Talk:BGD Lecture - Face and Ear Development

From Embryology
Jump to: navigation, search

Lecture Versions


2012

Stage16-18 face.jpg


Pharyngeal arch structure cartoon.gifStage13 pharyngeal arch excerpts.gif

Textbook References

Pharyngeal arch cartilages.jpg
  • The Developing Human: Clinically Oriented Embryology (8th Edition) by Keith L. Moore and T.V.N Persaud - Moore & Persaud Chapter Chapter 10 The Pharyngeal Apparatus pp201 - 240.
  • Larsen’s Human Embryology by GC. Schoenwolf, SB. Bleyl, PR. Brauer and PH. Francis-West - Chapter 12 Development of the Head, the Neck, the Eyes, and the Ears pp349 - 418.
  • Head Development


Animation of Face Development

This animation shows a ventral view of development of the human face from approximately week 5 through to neonate.

The separate embryonic components that contribute to the face have been colour coded.

  • Frontonasal Prominence (white)
  • Frontonasal Prominence - Lateral nasal (purple)
  • Frontonasal Prominence - Medial nasal (green)
  • Pharyngeal Arch 1 - Maxillary prominence (yellow)
  • Pharyngeal Arch 1 - Mandibular prominence (orange)
  • Stomodeum (black)

The stomodeum is the primordial mouth region and a surface central depression lying between the forebrain bulge and the heart bulge. At the floor of the stomodeum indentation is the buccopharyngeal membrane (oral membrane).

Note the complex origin of the maxillary region (upper jaw) requiring the fusion of several embryonic elements, abnormalities of this process lead to cleft lip and cleft palate.

See also the movie (Quicktime | Flash) showing a similar view of human embryo faces between Carnegie stage 16 to 18.


Sensory Development

Development Timing

Comparison of size at stage 14 to 23
  • Week 3 - otic placode, otic vesicle
  • Week 5 - cochlear part of otic vesicle elongates (humans 2.5 turns)
  • Week 9 - Mesenchyme surrounding membranous labryinth (otic capsule) chondrifies
  • Week 12-16 - Capsule adjacent to membranous labryinth undegoes vacuolization to form a cavity (perilymphatic space) around membranous labrynth and fills with perilymph
  • Week 16-24 - Centres of ossification appear in remaining cartilage of otic capsule form petrous portion of temporal bone. Continues to ossify to form mastoid process of temporal bone.
  • 3rd Trimester - Vibration acoustically of maternal abdominal wall induces startle response in fetus.

Embryonic Origin Overview

Adult hearing embryonic origins

External Ear

  • Auricle - Pharyngeal Arches 1 and 2 (ectoderm, mesoderm)
  • External Auditory Meatus - Pharyngeal Arch 1 groove or cleft (ectoderm)
  • Tympanic Membrane - Pharyngeal Arch 1 membrane (ectoderm, mesoderm, endoderm)

Middle Ear

  • Middle Ear Ossicles
    • Malleus and incus - Pharyngeal Arch 1 cartilage Neural crest (ectoderm)
    • Stapes - Pharyngeal Arch 2 cartilage Neural crest (ectoderm)
  • Middle Ear Muscles
    • Tensor tympani - Pharyngeal Arch 1 (mesoderm)
    • Stapedius - Pharyngeal Arch 2 (mesoderm)
  • Middle ear cavity - Pharyngeal Arch 1 pouch (endoderm)

Inner Ear

  • Inner Ear Labyrinth
    • Cochlea - Otic vesicle - Otic placode (ectoderm)
    • Semicircular canals - Otic vesicle - Otic placode (ectoderm)
    • Saccule and utricle - Otic vesicle - Otic placode (ectoderm)
  • Cranial Nerve VIII
    • Auditory component - Otic vesicle and neural crest (ectoderm)
    • Vestibular component - Otic vesicle and neural crest (ectoderm)

Sensory Placodes

Stage 14 sensory placodes
  • week 4 a series of thickened surface ectodermal patches form in pairs rostro-caudally in the head region.
    • Recent research suggests that all sensory placodes may arise from common panplacodal primordium origin around the neural plate, and then differentiate to eventually have different developmental fates. (More? Schlosser G.)
  • sensory placodes will later contribute key components of each of our special senses (vision, hearing and smell).
  • Other species have a number of additional placodes which form other sensory structures (fish, lateral line receptor).
  • Note that their initial postion on the developing head is significantly different to their final position in the future sensory system.

