Embryology and Anatomy of the Ear

2 Otology

2.0 Embryology and Anatomy of the Ear

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Embryology

Auricle

Week 6: Hillocks of His form from condensations of the first and second branchial arches. The first three hillocks are attributed to the first arch, and the second three hillocks are attributed to the second arch (Table 2.1).

Week 20: Adult configuration is achieved.

External Auditory Canal

Week 8: Ectoderm of the first pharyngeal groove (cleft) invaginates. Epithe- lial cells then grow as a solid core toward the middle ear. The cord dissolves at 20 to 21 weeks, and is complete by week 28. Medial epithelium forms the lateral surface of the tympanic cavity; lateral epithelium forms the skin of the bony external auditory canal (EAC). The EAC might not reach final adult size and shape until the early teenage years.

Middle Ear

Week 3: Endoderm from the first pharyngeal pouch forms the tubotympanic recess. This will become the middle ear and eustachian tube. Pneumatiza- tion begins at 10 weeks. The tympanic membrane (TM) forms after the canalization of the EAC. The outer layer is first groove endoderm, the fibrous layer is first arch mesoderm, and the innermost mucosal layer is first pouch endoderm.

Week 4: Mesenchyme from the first and second branchial arches fuses and begins to form the malleus and incus. The first arch is referred to as Meckel’s cartilage, and the second arch is referred to as Reichert’s cartilage.

The cartilaginous ossicles form adult size and shape by week 16. They then Table 2.1 Embryology of the Auricle

Hillock Arch Auricular Structure

1 1 Tragus

2 1 Crus of helix

3 1 Helix

4 2 Crus of antihelix

5 2 Antihelix

6 2 Antitragus and lobule

Note. There remains some controversy regarding the final contributions of hillocks 4, 5, and 6.

Table 2.2

Cartilage Arch Ossicular Structure

Meckel 1 Head and neck of malleus, body and short process of incus Reichert 2 Manubrium of malleus, long process of incus, stapes

suprastructure

Note. The manubrium of malleus never completely ossifies.

Fig. 2.1 Embryology of the inner ear. The otocyst forms from an epithelial thickening between the cutaneous ectoderm and neural groove in the third and fourth weeks of embryonic development. (A) This thickening invaginates and closes off to form a separate vesicle. (B) In the fifth week, the otocyst becomes infolded, forming the upper pars utriculovestibularis and the lower pars sacculocochlearis. (C) In the sixth week, the three semicircular canals form from the pars utriculovestibularis.

(D) In the seventh to ninth weeks, the cochlear duct forms as a tubular extension of the pars sacculocochlearis and becomes coiled. (From Probst R, Grevers G, Iro H. Basic Otorhinolaryngology: A Step-by-Step Learning Guide. Stuttgart/New York:

Thieme; 2006:158.) A

ossify by endochondral bone formation (Table 2.2). Reichert’s cartilage also goes on to form the stapes blastema between weeks 4 and 5, which gives rise to the stapes suprastructure. The footplate is of otic capsule origin.

Inner Ear

Week 3: The otic placode forms from ectoderm of the first pharyngeal groove.

It invaginates and is completely encircled in mesoderm and termed the otocyst by week 4. The pars superior (semi canals and utricle) develops prior to the pars inferior (saccule and cochlea). The membranous labyrinth is complete by week 15 or 16. Ossification occurs between weeks 20 and 25 (Fig. 2.1).

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Anatomy

Auricle

The auricle, with the external auditory canal known as the external ear, is made of elastic cartilage with perichondrium and skin tightly adherent anteriorly and loosely adherent posteriorly (Fig. 2.2). It is attached to the head by the extension of cartilage into the ear canal and an anterior, superior, and posterior ligament, and an anterior, superior, and posterior muscle.

External Auditory Canal

The EAC is one-third cartilaginous laterally and two-thirds osseus medially.

