EMBRYLOGY BY

DR. THAAER MAOHAMMED DAHER ALSAADM.B.Ch.B. (MBBS) F.I.B.M.S. (PhD)

SPECIALIST IN GENERAL SURGERYSENIOR LECTURER

IMS MSU

Ear

DEVELOPMENT

Ear

• In the adult, the ear forms one anatomical unit serving both hearing and equilibrium.

• In the embryo it develops from three distinctly different parts: 1. The external ear, the sound collecting organ;2. The middle ear, a sound conductor from the

external to the internal ear;3. The internal ear, which converts sound waves into

nerve impulses and registers changes in equilibrium.

Scanning electron micrograph of a mouse embryo equivalent to approximately 28 days of human development. The otic placodes, as shown in B, are invaginating to form the otic pits (arrows). Arrowhead, second arch; H, heart; star, mandibular prominence.

Region of the rhombencephalon showing the otic placodesin a 22-day embryo.

Internal Ear

• In embryos 22 days a thickening of the surface ectoderm on each side of the rhombencephalon, is the first indication of the developing ear.

• These thickenings, the otic placodes, invaginate rapidly and form the otic or auditory vesicles (otocysts).

• Each vesicle divides into;1. Ventral component that gives rise to the saccule and

cochlear duct .2. Dorsal component that forms the utricle, semicircular

canals, and endolymphatic duct (Figs. 16.3– 16.6). Together these epithelial structures form the membranous labyrinth.

Transverse sections through the region of the rhombencephalonshowing formation of the otic vesicles. Note the statoacoustic ganglia.

24 days 27 days 4.5 weeks

C to E. Cochlear duct at 6, 7, and 8 weeks, respectively. Note formation of the ductus reuniens and the utriculosaccular duct.

A and B. Development of the otocyst showing a dorsal utricular portionwith the endolymphatic duct and a ventral saccular portion.

SACCULE, COCHLEA, AND ORGAN OF CORTI 1/3

• In the sixth week of development, the saccule forms a tubular outpocketing at its lower pole.

• This outgrowth, the cochlear duct, penetrates the surrounding mesenchyme in a spiral fashion until at the end of the eighth week it has completed 2.5 turns.

• The ductus reuniens connects the remaining portion of the saccule with utricle.

Development of the scala tympani and scala vestibuli. A. The cochlear duct is surrounded by a cartilaginous shell. B. During the 10th week large vacuoles appear in the cartilaginous shell. C. The cochlear duct (scala media) is separated from the scala tympani and the scala vestibuli by the basilar and vestibular membranes, respectively. Note the auditory nerve fibers and the spiral (cochlear) ganglion.

• Mesenchyme surrounding the cochlear duct soon differentiates into cartilage.

• In the 10th week, this cartilaginous shell undergoes vacuolization, and two perilymphatic spaces, the scala vestibuli and scala tympani.

• The vestibular membrane separates cochlear duct from the scala vestibuli.

• The basilar membrane separates cochlear duct from the scala tympani by.

• The spiral ligament attaches lateral wall of the cochlear duct to the surrounding cartilage.

• The median angle of the cochlea angle is connected to and partly supported by a long cartilaginous process, the modiolus, the future axis of the bony cochlea.

SACCULE, COCHLEA, AND ORGAN OF CORTI 2/3

• The epithelial cells of the cochlear duct form two ridges:1. The inner ridge, the future spiral limbus,2. The outer ridge which forms the sensory cells, hair cells,

of the auditory system.A. One inner row,B. Three or four outer rows. They are covered by the tectorial membrane. The sensory cells and tectorial membrane together constitute

the organ of Corti. Impulses received by this organ are transmitted to the spiral

ganglion and then to the nervous system by the auditory fibers of cranial nerve VIII.

SACCULE, COCHLEA, AND ORGAN OF CORTI 3/3

Development of the organ of Corti. A. 10 weeks. B. Approximately 5 months. C. Full-term infant. Note the appearance of the spiral tunnels in the organ of Corti.