Otic Placode

  • stage 13/14 embryo (shown below) the otic placode has sunk from the surface ectoderm to form a hollow epithelial ball, the otocyst, which now lies beneath the surface surrounded by mesenchyme (mesoderm).
  • The epithelia of this ball varies in thickness and has begun to distort, it will eventually form the inner ear membranous labyrinth.

Lens Placode

  • lies on the surface, adjacent to the outpocketing of the nervous system (which will for the retina) and will form the lens.

Nasal Placode

  • 2 components (medial and lateral) and will form the nose olefactory epithelium.

Inner Ear

Stage 13 otocyst
Stage 22 ear
  • The inner ear is derived from a pair of surface sensory placodes (otic placodes) in the head region.
  • These placodes fold inwards forming a depression, then pinch off entirely from the surface forming a fluid-filled sac or vesicle (otic vesicle, otocyst).
  • The vesicle sinks into the head mesenchyme some of which closely surrounds the otocyst forming the otic capsule.
  • The otocyst finally lies close to the early developing hindbrain (rhombencephalon) and the developing vestibulo-cochlear-facial ganglion complex.

Links: Neuroscience - The Inner Ear

Middle Ear

Pharyngeal arch cartilages
  • The middle ear ossicles (bones) are derived from 1st and 2nd arch mesenchyme.
  • The space in which these bones sit is derived from the 1st pharyngeal pouch.

Links: Neuroscience - The Middle Ear

Outer Ear

  • The external ear is derived from 6 surface hillocks, 3 on each of pharyngeal arch 1 and 2.
  • The external auditory meatus is derived from the 1st pharyngeal cleft.
  • The newborn external ear structure and position is an easily accessible diagnostic tool for potential abnormalities or further clinical screening.

Links: Neuroscience - The External Ear

Pinna- Auricle

External ear stages 14-23 and adult (not to scale)
outer ear and external auditory meatus
  • develops from six aural hillocks
  • 3 on first arch
  • 3 on second arch
  • originally on neck, moves cranially during mandible development
Pharyngeal Arch Hillock Auricle Component
Arch 1 1 tragus
2 helix
3 cymba concha
Arch 2 4 concha
5 antihelix
6 antitragus
  • Outer- external auditory meatus
  • derived from first pharyngeal cleft
  • ectodermal diverticulum
  • week 5 - extends inwards to pharynx
  • until week 18 has ectodermal plug - plug forms stratified squamous epithelia of canal and outer eardrum
Embryonic period Ectodermal cells proliferate and fill the entire lumen forming a meatal plug
10 weeks Meatal plug extends in a disc-like fashion. In the horizontal plane the meatus is boot-shaped with a narrow neck and the sole of the meatal plug spreading widely to form the future tympanic membrane medially. Proximal portion of the neck starts to be resorbed.
13 weeks Disc-like plug innermost surface in contact with the primordial malleus, contributes to the formation of the tympanic membrane.
16.5 week Meatus is fully patent throughout its length, lumen is still narrow and curved.
18 week Meatus is already fully expanded to its complete form.

