DEVELOPMENT OF EAR Dr Samina Anjum

DEVELOPMENT OF EAR Ear is composed of three anatomical parts External ear (Collection of sound waves) auricle, external auditory meatus, external layer of tympanic membrane Middle ear (conduction of sound waves) 3 auditory ossicles, tympanic cavity, auditory tube Internal ear (hearing & balance) vestibulocochlear organ which converts waves into nerve impulses & register changes in equilibrium

DEVELOPMENT OF INTERNAL EAR Otic placodes -- on each side of rhombencephalon Inductive influence – notochord, paraxial mesoderm Otic pit otic/auditory vesicle (otocyst), loses connection from surface ectoderm

Otic vesicle membranous labyrinth Ventral component---- saccule cochlear duct Dorsal component---- utricle semicircular canals, & endolymphatic duct

DEVELOPMENT of COCHLEAR DUCT In 6thweek saccule forms a tubular out pocketing --- cochlear duct completes 2.5 turns in 8th week Connected with saccule through ductus reuniens

FORMATION OF SCALA VESTIBULI & SCALA TYMPANI The surrounding mesenchyme differentiates into cartilage, in 10th week vacuoles develop in the cartilage & two perilymphatic spaces formed Scala vestibuli & scala tympani & Vestibular & basilar membranes Spiral ligament, modiolus

DEVELOPMENT OF ORGAN OF CORTI Cells in wall of cochlear duct differentiate into organ of corti; forms two ridges inner ridge---- spiral limbus--- tectorial membrane Outer ridge---- 1 row of inner hair cells 3-4 rows of outer hair cells

Statoacoustic ganglion ---- wall of otic vesicle & neural crest cells

UTRICLE & SEMICIRCULAR CANALS During 6th week semicircular canals (SSC) appear as out pocketings of utricular part of otic vesicle Central parts of these diverticula fuse & disappear Peripheral unfused parts of diverticula become SSC

Cont… One end of each canal dilates to form crus ampullare while crus nonampullare, does not widen. Since two of latter type fuse, however, only five crura enter the utricle, 3 with an ampulla & 2 without

Cells in the ampulla forms a crest, crista ampullaris containing sensory cells for maintenance of equilibrium Similarly specialized receptor areas macula develop in utricle & saccule Sensory areas are stimulated by positional changes & impulses are transmitted via vestibular ganglion to brain through CN- VIII

DEVELOPMENT OF MIDDLE EAR Endoderm of 1st pharyngeal pouch forms the tubotympanic recess which comes in contact with 1st pharyngeal cleft to form the external auditory meatus Distal end is expanded -------Tympanic cavity Proximal end is narrow ----- auditory tube

EAR OSSICLES Malleus & incus ---- cartilage of 1st pharyngeal arch Stapes -----cartilage of 2nd pharyngeal arch Remain embedded in mesenchyme till 8th month, are free when surrounding tissue dissolves The endodermal epithelial lining of tympanic cavity then extends along the newly developing space When ossicles are entirely free of surrounding mesenchyme , the endodermal epithelium connects them in a mesentery like fashion to the wall of cavity

Tympanic cavity expands dorsally by vacuolization of surrounding tissue to form tympanic/mastoid antrum. After birth the epithelium of tympanic cavity invades the bone of developing mastoid process and epithelium lined air sacs are formed (pneumatization). Later most of the mastoid air sacs come in contact with the antrum and tympanic cavity.

DEVELOPMENT OF EXTERNAL EAR External auditory meatus --- 1st pharyngeal cleft By beginning of 3rd month ectodermal cells at the bottom of cleft proliferate to form a solid epithelial plate, the meatal plug In 7th month the plug dissolves to form internal part of external auditory meatus, if persists leads to congenital deafness Tympanic membrane is the 1st pharyngeal membrane b/w groove & pouch

AURICLE Develops from 6 mesenchymal proliferations in 1st & 2nd pharyngeal arches surrounding the first cleft These swellings are auricular hillocks 3 on each side of external meatus later fuse to form definite auricle As fusion of auricular hillocks is complicated developmental abnormalities of the auricle are common Lobule is the last part to develop As mandible develops the auricle moves to their normal position at the sides of head

CONGENITAL ANOMALIES Congenital deafness -maldevelopment of internal ear or auditory ossicles or ear drum External ear defects -chromosomal abnormalities

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