surrounding the developing sensory cells. Through signaling to these surrounding cells, FGF9 and FGF20 promote the growth of the sensory progenitor cells. A feedback loop is activated through this signaling that helps to direct proper development of the cochlea. Ornitz and Huh are of the opinion that future work will be focused on identifying the molecules involved in the feedback mechanism.
Ornitz said that it has been discovered by us that an FGF signal is instructive in forming the cochlea. These are the signals that tell the surrounding tissue to make a factor which regulates the progenitor cell growth. We need to find out what this factor is it is crucial to finding the key to restoring hearing.