UTRICLE AND SEMICIRCULAR CANALS 1/2

• During the sixth week of development, semicircular canals appear as flattened outpocketings of the utricular part of the otic vesicle. Central portions of the walls of these outpocketings eventually appose each other and disappear, giving rise to three semicircular canals.

• Whereas one end of each canal dilates to form the crus ampullare, the other, the crus nonampullare, does not widen.

• Five crura enter the utricle, three with an ampulla and two without. • Cells in the ampullae form a crest, the crista ampullaris, containing

sensory cells for maintenance of equilibrium. • Similar sensory areas, the maculae acusticae, develop in the walls of the

utricle and saccule.• Impulses generated in sensory cells of the cristae and maculae as a result

of a change in position of the body are carried to the brain by vestibular fibers of cranial nerve VIII.

16.6 Development of the semicircular canals. A. 5 weeks. C. 6 weeks. E. 8 weeks. B, D, and F. Apposition, fusion, and disappearance, respectively, of the central portions of the walls of the semicircular outpocketings. Note the ampullae in the semicircular canals.

16.7 A. Transverse section of a 7-week embryo in the region of the rhombencephalon, showing the tubotympanic recess, the first pharyngeal cleft, and mesenchymal condensation, foreshadowing development of the ossicles.

B. Middle ear showing the cartilaginous precursors of the auditory ossicles. Thin yellow line in mesenchyme indicates future expansion of the primitive tympanic cavity. Note the meatal plug extending from the primitive auditory meatus to the tympanic cavity.

Ear showing the external auditory meatus, the middle ear with its ossicles, and the inner ear.

• The statoacoustic ganglion forms during formation of the otic vesicle, .

• The ganglion splits into cochlear and vestibular portions,

• Supply sensory cells of the organ of Corti and those of the saccule, utricle, and semicircular canals, respectively.

UTRICLE AND SEMICIRCULAR CANALSStatoacoustic ganglion

2/2

Middle EarTYMPANIC CAVITY AND AUDITORY TUBE

• The tympanic cavity is derived from the first pharyngeal pouch.

• This pouch expands in a lateral direction and comes in contact with the floor of the first pharyngeal cleft.

• The distal part of the pouch gives rise to the tubotympanic recess.

• The proximal part gives rise to the auditory tube (Eustachian tube).

B. Middle ear showing the handle of the malleus in contact with the eardrum. The stapes will establish contact with the membrane in the oval window. The wall of the tympaniccavity is lined with endodermal epithelium.

A. Derivatives of the first three pharyngeal arches. Note the malleus andincus at the dorsal tip of the first arch and the stapes at that of the second arch.

OSSICLES 1/2

• The malleus and incus are derived from cartilage of the first pharyngeal arch,• The stapes is derived from that of the second arch.• The ossicles appear during the first half of fetal life,• they remain embedded in mesenchyme until the eighth month.• When the ossicles are entirely free of surrounding mesenchyme, the endodermal

epithelium connects them in a mesentery-like fashion to the wall of the cavity.• The supporting ligaments of the ossicles develop later within these mesenteries• Since the malleus is derived from the first pharyngeal arch, its muscle, the

tensor tympani, is innervated by the mandibular branch of the trigeminal nerve.

• The stapedius muscle, which is attached to the stapes, is innervated by the facial nerve, the nerve to the second pharyngeal arch. ts of the ossicles develop later within these mesenteries.

• During late fetal life, the tympanic cavity expands dorsally by vacuolization of surrounding tissue to form the tympanic antrum.

• After birth, epithelium of the tympanic cavity invades bone of the developing mastoid process, and epithelium-lined air sacs are formed (pneumatization).

• Later, most of the mastoid air sacs come in contact with the antrum and tympanic cavity.

• Expansion of inflammations of the middle ear into the antrum and mastoid air cells is a common complication of middle ear infections.

