Mapping the distribution of stem/progenitor cells across the mouse middle ear during homeostasis and inflammation

Abstract

The middle ear epithelium is derived from neural crest and endoderm, which line distinct regions of the middle ear cavity. Here, we investigate the distribution of putative stem cell markers in the middle ear, combined with an analysis of the location of label-retaining cells (LRCs) to create a map of the middle ear mucosa. We show that proliferating cells and LRCs were associated with specific regions of the ear epithelium, concentrated in the hypotympanum at the base of the auditory bulla and around the ear drum. Sox2 was widely expressed in the endodermally derived ciliated pseudostratified epithelium of the hypotympanum. This part of the middle ear showed high levels of Wnt activity, as indicated by the expression of Axin2, a readout of Wnt signalling. Keratin 5 showed a more restricted expression within the basal cells of this region, with very little overlap between the Sox2- and keratin 5-positive epithelium, indicating that these genes mark distinct populations. Little expression of Sox2 or keratin 5 was observed in the neural crest-derived middle ear epithelium that lined the promontory, except in cases of otitis media when this epithelium underwent hyperplasia. This study lays the foundation for furthering our understanding of homeostasis and repair in the middle ear.

The transcriptional landscape of the murine middle ear epithelium in vitro

“…This gene signature is consistent with our previous IF studies which showed the presence of ciliated, secretory and basal cells in the cultures [16]. This in vitro data is also consistent with in vivo studies which have shown the presence of cuboidal epithelial cells expressing secretory proteins alongside regions of ciliated cells [6,7]. Although some of the ciliated cell signature genes are reasonably well studied such as; Spata18 [41] and Cdhr3 [42], none have been identified in the ear previously.…”

“…The mucosal lining of the middle ear cavity covers the entire surface and varies according to the location [6,7]. Cellular morphology can be simple squamous, cuboidal or columnar depending on the location and can have tracts of ciliated cells interspersed with secretory cells.…”

“…The epithelial lining of the middle ear cavity varies according to the location [6,7]. The attic, or epitympanum, is lined by squamous epithelium while the middle ear proper is lined by cuboidal epithelium and the hypotympanum by ciliated columnar epithelium.…”

“…Phenotypic changes throughout the middle ear epithelium are key to the pathophysiology of OM [7,8]. Current knowledge suggests that OM is caused by an unrestrained response by the middle ear epithelium to an exogenous trigger, often a pathogen [9,10].…”

Developmental aspects of the tympanic membrane: Shedding light on function and disease

“…Lineage tracing experiments suggest that the inner layer of the pars tensa is derived from first pouch endoderm and is thus continuous with middle ear mucosa (Thompson & Tucker, 2013). Keratin 5 (K5) positive cells have been identified in the basal layer of the middle ear mucosa and play a role in adult middle ear epithelial maintenance (Luo et al, 2017;Tucker et al, 2018). Interestingly, a combination of scRNAseq and lineage tracing has highlighted Keratin 19 (K19) as a marker of mucosal stem/progenitor cells in the murine TM, rather than K5 (Frumm et al, 2019).…”