(Nishimura, 1992)

Middle

tympanic cavity

  • derived from first pharyngeal pouch
  • extends as tubotympanic recess - during week 5 recess contacts outer ear canal
  • mesoderm between 2 canals forms tympanic membrane
  • expands to form tympanic recess
  • stalk of recess forms auditory tube(eustachian tube, pharyngotympanic tube)

Ossicles

Pharyngeal arch cartilages
  • develop from first and second pharyngeal arches
  • tympanic cavity enlarges to incorporate
  • coats with epithelia
  • first arch mesoderm
  • tensor tympani muscle
  • malleus and incus
  • second arch mesoderm
  • stapedius muscle and stapes

Middle Ear Genes - gooscoid, RARs, Prx1, Otx2, Hoxa1, Hoxb1, endothelian related molecules


Inner

Otic placodes (Stage 11 dorsal view)

Otocyst

Carnegie Stage 12 otic placode
Carnegie Stage 13 otic vesicle
  • week 3 otic placode forms on surface ectoderm
  • otic placode sinks into mesoderm
  • forms otocyst (otic vesicle)
  • branches form and generate endolymphatic duct and sac
  • forms vestibular (dorsal) and cochlear (ventral) regions
  • differentiation of otic vesicle to membranous labyrinth

Vestibular Sac

  • generates 3 expansions - form semicircular ducts
  • remainder forms utricle
  • epithelia lining generates - hair cells, ampullary cristae, utricular macula
  • Vestibular - Otoconia, otoconin- inner ear biominerals

Cochlear sac

  • generates coiled cochlear duct (humans 2 1/2 turns)
  • remainder forms saccule
  • epithelia lining generates
  • hair cells
  • structures of organ of corti
  • saccular macula
Inner ear hair cells

Bony Labyrinth

  • formed from chrondified mesoderm
  • Periotic Capsule
  • mesenchyme within capsule degenerates to form space filled with perilymph

Vestibulocochlear Nerve

  • forms beside otocyst
  • from wall of otocyst and neural crest cells
  • bipolar neurons
  • vestibular neurons
    • outer end of internal acoustic meatus
    • innervate hair cells in membranous labyrinth
    • axons project to brain stem and synapse in vestibular nucleus
  • cochlear neurons
    • cell bodies lie in modiolus
    • central pillar of cochlear
    • innervate hair cells of spiral organ
    • axons project to cochlear nucleus

Semicircular canal

Postnatal Changes

Eustacian tube angle changes

Newborn to adult Eustachian (auditory, otopharyngeal or pharyngotympanic) tube.

  • Connects middle ear cavity to nasopharynx portion of pharynx

Functions

  • Ventilation - pressure equalization in the middle ear
  • Clearance - allow fluid drainage from the middle ear Tube is normally closed and opened by muscles

At birth

  • shorter (17-18 mm), narrower and runs almost horizontal Tube is opened by a single muscle, tensor palati muscle

Adult

  • longer (twice as long), wider and runs at approximately 45 degrees to the horizontal. Tube is opened by two separate muscles, tensor palati and levator palati


Outer Ear Abnormalities

Microtia
Preauricular sinus
  • Microtia - abnormally small external ear
  • Preauricular sinus - occurs in 0.25% births, bilateral (hereditary) 25-50%, unilateral (mainly the left), duct runs inward can extend into the parotid gland, Postnatally sites for infection

Fetal Alcohol Syndrome

Fetal Alcohol Syndrome Face
  • Postion- Lower or uneven height, "railroad track” appearance, curve at top part of outer ear is under-developed, folded over parallel to curve beneath


Congenital Deafness

Sensorineural - cochlear or central auditory pathway

  • Hereditary
  • recessive- severe
  • dominant- mild
    • can be associated with abnormal pigmentation (hair and irises)
  • Acquired
    • rubella (German measles), maternal infection during 2nd month of pregnancy, vaccination of young girls
    • streptomycin
    • antibiotic
    • thalidomide

Conductive - disease of outer and middle ear

Eustacian tube angle
  • produced by otitis media with effusion, is widespread in young children.
  • temporary blockage of outer or middle ear

Bionic Ear

Cochlear Implant - Professor Graeme Clark (1960s, Australia) Array of electrodes implanted within cochlea, direct electrical stimulation to auditory nerve fibres