OSSICLES 2/2

• EXTERNAL AUDITORY MEATUS.• EARDRUM OR TYMPANIC MEMBRANE.• AURICLE.

External Ear

External EarEXTERNAL AUDITORY MEATUS

• The external auditory meatus develops from the dorsal portion of the first pharyngeal cleft.

• At the beginning of the third month, epithelial cells at the bottom of the meatus proliferate, forming a solid epithelial plate, the meatal plug.

• In the seventh month, this plug dissolves and the epithelial lining of the floor of the meatus participates in formation of the definitive eardrum.

• Occasionally the meatal plug persists until birth, resulting in congenital deafness.

B. Middle ear showing the handle of the malleus in contact with the eardrum. The stapes will establish contact with the membrane in the oval window. The wall of the tympaniccavity is lined with endodermal epithelium.

A. Derivatives of the first three pharyngeal arches. Note the malleus andincus at the dorsal tip of the first arch and the stapes at that of the second arch.

EARDRUM OR TYMPANIC MEMBRANE

• The eardrum is made up of;1. Ectodermal epithelial lining at the bottom of the auditory

meatus,2. Endodermal epithelial lining of the tympanic cavity.3. Intermediate layer of connective tissue that forms the

fibrous stratum. The major part of the eardrum is firmly attached to the

handle of the malleus, and the remaining portion forms the separation between

the external auditory meatus and the tympanic cavity

AURICLE

• The auricle develops from six mesenchymal proliferations at the dorsal ends of the first and second pharyngeal arches, surrounding the first pharyngeal cleft.

• These swellings (auricular hillocks), three on each side of the external meatus, later fuse and form the definitive auricle.

• As fusion of the auricular hillocks is complicated, developmental abnormalities of the auricle are common.

• Initially, the external ears are in the lower neck region, but with development of the mandible, they ascend to the side of the head at the level of the eyes.

B to D. Fusion and progressive development of the hillocks into the adult auricle

A. Lateral view of the head of an embryo showing the six auricular hillocks surrounding the dorsal end of the first pharyngeal cleft

G. External ear nearly complete. Growth of the mandible and neck region places the ears in their permanent position.

E. The six auricular hillocks fromthe first and second pharyngeal arches. H, heart; NP, nasal placode

F. The hillocks becoming more defined. Note the position of the ears with respect to the mouth and eyes (e).

C L I N I C A L C O R R E L A T E SDeafness and External Ear Abnormalities

• Congenital deafness, usually associated with deaf-mutism, may be caused – Abnormal development of the membranous and bony labyrinths,– Malformations of the auditory ossicles and eardrum.– In the most extreme cases the tympanic cavity and external meatus are absent. Most forms of congenital deafness are caused by genetic factors, but environmental factors may also interfere with normal development of the

internal and middle ear;1. Rubella virus, affecting the embryo in the seventh or eighth week, may cause

severe damage to the organ of Corti.2. It has also been suggested that poliomyelitis,3. Erythroblastosis fetalis,4. Ddiabetes, 5. Hypothyroidism, 6. Toxoplasmosis can cause congenital deafness.

• External ear defects are common; they include minor and severe abnormalities

They are significant from the standpoint of the psychological and emotional trauma they may cause and for the fact they are often associated with other malformations.

Thus, they serve as clues to examine infants carefully for other abnormalities.

All of the frequently occurring chromosomal syndromes and most of the less common ones have ear anomalies as one of their characteristics.

External Ear Defects 1/2

16.11 A. Microtia with preauricular pit (arrow). B. Preauricular pits (arrows).C and D. Preauricular appendages (skin tags). Note the low position of the tag in D.

• Preauricular appendages and pits are skin tags and shallow depressions, respectively, anterior to the ear.

• Pits may indicate abnormal development of the auricular hillocks,

• whereas appendages may be due to accessory hillocks.

• Like other external ear defects, both are associated with other malformations.

External Ear Defects 2/2