1 Embryology and Anatomy of the Ear

1.1.1 Embryology (Fig. 1.1)

• External auditory meatus from 1st pharyngeal cleft; at 3/12 cells at bottom of cleft form meatal plug of proliferating cells, that by 7/12 dissolves; epithelium forms outer part of tympanic membrane (TM)

• TM has ectodermal outer, endodermal inner lining, and mesodermal middle layer

• Auricle develops from six hillocks of His from 1st (mandibular) and 2nd (hyoid) pharyngeal arches, around the 1st pharyngeal groove during 5th week; rudimentary pinna by ~8/40

• Bulk of auricle derived from the mesenchyme of 2nd arch that extends around the top of the groove to become what will be the helix

• As the face develops the auricle gradually translocates from its original location low on the side of the neck to a more lateral and cranial site

1.1.2 Anatomy

• Auricle projects at ~30° angle from the side of the head; angle of inclination is 20 to 30° (generally parallel to nasal profile); normal angle between concha and scapha is ~90°

• Pinna has a skeleton of thin elastic fibrocartilage

Curved rim is the helix, which at its postero-superior aspect a small tubercle (Darwin) may be found (homologous with the tip of the mammalian ear)

Parallel to the helix is the antihelix that divides superiorly into two crura, between which is the triangular fossa; scaphoid fossa lies above the superior of the two crura

Anterior to antihelix is the concha; in its anterosuperior aspect the crus of the helix forms a divide, such that the cymba conchae lies superiorly (direct lateral relation to the suprameatal triangle of the temporal bone)

Tragus partly covers the meatus

At the inferior aspect of the antihelix is the antitragus, which has the intertragic notch between it and the tragus

Medial (cranial) aspect of auricle has elevations that correspond to depressions on lateral surface, and have corresponding names, e.g., the eminentia conchae

Caudal aspect of the pinna is cartilage free, and is known as the lobule; contains fibrofatty tissue

• Cartilage covered by thin skin to which it is closely adherent to perichondrium; sebaceous glands most numerous in the scaphoid fossa and concha; thicker hairs may develop in the intertragic notch in older men

• Cartilage continues inwards in a tubular fashion to become the cartilaginous part of the external auditory meatus; attaches to bone and so stabilizes the auricle in position; further fixation from ligaments

• Muscles: auricularis anterior and superior supplied by temporal branch of VII nerve (n); auricularis posterior by posterior auricular branch

• Blood supply of pinna is derived from branches of the external carotid; posterior auricular supplies cranial aspect, anterior auricular branches of superficial temporal artery supply most of lateral surface; small auricular branch of occipital helps supply medial aspect; arteries have corresponding veins

• Cartilage relies on its overlying perichondrium for supply of nutrients and removal of by-products

• Lymphatic drainage from the posterior part is to nodes at the mastoid tip, from the tragus and upper anterior part of the auricle is to the preauricular nodes, and from the rest of the pinna to upper cervical nodes

• Cutaneous nerve supply is variable and with overlap

Greater auricular n (from cervical plexus C2, 3) supplies cranial surface of auricle and lower part of lateral surface

Auriculotemporal (from mandibular division of V n) supplies upper part of lateral surface and most of meatal skin and tragus

Auricular branch (from vagus) supplies small areas of skin in cranial auricular surface, posterior wall of the concha and floor of meatus and adjoining part of TM

Lesser occipital n (from cervical plexus C2, 3) supplies small area of medial aspect at top of pinna

Fig. 1.1a, b Developmental stages of the external auditory canal, middle ear, and labyrinth.

a Approx. 8th week. 1, Otic vesicle; 2, primary auditory canal; 3, tubotympanic recess; 4, mesenchymal condensation; 5, acousticofacial ganglion.

b Approx. 7th month. 2, Primary auditory canal; 6, primordium of the auditory ossicles; 7, tympanic cavity; 8, primordium for the labyrinth.

1.2.1 Embryology (Fig. 1.1)

• Tympanic cavity endodermal in origin from 1st pharyngeal pouch; pouch grows laterally, so lateral part forms middle ear (ME) and medial section forms eustachian tube

• Malleus and incus from Meckel cartilage of 1st pharyngeal arch (dorsal end); stapes (suprastructure) from 2nd (Reichert cartilage); from 4/40; remain embedded in mesenchyme till 8/12, when dissolves save for supporting ligaments

• Tensor tympani (to malleus) supplied by mandibular branch V n; stapedius by VII n

• Tympanic antrum formed by birth; subsequent pneumatization of mastoid after birth

1.2.2 Anatomy

• Epitympanum (attic; above level of malleolar folds); mesotympanum; hypotympanum (below level of inferior part of tympanic sulcus)

• The posterior wall of ME has open upper part—the additus, going from epitympanum to mastoid antrum; the lower aspect of the additus corresponds to line going between pars tensa and flaccida, and on to eustachian tube

• Medial wall of additus corresponds to the lateral Scc prominence with fallopian canal housing VII n below

• Lower part of posterior wall is bony with the pyramid; fossa incudis (where ligament from short process incus attaches to buttress) above and sinus tympani below

• Fossa incudis is small depression in lower/post part epitympanic recess that houses short process incus

• Lateral to the pyramid is the facial recess: collection of air cells immediately lateral to VII n at 2nd genu; relationships are:

Medial: facial n

Superior: fossa incudis/ponticulus

Inferior: subiculum/then round window below this

Lateral: chorda tympani

Anterior (and lateral): TM

• Opening into facial recess from mastoid cavity = posterior tympanotomy; allows additional aeration route for mastoid ventilation, better visualization of horizontal part VII n, round window (i.e., for cochlear implant insertion), access to otherwise hidden cholesteatoma, lenticular process of incus going to ISJ, stapedial tendon

• Sinus tympani: medial (deep) to VII n

• Processus cochleariformis: bony process anterior to oval window to which the tensor tympani attaches via a pulley; rarely destroyed by disease

• Macewen triangle: temporal line (continuous with superior aspect of zygomatic arch); perpendicular drop from this down through mastoid tip; diagonal through spine of Henle; triangle corresponds to cribriform area of temporal bone (through which infection can spread from antrum to subperiosteum); deep relationship corresponds to mastoid antrum

1.3.1 Embryology (Fig. 1.1)

• 3/40 otic placode forms a thickening on the surface ectoderm

• Invaginates to form otic vesicles or otocyst (via otic pit) by 4/40, that divides later into saccule and cochlear duct (ventral) and utricle, Sccs and endolymphatic duct: i.e., the membranous labyrinth

• As vesicle forms, some cells break offto form statoacoustic ganglion (also gets cells from neural crest) that supplies sensory cells to cochlear and vestibular parts

• By 8/40 cochlear duct has grown and completed 2.5 turns; connection to saccule remains as ductus reuniens

• Mesenchyme around cochlear duct forms cartilage; by 10/40 vacuolizes into scala vestibuli and tympani

• Epithelial cells of cochlear duct develop into two ridges on the basilar membrane; inner ridge forms spiral limbus with attached tectorial membrane; outer forms hair cells (all together = organ of Corti) by ~25/40

• By 6/40 Sccs are flattened outpockets of utricular part of otic vesicle, at right angles to each other; central parts disappear to give canals, and one end of each dilates to ampulla

• Cells of ampulla form crest (crista ampullaris) of sensory cells; similar in saccule and utricle

1.3.2 Anatomy (Figs. 1.2, 1.3, and 1.4)

• Inner ear or labyrinth has bony and membranous components

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