Conductive Hearing Loss

  • Conductive Hearing Loss Produces a Reversible Binaural Hearing Impairment David R. Moore, Jemma E. Hine, Ze Dong Jiang, Hiroaki Matsuda, Carl H. Parsons, and Andrew J. King J. Neurosci. 1999;19 8704-8711 http://www.jneurosci.org/cgi/content/abstract/19/19/8704
    • tested ferrets by lon-term plugging of ear canal
    • Repeated testing during the 22 months after unplugging revealed a gradual return to normal levels of unmasking.
    • Results show that a unilateral conductive hearing loss, in either infancy or adulthood, impairs binaural hearing both during and after the hearing loss.
    • Show scant evidence for adaptation to the plug and demonstrate a recovery from the impairment that occurs over a period of several months after restoration of normal peripheral function.

References

Textbooks

  • Before We Are Born (5th ed.) Moore and Persaud Chapter 20: p460-479
  • Essentials of Human Embryology, Larson Chapter 12: p252-272

Online Textbooks

Search

Links

Terms

altricial animal - Term used to describe an animal born in a helpless state, with incomplete development of sensory systems at birth. For example rats and mice are born with incomplete development of visual and auditory systems.

ampulla - Term used to describe an anatomical dilation of a tube or canal lumen. Anatomical description of the opening end of the uterine tube lying above the ovary and the enlarged initial segmeny of the semicircular canals of the inner ear vestibular system. (More? [ear6.htm Inner Ear] | [genitalXXuterus.htm Genital System - Female Uterus])

aneurism - (Greek, aneurysma = a widening, aneurysm) A term used to describe an abnormal widening of a vessel or anatomical tubal structure.

aquaeductus vestibuli - see vestibular aqueduct

auditory neuropathy - (AN) abnormality of transmission of sound information to the brain.

auditory tube - (eustachian tube) between the middle ear and oral cavity, has a bony (tympanic 1/3) and cartilaginous (pharyngeal 2/3) portion. The main role is equalization of pressure and fluid drainage in the middle ear.

auricular hillock - see hillock

atresia - narrowing, usually of an anatomical tube or cavity.

autophagocytosis - (Greek, auto = self, phagy = eating, also called autophagy) a cell death mechanism that uses the cell's own lysosomes to self digest.

border cells - columnar cells within the organ of Corti on the medial portion of the basilar membrane.

canalis reuniens - (ductus reuniens, canaliculus reuniens, canalis reuniens, Hensen's canal, Hensen's duct, uniting canal, canalis reuniens of Hensen) short narrow canal connecting the cochlea duct to the saccule. (Victor Hensen, 1835-1924)

cerumen - (ear wax) produced by glands in the skin of the outer portion of the ear canal.

chondrified - the developmental differentiation of cartilage from mesenchye, an embryonic connective tissue.

cristae ampullaris - located in the ampulla of the membranous semicircular canals a region with both supporting and hair cells. The hair cell cilia are embedded in the gelatinous cupula.

claudius cells - (cells of Claudius) columnar cells with microvilli overlying the basilar membrane and extend from Hensen's cells to the spiral prominence. Barrier cells that lie external to the organ of corti in endolymph.

cochlear sac - embryonic structure, which will form the coiled cochlear duct and contribute to the saccule.

cochlear aqueduct - a bony channel containing the fibrous periotic duct. It connects the basal turn of the cochlea perilymphatic space with the subarachnoid space of the posterior cranial cavity.

cochlin - major constituent of the inner ear extracellular matrix.

collagen type II - major constituent of the inner ear extracellular matrix.

conductive loss - term used to describe one of the two major classes of hearing loss involving external and middle ear abnormalities (other form is Sensorineural loss).

connexins - channel proteins of the gap junctions that allow rapid communication between adjacent cells. The two connexins Cx26 and Cx30 are the major proteins of cochlear gap junctions.

connexin 26 - A strikingly high proportion (50%) of congenital bilateral nonsyndromic sensorineural deafness cases have been linked to mutations in the GJB2 coding for the connexin26

cupular deposits - basophilic material on the cupulae of the semicircular ducts, an postnatal ageing phenomenon seen in some vestibular labyrinth.

clinical weeks - taken from last menstrual period (LMP) and therefore approximately two weeks before fertilization occurs.

