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

Abstract

The mammalian ear is made up of three parts (the outer, middle, and inner ear), which work together to transmit sound waves into neuronal signals perceived by our auditory cortex as sound. This review focuses on the often-neglected outer ear, specifically the external auditory meatus (EAM), or ear canal. Within our complex hearing pathway, the ear canal is responsible for funneling sound waves toward the tympanic membrane (ear drum) and into the middle ear, and as such is a physical link between the tympanic membrane and the outside world. Unique anatomical adaptations, such as its migrating epithelium and cerumen glands, equip the ear canal for its function as both a conduit and a cul-de-sac. Defects in development, or later blockages in the canal, lead to congenital or acquired conductive hearing loss. Recent studies have built on decades-old knowledge of ear canal development and suggest a novel multi-stage, complex and integrated system of development, helping to explain the mechanisms underlying congenital canal atresia and stenosis. Here we review our current understanding of ear canal development; how this biological lumen is made; what determines its location; and how its structure is maintained throughout life. Together this knowledge allows clinical questions to be approached from a developmental biology perspective.

Ecological correlates of the morphology of the auditory bulla in rodents: Application to the fossil record

“…The short EAMs are interesting in light of their lateral expansion in fossorial taxa; semifossorial taxa are more similar to terrestrial ones in this aspect of bullar morphology. This hints at additional support for the fact that the length of the EAM in burrowing rodents is a consequence of the rearrangement of their skull morphology rather than an adaptation to acoustic performance (see also Mozaffari et al, 2021).…”

“…This large opening may be associated with an improved intake of auditory information into the EAM for transmission to the middle ear. Indeed, the wide unobstructed opening of the EAM is critical to hearing (Mozaffari et al, 2021) and, presumably, the identification of sounds by prey, predators, and conspecifics. The round shape of the bulla and the ventral opening of the meatus represent ancestral conditions in the absence of asymmetric inflation and the relocation of the external ear in a more dorsal position observed in fossorial and semi-aquatic taxa (see above).…”

Variants and Modifications of the Retroauricular Approach Using in Temporomandibular Joint Surgery: A Systematic Review

“…Stenosis of the external auditory canal is found in about 1 in 3 patients, but it is no greater than 1.5 mm of reduction in diameter [ 4 ]. The typical external auditory canal diameter in an adult is about 8 mm, which means the narrowing does not exceed 20% in the classic version of RA [ 4 , 24 ].…”

The EDA-deficient mouse has Zymbal's gland hypoplasia and acute otitis externa

“…Rodents lack the apocrine ceruminous glands that are found in human, dog, goat and pig ear canals (Wang et al, 2021) but have a specialised large multilobulated auditory sebaceous gland, also known as the Zymbal's gland (Rudmann et al, 2012), which opens via a duct into the ear canal close to the tympanic membrane. The Zymbal's gland is also called the ear-wax gland (glandula ceruminosa) (Grüneberg, 1971) or ceruminous gland (Berry et al, 1994;Mozaffari et al, 2021). Hereafter we use the term Zymbal's gland to draw a distinction between this specialised sebaceous holocrine gland and the human apocrine ceruminous gland.…”