In mammals, loss of hearing is irreversible because neurons in the cochlea and so-called “hair cells” do not regenerate. More than half of the population over the age of 60 suffers from severe hearing loss. Replacing cells in the inner is an important goal for regenerative medicine.
The ear consists of three main compartments. The outer ear consists of the cup-like structure on the sides of our heads called “pinnae,” and the opening to the middle called “external auditory meatus” (EAM). The EAM terminates at the eardrum. Behind the ear lies the middle ear, inside which is housed three small bones called the “auditory ossicles.” The auditory ossicles are attached to the eardrum and when the eardrum vibrates as a result of air pressure disturbances caused by sound waves traveling through the air, the ossicles vibrate with the eardrum and set up a series of vibrations on the other side of the middle ear where the ossicles are attached to the so-called oval window.
The vibrations that occur at the oval window are passed into the cochlea, which is derived from the Latin word for snail-shell.
This coiled structure contains two main compartments, one of which extends throughout the cochlea, and the other of which surrounds this central compartment. The central compartment is called the scala media, and the surrounding compartments are the scala vestibuli (above) and the scala timpani (below). The scala media contains an organ called the Organ of Corti, and this structure is responsible for producing the signals that are interpreted by the brain as hearing.
The organ of Corti consists of a series of cells with hair-like extensions that called “hair cells.” When the vibrations from the oval window are transmitted to the cochlea, the fluid in the scala vestibuli and scala timpani vibrates and these vibrations are transmitted into the scala media. The vibrations of the scala media causes a membrane above the hair cells, called the “tectorial membrane” to vibrate and this vibrating tectorial membrane, into which the tips of the hair cells are embedded, moves the hair cell extensions back and forth. The louder the sound, the greater the degree to which the tectorial membrane vibrates. Also, the frequency of the sound varies the type of vibrations in the scala media and each hair cell releases neurotransmitters to the neurons that connect with them only when vibrations of the proper frequency activate them. These activated hair cells release neurotransmitters to the connecting neurons and these neurons take those signals to the brain where they are interpreted and turned into sound sensations.
Sensoineural deafness results from the death of neurons that innervate the hair cells, or the hair cells themselves. Replacing the hair cells or the connecting neurons is one of the main goals of regenerative medicine. To that end, several labs have injected neural stem cells, embryonic stem cells or neural stem cells made from embryonic stem cells into the cochleas of deaf animals. These experiments have shown that injected embryonic stem cells can survive when injected into the cochlea (see Hildebrand MS, et al., J Assoc Res Otolaryngol.2005 Dec;6(4):341-54), and injected cells can even differentiate into cell types that are specific for the cochlea (see Coleman B, et al., Cell Transplant.2006;15(5):369-80). Inject neural stem cells made from embryonic stem cells can even extend axons that move into the cochlea and make contact with hair cells (Corrales CE,, et al. J Neurobiol. 2006 Nov;66(13):1489-500). The difficulty with these cells is that they are not originally from the ear and might not differentiate fully into the tissue they are trying to repair.
What is a better cell type for repairing the inner ear? Fetal ears contain a stem cell population that was identified in 2007 by Wei Chen in Marcelo Rivolta’s lab from the University of Sheffield (Chen et al., Hear Res. 2007 Nov;233(1-2):23-9). Rivolta’s lab has also designed protocols for isolating and expanding these cells in vitro. These fetal auditory stem cells form structures that resembled those found in organ of Corti. The electrophysiological profiles of these cells also greatly resembled those observed in organ of Corti cells. Also, the auditory stem cells expressed a great many of the genes found in developing inner ear cells when induced with various growth factors and small molecules (Chen W, et al., Stem Cells. 2009 May;27(5):1196-204).
These cell lines be used to cure deafness? That is a different question, but they can certainly be used as a model system for drug screening, toxicity, and testing therapies to cure hearing loss. Treating defects of the inner ear have many challenges, and while such cells are a first start, they represent the beginning of what might be a viable source of treatment for hearing loss.
There is also much to say about the fetal source of these cells. Fetal cells were used to treat Parkinson’s disease and Huntington’s chorea. The use of brain tissue from aborted fetuses is gruesome to say the least, and while these experiments did not cause the death of the pre-born baby, they represent an acceptance of the killing of unborn children that is execrable. While the knowledge that has been gained from these experiments is certainly useful, it was gained over the bodies of innocent victims of children who were killed because they were an inconvenience. This should disturb and sicken us. The fact that it often doesn’t is a testimony of our moral deafness as a nation.