EMBRYOLOGY OF THE EAR
Embryology -
Introduction :
•
The word embryo has been come from a Greek word meaning “to
swell”.
• Study of embryo is known as Embryology.
• A developing baby during its 1st
8 week of gestation is known as
embryo.
Relevance to audiology :
•
An understanding of embryologic development & its relationship helps
to the audiologist/physician for early identification, intervention & for
prevention.
• The timetable of prenatal development & the association of the various
structures with each other helps to the audiologist/physician for any
kind of suspicion of deafness, its subsequent diagnosis & management.
• Development :
•
Major changes in the development of the ear takes place in the
mother’s womb, first as an embryo and later as a fetus.
• However development of auditory structure doesn’t stop nor it is totally
completed at the time of birth.
• Knowledge of the origins of auditory structures can be diagnostically
significant to the clinician.
Eg: An infant presents with a congenital skin disorder, as the skin and
otocyst both originate from the structure called ectoderm.
Hence
anomalies of the cochlear structures could have link contiguously with
the skin disorder.
• The timing of development of the various organs system guides the
clinician about the occurrence of HL.
• Eg: A noxious influence occurs at 2 months of gestation may result in
malformation of pinna that is developing at that time.
• However, the pinna malformation doesn’t necessarily imply
malformation of the middle ear ossicles, although they are also
developing at the same time but the origins of the structures are
different.
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• Prognosis of auditory function can be estimated from the origin & the
expected pathology.
• All growth is the the result of cell division of the pre-existing cell called
zygote.
• A zygote is a cell which is an ovum fertilized by sperm.
(Ovum + Sperm cell = zygote)
• This zygote/pre-existence cell get divided into 2 main cells through the
process called mitosis .
• Thus mitosis results in the changes in the nucleus of a cell which
produces a specific no. of double structures i.e the cell & nucleus then
subdivided into 2 identical daughter cells. (Mitosis)
• Organization in the embryo stimulate development of associated areas
& create specific differentiation of cells during the developmental
process.
• One of the earliest organizational developments of embryo is the
formation of 3 super-imposed cellular plates known as ‘germ layers’.
• However each of these layer is also a key structure for development of
the various portions & part of the ear & the auditory system.
• The ear begins its development during the early life of embryo.
• Ear begins its development at about 25 hours of the embryonic
development, when embryonic disc split & leads to the formation of a
structure called primitive groove.
• Primitive groove is underlined by ectoderm layer.
• This primitive groove then further develops into a structure called
‘primitive pit’ which ultimately results to form the ‘neural groove’ &
‘neural fold
• This neural fold along with the neural grooves come together to form a
structure called ‘neural tube’.
• This neural tube remarks the beginning of the ear development.
Development of the inner ear :
• The earliest demarcation of the ear in the human embryo occurs as
early as in the 3
rd
week of gestation.
• -At this time there is thickening of the superficial ectoderm on the
either side of the ‘open neural plate’.
• Each thickening are known as auditory/otic placodes which are the
obvious by middle of 3
rd
week of gestation.
• Around 23
rd
day of gestation, the auditory placodes get mixed with the
surface ectoderm & forms auditory/otic pits.
• Around 30th day of gestation the mouth of each auditory pit closes off &
forms a structure called auditory vesicle and otocyst which remarks the
development of the inner ear
.
• The auditory vesicle takes elongated shape which is divided into 2 parts
i.e.
1. Utricular-saccule area
2. Tubular extension / Endolymphatic duct
• By 4½ weeks of gestation, the portion of auditory vesicle get connected
to the endolymphatic duct which is the future vestibular apparatus of
membranous labyrinth, whereas there is more slender portion of the
auditory vesicle begins to form the future cochlea which is originated
from the saccular area.
• At the end of 6
th
week of gestation, 3 arch like pockets are visible which
are destined to form future of 3 semicircular canal.
• At the same time, utricle and saccule are developed as a part of
vestibular portion of the auditory system.
• By the end of the 7
th
week of gestation, the elongated portion of the
saccule of the auditory vesicle completes one turn / one coil of the
future cochlea.
• -The 2½ turns / 2¾ turns of cochlea get completed from 8-11
th
week of
gestation.
• At this time, the cochlear duct gets attached to vestibular portion by
means of narrow tube called ‘ductus reuniens’.
• During 8-11 weeks of gestation, the cochlear division of the 8
th
nerve
follows the coiling of the cochlear duct i.e. it supplies its fiber to the
cochlear duct.
• During the 7
th
week of gestation the bony labyrinth continue to
developed into sensory end organs with the appearance of localized
thickening of epithelium layers in the utricle & saccule.
• Similar localized epithelial thickening in the ampullated ends of the
semicircular canals during the 8
th
week of gestation.
• These epithelium thickening shows the differentiation into 2 types of
cell .i.e.
1. Sensory cells with bristle like hairs (OHC & IHC)
2. Supporting cells (dieter cells)
• However complete maturation of these cells in cochlea doesn’t occur
until 5
th
month of gestation when entire cochlear duct results
considerable growth & expansion
.
• The membranous labyrinth of the inner ear reaches its full adult
configuration by the early part of the 3
rd
month of gestation.
• At this time the otic capsule / bony labyrinth gets started / begins to
ossify.
• Thus the inner ear is the only sense organ in the entire human body to
reach full adult size & differentiation by fetal midterm means 5 months
of pregnancy
• However it should be noted that, the cochlear portion of the inner ear is
the last inner ear end organ to differentiate and mature.
• Thus the cochlea may be subject to more possible developmental
deviations, malformations, acquired disease than the vestibular end
organs.
• Development of middle ear :
• During the period time when the inner ear developing, at the same
time the transmission portion of the auditory mechanism is developed
as the middle ear.
