Introduction

•        Outer ear transmits sound from the world to the middle ear
•        Inner ear produces electrical impulses both for sound
•        Important Concept: ear develops backwards: inner ear first, then middle, then outer ear
•        Development of the inner ear starts with a patch of ectoderm-these cells eventually become elaborated mechanoreceptors, sensing mechanical stimulation in interesting ways

Middler Ear: First Branchial Pouch

•          middle ear is an invagination of the endoderm b/t the 1^st and 2^nd branchial arch-early on, this forms the tubotympanic recess which eventually forms the eustachian tubes
•          the eustachian tube grows wider at distal end to from middle ear cavity
•          Ossicles from 1^st and 2^nd arch condense to from connection from tympanic membrane to oval window-allows vibrations to get to the inner ear

Outer Ear-formed from the 1^st branchial cleft-invagination of ectoderm from 1^st gill arch

•          At junction of 1^st and 2^nd arch, there arises little hillocks that develop the pinnicle, or auricle

CLINICAL: congenital/aural atresia-failure of the external auditory meatus to form and failure of auricle to form

INNER EAR IS FORMED FROM ECTODERM-remember development is backwards of what it would seem

Imagine: Ectoderm is outer layer with sensory endings that sense mechanical stimulation (sound waves) and in fish it’s sensitive to water vibrations. REMEMBER: It’s an INVAGINATION OF ECTODERM descending into MESENCHYME that form membranous labyrinth of the inner ear

•          B/t 1^st arch(forms maxillary & mandibular) and the 2^nd arch(forms side of neck) there’s a patch of ectoderm where a little thickening develops-this thickening is the OTIC PLACODE
•          OTIC PLACODE invaginates into the mesenchyme of head and neck to form OTIC PITS

Imagine: Sensory cells that were on the outside have turned inward w/ the invagination and thus lose connection with the outside world to form OTIC VESSICLE and this develops to form inner ear-and we aren’t fish anymore eh….

Ectodermal thickening -> otic placode -> otic pits > otic vesicle > inner ear structures

•          Otic vesicle descends into mesenchyme and forms membranous labyrinth-coils 2.75 times-coils & forms cochlea
•          Semicircular ducts-at one end, there’s a dilation, the ampullae-this is where the sensory receptors live

Ampullae + Utricle + Saccuole = Semicircular Duct

•          Receptors of Semicircular ducts-sense acceleration-are found in the AMPULLAE
•          Receptors of the Utricle and Saccule-sense gravity-are found in patches of sensory epithelia, MACULAE
•          Receptors in the Cochlear Duct-sense sound-found in ORGAN OF CORTI
•          Primordial hair cells-the aforementioned ectodermal mechnoreceptors that invaginate with the otic placode
•          Hair cells in Organ of Corti sense sound vibration in the inner ear fluid
•          hair cells bend, starting electrical impulses that go to the brain-YOU HEAR
•          Maculae (of utricle and saccule) secrete otoconia-puts weight on hair cells, bending them, allowing you sense the position of your head relative to gravity
•          Semicircular ducts are like “gelatinous veins” running on top of these hair cells-if the fluid in one of these ducts moves, the cells sense fluid’s acceleration-this is what happens when you move your head-tells where you head goes

Temporal Bone-densest bone in body-allows gravity and acceleration into our head, not sound

Impedence Matching-purpose of middle ear is to match the impedence of air and fluid such that sound can be transmitted to the inner air without much energy lost-physics….what were those equations Gary?

Tympanic Membrane-acts as a wonderful impedence matcher from different media(air to air)

•        2nd arch covers the 3rd and 4th arch
• REVIEW: Groove between the 1st and 2nd arch-these form the invagination, OTIC PIT; then three hillocks develop on the outside of the 1st and 2nd arches to from the PINNA, or AURICLE. As pinna is formed, there’s also  a condensation of mesenchyme-this becomes MIDDLE EAR OSSICLES
•        1st pharyngeal pouch elongates to form the TUBOTYMPANIC RECESS-this eventually differentiates to form the expanded tympanic cavity of middle ear and eustachian tube
•        The pharyngeal membrane separating tympanic cavity from external auditory meatus develops into tympanic membrane
•        Three layers of tympanic membrane: 1) ectodermal layer 2) mesenchymal layer that holds tympanic membrane and allow it to vibrate when sound waves hit it 3) endodermal epithelium

First Branchial Cleft becomes the EXTERNAL AUDITORY MEATUS

As the external auditory meatus forms, it fills with meatal plug-cells of this plug disappear to form patent ext. aud. meatus

CLINICAL: If the meatal plug ossifies and forms bone that’s dense like temporal-Maximum Conductive Hearing Loss

•          The tubotympanic recess grows after birth, surrounding the ossicles which themselves, have suspensory ligaments allowing them to vibrate
•          FIRST ARCH-MALLEUS & INCUS
•          SECOND ARCH-STAPES & also middle ear space & rest of tubotympanic recess that become eustachian tube

In children-angle of eustachian tube fairly flat, allowing pathogens into middle ear, leads to infection & hearing impairment

In adults-middle ear space larger & eustachian tube at more an upright angle-prevents pathogen movement

Cupola-dome-like apex of cochlea-in the semicircular ducts, there’s cartilaginous vein with the cupola-if there’s acceleration like when you move your head and the cupola bends, an electrical impulse goes to brain

Caloric Testing-squirt hot/cold water into ear (this creates convection current)-tests vestibular function

INTERESTING FACTOID-Ethanoal diffuses through the ampullae to the semicircular canals and sets up convection currents so that all semicircular canals sense “spinning” can make you feel nice and groovy, or it can make you quite sick-neither makes you compatible with driving.

Condensation of Ossicles-growth of the tumbo tympanic process

•          Meatal plug forms in the external auditory meatus which involutes to form the normal ear canal-forms MIDDLE EAR
•          Stapes is forms around the artery of the 2^nd arch-artery is supposed to disappear-the stirrups of the stapes, the crurae, form because their cartilage forms around this artery

DEVELOPMENTAL DEFECTS

•          1^st branchial clefts can form-usually in front of the ears. These can usually be fixed.

Congenital Aural Atresia-1^st branchial arch syndrome leading to microtia small ear-smallness of the pinna or auricle

Tags: ampullae, aural atresia, cochlea, conductive hearing loss, ear development, eustachian tubes, first branchial cleft, first gill arch, macula, membranous labrynth, otic pit, otic placode, otic vessicle, otoconia, saccuole, semicircular ducts, utricle

This entry was posted on Monday, January 12th, 2009 at 5:06 am and is filed under Embryology. You can follow any responses to this entry through the RSS 2.0 feed. Responses are currently closed, but you can trackback from your own site.