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As printed in Hearing Health, volume 19:2,
Summer 2003
By Dwayne D. Simmons, Ph.D.
The development of the ear, even when compared to
the multitude of other bodily systems, organs and intricacies,
ranks among the most remarkable examples of physiological
engineering in vertebrates.
In a human embryo, the ear begins to form soon after
three weeks of gestation and is complete before the
end of the third trimester. During those approximately
eight months, the three distinct anatomical areas of
the ear – inner, middle and outer – emerge,
mature and start to function as the auditory system.
Inner Ear
During the first and second trimesters of pregnancy,
rudimentary gene networks play fundamental roles in
the initial patterning of the inner ear. Signals are
released from the developing hindbrain, the area that
will become the cerebellum and medulla oblongata, and
stimulate the exterior lining of the embryo to form
a thickened concentration of embryonic cells on each
of side. Continuing into week four, these areas rapidly
turn inward and form hollow vesicles called otocysts.
Over the next seven to 14 days, different parts of the
otocysts give rise to the cochlear duct, semicircular
canals and the other structures collectively known as
the membranous labyrinth, the precursor of the inner
ear.
Next, the cochlear duct begins to coil until by the
end of the eighth week of gestation, it has completed
two and one half turns and resembles the signature shell
shape we call the cochlea. Further stimulation induces
a series of differentiations among the tissues surrounding
the cochlear duct to form the protective bony labyrinth
and the membranes that separate different areas of the
inner ear.
At this stage, the ear is ready to be wired for sound.
Cells of the cochlear duct separate into outer and inner
ridges. Then the outer ridge further differentiates
into the very important sensory hair cells that will
soon transmit sound to the brain through neural impulses.
A gelatinous substance, the tectorial membrane, eventually
covers the hair cells, thereby completing the ear’s
sensory structure called the organ of Corti. All that
remains is connecting the inner ear to the brain.
The finishing touch comes when cells of the eighth
cranial nerve migrate and collect along the coils of
the cochlea. From here, nerve fibers extend out to the
organ of Corti where they terminate on the hair cells.
At about the same time that this innervation occurs,
fibers originating from the brainstem project to the
organ of Corti, allowing the brain to receive sensory
input.
All of this structural and neural activity happens
by the middle of the fetal period at 20 to 22 weeks
of gestation – the inner ear has reached its full
adult size and shape while the fetus is still only eight
to 10 inches long and weighs under one pound!
Middle and Outer Ear
A wholly different process applies to how the middle
ear forms. First, embryonic neck structures called pharyngeal
arches, which are bars of embryonic cellular tissue
separated by deep grooves or clefts, must already be
in place, a developmental feat that is accomplished
by week five. At that time, the middle ear or tympanic
cavity derives from the first pharyngeal pouch, a balloon-like
expansion between arches, and opens as a tubular indentation.
Tissue located at the tip probably plays a role in the
early development of the middle ear cavity by inducing
a process called programmed cell death. The stalk of
the indentation remains narrow and forms the Eustachian
tube through which the tympanic cavity communicates
with the nasopharynx, the back of the nose.
Meanwhile, the three tiny bones of the middle ear form
from the cartilage of the first and second pharyngeal
arches. The tympanic cavity enlarges to incorporate
the bones, although they remain embedded in tissue until
the eighth month of pregnancy.
The external ear also originates from the pharyngeal
arches. During the sixth week, six tissue swellings
termed auricular hillocks become apparent on the embryonic
neck area below the lower jaw. As the jaw develops,
the external ear moves relatively higher with a more
vertical orientation. By week nine, each of the auricular
hillocks has formed a distinctive portion of the external
ear.
The ear canal develops earlier, around week five, from
the back of the first pharyngeal groove. Toward the
end of the first trimester, cells at the bottom of the
canal proliferate, forming a solid plate or plug. In
the seventh gestational month, this plug dissolves and
the lining that is left behind participates in forming
the tympanic membrane or eardrum.
Genetic and Environmental Variables
About one in every 1,000 babies arrives in the world
with a significant hearing loss and even more have some
degree of hearing impairment. Recessive inherited genetic
material or influences cause most types of congenital
deafness. A malfunctioning gene is often the root of
abnormal development of the membranous and bony labyrinths
and the malformation of the middle ear bones and eardrum.
In extreme cases, the middle ear cavity and ear canal
are absent.
In addition, several environmental factors can interfere
with normal inner and middle ear development: An infection
of the rubella virus, or measles, during the seventh
or eighth week may cause severe damage to the organ
of Corti and result in sensorineural deafness. It has
also been suggested that polio, diabetes, hypothyroidism
and certain disorders of the blood or central nervous
system can cause congenital deafness.
Outer ear defects are common and may serve as indicators
of a specific pattern of congenital anomalies. The external
ears are often abnormal in shape and low-set in infants
with chromosomal syndromes, such as Down’s syndrome,
and in infants affected by certain drugs, i.e., trimethadione,
ingested by their mothers.
Auricular appendages, or extra folds of skin around
the ear, are also common. Small external ears, a condition
known as microtia, results from suppressed development
of their embryonic tissue beginnings. This anomaly is
often an indicator of and associated with others, such
as atresia, the obstruction or absence of the ear canal,
and middle ear abnormalities. All of the frequently
occurring chromosomal syndromes and most of the less
common ones have ear anomalies as one of their characteristics.
Learning about the intricate processes that take place
as the ear forms during gestation introduces us to the
remarkable level of sophistication of the human auditory
system. Perhaps this awareness can also serve to increase
our appreciation of the complex and central role that
hearing plays in many facets of human development and
encourage us to provide its benefits to as many individuals
as science and technology allow.
Dr. Dwayne D. Simmons earned a Ph.D. in medical sciences
from Harvard and has conducted extensive research on
the development of neural connections within the brain
and inner ear. He holds faculty positions at Central
Institute for the Deaf and Washington Univ. School of
Medicine in St. Louis, Mo.
Related article:
Prenatal Auditory
Stimulation: Truth, Fiction or Moot Point?
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