Development of the ears
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Ear embryology concerns the development of the ear, the organ responsible for hearing and equilibrium processes. During embryotic formation, the ear develops into three different structures: the inner ear, the middle ear and the outer ear. The inner ear consists of the vestibulocochlear organ. The middle ear consists of the tympanic cavity and the epytimpanic recess. The external ear consists of the auricule and the external acoustic meatus. Each structure originates from different germinative layers or tissues: the ectoderm, endoderm and mesenchyme. The ear begins to appear during the 22nd day of embryotic development.
Development of parts of the ear
The first part of the ear to develop is the inner ear. Its appearance occurs around the 22nd day of the embryo’s development, originating from the ectoderm germinative layer. Specifically the inner ear derives from a set of placodes called otic placodes. Each otic placode forms the otocyst or otic vesicle. This epithelial mass will immerse itself and eventually be surrounded by mesenchyme to form the otic capsule.
Before any of the vital components of the inner ear can be formed, a group of sensory cells called a saccule forms the inner ear’s epithelium. Part of the saccule will eventually give rise and connect to the cochlear duct. This duct appears approximately during the sixth week and connects to the saccule through the ductus reuniens. As the cochlear duct’s mesenchyme begins to differentiate, three cavities are formed: the scala vestibule, the scala tympani and the scala media. Both the scala vestibule and the scala tympani contain an extracellular fluid called perilymph. The scala media contains endolymph. A set of membranes called the vestibular membrane and the basilar membrane separate the cochlear duct from the scala vestibule and the scala tympani. A spiral ligament and a cartilaginous process called the modiolus connect and support the cochlear duct to the rest of the cartilaginous structures that surround it. The organ of Corti is made up of sensory cells and a tectorial membrane. The utricule and saccule generate sensory areas called the maculae acusticae. The otic vesicle in turn forms the statoacoustic ganglion. The structures of the inner part work together in the adult ear to convert the signals that they receive from the middle and external ears and transfer them to the brain where they can be processed.
The middle ear, which includes the tympanic cavity and the auditory tube, originates from the first pharyngeal pouch. More specifically, the tubotympanic recess originates from the distal part of the pouch while the eustachian tube from the part next to it. This last structure will establish the final connection between the tympanic cavity and the nasopharynx. The auditory ossicles (malleus, incus and stapes), originate to form the second pharyngeal pouch are embedded in the tympanic cavity and normally appear during the first half of fetal life. The first two (malleus and incus) derive from the first pharyngeal arch and the stapes from the second. Eventually cells from the tissue surrounding the ossicles will experience apoptosis and a new layer of endodermal epithelial will constitute the formation of the tympanic cavity wall. The mastoid process will appear as the tympanic cavity continues to grow.
Unlike structures of the inner and middle ear, which develop from pharyngeal pouches, the external auditory meatus originates from the dorsal portion of the first pharyngeal cleft. It is fully expanded by the end of the 18th week of development. The tympanic membrane or eardrum is made up of three layers (ectoderm, endoderm and connective tissue) all of which form the outer layer of the articular capsule (fibrous stratum). The auricule originates as a fusion of six proliferations or auricular hillocks of His from the first and second pharyngeal pouches. The external ears are firstly situated in the lower neck region. As the mandible forms they move towards their final position leveled with the eyes. Once it is fully developed, the external ear functions both to capture sound from the outside and to conduct it through the external auditory meatus towards the tympanic membrane.
Most of the genes responsible for the regulation of inner ear formation and its morphogenesis are members of the homeobox gene family such as Pax, Msx and Otx homeobox genes. The development of inner ear structures such as the cochlea is regulated by Dlx5/Dlx6, Otx1/Otx2 and Pax2, which in turn are controlled by the master gene Shh. Shh is secreted by the notochord.
Approximately one out of one thousand children suffer some type of congenital deafness related to the development of the inner ear. Inner ear congenital anomalies are related to SNHL (sensorineural hearing loss) and are generally diagnosed with a computed tomography (CT) scan or a magnetic resonance imaging (MRI) scan. Hearing loss problems also derive from inner ear anomalies because its development is separate from that of the middle and external ear. Middle ear anomalies can occur because of errors during head and neck development. The first pharyngeal pouch syndrome associates middle ear anomalies to the malleus and incus structures as well as to the non-differentiation of the annular stapedial ligament. Temporal bone and external auditory meatus anomalies are also related to this structure of the ear and are known to be associated with SNHL and conductive hearing loss (CHL). Auricule anomalies and minor malformations are common external ear anomalies. These types of anomalies include chromosome syndromes such as gene 18 duplications. Children may also present cases of abnormal auditory meatus and low ear implantation. Small auricules can develop when the auricular hillocks do not develop properly. Atresia of the external auditory meatus can occur if the meatus does not channelize properly or if there is an obstruction. Reconstructive surgery to treat hearing loss is considered as an option for children older than five.
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