Development and age features of the pre-cochlear organ
Last reviewed: 23.04.2024
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The organ of hearing and balance is laid in a person in embryogenesis rather early. The ear of the membranous labyrinth appears on the third week of intrauterine life in the form of an ectoderm thickening on the surface of the head of the embryo, along the sides of the neural plate. At the 4th week the ectodermal plate flexes, forming an auditory fossa, which then becomes an auditory vesicle. Later the vial is separated from the ectoderm and immersed in the underlying mesenchyme (6th week). By a complex differentiation from the vesicle, three semicircular ducts, a martha and a sac are formed. In each formation a specialized area develops: in the semicircular ducts - scallops, in the uterus and sac - spots containing sensitive cells of the neuroepithelium. A cochlear duct is formed from the anterior part of the vesicle by elongation and spiral coagulation. At the 3rd month of embryogenesis, the membranous labyrinth is basically formed. At this time, the sound-receiving apparatus-the spiral (corti) organ begins to form. From the thickening of the epithelium of the cochlear duct, a cover membrane is formed, under which the neuroepithelial (hair) sensory cells are differentiated. By the 6th month the structure of the spiral organ is gradually becoming more complicated. With the sensitive cells of the spots and scallops, as well as with the spiral organ, branchings of the peripheral part of the VIII pair of cranial nerves are connected. Simultaneously with the development of the membranous labyrinth around it, the mesenchyme (auditory capsule) is concentrated, which then turns into cartilage. Between the cartilage and the labyrinth appears a fluid-filled perilymphatic space. Subsequently, the cartilaginous capsule of the labyrinth turns into bone.
In parallel with the development of the sound-receiving apparatus (inner ear), a sound producing apparatus (middle ear) is formed. From the first gill (visceral) pocket and the walls of its distal part, a drum lining is formed, and the proximal part narrows and becomes an auditory tube. Emerging opposite the forming drum cavity protrusion - the gill furrow is further transformed into an external auditory canal. Auditory ossicles develop from the cartilages of the first and second visceral arches. The auricle is formed from the mesenchymal islets adjacent to the ectodermal sulcus.
The ear conch of the newborn is flattened, its cartilage is soft, its skin is thin. The lobule of the auricle (lobe) is small in size. Most quickly the auricle grows during the first 2 years of a child's life and after 10 years. In length, it grows faster than width. The external auditory meatus of the newborn is narrow, long (about 15 mm), steeply curved, narrowed at the border of the enlarged medial and lateral divisions. The walls of the external auditory canal are cartilaginous, with the exception of the tympanic ring. The skin that lines the external passage is thin, delicate. At the child of 1 year its length about 20 mm, at the child of 5 years - 22 mm.
The tympanic membrane of the newborn is relatively large. Its height is 9 mm. The eardrum of the newborn is tilted more than the adult. The angle that it forms with the lower wall of the external auditory meatus is 35-40 °.
The tympanic cavity of a newborn in size differs little from that of an adult, but it seems narrow because of the thickened mucosa at that age. At the time of birth, there is a liquid in the tympanic cavity, which, with the onset of breathing through the auditory tube, enters the pharynx and is swallowed.
The walls of the tympanum are thin, especially the upper. The lower wall is sometimes represented by a connective tissue. The posterior wall has a wide opening leading to the mastoid cave. Posterous cells in the newborn are absent because of the mild development of the mastoid process. Auditory ossicles have dimensions close to those of an adult. The auditory tube of the newborn is straight, wide, short (17-21 mm). The cartilaginous part of the auditory tube is poorly developed. During the first year of life, the auditory tube grows slowly, in the second year faster. The length of the auditory tube in a child of 1 year is 20 mm, 2 years - 30 mm, 5 years - 35 mm, in an adult is 35-38 mm. The lumen of the auditory tube gradually narrows: from 2.5 mm in 6 months to 2 mm in 2 years and up 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. Bone walls of semicircular canals thin, gradually thicken due to fusion of ossification nuclei in the pyramid of the temporal bone.
Anomalies in the development of the pre-cochlear organ
Disturbances in the development of the receptor apparatus (spiral organ), hypoplasia of the auditory ossicles, which impede their movement, lead to congenital deafness. Defects of the position, shape and structure of the external ear (ugliness), as a rule, are associated with underdevelopment of the lower jaw (micrognathia) or even with its absence (agnathia).
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