Therapeutic potential of cell therapy in the repair of hair cells and spiral ganglion neurons: review article

The mammalian cochlea is a highly complex structure which contains several cells, including sensory receptor or hair cells. The main function of the cochlear hair cells is to convert the mechanical vibrations of the sound into electrical signals, then these signals travel to the brain along the auditory nerve. Auditory hair cells in some amphibians, reptiles, fish, and birds can regenerate or replace by new cells, but irreversible damage to the mammalian hair cells are not being replaced through differentiation of the internal epithelial cells in the inner ear. Indeed, mammalian auditory hair cells do not spontaneously repair or regenerate after development. Sometimes, functions of damaged hair cells may be restored, but in most cases, there is no such possibility and permanent hearing loss occurs. Several factors such as chronic ear infections, genetic disorders, drug abuse, acoustic trauma and aging can damage the cochlea, resulting in permanent hearing loss. More than 250 million people in the world have disabling hearing impairment. Deafness is caused by damage to sensory hair cells or spiral ganglion neurons. Although hearing aids and cochlear implants were used for improvement of hearing loss, but they do not restore normal hearing. In addition, application of new biological approaches to induce auditory hair cell regeneration provides more comprehensive treatment for hearing loss. Cell therapy is considered a promising way in the treatment of several diseases such as Parkinson, diabetes and cardiac diseases. According to recent research, cell therapy can be useful in hair cell regeneration. Cell therapy is effective in hearing loss when stem cell differentiates into hair cells with appropriate morphology, electrical activity and capacity for suitable innervations with inner ear tissues. In fact, stem cell-derived neurons need to project neural processes toward the sensory hair cells and the cochlear nucleus neurons. In this regard, studies focus on methods in which hair cells can be provided from exogenous and endogenous stem cells. Here, we review cell therapy approaches in repair damaged cochlear hair cells, as well as imitations and problems of its clinical application.