Development of the Ear

Internal Ear • First indication - found in embryo at approximately 22 days as thickening of the surface ectoderm on each side of the rhombencephalon • These thickenings – otic placodes – they invaginate rapidly forming otocysts • Later each vesicle divides into o Ventral Component – gives rise to the saccule, cochlear duct and ducuts reuniens o 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 o This outgrowth is – cochlear duct o It penetrates the surrounding mesenchyme in a spiral fashion till 8th week o Ends until it has completed 2.5 turns o Connection with saccule later narrows and remains as the ductus reuniens • Mesenchyme surrounding the cochlear duct differentiates into cartilage • 10th week – 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 o NB: statoacoustic ganglion derived from cells of the neural crest o 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 o 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 – 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 = 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 o NB: 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 • 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 • NB: 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 o these swellings – auricular hillocks • three on each side of ex ac meatus • fuse to form definitive auricle • A – six week old embryo • Labels 1,2,3 – 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 o As mandible grows anteriorly and posteriorly, the ears are repositioned to the side of the head in line with the eyes