Stem Cells and the Inner Ear
By Richard Salvi, Ph.D

The inner ear is composed of two main parts. The auditory portion includes the cochlea, which is involved in hearing.  The vestibular system is involved in balance.   Our ability to hear and our sense of balance are critically dependent on specialized sensory receptors called hair cells, which develop in the inner ear during the last trimester of pregnancy. These cells have structures called stereocilia, which sense sounds by bending back and forth, converting mechanical vibrations into electrical, or neural, signals that are then passed to the brain through the auditory nerve.  

Anterior semicircular canal Ampulla (superior canal) Ampulla (lateral canal) Sacculus Cochlear duct Helicotrema Lateral (horizontal) canal Posterior canal Ampulla (posterior canal) Oval window Round window Vestibular duct (scala vestiuli) Tympanic duct (scala tympani) Utricule Two sensory organs are located in the inner ear. The vestibule is the organ of equilibrium and the cochlea the organ of hearing. The schematic drawing here represents the osseous (top left) and membranous (seen by transparency in the main drawing) labyrinths. Modified from: http://www.iurc.montp.inserm.fr/cric/audition/english/ear/inear/inear.htm

Auditory hair cells, like a microphone, are designed to convert sounds into neural activity. Vestibular hair cells, those located in structures called the semicircular canals, detect head rotation, whereas other vestibular hair cells, located in structures called the utricle and saccule, sense head orientation with respect to gravity.  

The destruction of the hair cells due to certain drugs, acoustic overstimulation, aging or infection results in permanent hearing loss or balance problems. Severe or profound hearing loss is particularly devastating because our inability to communicate through spoken language leads to social isolation and depression.  

Finding ways to cure deafness and balance problems would represent a major scientific and clinical breakthrough. Recently, stem cells from the inner ear of adult mice have been identified.  These adult stem cells are found in the utricle of the vestibular region of the inner ear. They have the characteristic features of stem cells in that they have the capacity for self renewal and expansion (they divide and multiply).   They form spheres, which begin to differentiate into new cell types termed progenitor cells.  

Some progenitor cells differentiate into cells that express proteins and genes present in the developing inner ear and nervous system. Under appropriate conditions, some cells differentiate into cells resembling hair cells, which have stereocilia hair bundles protruding from their surface and express specific hair cell marker proteins. The discovery of such cells is a first step towards the promise of restoring hearing and balance function.  

What is the potential for stem cells? From a scientific perspective, scientists will first need to identify compounds and conditions that can increase the growth of stem cells and promote their differentiation into hair cells or supporting cells.  From a clinical perspective, surgical and technical procedures need to be developed to successfully transplant stem cells into the inner ear. The critical question to be answered is whether transplanted stem cells can migrate into the correct location, differentiate into hair cells and restore hearing or balance.  

By Richard Salvi, Ph.D, an ear stem cell specialist at the University of Buffalo.

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Updated: February 2, 2005