Hearing loss is classified into two different categories. First, conductive hearing loss is the hearing loss originating from the middle or external ear. Most of these conditions are surgically correctable and shows good prognosis with hearing rehabilitation devices such as bone conduction hearing aids. On the other hand, sensorineural hearing loss is the hearing loss originating from the inner ear. Inner ear is composed of cochlea (hearing organ) and vestibule (balance organ). In cochlea, crucial cochlea hair cells which can transfer mechanical stimuli to neural signal exist. One of the noticeable findings discovered is that these cochlear hair cells are extremely weak to various insults such as noise, drugs, and aging. Furthermore, these cells do not regenerate once they are lost (Fig. 1A). Therefore, sensorineural hearing loss which could be defined as hearing loss due to loss of sensory cells and relating neural structures has huge impact in current socioeconomic environment. Trials to regenerate the sensory cochlea hair cell or sensory neural structures are on-going. But still, there has not been enough evidence to apply any innovative therapeutic options for this disease. Examples for this innovative therapeutic could be gene and cell therapy.

Figure 1. Illustrations of secondary degeneration of spiral ganglion and therapeutic application of stem cells using laser. (A) Secondary degeneration of spiral ganglion, progressive damage of nervous structures after loss of sensory hair cell (HC) is demonstrated. (B) Therapeutic application of stem cell using laser, laser induced (by release of growth factor and homing factor incapsulated with stem cell [SC]s) differentiation into neurons and migration into Rosenthal’s canal is illustrated.

Stem cell therapy has been proposed to be one of the promising therapeutic option for hearing restoration for sensorineural hearing loss. There are two main targets for this regenerative therapy using stem cell. One would be the cochlear hair cell which performs the primary role in sound transduction to brain. As this cell type does not regenerate once lost, there have been many efforts to differentiate fully mature cochlear hair cells from embryonic stem cells or induced pluripotent stem cell (iPSC)s. Several recent studies have introduced the technique to differentiate the inner ear (not specific enough to distinguish between cochlea and vestibule) from both stem cells (embryonic and induced pluripotent) using the inner ear organoid generation technique.^1-5 However, with this very recent technology, final differentiation outcome was not specific enough to show characteristics of cochlear hair cell.⁶ Furthermore, time and procedures required to generate inner ear organoid is too long and complicated to apply this clinically. Lesser complicated method which is using the progenitor cells within the organ of Corti (functional structure within cochlea; composed of hair cells and supporting cells) to differentiated into the organoid including hair cell is introduced.^7,8 However. this protocol is only proved for mouse premature progenitor supporting cells. Therefore, much more data have to be discovered in the future (Table 1).

Table 1 . Inner ear (otic) organoid generation using stem cells

Year	Reference	Source cell	Final target specificity	Journal
2012	Shi et al.7	Mouse Progenitor	Otic (not specific)	Journal of Neuroscience
2014	Koehler and Hashino1	Mouse ESC	Otic (not specific)	Nature Protocols
2017	Koehler et al.3	iPSC	Otic (not specific)	Nature Biotechnology
2018	Schaefer et al.6	Mouse ESC	Otic (not specific)	Stem Cells and Development
2020	Chang et al.15	Mouse ESC	Otic (not specific)	Molecular Therapy Methods & Clinical Development
2021	Abdul-Aziz et al.8	Mouse progenitor	Otic (not specific)	Stem Cell Reports
2022	Ueda et al.5	iPSC	Otic (not specific)	Methods in Molecular Biology
2021	Zine et al.4	iPSC	Otic (not specific)	Stem Cells

ESC, embryonic stem cell; iPSC, induced pluripotent stem cell.

Secondary target would be the neural structures connecting to organ of Corti (specifically inner hair cell). As the loss of cochlear hair cell maintains, connecting neural structures consequently degenerate.⁹ This secondary degeneration of neural structure is very critical for current hearing rehabilitation strategies (Fig. 1A). Currently in profound hearing loss, since there are no alternative ways to regenerate cochlear hair cells, cochlear implant which directly stimulating the peripheral sensory ganglion (spiral ganglion) within the cochlea is in use worldwide. Since these applications of cochlear implant are very recent, performance of hearing rehabilitation in patients who have been deaf for long duration was not as good as the short deaf duration patients. To provide better rehabilitation to these long deaf duration patients or to the patients with neural hearing loss who have deformed neural structures due to genetic or other inflammatory causes, innovative approach to regenerate this specific neural structure is necessary. Neural stem cells are currently available in market. These stem cells are derived from various origin and induced by specific technology to reprogram the mature cells. I believe that the neural reprogramming technique will be advanced soon enough and stem cells could be applied with cochlear implants or to the patients with neural hearing loss.

These outlooks of stem cell applications for sensorineural hearing loss seem very promising. There are several obstacles for these applications to be accomplished. First, survival of stem cells within the cochlea which is very hostile environment to external cells is very difficult. Sufficient numbers of cells are required to perform the possible role inside the target organ and sufficient time is required to transplanted cells to be accustomed to target environments and to be differentiated into target cells. Conditioning or modifying the transplanted or to be transplanted cells is highly necessary. Secondly, differentiation protocol, specially for the sensory cochlear hair cell, is extremely complicated and not effective. Therefore, as noted above, sufficient survival is necessary and this complicated protocol should be simplified and delivered efficiently to the target.