Deiters' cells

discoidin domain receptor 1 - (DDR1) a tyrosine kinase receptor activated by native collagen, expressed in the basement membrane and with fibrillar collagens. Found in basal cells of the stria vascularis, type III fibrocytes, and cells lining the basilar membrane of the organ of Corti. {Meyer zum Gottesberge, 2008 #1877}

ductus utriculosaccularis -

endochondral ossification - the process of bone formation from a pre-existing cartilage template.

endoderm -

endolymphatic fluid -

endolymphatic sac - inner ear structure that has anatomically both an intraosseous and extraosseous component. Th e sac has functions regulating endolymph that are both secretory and absorptive. Also the site of endolymphatic sac tumors either sporadical occurring or associated with the autosomal-dominant von Hippel-Lindau (VHL) disease, due to a germ line mutation.

embryological weeks - taken from the time of fertilization which typically occurs around the middle (day 14), or just after, of the typical 28 day menstrual cycle.

Emx2 - homeobox gene affecting middle ear and inner ear development.

eustachian tube - (auditory tube) A cavity linking the pharynx to the middle ear, which develops from the first pharyngeal pouch. Named after Bartolomeo Eustachi (1500 - 1574) an Italian anatomist.

external auditory meatus - (ear canal) develops from the first pharyngeal cleft.

ear wax - see cerumen.

epithelia -

espins - calcium-resistant actin-bundling proteins enriched in hair cell stereocilia and sensory cell microvilli and spiral ganglion neurons (SGNs)

eustachian tube - (auditory tube) between the middle ear and oral cavity, equalization of pressure in the middle ear.

external auditory canal -

fenestra ovalis - (oval window) separates the tympanic cavity from the vestibule of the osseous labyrinth.

fenestra rotunda - (round window) separates the tympanic cavity from the scala tympani of the cochlea.

fetus - (foetus) term used to describe human development after the 8th week (10th clinical week, LPM) and covers the developmental periods of second and third trimester.

fibroblast growth factor 1 - (Fgf-1) a growth factor released from cochlea sensory epithelium which stimulates spiral ganglion neurite branching.

fibroblast growth factor 8 - (Fgf-8) a growth factor released by inner hair cells which regulates pillar cell number, position and rate of development.

fibroblast growth factor receptor 3 - (Fgfr-3) a tyrosine kinase receptor with a role in the commitment, differentiation and position of pillar cells in the organ of corti

fundamental frequency - (natural frequency) the lowest frequency in a harmonic series, for the female voice this is about 225 Hz.

helicotrema - term used to describe the cochlear apex.

Hes - (hairy and enhancer of split) family of factors, which has been shown to be a general negative regulator of neurogenesis {Zheng, 2000 #1936}.

hillock - a small hill, used to describe the six surface elevations on pharyngeal arch one and two.

Hindbrain - Invaginate -

Incus - (anvil) auditory ossicle

inner phalangeal cells

inner pillar cells - organ of Corti cells arranged in rows and form a boundary between the single row of inner hair cells and three rows of outer hair cells. These cells have surface-associated microtubule bundles.

inner sulcus - area of the cochlear duct

interdental region -

internal auditory meatus - (internal acoustic meatus, IAM) Anatomical canal in which CN VII and CN VIII ganglia reside and pass through to the brainstem. This bony canal lies between the posterior surface of the petrous pyramid and the bony labyrinth within the dense petrous bone. Also associated clinically with the site where acoustic neuromas may occur.

