Embryonic Development Of The Ear

The ear consists of three sections:


 * 1) inner,
 * 2) middle,
 * 3) external.

Inner Ear
Auditory placodes are formed 22. den from the ectoderm on both sides of the rhombencephal. They gradually sink in and form auditory sacs (otocysts). The ventral part' of the cam provides the basis for the formation of the sacculus and the ductus cochlearis. 'Dorsal is for the utriculus and the ductuli semicirculares and the ductus endolymphaticus. Together, therefore, it provides the basis for the membranous labyrinth. From the mesenchyme around the ductus cochlearis cartilage arises, which later becomes vacuolated - the scala tympani and the scala vestibuli are formed. Gradually, other structures of the inner ear are formed.



During the 6. week 3 semicircular ducts (ductus semicirculares) are formed as flattened protrusions of the utricle. The widened part of the ducts is called the ampulla. The two ampoules then merge into one. In the ampoules there is the crista ampularis = component of the kinetic part of the balance organ.

The static part develops in the saccule and utricle - maculae staticae. The impulses are conducted by the vestibulocochlearis nerve, which forms the ganglion vestibulocochleare.

Otocyst
Around 22. on' of embryonic development, an otic placode in the form of condensed ectoderm is present on each side of the future rhombencephalon (in the region of the 3rd brain pouch). In 4. week, the otic placode, after being induced from the mesoderm and rhombencephalon by the growth factor FGF8, begins to snap inward as an otic pouch, which is called an 'otocyst after decompression. The otocyst is formed by the ectoderm epithelium, which after invagination gets closer to the rhombencephalon and the statoacoustic ganglion begins to form in the mesenchyme.

Around 5. week, the otocyst divides into 2 parts:

a) Ventral area' (induced from the ventral part of the rhombencephalon by SHH) which will give rise to the future sacculus and the ductus cochlearis (cochlear part).

b) The Dorsal area' (induced from the dorsal part of the rhombencephala via the WNT) which gives rise to the future utriculus, ductus endolymphaticus and the ductus semicirculares (vestibular part).

Ductus cochlearis

 * In 6. week a canal grows from the lower part of the saccule – the ductus cochlearis, which grows to a length of 2.5 turns and remains connected to the saccule through a narrow 'ductus reuniens'.
 * Ductus cochlearis is encased in mesenchyme, which turns into cartilage, which is the basis for the future bony labyrinth.
 * In '10. week, vacuolization occurs in this cartilaginous base and 2 perilymphatic spaces are formed:


 * 1) scala vestibuli - a space separated by the vestibular membrane,
 * 2) scala tympani - space separated by the basilar membrane

The ductus cochlearis is laterally connected to the cartilage by means of a ligamentum spirale, it is medially supported by a cartilage projection = modiolus'.

The cells in the ductus cochlearis are initially the same size, but gradually differentiate into cylindrical cells that form 2 mounds:


 * Internal - future limbus spiralis.
 * External – hair cells arranged in parts:


 * 1) outer (1 row of cells),
 * 2) inner (3-4 rows of cells)

Hair cells

 * Hair cells are actually hearing sensory cells, their projections touch the tectorial membrane, which has the character of a jelly-like substance and is attached to the limbus spiralis. The stereocilia of the hair cells protrude above the level of the tunnel of Corti and penetrate into the membrana tectoria.


 * Sensory cells Organ of Corti are secondary sensory cells that convert mechanical stimuli (which originate from sound waves) into nerve impulses. These are further transmitted by dendrites of ggl cells. spirals, which are at the base of the sensory cells and go to the modiolus to the fundus meatus acustici interni to the ggl. spiral Axons of cells from ggl. spirals emerge from the fundus meatus acustici interni as 'n. cochlearis.

Ductus semicirculares

 * In the utricular part, in 6. week 3 flattened projections appear. The central parts of the walls of these protrusions are attached to each other, merge and eventually disappear apoptotically, creating 3 semicircular canals - ductus semicirculares (anterior, posterior, lateral).
 * The semicircular canals have a dilated part - the ampulla and a 'non-dilated part. The unexpanded parts of the anterior and posterior semicircular canals merge together, and thus the canals open into the utriculus with 5 (not 6) openings (3 contain an ampulla, 2 are unexpanded).
 * There are other sensory structures in the ampullae - ''cristae ampullares'. They are raised ridge-like edges that stand out from the s