Hair cells are sensory cells found in the auditory and vestibular systems, named due to the filamentous structures called stereocilia located at their apex. In the cochlea, two types of hair cells are identified: inner hair cells (IHCs) and outer hair cells (OHCs). Humans have approximately 3,500 inner hair cells, with their size gradually increasing towards the apex of the cochlea.
The outer hair cells, arranged in three rows, number approximately 12,500 and are cylindrical in shape. The apical surface of these cells and their stereocilia are immersed in endolymph. The sensory or auditory cells, supporting cells, basilar membrane, spiral groove, and tectorial membrane together form the organ of Corti.
Hair cells are responsible for converting the hydromechanical energy transferred to the cochlea by the movements of the eardrum and ossicle chain into bioelectric signals. These signals are then transmitted via the auditory nerve fibres to the auditory regions of the brain. Within the cochlea, hair cells are organised in rows of varying sizes, each corresponding to different frequencies in the sound spectrum that humans are capable of hearing.
The structure of the hair cells, along with the unique properties of their cilia, enables them to detect minute displacements on the nanometre scale, which can occur in response to even very low-intensity acoustic stimuli. Although outer hair cells vastly outnumber inner hair cells, 95% of auditory nerve fibres innervate the inner hair cells, with only 5% connecting to the outer hair cells.
The inner hair cells serve as the primary vibration sensors of the basilar membrane. Their depolarisation activates the afferent nerve fibres, allowing them to transmit sound information to the auditory nerve fibres and, subsequently, to the central nervous system.
The depolarisation of inner hair cells is directly proportional to both the intensity of the sound stimulus and the amplification provided by the motility of the outer hair cells. Inner hair cells are generally more resilient than outer hair cells, which are more susceptible to damage from noise exposure, ototoxic drugs, and antibiotics.
At birth, the hair cells in the organ of Corti are present in large numbers, but they cannot regenerate. Various factors can damage these cells, reducing their number and leading to deafness. The most common causes of hearing loss due to damage to cochlear cells include:
Consequently, damage or loss of hair cells in the inner ear can lead to varying degrees of hearing impairment, affecting sound perception and speech comprehension.
Hair cells do not naturally regenerate. However, research into pharmaceutical treatments for deafness is advancing, with one promising area being the regeneration of hair cells in the inner ear. Scientists at the Massachusetts Eye and Ear Infirmary (USA) have discovered that it is possible to reactivate the cellular mechanism that enables supporting cells in the organ of Corti to differentiate into hair cells. This is achieved by reactivating two proteins, MYC and NOTCH1. These interventions result in the proliferation of cells with certain characteristics of hair cells, such as transduction channels and the ability to connect with spiral ganglion neurons.
The Belgian researcher Brigitte Malgrande and her team in Liège have been exploring this concept for several years: modifying other cells in the organ of Corti without impacting the overall population of auditory cells. The Ephrin-B2 protein is exclusively found in supporting cells; when this protein is rendered inactive, the supporting cell transforms into a hair cell. Current research is focused on establishing connections between these newly formed hair cells and neurons, which is crucial for transmitting external sounds to the brain. This research is being conducted at GIGA-Neurosciences. If the researchers succeed in 'creating' hair cells and connecting them to the neurons, it could lead to a partial restoration of impaired hearing.
Learn more on how your hearing works, thanks to our experts contents. You can also have a look at our blog, with our latest news.