Development of the Ear: Difference between revisions

Revision as of 00:40, 5 February 2012

Internal Ear

Firstly at approximately 22 days of development a thickening of the surface ectoderm on each side of the rhombencephalon can be seen. These are defined as the Otic Placode, with further development they invaginate forming otocysts. Moreover,each vesicle splits into a Ventral Component, gives rise to the saccule, cochlear duct and ducuts reuniens The Primitive Dorsal Component gives rise to the utricle, endolymphatic duct and semicircular canals.

Cochlea, saccule and organ of corti

On the 6th week of embryonic development, the saccule forms a tubular outgrowth at its lower border. This is the primitive Cochlear Duct. The surrounding mesenchyme is penetrated by this duct till the end of the 8th week of development. The ductus reuniens is the narrowing that forms of this duct connecting it to the Saccule. Later Mesenchymal condensation occurs surrounding the cochlear duct, this will differentiates into cartilage later forming the bony labirynth. Moreover, During the 10th week within this cartilaginous shell vacuolization occurs that gives rise to the perilymphatic spaces these are the scala tympani and scala vestibule. Vestibular membrane also known as Reissners membrane is known to seperate the scala vestibuli from the cochlear duct. And the Basilar membrane seperates scala tympani and cochlear duct. Moreover, The Lateral wall of the cochlear duct remains is attached to the cartilage by the spiral ligament, today some authors believe that the cells within spiral ligament have neural crest cell origins.

Utricle and Semicircular Canals

Approximately during the 6th week, impulses generate within cristae and maculae triggered by changes in body and head position, these are carried to the brain via vestibular fibers of cranial nerve VIII. Moreover, the statoacoustic ganglion derived neural crest cells has also fully developed by this embryonic stage. This Ganglion subsequently divides into the cochlear and vestibular divisions that supply sensory cells in organ of corti, saccule, utricle and semicircular canals.

Middle Ear- Tympanic Cavity and Auditory Tube

Endoderm from 1st pharyngeal pouch gives rise to the Primitive Tympanic Cavity. This pouch found laterally to the primitive pharynx later expands laterally coming into close contact with the floor the 1st pharyngeal cleft. With further development, the distal portion of this primitive cavity expands to form the tubotympanic recess. And the Proximal portion stays narrow to give rise to the auditory tube which allows the nasopharynx to communicate with the tympanic cavity.

Ossicles

As the Pharyngeal arches begin to form during the 4th week of embryonic development, it is visible to see the 1st and 2nd pharyngeal arches. The cartilage within these arches give rise to the ossicles of the ear. The Malleus, Incus and Stapes. Ossicles appear during the 1st half of fetal life however in contrast to their position they remain embeded in this ectomesenchymal tissue until the 8th month. Once complete mesenchymal condesation has occured around the ossicles, the endodermal epithelium connects them in a mesentery like fashion to the wall of the cavity. This is the supporting ligaments of the ossicles develop later within these mesenteries. Tympanic cavity grows dorsally by vacuolization of surrounding tissue giving rise to the primitive tympanic antrum. External Ear- External Auditory Meatus

Eardrum/ tympanic membrane

As the 1st Pharyngeal Cleft and its complementary pouch never come into complete contact, the tympanic membrane forms which is a Tridermal Layer. However in the case of parahyletic organisms most commonly Fish, this cleft and pouch come into contact obliterating the mesenchyme inbetween and thus giving rise to gills.

Auricle

During approximately the 6th and 8th months the auricles forms from 6 Mesenchymal condensations in the 1st and 2nd Pharyngeal Arches , these are known as the auricular hillocks. Around the external acoustic meatus, three of each of these auricular hillocks can be found on each side. With Further Development they form the definitive auricle.

