Defining the Cellular Environment in the Organ of Corti following Extensive Hair Cell Loss: A Basis for Future Sensory Cell Replacement in the Cochlea

Abstract

BackgroundFollowing the loss of hair cells from the mammalian cochlea, the sensory epithelium repairs to close the lesions but no new hair cells arise and hearing impairment ensues. For any cell replacement strategy to be successful, the cellular environment of the injured tissue has to be able to nurture new hair cells. This study defines characteristics of the auditory sensory epithelium after hair cell loss.Methodology/Principal FindingsStudies were conducted in C57BL/6 and CBA/Ca mice. Treatment with an aminoglycoside-diuretic combination produced loss of all outer hair cells within 48 hours in both strains. The subsequent progressive tissue re-organisation was examined using immunohistochemistry and electron microscopy. There was no evidence of significant de-differentiation of the specialised columnar supporting cells. Kir4.1 was down regulated but KCC4, GLAST, microtubule bundles, connexin expression patterns and pathways of intercellular communication were retained. The columnar supporting cells became covered with non-specialised cells migrating from the outermost region of the organ of Corti. Eventually non-specialised, flat cells replaced the columnar epithelium. Flat epithelium developed in distributed patches interrupting regions of columnar epithelium formed of differentiated supporting cells. Formation of the flat epithelium was initiated within a few weeks post-treatment in C57BL/6 mice but not for several months in CBA/Ca's, suggesting genetic background influences the rate of re-organisation.Conclusions/SignificanceThe lack of dedifferentiation amongst supporting cells and their replacement by cells from the outer side of the organ of Corti are factors that may need to be considered in any attempt to promote endogenous hair cell regeneration. The variability of the cellular environment along an individual cochlea arising from patch-like generation of flat epithelium, and the possible variability between individuals resulting from genetic influences on the rate at which remodelling occurs may pose challenges to devising the appropriate regenerative therapy for a deaf patient.

How to Bury the Dead: Elimination of Apoptotic Hair Cells from the Hearing Organ of the Mouse

“…Swelling has been previously reported following ototoxic and noise traumas (Fredelius 1988;Flock et al 1999;Ahmad et al 2003;Abrashkin et al 2006;Taylor et al 2012;Forge et al 2013). A possible trigger for this event is altered ion transport coupled with intercellular signaling between dying hair cells and supporting cells (Piazza et al 2007;Forge et al 2013).…”

Spontaneous regeneration of cochlear supporting cells after neonatal ablation ensures hearing in the adult mouse

“…The remarkably mild effect of neonatal IBC/IPhC ablation on the adult cochlea was surprising, because a number of studies suggest that these cells play important roles in cochlear development (1,(2)(3)(4)(5)(6). Therefore, we thought it was necessary to test whether IBC/IPhC ablation at later stages had a different effect.…”

“…S1A). IBCs and IPhCs, together with other SCs, are known to play critical roles during the development and maturation of the organ of Corti, in processes such as patterning of the epithelium, synaptogenesis, and initiation of electrical activity in auditory nerves before the onset of hearing and formation of extracellular matrices (1)(2)(3)(4)(5)(6)(7). SCs also are essential for the function of the mature organ of Corti, where they contribute to the maintenance of the reticular lamina at the apical surface of the epithelium (8), control the extracellular concentration of ions (e.g., K + ) (9, 10) and neurotransmitters (e.g., glutamate) (11), and support hair cell (HC) and auditory sensory neuron survival (5,(12)(13)(14)(15).…”

Connexin30 mediated intercellular communication plays an essential role in epithelial repair in the cochlea

“…This pattern was displayed in the organs of Corti of control littermates of Cx30 null mice that had been treated with a combination of the loop diuretic, bumetanide, and kanamycin, an aminoglycoside antibiotic (Fig. 5B), a treatment regime which causes loss of almost all OHC within 48 hours (Taylor et al, 2008;Taylor et al, 2012). At the reticular lamina, each OHC in the first (innermost) row was replaced by the expanded heads of three cells (2 adjacent outer pillar and single 1st row Deiters' cells); each OHC in the second row was replaced by 4 supporting cells (outer pillar and three adjacent Deiters' cells); and each third row OHC, was also replaced by four cells (4 adjacent Deiters' cells).…”

“…We have shown previously (Jagger and Forge, 2006;Taylor et al, 2012) that in the normal organ of Corti there is extensive gap junctional communication between the supporting cells, and that in the functionally mature organ of Corti, there are two coupled compartments. One consists of all the supporting cells from the inner pillar cells inwards; the other consists of outer pillar cells, Deiters' cells and all those cells to the outer side.…”