Embryology of the Human Ear Ashley Balaker, M.D. March 17, 2010

Outline Inner Ear Middle Ear External Ear

Inner Ear Ectoderm = membranous labyrinth Mesoderm = bony labyrinth 3rd week Surface ectoderm = otic placode Invaginates --> otic pit 4th week Pit edges fuse to become otocyst

Inner Ear Dorsal utricular portion, ventral saccular portion Utricular portion –3 diverticula for semicircular canals Saccular portion –Tubular diverticulum (cochlear duct) grows in spiral fashion to become membranous cochlea –The organ of Corti differentiates from cells along the wall of the cochlear duct.

Inner Ear 6th week neuroectoderm --> spinal and vestibular ganglia and corresponding sensory nerves Mesoderm around otocyst soon forms a cartilaginous otic capsule. Ossifies by 25 weeks

Inner Ear Vacuoles containing the perilymph develop within the otic capsule. The vacuoles enlarge and unite to form the perilymphatic space Divides into the scala tympani and the scala vestibuli. The cartilaginous otic capsule ossifies to form the bony labyrinth of the inner ear (mesoderm).

Middle Ear 1st pharyngeal pouch endoderm –Lining of middle ear (tympanic cavity) –Connection to pharynx elongates and forms eustachian tube

Middle Ear 1st and 2nd pharyngeal arch cartilage (mesoderm) --> ossicles 1st (Meckel’s): epitypanum ossicles –Head of malleus, body and short process of incus 2nd (Reichert’s): mesotympanum ossicles –Long process malleus, long process incus, stapes superstructure Stapes footplate: otic capsule

Middle Ear Ossicles full sized by 15 weeks Ossify by 25 weeks

Middle and External Ear

External Ear EAC develops from the surface ectoderm that covers the dorsal end of the first pharyngeal grove. A solid epithelial plate meatal plug develops at the bottom of the funnel- shaped pharyngeal groove.

External Ear TM –Inner layer endoderm –Middle layer mesoderm –Outer layer ectoderm

Auricle 6 Hillocks of His (mesoderm) 1st pharyngeal arch –1: Tragus –2: Helical crus –3: Helix 2nd pharyngeal arch –4: antihelix –5: antihelix –6: antitragus

Auricle

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