Kolliker's organ - (Kollicker's organ, greater epithelial ridge) Developing cochlear structure consisting of columnar-shaped supporting cells filling the inner sulcus and lying directly under the tectorial membrane. This transient organ regresses and generates the space of the inner sulcus. Rudolph Albert von Kolliker (1817-1905)??

lateral semicircular duct - Limbus -

LMP - acronym for last menstrual period, used to clinically measure gestation.

malleus - (hammer) auditory ossicle

mastoid process - of temporal bone

Math1 - homolog of the Drosophila proneural gene atonal, necessary and sufficient for the production of hair cells in the mouse inner ear. {Chen, 2002 #1932}Negatively regulated by Hes1 and Hes5

meatal plug - temporary blockage of the external auditory meatus which forms at the end of the embryonic period and remains present until the seventh month.

meatus - anatomical opening, cavity or space (external acoustic meatus,internal auditory meatus)

Meckel's cartilage - first pharyngeal ach cartilage, located within the mandibular prominence. This cartilage first appears at stage 16, stage 20 the beginning of membranous ossification. Named after Johann Friedrich Meckel, (1781 - 1833) a German anatomist. (http://www.whonamedit.com/doctor.cfm/1840.html)

membranous labyrinth - Mesenchyme - Mesoderm - Microtia - Modiolus -

mucopolysaccharidosis - (MPS IIIB, Sanfilippo Syndrome type B) abnormality caused by a deficiency in the lysosomal enzyme N-acetyl-glucosaminidase (Naglu). Children with MPS IIIB develop abnormal hearing, and mental functioning culminating in early death.

netrin-1 - secreted growth factor, expressed in the organ of Corti and spiral ganglion cells, role in process outgrowth.

neural tube -

olivocochlear - brainstem cholinergic and GABAergic efferent system that innervates sensory cells and sensory neurons of the inner ear.

organ of Corti - organ of Corti protein II - (OCP-II) cytosolic protein or transcription factor?

otolithic membrane - extracellular matrix that cover the sensory epithelia of the inner ear.

ossicle - (small bone) the individual bone of the three middle ear bones (auditory ossicles), which reduce vibrational amplitude but increase force to drive fluid-filled inner ear.

ossify -

otic capsule -

otic cup

otic placode -

otic vesicle -

otoconin - inner ear biominerals required for vestibular apparatus function.

otogelin - (Otog) an inner ear specific glycoprotein expressed in cochlea cells at different developmental times.

otolithic membrane - a membrane within the utricle and saccule containing embedded hair cell cilia and small crystalline bodies of calcium carbonate (otoliths). Functions to detect head motion.

otoliths - small crystalline bodies of calcium carbonate found within the otolitic membrane of the utricle and saccule.

ototoxic - compound or drug causing temporary or permanent hearing loss.

outer hair cells - (OHCs) three rows of hair cells that function to increase basilar membrane motion through a local mechanical feedback process within the cochlea, the "cochlear amplifier".

outer pillar cells - arranged in rows and form a boundary between the single row of inner hair cells and three rows of outer hair cells.

paratubal musculature - muscles lying beside the auditory (Eustachian) tube. The tensor veli, palatini (TVP) and tensor tympani muscles.

perilymph - perilymphatic space - Periotic Capsule - petrous portion - of temporal bone

pejvakin gene - in humans, two missense mutations in this gene cause nonsyndromic recessive deafness (DFNB59) by affecting the function of auditory neurons.

pharyngeal archpharyngeal pouchpharyngeal membranePharynx

pillar cells - (PC) form an inner and outer row of support cells that form a boundary between inner and outer hair cells.

Placode

preyer reflex - ear flick in mouse in response to sound.

presbyacusis

prestin - a motor protein structurally similar to the anion transporter family expressed in cochlear outer hair cells.

preauricular tag - skin tags located in front of the external ear opening, are common in neonates and in most cases are normal, though in some cases are indicative of other associated abnormalities.

primordium-

protocadherin 15 - (Pcdh15) required for initial formation of stereocilia bundles and changes in the actin meshwork within hair cells. The Ames waltzer (av) mouse mutant has both auditory and vestibular abnormalities from a mutation in this gene.

Reichert's cartilage - pharyngeal ach 2 cartilage, named after Karl Bogislaus Reichert (1811 - 1883) a German anatomist.

