Hair Cells

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 - a special fluid inside the inner ear that helps detect balance and sound.. 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.

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.