1. EMBRYLOGY BY DR. THAAER MAOHAMMED DAHER ALSAADM.B.Ch.B. (MBBS) F.I.B.M.S. (PhD)SPECIALIST IN GENERAL SURGERYSENIOR LECTURERIMS MSU

2. Ear DEVELOPMENT

3. 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: Theexternal ear, the sound collecting organ; Themiddle ear, a sound conductor from the external to the internal ear; Theinternal ear, which converts sound waves into nerve impulses and registers changes in equilibrium.

4. 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 placodes in a 22-day embryo.

5. 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 oticor auditory vesicles (otocysts). Each vesicle divides into; Ventral component that gives rise to thesaccule and cochlear duct . Dorsal component that forms the utricle, semicircular canals, and endolymphatic duct (Figs. 16.3– 16.6). Together these epithelial structures form the membranous labyrinth.

6. 24 days 4.5 weeks 27 days Transverse sections through the region of the rhombencephalon showing formation of the otic vesicles. Note the statoacoustic ganglia.

7. 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 portion with the endolymphatic duct and a ventral saccular portion.

8. SACCULE, COCHLEA, AND ORGAN OF CORTI 1/3 In the sixth week of development, the saccule forms a tubular outpocketingat 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.

9. 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.

10. 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

12. The sensory cells and tectorial membrane together constitute the organ of Corti.

13. 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

14. 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.

15. 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. 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.

17. 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. 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.

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

19. UTRICLE AND SEMICIRCULAR CANALS Statoacoustic ganglion 2/2 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.

20. 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).

21. A. Derivatives of the first three pharyngeal arches. Note the malleus and incus at the dorsal tip of the first arch and the stapes at that of the second arch. 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 tympanic cavity is lined with endodermal epithelium.

22. 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.

23. 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

24. EXTERNAL AUDITORY MEATUS. EARDRUM OR TYMPANIC MEMBRANE. AURICLE. External Ear

25. 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.

26. A. Derivatives of the first three pharyngeal arches. Note the malleus and incus at the dorsal tip of the first arch and the stapes at that of the second arch. 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 tympanic cavity is lined with endodermal epithelium.

29. A. Lateral view of the head of an embryo showing the six auricular hillocks surrounding the dorsal end of the first pharyngeal cleft B to D. Fusion and progressive development of the hillocks into the adult auricle

30. E. The six auricular hillocks from the 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). G. External ear nearly complete. Growth of the mandible and neck region places the ears in their permanent position.

32. but environmental factors may also interfere with normal development of the internal and middle ear;Rubella virus, affecting the embryo in the seventh or eighth week, may cause severe damage to the organ of Corti. It has also been suggested that poliomyelitis, Erythroblastosis fetalis, Ddiabetes, Hypothyroidism, Toxoplasmosiscan cause congenital deafness.

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

35. 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

36. 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.

37. 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