Development of the Ear

Internal Ear

On First indication found in embryo at approximately 22 days as thickening of the surface ectoderm on each side of the rhombencephalon. These thickenings are the otic placodes. They invaginate rapidly forming otocysts. Later each vesicle divides into. Ventral Component, gives rise to the saccule, cochlear duct and ducuts reuniens Dorsal Component gives rise to the utricle, semicircular canals and endolymphatic duct. Together these epithelial cells form the membranous labyrinth

Cochlea, saccule and organ of corti

6th week of development saccule forms a tubular outpocketing at its lower pole. This outgrowth is the cochlear duct. It penetrates the surrounding mesenchyme in a spiral fashion till 8th week. Ends until it has completed 2.5 turns. Connection with saccule later narrows and remains as the ductus reuniens. Mesenchyme surrounding the cochlear duct differentiates into cartilage. 10th week the cartilaginous shell undergoes vacuolization forming the perilymphatic spaces scala tympani and scala vestibule. Vestibular membrane seperates the scala vestibuli and cochlear duct. Basilar membrane seperates scala tympani and cochlear duct. Lateral wall of the cochlear duct remains attached to the surrounding cartilage by the spiral ligament. Median angle is connected by the cartilaginous process the modiolus

Utricle and Semicircular Canals

During the 6th week, impulses generate in cristae and maculae due to change in position of the body are carried to the brain via vestibular fibers of cranial nerve VIII. Also, statoacoustic ganglion derived from cells of the neural crest. The Ganglion subsequently splits into cochlear and vestibular portions supplying sensory cells in organ of corti, saccule, utricle and semicircular canals.

Middle Ear- Tympanic Cavity and Auditory Tube

Tympanic cavity originates in the endoderm from 1st pharyngeal pouch. Pouch expands in lateral direction and comes in contact with the floor of the 1st pharyngeal cleft. Distal part of the pouch forms – tubotympanic recess – widens and gives rise to the primitive tympanic cavity. Prox part remains narrow and forms auditory tube through which the tympanic cavity communicates with the nasopharynx

Ossicles

Malleus + Incus derived from cartilage of 1st pharyngeal arch. Stapes is derived from the 2nd arch. Ossicles appear during the 1st half of fetal life they remain embedded in mesenchyme until the 8th month. When ossciles are entirely free of surrounding mesenchyme, the endodermal epithelium connects them in a mesentery like fashion to the wall of the cavity. This is the supporting ligaments of the ossicles develop later within these mesenteries. Tympanic cavity exands dorsally by vacuolization of surrounding tissue to form the tympanic antrum. After birth – tympanic cavity epithelium invades bone of the developing mastoid process, and epithelium lined are sacs are formed (pneumatisation)

External Ear- External Auditory Meatus

This Develops from dorsal portion of the 1st pharyngeal cleft ectodermal. Beginning of 3rd month epithelial cells at bottom of meatus proliferate forming meatal plug. 7th month plug dissolves and epithelial lining of meatus floor participates in formation of definitive eardrum. Also meatal plug may persist resulting in congenital deafness

Eardrum/ tympanic membrane

Tridermal layer

Auricle

Auricle develops from six mesenchymal proliferations at dorsal ends of 1st and 2nd pharyngeal arches, these swellings are auricular hillocks. Three on each side of ex ac meatus. Fuse to form definitive auricle. These hillocks are part of the mandibular portion of the 1st pharyngeal arch and that the ear lies horizontally a the side of the neck. As mandible grows anteriorly and posteriorly, the ears are repositioned to the side of the head in line with the eyes