As has been introduced earlier by our group, PBM which is non-invasive stimulation with varying wavelength has many beneficial effects such as differentiation, proliferation and migration.¹⁰ Recently, PBM is thought to have clinical potential for regenerative medicine. PBM modulation on stem cells were published for animal derived mesenchymal stem cell (MSC)s, umbilical cord blood-derived MSCs, human bone marrow stem cells, human dental pulp stem cells and human adipose derived stem cells. Please refer to the paper published 2020 by Chang et al.¹⁰ for detailed tables for each studies. Considering these publications, it is feasible to believe that PBM could strengthen the stem cell and might overcome the obstacles which were raised previously (small survival and differentiation possibility).

To achieve the goal of fully functional regenerative hair cells in cochlea, first we have to transplant the stem cells into scala media (one of the fluid filled compartment which is hostile for external cells). To transplant into scala media, small hole in the bony cochlea has to be made to penetrate into scala media and fluid in the scala media have to be preconditioned to increase the survival rate.¹¹ Indeed this preconditioning process increased the survival and possible differentiation. Furthermore, it has been demonstrated that with PBM transplanted stem cells were likely to be aggregated forming cellular spheroid within the cochlea suggesting the possibility of cellular connection further indicating presumptive differentiation.¹² These results indicate that PBM is modulating the transplanted stem cells in cochlea but detailed effect was not clear at this point.

Secondly, and most importantly, stem cell should differentiate into target organ in this case cochlear hair cells. As has been noted earlier in this manuscript, inner ear organoid which contains functional hair cells has been introduced by several reports.^1-8,13,14 Moving along, we have applied PBM in the protocol introduced by prior reports and demonstrated that PBM with light emitting diode (LED) wavelength around 600 nm has increased the differentiation efficiency possibly downregulating the Hes5, which is gene limiting the inner ear hair cell development.¹⁵ This positive effect was confirmed by PBM with laser and LED wavelength around 800 nm. But exact pathomechanisms of PBM with 800 nm was not further investigated.¹² These results along with the outcomes mentioned earlier indicate that PBM is surely modulating the stem cell both in organoids and within cochlea. Possibly this modulation is thought to be towards differentiation. However even with PBM, cochlear specific hair cells were not demonstrated. Thus further study should be focused in generating the cochlear specific hair cell with possible facilitation by PBM. Table 2 includes the papers regarding PBM, stem cells and cochlear hair cell.

Table 2 . Publications of inner ear (otic) organoid and photobiomodulation

Year	Reference	Source cell	Photobiomodulation			Journal
			Type	Wave­length	Duration
2020	Chang et al.15	mESC	LED	630 nm	For 3 days (at 40 mW × 750 s/day)	Molecular Therapy Methods & Clinical Development
2021	Chang and Lee12	mESC	LED	850 nm	For 3 days (at 40 mW × 750 s/day)	Lasers in Medical Science
			Laser	808 nm	250 mW for 90 min

mESC, mouse embryonic stem cell; LED, light emitting diode.

As explained earlier, neural regeneration with source of stem cell could be a breakthrough, could expand the surgical candidacy of cochlear implantation (see stem cell therapy for sensorineural hearing loss [neural structure]). To deliver the stem cells to cochlear neural ganglion, round window which is the only membranous opening without any obstacle is the candidate route. Round window connects to scala tympani which is also fluid filled cavity. But scala tympani is relatively less hostile compared to scala media. Therefore, better survival of transplanted cells are expected. Indeed we have transplanted the cells through the round window and observed the varying morphologies of stem cells in animal cochlea of secondary neural degeneration.¹⁶ In this study, very small cell transplantation into the neural ganglion was observed. We have applied PBM to increase the survival of stem cell and indeed robust survival and proliferation was observed in a case but failed to show differentiation into functional neural structures (no electrically evoked auditory brainstem response [eABR]; unpublished data). With this data in more hospitable environment, effect of PBM might be more robust but targeted differentiation was not feasible. Fortunately, evidences of tumorigenesis were not observed (unpublished data). However, more detailed used of PBM such as using hydrogels containing homing and differentiation factors to be released by light stimulus has to be performed (Fig. 1B).

In summary, the stem cells are very promising source for hearing restoration for sensorineural hearing loss. Both sensory inner hair cell and neural structures could be the target for final differentiation. But there are several obstacles such as low survival rate and complicated differentiation process. PBM could be the candidate tool to overcome these obstacles and several studies showed promising outlook. But more detailed and well-constructed experiment to prove and upgrade the role of PBM is necessary.

I would like to thank Eunkyu Lee for providing the illustrations shown in Fig. 1.

No potential conflict of interest relevant to this article was reported.

This research was supported by a grant from a Creative Materials Discovery Program through the National Research Foundation (2019M3D1A1078943) funded by the National Research Foundation of Korea (NRF).