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 '''Internal Ear'''
-On First indication found in embryo at approximately 22 days as thickening of the surface ectoderm on each side of the rhombencephalon. These thickenings are the otic placodes. They invaginate rapidly forming otocysts. Later each vesicle divides into. Ventral Component, gives rise to the saccule, cochlear duct and ducuts reuniens
+Firstly at approximately 22 days of development a thickening of the surface ectoderm on each side of the rhombencephalon can be seen. These are defined as the Otic Placode, with further development they invaginate forming otocysts. Moreover,each vesicle splits into a Ventral Component, gives rise to the saccule, cochlear duct and ducuts reuniens
-Dorsal Component gives rise to the utricle, semicircular canals and endolymphatic duct. Together these epithelial cells form the membranous labyrinth
+The Primitive Dorsal Component gives rise to the utricle, endolymphatic duct and semicircular canals.
 '''Cochlea, saccule and organ of corti'''
-th week of development saccule forms a tubular outpocketing at its lower pole. This outgrowth is the cochlear duct. It penetrates the surrounding mesenchyme in a spiral fashion till 8th week. Ends until it has completed 2.5 turns. Connection with saccule later narrows and remains as the ductus reuniens. Mesenchyme surrounding the cochlear duct differentiates into cartilage. 10th week the cartilaginous shell undergoes vacuolization forming the perilymphatic spaces scala tympani and scala vestibule. Vestibular membrane seperates the scala vestibuli and cochlear duct. Basilar membrane seperates scala tympani and cochlear duct. Lateral wall of the cochlear duct remains attached to the surrounding cartilage by the spiral ligament. Median angle is connected by the cartilaginous process the modiolus
+On the 6th week of embryonic development,  the saccule forms a tubular outgrowth at its lower border. This is the primitive Cochlear Duct. The surrounding mesenchyme is penetrated by this duct till the end of the 8th week of development. The ductus reuniens is the narrowing that forms of this duct connecting it to the Saccule. Later Mesenchymal condensation occurs surrounding the cochlear duct, this will differentiates into cartilage later forming the bony labirynth. Moreover, During the 10th week within this cartilaginous shell vacuolization occurs that gives rise to the perilymphatic spaces these are the scala tympani and scala vestibule. Vestibular membrane also known as Reissners membrane is known to seperate the scala vestibuli from the cochlear duct. And the Basilar membrane seperates scala tympani and cochlear duct. Moreover, The Lateral wall of the cochlear duct remains is attached to the cartilage by the spiral ligament, today some authors believe that the cells within spiral ligament have neural crest cell origins.
 '''Utricle and Semicircular Canals'''
-During the 6th week, impulses generate in cristae and maculae due to change in position of the body are carried to the brain via vestibular fibers of cranial nerve VIII. Also, statoacoustic ganglion derived from cells of the neural crest. The Ganglion subsequently splits into cochlear and vestibular portions supplying sensory cells in organ of corti, saccule, utricle and semicircular canals.
+Approximately during the 6th week, impulses generate within cristae and maculae triggered by changes in body and head position, these are carried to the brain via vestibular fibers of cranial nerve VIII. Moreover, the statoacoustic ganglion derived neural crest cells has also fully developed by this embryonic stage. This Ganglion subsequently divides into the cochlear and vestibular divisions that supply sensory cells in organ of corti, saccule, utricle and semicircular canals.
 '''Middle Ear- Tympanic Cavity and Auditory Tube'''
-Tympanic cavity originates in the endoderm from 1st pharyngeal pouch. Pouch expands in lateral direction and comes in contact with the floor of the 1st pharyngeal cleft. Distal part of the pouch forms – tubotympanic recess – widens and gives rise to the primitive tympanic cavity. Prox part remains narrow and forms auditory tube through which the tympanic cavity communicates with the nasopharynx
+Endoderm from 1st pharyngeal pouch gives rise to the Primitive Tympanic Cavity. This pouch found laterally to the primitive pharynx later expands laterally coming into close contact with the floor the 1st pharyngeal cleft. With further development, the distal portion of this primitive cavity expands to form the tubotympanic recess. And the Proximal portion stays narrow to give rise to the auditory tube which allows the nasopharynx to communicate with the tympanic cavity.
 [[File:HumanEar.jpg‎]]
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 '''Ossicles'''
-Malleus + Incus derived from cartilage of 1st pharyngeal arch. Stapes is derived from the 2nd arch. Ossicles appear during the 1st half of fetal life they remain embedded in mesenchyme until the 8th month. When ossciles are entirely free of surrounding mesenchyme, the endodermal epithelium connects them in a mesentery like fashion to the wall of the cavity. This is the supporting ligaments of the ossicles develop later within these mesenteries. Tympanic cavity exands dorsally by vacuolization of surrounding tissue to form the tympanic antrum. After birth – tympanic cavity epithelium invades bone of the developing mastoid process, and epithelium lined are sacs are formed (pneumatisation)
+As the Pharyngeal arches begin to form during the 4th week of embryonic development, it is visible to see the 1st and 2nd pharyngeal arches. The cartilage within these arches give rise to the ossicles of the ear. The Malleus, Incus and Stapes. Ossicles appear during the 1st half of fetal life however in contrast to their position they remain embeded in this ectomesenchymal tissue until the 8th month. Once complete mesenchymal condesation has occured around the ossicles, the endodermal epithelium connects them in a mesentery like fashion to the wall of the cavity. This is the supporting ligaments of the ossicles develop later within these mesenteries. Tympanic cavity grows dorsally by vacuolization of surrounding tissue giving rise to the primitive tympanic antrum.
 '''External Ear- External Auditory Meatus'''
-This Develops from dorsal portion of the 1st pharyngeal cleft ectodermal. Beginning of 3rd month epithelial cells at bottom of meatus proliferate forming meatal plug. 7th month plug dissolves and epithelial lining of meatus floor participates in formation of definitive eardrum. Also meatal plug may persist resulting in congenital deafness
 '''Eardrum/ tympanic membrane'''
-Tridermal layer
+As the 1st Pharyngeal Cleft and its complementary pouch never come into complete contact, the tympanic membrane forms which is a Tridermal Layer. However in the case of parahyletic organisms most commonly Fish, this cleft and pouch come into contact obliterating the mesenchyme inbetween and thus giving rise to gills.
 '''Auricle'''
-Auricle develops from six mesenchymal proliferations at dorsal ends of 1st and 2nd pharyngeal arches, these swellings are auricular hillocks. Three on each side of ex ac meatus. Fuse to form definitive auricle. These hillocks are part of the mandibular portion of the 1st pharyngeal arch and that the ear lies horizontally a the side of the neck. As mandible grows anteriorly and posteriorly, the ears are repositioned to the side of the head in line with the eyes
+During approximately the 6th and 8th months the auricles forms from 6 Mesenchymal condensations in the 1st and 2nd Pharyngeal Arches , these are known as the auricular hillocks. Around the external acoustic meatus, three of each  of these auricular hillocks can be found on each side. With Further Development they form the definitive auricle.