development of ear by dr samina anjum. development of ear ear is composed of three anatomical parts...
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DEVELOPMENT OF EAR
ByDr Samina Anjum
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DEVELOPMENT OF EAREar 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
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DEVELOPMENT OF INTERNAL EAR
• Otic placodes -- on each side of myelencephalon• Inductive influence – notochord, paraxial mesoderm• Otic pit, otic vesicle, loses connection from surface
ectoderm• Two parts of otic vesicle---- memb. labyrinth• Ventral component---- saccule & cochlear duct• Dorsal component---- utricle, semicircular canals, &
endolymphatic duct
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SACCULE, COCHLEA, & ORGAN OF CORTI
• In 6thweek saccule forms a tubular out pocketing --- cochlear duct completes 2.5 turns in 8th week
• Connected with saccule through ductus reuniens• Surrounding mesenchyme differentiates into cartilage• In10th week vacuoles develop in cartilage & two perilymphatic
spaces formed• Scala tympani & scala vestibuli • Vestibular & basilar membranes• Spiral ligament, modiolus• 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
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DERIVATIVES OF OTIC VESICLE
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FORMATION OF SCALA VESTIBULI & SCALA TYMPANI
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DEVELOPMENT OF ORAN OF CORTI
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UTRICLE & SEMICIRCULAR CANALS• 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
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• 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 re transmitted via vestibular ganglion to brain through CN- VIII
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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
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EAR OSSICLES• Malleus & incus---cartilage of 1st pharyngeal arch• Stapes-----cartilage of 2nd pharyngeal arch• Remain embedded in mesenchyme till 8th month when
surrounding tissue dissolves
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Cont…• 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 antrum
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DEVELOPMENT OF EXTERNAL EAR• Begins at 3rd month• External auditory meatus --- 1st pharyngeal cleft• Ectodermal cells at the bottom of cleft proliferate to form
a solid epithelial plate, the meatal plug• During 7th month the plug dissolves to form internal part
of external auditory meatus• Tympanic membrane is the 1st pharyngeal membrane
b/w cleft & pouch
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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
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Cont…
• These swellings 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
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CONGENITAL ANOMALIES
• Congenital deafness
Mal development of internal ear or auditory ossicles or ear drum
• External ear defects
Due to chromosomal abnormalities
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