Human Anatomy, Part 2 - K. A. Dyubenko, A. K. Kolomiyytsev, Yu. B. Chaykovsky 2008

Special Part
Sense Organs, organa sensuum
Vestibulocochlear Organ - Development of the Organ of Hearing and Balance

During the 3rd week of intrauterine development, the primordium of the membranous labyrinth appears. The membranous labyrinth develops from the ectoderm lateral to the primordium of the Medulla Oblongata. This primordium is called the otic placode. By the 4th week of intrauterine development, the otic placode gradually invaginates into the mesenchyme to form the otic vesicle, which produces endolymph. As development proceeds, the otic vesicle divides into two parts: one transforms into the utricle with three semicircular ducts, while the other becomes the saccule with the cochlear duct. The otic vesicle maintains contact with the embryonic vestibulocochlear ganglion, which subsequently splits into the vestibular and spiral ganglia. At the site of contact between the otic vesicle and the ganglion, the wall of the vesicle thickens to form a common macula, which then subdivides into superior and inferior portions. The superior macula gives rise to the static maculae of the utricle and the ampullary crests, whereas the inferior macula forms the static maculae of the saccule and the spiral organ. By the third month of Prenatal ontogenesis, the membranous labyrinth is nearly fully formed. Around this time, the sound-receptive apparatus—the spiral organ (organ of Corti)—begins to develop.

Concurrently with The Development of the membranous labyrinth, the surrounding mesenchyme (otic capsule) condenses and subsequently transforms into Cartilage. A perilymph-filled space appears between the cartilage and the membranous labyrinth. Gradually, the cartilaginous otic capsule ossifies. The sound-conducting apparatus (Middle ear) also forms. The tympanic cavity develops from the distal part of the first pharyngeal pouch, while the pharyngotympanic (auditory) tube originates from its proximal part. The pharyngeal groove, which is an ectodermal outgrowth, turns into the external acoustic meatus. The tympanic membrane is formed by the apposition of the endoderm of the first pharyngeal pouch with the mesenchyme of the first pharyngeal arch and the ectoderm of the first pharyngeal groove. The auditory ossicles develop from the cartilages of the First and Second pharyngeal arches. The auricle (pinna) is formed from mesenchymal nodules adjacent to the ectodermal groove. The tensor tympani Muscle develops from the first pharyngeal arch, and the stapedius muscle develops from the second pharyngeal arch.

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Fig. 231. Development of the Vestibulocochlear Organ

Developmental anomalies of the Ear

Congenital anomalies of The Organ of Hearing and Equilibrium include: disorders in the development of the receptor apparatus (e.g., the spiral organ), underdevelopment of the auditory ossicles leading to congenital deafness, as well as defects of the auricle associated with congenital tissue deficiency or malformations caused by its deformation.

Malformations of the auricle caused by congenital tissue deficiency (according to G. I. Lazyuk, 1991): anotia, gnatia-associated auricle, persistence of only the lobule, cutaneous-cartilaginous fold, microtia, low-set ear, constricted ( cup) ear, folded ear, flat ear, corrugated ear, lobule defect.

Malformations of the auricle caused by its deformation (according to G. I. Lazyuk, 1991): pointed ear (satyr ear), macaque ear, unfolded helix, crus superius of antihelix, macrotia, large earlobe, accessory auricles, polyotia, transverse cleft of the auricle, longitudinal cleft of the earlobe.