Epithelial dynamics shed light on mechanisms underlying ear canal defects

Abstract

Defects in ear canal development can cause severe hearing loss as sound waves fail to reach the middle ear. Here we reveal new mechanisms that control human canal development and highlight for the first time the complex system of canal closure and reopening. These processes can be perturbed in mutant mice and in explant culture, mimicking the defects associated with canal aplasia. The more superficial part of the canal forms from an open primary canal that closes and then reopens. In contrast, the deeper part of the canal forms from an extending solid meatal plate that opens later. Closure and fusion of the primary canal was linked to loss of periderm, with failure in periderm formation in Grhl3 mutant mice associated with premature closure of the canal. Conversely, inhibition of cell death in the periderm resulted in an arrest of closure. Once closed, re-opening of the canal occurred in a wave, triggered by terminal differentiation of the epithelium. Understanding these complex processes involved in canal development sheds light on the underlying causes of canal aplasia.

An Essential Requirement for Fgf10 in Pinna Extension Sheds Light on Auricle Defects in LADD Syndrome

“…No positive cells were identified in the developing pinna in mutants or littermates controls at E14.5 and E15.5 ( Supplementary Figure 5 ) ( N = 3). As a positive control, apoptotic cells were observed in the ear canal during normal development at E15.5 ( Supplementary Figure 5F arrow), agreeing with the literature ( Nishizaki et al, 1998 ; Fons et al, 2020 ). A reduction of proliferation, rather than an upregulation of cell death, therefore, appeared to underlie the defect.…”

Anatomy and Development of the Mammalian External Auditory Canal: Implications for Understanding Canal Disease and Deformity

“…The bony canal skin, does not possess ceruminous glands or hair follicles ( Standrig et al, 2016 ). Our observations of human cranial skin verses ear canal skin development suggest the latter develops precociously in relation to skin ( Fons et al, 2020 ). This has interesting implications for experimental comparisons drawn between ear canal skin and skin elsewhere, as well as clinical implications, where skin from the limbs is used to reconstruct the canal lumen.…”

“…The ear canal develops in two parts, the outer part or primary canal, and the inner part or meatal plug ( Fons et al, 2020 ; Figure 3 ). At around Carnegie stage 17 (6 weeks post conception and equivalent to E12.0 in mice), the primary canal begins to form either by the first cleft deepening or as a new invagination within the 1st arch.…”

“…This plug of cells will continue to extend inward into the surrounding mesenchyme becoming a finger-like projection pointing to the developing middle ear cavity (MEC) ( Figure 3B ; Nishimura and Kumoi, 2017 ). Temporal and situational differences in cell proliferation appear to influence the extension and directional growth of the meatal plug as it begins to develop ( Fons et al, 2020 ). Both human and mouse display a distinctive duality in EAM development: the outer, primary canal (which will become the cartilaginous canal) develops differently to the inner, meatal plug (which will become the osseous canal) ( Fons et al, 2020 ).…”

“…Temporal and situational differences in cell proliferation appear to influence the extension and directional growth of the meatal plug as it begins to develop ( Fons et al, 2020 ). Both human and mouse display a distinctive duality in EAM development: the outer, primary canal (which will become the cartilaginous canal) develops differently to the inner, meatal plug (which will become the osseous canal) ( Fons et al, 2020 ). Initially open, the primary canal closes for some time before opening again along with the meatal plug, to form one long lumen ( Fons et al, 2020 ) ( Figures 3C,E ).…”