Introduction: Hearing impairment can be classified into two categories; Conductive hearing loss & Neurosensory hearing loss. The currently viable solutions for overcoming these losses to some extent are external hearing aids and cochlear implants. Neurosensory hearing impairment results from the loss of auditory hair cells present in the cochlea of the inner ear. Cochlea is a complex part of the inner ear which houses many different types of cells among these are the auditory hair cells. Damage to these cells during episodes of injury, trauma, exposure to loud noise or aging may result in permanent hearing loss. The current advances in the field of stem cell therapy for neurosensory hearing loss show a new hope in paving a way for a permanent solution. Although the use of stem cells as regenerative medicine has shown significant results in other fields yet it has various prerequisites and limitations in this area of research.
CURRENTLY ADMISSIBLE ROUTE TO REGENERATION OF AUDITORY HAIR CELLS 1) SELECTION OF TARGET AREA IN THE COCHLEA The target area of the application of stem cell may differ depending upon the part of cochlea damaged [2]. All types of hearing losses (except some of genetic origin ) are analogous to damaged hair cells and nerve fibers [2] . So, these can be the main target areas of application.
2) CHOOSING THE TYPE OF STEM CELLS Choice of species and developmental stage that will be used for transplantation is considered depending upon the target area selected [2]. The developmental stage of that selected species is considered i.e. whether to transplant differentiated, partly differentiated or undifferentiated cells [1][2][3].
3) METHOD OF TRANSPLANTATION Transplantation of stem cells in cochlea is complicated because of its structure. A surgical approach such as cochleostomy can be used yet this can cause disruption of cochlear homeostasis resulting in residual hearing loss & vertigo [2]. There have been reports of stem cell transplantation in animal models [4][5] still there is a high risk factor for its consideration on homo sapiens
4) INTEGRATION INTO TARGET AREA The existence of inter-cellular junctions in the auditory epithelium serves as a major barrier in integration of the transplanted cell. Certain inter-cellular junction modulations can be done by disrupting them using sodium caprate (widely used to increase intestinal drug absorption) [2]
[Fig. 1] Auditory Hair Cells embedded in basilar membrane.
5) ENHANCING CELL SURVIVAL High potassium concentration & presence of tight junction barrier in endolymph makes the survival of foreign cells very limited [1][2][3]. The different ways reported to lower the potassium concentration include flushing out of scala media fluid &replacing its content with more congenial medium such as stem cell media [3][6]. Hence such an approach can help enhance cell survival post its transplant.
LIMITATIONS & CONCLUSION Though using stem cells to restore neurosensory hearing loss is a tempting area of research yet many foreseeable obstacles remain in its way. There is a high possibility of tumor formation if the proliferation of cells is not controlled. Uncontrolled cell cycle can lead to further mutilation of auditory passage. If the polarity and positioning of all auditory hair cells is non-synchronized then the process will be rendered useless. However, combining stem cell therapy with gene therapy or trans differentiation can produce promising results. Owing to the recent advancements in the field of biotechnology I would conclude that clinical application of this approach is feasible yet challenging.
STEM CELL THERAPY FOR CURING NEUROSENSORY HEARING LOSS : The current scenario. ( Author: HEMANTIKA hemantika.hb@gmail.com )
ABSTRACT: Hearing loss is a major public health problem caused mainly due to loss of auditory hair cells in the inner ear occurring as an account of exposure to loud noise, acoustic trauma or simply aging. The auditory system of humans is very complex. Cochlea, a part of the inner ear plays an important role in the process of hearing & auditory transduction. Auditory hair cells, present in the cochlea are specialized cells which do not possess any regenerative power after their development. Currently there are two technologically advanced solutions to cure hearing impairment. These are external hearing aids and cochlear implants. The former amplifies the received sound to overcome conductive hearing loss whereas the latter helps to subdue neurosensory hearing losses. Despite these advancements the sound quality provided is not satisfactory to the patient. So the sole aim that remains is the regeneration of cochlear hair cells. One such approach for regeneration of auditory hair cells is the use of stem cell therapy. Stem cells are unspecialized cells which can further give rise to a wide range of specialized cells without losing their dividing potential overtime. The use of stem cells in regenerative medicine for curing hearing impairment is highly revolutionary in the field of medical science. This review discusses current advancements and future directions in using stem cell therapy for neurosensory hearing loss.
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