Since in mammals, the hair cells or the spiral ganglion neurons (SGNs) in the
inner ear do not regenerate, damage to these cells is an irreversible process.
Presently the only aid for patients with severe to profound hearing impairment
due to damaged hair cells is a cochlear implant (CI). A CI converts sound to
electrical signals that stimulate the SGNs via an electrode that is implanted
into the cochlea. Hence, for a successful outcome the CI is dependant on the
activation of the auditory nerve. There are several conditions, diseases or
even traumatic events that primarily may impair the function of the SGNs in
the auditory nerve. It is also known that in the absence of nerve stimuli due
to hair cell damage, the SGNs will eventually degenerate. Lately there has
been an increasing interest in regenerative medicine and bioengineering. This
thesis presents results from in vivo experiments aiming to replace or repair
the injured SGNs with the use of transplanted stem cells or neuronal tissue.
All transplanted cells were labeled with a green fluorescent protein
facilitating identification in the host animal.
Paper I presents a new animal model of selective auditory nerve injury with
preserved hair cells. The lesion was induced in rats by the application of
β-bungarotoxin to the round window niche. Immunohistochemical straining
confirmed the loss of SGN while the hair cells were kept intact. The induced
hearing impairment was verified by auditory brain stem response (ABR).
Paper II presents a surgical approach for the injection of stem cells to the
auditory nerve by the internal auditory meatus (IAM). It was shown that this
approach does not significantly affect the hearing as verified by ABR.
Further, neuronal tract tracing with the enzyme horseradish peroxidase
illustrated that injection of selected substances may be distributed by
intra-axonal transportation centrally to the brain stem as well as
peripherally to the cochlea. Furthermore it was illustrated that statoacoustic
ganglions transplanted by the IAM survived for up to five weeks, though in low
numbers. No cells had migrated through the Schwann-glia transitional zone into
the cochlea.
Paper III presents an assessment of mouse tau-GFP embryonic stem cells
transplantated to the auditory nerve trunk by the IAM or into the modiolus in
previously deafened rats. It was shown that supplementary treatment of BDNF in
a bioactive peptide amphiphile (PA) nanogel increased survival and neuronal
differentiation of the transplanted cells. It was also demonstrated that
supplement of the enzyme chondroitinase ABC in the PA gel facilitated
migration of transplanted cells through the transitional zone.
Paper IV presents the use of human neural progenitor cells for transplantation
to the auditory nerve by the IAM. We further assessed supplement of BDNF in
the PA gel. After three weeks, survival and differentiation of the
transplanted cells were observed. After six weeks of survival the majority of
the surviving cells had differentiated into neurons. The addition of BDNF in
PA gel significantly increased both survival and differentiation. The
transplanted cells migrated to the brainstem and formed neuronal profiles
including extensive arborisation of nerve fibers in the vicinity of the
cochlear nucleus.
In conclusion, this thesis presents a new animal model for a selective lesion
of the auditory nerve. Further, promising results were demonstrated regarding
the possibility of replacing auditory SGNs including increased rates of
survival and neuronal differentiation of the transplanted cells in the
presences of BDNF. These results suggest for further studies on auditory nerve
replacement but also for functional assessment of the transplanted cells