Reissner's membrane - (vestibular membrane, vestibular wall) is a membrane located inside the cochlea separating the scala media from scala vestibuli. Named after Ernst Reissner (1824-1878) a German anatomist. “It primarily functions as a diffusion barrier, allowing nutrients to travel from the perilymph to the endolymph of the membranous labyrinth.

rhombomere -

Saccular macula -

Saccule - (Latin, sacculus = a small pouch)

sacculocollic reflex -

scala tympani - one of the three Cochlea cavities, it is filled with perilymph.

Scarpa's ganglion - (vestibular ganglion) primary afferent vestibular neuron ganglion of the vestibular nerve. Located within the internal auditory meatus.

semicircular canals - series of fluid-filled loops of the inner ear required for balance and sensing acceleration.

sensorineural - term used to describe one of the two major classes of hearing loss involving the central pathway from the cochlear (other form is conductive loss).

space of Nuel - within the cochlea, an organ of Corti space between the outer pillar cells and the phalangeal and hair cells. Named after Jean-Pierre Nuel (1847-1920) a Belgian ophthalmologist.

spiral ganglion neurons - (SGN) innervate the inner (Type I) and outer (Type II) hair cells of the cochlea.

stapedius muscle - (innervated by CN VII tympanic branch) one of the two muscles in the middle ear, contraction of this muscle pulls the stapes and dampens auditory ossicle movement.

stapes - (stirrup) a middle ear auditory ossicle (bone).stapes footplate - startle response -

stereocilia -finger-like projections from the apical surface of sensory hair cells forming the hair bundle in the cochlea. Formed by tightly cross-linked parallel actin filaments in a paracrystalline array with cell surface specializations (tip links, horizontal top connectors, and tectorial membrane attachment crowns).

stratified squamous epithelia - classification of epithelium which transiently forms a plug in external ear canal to the outer eardrum.

stria vascularis - forms the outer wall of the cochlear duct of the mammalian cochlea is composed primarily of three types of cells. Marginal cells line the lumen of the cochlear duct and are of epithelial origin. Basal cells also form a continuous layer and they may be mesodermal or derived from the neural crest. Intermediate cells are melanocyte-like cells, presumably derived from the neural crest, and are scattered between the marginal and basal cell layers. The stria forms endolymph and also contains a rich supply of blood vessels.

sulcus -

synostotically - anatomically normally separate skeletal bones fused together.

tectorial membrane - extracellular matrix that cover the sensory epithelial hair cells of the organ of corti within the cochlea.

alpha-tectorin and beta- (TECTA, TECTB) major non-collagenous protein component of the tectorial membrane forming a striated-sheet matrix. Synthesized as glycosylphosphatidylinositol-linked, membrane bound precursors.

temporal bone -

tensor tympani - (innervated by CN V mandibular nerve) one of the two muscles in the middle ear, contraction of this muscle pulls the malleus and tenses the tympanic membrane, dampening auditory ossicle movement. The muscle arises from auditory tube (cartilaginous portion) and is inserted into the malleus (manubrium near the root).

teratogens - trilaminar embryo -

tonotopy - term describing the mapping along the tectorial membrane within the cochlea of the different sound frequencies.

tympanic cavity - tympanic membrane -Utricle -Vacuolization - Vesicle - vestibular apparatus - vestibular evoked myogenic potential (VEMP) test

vestibular ganglion - (Scarpa's ganglion) primary afferent vestibular neuron ganglion of the vestibular nerve. Located within the internal auditory meatus.

vestibular membrane - (Reissner's) extends from the spiral lamina to the outer wall and divides the cochlea into an upper scala vestibuli, a lower scala tympani.

Vestibulocochlear Nerve - Cranial Nerve VIII

Whirlin - A PDZ scaffold protein expressed in hair cells at the stereocilia tips, essential for the stereocilia elongation process. The DFNB31 gene mutations cause hearing loss in human and mouse. This protein can interact with membrane-associated guanylate kinase (MAGUK) protein, erythrocyte protein p55 (p55).

Wnt7a - signaling through the Wnt pathway regulates the development of hair cell unidirectional stereociliary bundle orientation.