Development and age-specific features of the preauriculo-cochlear organ

The organ of hearing and balance is laid down in humans quite early in embryogenesis. The rudiment of the membranous labyrinth appears in the 3rd week of intrauterine life as a thickening of the ectoderm on the surface of the head of the embryo, on the sides of the neural plate. In the 4th week, the ectodermal plate bends, forming the auditory fossa, which then turns into the auditory vesicle. Later, the vesicle separates from the ectoderm and immerses in the underlying mesenchyme (6th week). By means of complex differentiation, three semicircular ducts, the utricle and the saccule are formed from the vesicle. In each formation, a specialized area develops: in the semicircular ducts - combs, in the utricle and saccule - spots containing sensitive cells of the neuroepithelium. From the anterior part of the vesicle, by means of elongation and spiral folding, the cochlear duct is formed. By the 3rd month of embryogenesis, the membranous labyrinth is mostly formed. At this time, the sound-perceiving apparatus, the spiral (organ of Corti), begins to form. The integumentary membrane is formed from the thickening of the epithelium of the cochlear duct, under which neuroepithelial (hair) sensory cells differentiate. By the 6th month, the structure of the spiral organ gradually becomes more complex. The branches of the peripheral part of the VIII pair of cranial nerves connect with the sensitive cells of the spots and combs, as well as with the spiral organ. Simultaneously with the development of the membranous labyrinth, mesenchyme (auditory capsule) is concentrated around it, which then turns into cartilage. A fluid-filled perilymphatic space appears between the cartilage and the labyrinth. Subsequently, the cartilaginous capsule of the labyrinth turns into a bone capsule.

In parallel with the development of the sound-perceiving apparatus (inner ear), the sound-producing apparatus (middle ear) is formed. From the first branchial (visceral) pocket and the walls of its distal part, the rudiment of the tympanic cavity arises, and the proximal part narrows and turns into the auditory tube. The protrusion that appears opposite the forming tympanic cavity - the branchial groove is later transformed into the external auditory canal. The auditory ossicles develop from the cartilages of the first and second visceral arches. The auricle is formed from mesenchymal islets adjacent to the ectodermal groove.

The auricle of a newborn is flattened, its cartilage is soft, the skin covering it is thin. The lobe of the auricle (lobe) is small. The auricle grows most rapidly during the first 2 years of a child's life and after 10 years. It grows in length faster than in width. The external auditory canal of a newborn is narrow, long (about 15 mm), steeply curved, has a narrowing at the border of the expanded medial and lateral sections. The walls of the external auditory canal are cartilaginous, with the exception of the tympanic ring. The skin lining the external canal is thin and delicate. In a 1-year-old child, its length is about 20 mm, in a 5-year-old child - 22 mm.

The eardrum of a newborn is relatively large. Its height is 9 mm. The eardrum of a newborn is inclined more than that of an adult. The angle it forms with the lower wall of the external auditory canal is 35-40°.

The tympanic cavity of a newborn differs little in size from that of an adult, but it seems narrow due to the thickened mucous membrane at this age. At the time of birth, there is fluid in the tympanic cavity, which, with the onset of breathing, enters the pharynx through the auditory tube and is swallowed.

The walls of the tympanic cavity are thin, especially the upper one. The lower wall is represented in places by connective tissue. The posterior wall has a wide opening leading to the mastoid cave. The mastoid cells are absent in a newborn due to the weak development of the mastoid process. The auditory ossicles are similar in size to those in an adult. The auditory tube in a newborn is straight, wide, short (17-21 mm). The cartilaginous part of the auditory tube is poorly developed. During the first year of a child's life, the auditory tube grows slowly, and faster in the 2nd year. The length of the auditory tube in a 1-year-old child is 20 mm, 2 years old - 30 mm, 5 years old - 35 mm, in an adult it is 35-38 mm. The lumen of the auditory tube narrows gradually: from 2.5 mm at 6 months to 2 mm at 2 years and to 1-2 mm in a 6-year-old child.

The inner ear of a newborn is well developed, its dimensions are close to those of an adult. The bone walls of the semicircular canals are thin, gradually thickening due to the fusion of ossification nuclei in the pyramid of the temporal bone.

Anomalies in the development of the vestibulocochlear organ

Developmental disorders of the receptor apparatus (spiral organ), underdevelopment of the auditory ossicles, which prevents their movement, lead to congenital deafness. Defects in the position, shape and structure of the outer ear (deformities) are usually associated with underdevelopment of the lower jaw (micrognathia) or even its absence (agnathia).

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