• Middle ear develops from endodermal structure
.
• Middle ear cavity begins its development during the 3
rd
week of
gestation, when the auditory pit is sinking into neural plate to become
the auditory vesicle.
• The tympanic cavity & auditory tube originate from an elongation of the
lateral superior edge of the 1
st
pharyngeal pouch known as ‘tubotympanic recess’.
• By that time, the human embryo is in its 4
th
week of gestation when a
series of 5 branchial grooves known as “gill slits” have appeared.
• These grooves are in the lower head & neck region on the outside of
the embryo.
Pharyngeal pouches :
• At the same time, on the inside of embryo a corresponding series of 5
phryngeal pouches develops & these structures collectively identified as
‘arches’.
• In fish, these grooves are ultimately meets the corresponding pouches
to form gills as a part of their respiratory mechanism whereas in
humans one of the gill pouches does actually become perforated
forming a passage way which becomes the external ear canal & the
eustachian tube.
• The tympanic membrane forms a barrier between these 2 portions of
the passage-way.
• .Occasionally , an additional opening will occur forming a structure
called ‘cervical fistula’ or ‘branchial cyst’ which an opening on the
throat between pharynx & the surface of the neck.
• During the 2
nd
month, the tubo-tympanic recess approaches the
embryo, surface between the 1
st
& 2
nd
branchial arches known as
Meckel’s / Mandibular and Reichert’s /Hyoid cartilages, respectively
• By the 8
th
week of gestation the tympanic cavity is developed in the
lower half of the future middle ear, while the upper half is filled with
‘cellular mesenchyme’.
• According to the classical theory of ossicles origin, malleus & incus
develops from Meckel’s cartilage while stapes developed from
Reichert’s cartilage.
• According to Pearson et al (1970) suggests the most of the part of
malleus & incus are originated from the 1
st
branchial arch while the
lenticular process of the incus, the handle of malleus & the stapes
originates from 2
nd
branchial arch.
• Thus the middle ear cavity has a dual origin with the anterior area
developing from the 1
st
arch while the posterior area from the 2
nd
arch.
• The incus & malleus have attained a complete cartilaginous form like an
adult by 8½ weeks, whereas stapes attained the same by 15
th
week of
gestation.
• The ossification of malleus & incus begins by end of the 15
th
week of
gestation & reach its completion by 32
nd
week of gestation.
• However, the ossification of stapes begins by 18
th
week & it continue to
develop further during the life
.
• As the ossicles begin to ossify, the surrounding mesenchymal tissue
becomes loose, less cellular, & connects each ossicle to the walls of the
middle ear cavity.
• While the 30
th
week of gestation the development of the tympanum
proper is almost complete
.
• The middle ear cavity with antrum is pneumatized during the last 34
th
to
35
th
week of gestation.
• While the epitympanum is pneumatized during the last fetal month 36
th
to 38
th
week of gestation.
Development of the outer ear :
• 2 main parts of outer ear i.e.
1. Auricle / pinna
2. External Auditory Meatus (EAM)
1. Auricle :
• Auricle develops from the 1
st
& 2
nd
branchial arches during the 3
rd
or
4
th
week of gestation.
• However, the auricle actually derived primarily from 2
nd
branchial arch
while only tragus seems to originate from the 1
st
branchial arch.
• This is occurring at the same time, when auditory vesicle is formed in
the development of inner ear.
• During the 6
th
week of gestation 6 hillocks or tissue thickenings form on
both sides of the 1
st
branchial groove.
• They are arranged as 3 hillocks on each facing border.
• The ultimate shape & configuration of the adult auricle depends on the
development of these 6
th
growth centers / hillocks
.
• During the 6
th
week of gestation, the ‘mesenchymal folds’ of the auricle
are beginning to become cartilage.
• From the 7
th
to 20
th
week of gestation, the auricle moves from its
original ventro-medial position to be slowly displaced laterally, so as get
match to growth of mandible & face.
• At the 20
th
week of gestation, the auricle attains an adult shape but
continues to growth until the individual becomes 9 years of age.
2. External auditory meatus (EAM) :
• The EAM is derived from the 1
st
branchial groove during 4
th
to 5
th
week
of gestation.
• At this time the ectodermal lining of the 1
st
branchial groove is in brief
contact with the endodermal lining of the 1
st
pharyngeal pouch.
• Mesodermal tissue grows between this 2 layers & separate the
pharyngeal pouch from that of branchial groove.
Pharyngeal pouches
• In the 8
th
week of gestation, the primary auditory meatus sinks towards
the middle ear cavity & forms outer 1/3
rd
of auditory canal which is
surrounded ultimately by cartilage.
• The ectodermal groove continues to deepen toward the tympanic cavity
from the external surface until it meets a thickening of epithelial cells
known as the meatal plug which is developed from surface ectoderm.
• Mesenchyme grows between the meatal plug and the epithelial cells of
the tympanic cavity.
• Thus 3 layers of tissues are formed which are combined to form the
tympanic membrane. These 3 layer includes ;
1. The inner circular fiber layer
2. Fibrous middle layer of tissue
3. Outer radial fiber layer
• These entire development occurred before 9
th
week of gestation.
• The solid meatal plug keeps the external auditory canal closed until 21
st
week of gestation.
• By this time the inner & middle ear structures are well formed &
ossified.
• The meatal plug disintegrates & forms a canal, with the inner most layer
of epithelium which becomes the squamous epithelial layer of the
tympanic membrane.
• The EAM, continues to grow until the 9 year of age.
• At birth the floor of the external auditory canal has no bony portion
which develops after the birth till the age of 9 years.
• Hence the EAC is short & straight in infants while in adult it is longer &
curved
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