Embryology and anatomy of the human ear. Pptx

Anatomy made easy

1. Embryology of human ear & Anatomy of external ear

2. Development of the human ear Auricle – First branchial cleft is the precursor of external auditory canal. At around 6 th week of IUL, six tubercles appear around 1 st branchial cleft. They fuse to form the auricle. Tragus – develops from the tubercle of the first arch while the rest of the pinna develops from the remaining 5 tubercles of the second arch Faulty fusion between the first and second arch tubercles causes preauricular sinus or cyst.

3. External auditory meatus – it develops from the first branchial cleft by 16 th embryonic week. Cells proliferate from the bottom of ectodermal cleft and form a meatal plug. Recanalization of this plug forms the epithelial lining of the bony meatus , it begins from the deeper part near the Tympanic membrane and progresses outwards

4. Tympanic membrane – it develops from all the three germinal layers Outer epithelial layer is formed by ectoderm, Inner mucosal layer by endoderm and middle fibrous layer by mesoderm. Middle ear cleft – First and partly second pharyngeal pouch form the tubo tympanic recess the endoderm of tubo tympanic recess forms the eustachian tube, tympanic cavity, attic, antrum and mastoid air cells. Malleus and incus are derived from mesoderm of the first arch .

5. Stapes develops from the second arch except its footplate and annular ligament which are derived from the otic capsule.

6. Membranous inner ear – development of the inner ear starts at 3 rd week and completes by 16 th week of IUL. The ectoderm region of hindbrain thickens to form an AUDITORY PLACODE, which is invaginated to form auditory vesicle or OTOCYST. Otocyst - differentiates into endolymphatic duct and sac ; the utricle, the semicircular ducts ; and saccule and cochlea.

7. Pars superior (semicircular canals and utricle) takes place earlier than pars inferior (saccule and cochlea).

8. Organ of corti - it starts developing as a single block of heaped up ectodermal cells at about 11 weeks within this mass develop inner , outer hair cells & specialized supporting cells. The tunnel of corti appears between inner and outer pillar cells as clusters of stereocilia and single kinocilium are developing on each hair cell. The cochlear kinocilium regresses The dark cell region- epithelium close to sensory region develop into specialised cell groups which maintain ionic and electrical stability of the endolymph .

9. The bony labyrinth – mesenchyme enclosing the otocyst becomes chondritified to form the otic capsule. The membranous labyrinth expands, the otic capsule remodels and in places undergoes dedifferentiation to form fluid filled spaces called perilymphatic spaces. The perilymphatic space become continuous and a communication with csf is formed by development of cochlear aqueduct, which runs to the post. Cranial fossa. Most important channel that remains within otic capsule is oval window, where part of otic capsule becomes stapes footplate and annular ligament.

10. The temporal bone – is formed by 1) petromastoid complex 2)squamous portion 3)tympanic bone 4) styloid process. Petromastoid complex- derived from petrous bone, it forms the roof of middle ear, layeral wall of eustachian tube, floor of middle ear, canal of facial nerve & petrous apex. The squamous bone – it develops from mesenchyme, it forms the lateral wall of foetal mastoid antrum.

11. The tympanic bone develops from mesenchyme, but from several centres around the external Meatus. These form the major part of a ring with a groove on the inner aspect that becomes the sulcus for TM. The styloid develops from 2 centres at the cranial end of the second arch. ( reicherts ) cartilage. The petrous bone undergoes generalized downward rotation, this makes the neonatal Horizontal TM to an angle of 55 ° in adults.

12. Anatomy of external ear The external ear consists of 1)auricle or pinna, 2)external acoustic canal & 3)tympanic membrane. Auricle /Pinna – The entire pinna except its lobule and outer part of external auditory canal are made up of a single piece of yellow elastic cartilage covered with skin.

13. There is no cartilage between the tragus and crus of helix, and this area is called incisura terminalis . An incision in this area will not cut through cartilage used in endaural approach in surgery of EAC or mastoid. External auditory canal – it extends from the bottom of concha to the tympanic membrane and measures about 24mm along its posterior wall. It is not a straight tube, its outer part is directed upwards, backwards and medially while its inner part is directed downwards, forwards and medially The canal is divided into two parts 1)cartilaginous & 2)bony.

14. Cartilaginous part – it forms outer 1/3 of the canal 8mm. Cartilage is continuation of the cartilage which forms the framework of the pinna. It has 2 deficiencies – 1) fissures of Santorini through them the parotid or superficial mastoid infection appear in the canal or vice versa. The skin covering the cartilaginous canal is thick and contains ceruminous and pilosebaceous glands which secrete wax, hair is only confined to outer canal and hence furuncles are only seen in outer 1/3 of the canal

15. Bony part – it forms inner 2/3 (16mm).Skin lining the bony canal is thin and continuous over the tympanic membrane. It is devoid of hair and ceruminous glands. About 6 mm lateral to tympanic membrane, the bony meatus presents a narrowing called isthmus, foreign bodies, lodged medial to the isthmus, get impacted. Anterior recess – it is present in anteroinferior part of the deep meatus, beyond the isthmus, it acts like a cesspool for discharge and debris in cases of external and middle ear infections.

16. Foramen of huschke – it is deficiency present in the anteroinferior part of the bony canal. Tympanic membrane – it forms the partition between external acoustic canal and middle ear. It is obliquely set and as a result, its posterosuperior part is more lateral than its anteroinferior part. Tympanic membrane can be divided into 2 parts – 1)pars tensa 2)pars flaccida

17. Pars Tensa – forms most of the tympanic membrane, its periphery is thickened to form a fibro cartilaginous ring called annulus tympanicus , which fits the tympanic sulcus. The central part of pars tensa is tented inwards at the level of tip of malleus and is called umbo A bright cone of light can be seen radiating in the anteroinferior quadrant.

18. Pars flaccida ( Shrapnell’s Membrane) – This is situated above the lateral process of malleus between notch of Rivinus and the anterior and posterior malleal folds (earlier called malleolar folds) Layers of tympanic membrane – 1) Outer epithelial layer 2)Inner mucosal layer 3)Middle fibrous layer – encloses handle of malleus and has 3 fibres – the radial, circular and parabolic. Outer epithelial layer is continuous with the skin lining of meatus, inner mucosal layer is continous with mucosa of middle ear

19. Relations of the external acoustic meatus Superiorly – Middle cranial fossa Posteriorly – Mastoid air cells and the facial nerve Inferiorly – Parotid gland Anteriorly – Temporomandibular joint.

20. Nerve supply of the external ear. Pinna – 1)Greater auricular nerve (C2, 3) supplies most of the medial surface of pinna and only posterior part of lateral surface. 2)Lesser occipital (C2) supplies upper part of the medial surface. 3) Auriculotemporal (V3) supplies the tragus, crus of helix and the adjacent part of the helix. 4) Auricular branch of vagus (CN X), also called Arnold’s nerve, supplies the concha and corresponding eminence of medial surface. 5) Facial nerve – supplies the concha and retroauricular groove.

21. External auditory canal – 1)Anterior wall and roof is supplied by Auriculotemporal (V3) 2) Posterior wall and floor : Auricular branch of vagus (CN X) 3) Posterior wall of the auditory canal also receives sensory fibres of CN VII through auricular branch of vagus . Tympanic membrane – 1)Anterior half of lateral surface : Auriculotemporal (V3) 2) Posterior half of the lateral surface :Auricular branch of vagus (CN X) 3) Medial surface :tympanic branch of CN IX (Jacobson’s nerve)