The cartilage is fibrocartilage, not elastic cartilage. The skin over the lateral

Triangular fossa Crura of antihelix

Cymba concha External auditory canal Tragus Intertragic incisure Antitragus Scaphoid

fossa

Helix

Antihelix

Concha Earlobe

Fig. 2.2 Anatomy of the auricle. (From THIEME Atlas of Anatomy, Head and Neuroanatomy, © Thieme 2007, Illustration by Karl Wesker.)

one-third is thick with hair follicles, sebaceous and ceruminous glands.

The medial skin is thin without any adnexal structures or subcutaneous tissue. It is the only site in the body with skin directly onto periosteum, and is extremely pain sensitive. The canal is innervated by contributions by the trigeminal nerve (cranial nerve [CN] V, auriculotemporal branch), the cervical plexus (C3, great auricular nerve), the vagus nerve (CN X, Arnold’s nerve) and the facial nerve (CN VII). C2 and C3 also form the lesser occipital nerve, which helps innervate the anterior and posterior inferior surface of the auricle, but not the canal.

Arnold’s reflex is a cough during manipulation of the ear canal via Arnold’s nerve, and the Hitselberger sign is numbness over the postportion of the external meatus secondary to a cerebellopontine angle tumor affecting the function of the facial nerve.

Middle Ear

The tympanic membrane is made up of three layers. From lateral to medial:

squamous epithelium, a radiating and circular fibrous layer, an inner mucosal layer. It is ⬃10 mm in height and 9 mm in width. Circumferentially, the fibrous annulus sits within the bony annulus, but is discontinuous superiorly at the notch of Rivinus. Inferiorly to the notch of Rivinus, the TM has a well-organized fibrous middle layer, and is known as the pars tensa.

At the level of the notch of Rivinus and above, the middle fibrous layer is less well organized, and is known as the pars flaccida. Prussak’s space is at the level of the pars flaccida, bounded laterally by the TM and medially by the neck of the malleus. It is a frequent site of retraction and cholesteatoma formation.

The boundaries of the middle ear space are as follows:

● Lateral: tympanic membrane

● Superior: tegmen

● Inferior: jugular dome

● Anterior: carotid canal, eustachian tube

● Posterior: mastoid via facial recess or retro facial cells

● Medial: cochlear promontory and labyrinthine wall Ossicles (Fig. 2.3)

The malleus consists of a manubrium, anterior and lateral processes, neck and head. It is incorporated into the TM from the lateral process to the tip of the manubrium (umbo). The tensor tendon runs from the cochleariform process of the middle ear to the medial surface of the neck and manubrium of the malleus.

The incus is the largest ossicle, and articulates with the malleus head in the epitympanum. It has a body and short, long, and lenticular processes.

The short process is held in place by the posterior incudal ligament.

The stapes is the smallest ossicle, and articulates with the incus at the lenticular process. It has a capitulum, anterior and posterior crus, and a footplate.

Fig. 2.3 Middle ear ossicles. Malleus, posterior (A) and anterior (B) views. Incus, medial (C) and anterolateral (D) views. (E) Stapes, superior and medial views.

(F) Medial view of the ossicular chain. (From THIEME Atlas of Anatomy, Head and Neuroanatomy, © Thieme 2007, Illustration by Karl Wesker.)

Articular surface for incus

Handle Neck

Lateral process

Neck Head

Lateral process

Anterior process Body

Short process

Body

Short process Long process

Lenticular process Articular surface for

malleus A,B

C,D

Neck of malleus

Base

Incudomalleolar joint

Incudostapedial joint

Head of malleus

Handle Anterior process

Anterior crus Posterior

crus Body of incus Short process

F Neck

Ba Basese Head

Anterior A crus c Posterior

crus

The stapedius tendon runs from the pyramidal eminence to the posterior surface of the capitulum, or the posterior crus. Both incudomalleolar and incudostapedial joints are synovial diarthrodial joints. The footplate is kept in position by the annular ligament.

Eustachian Tube

It measures ⬃18 mm at birth, and 35 mm in adulthood. It is one-third bony laterally at the anterior mesotympanum, and is lined with ciliated cuboidal epithelium. Two-thirds of the eustachian tube is cartilaginous as it travels medially to end at the torus tubarius of the nasopharynx, and this portion is lined with pseudostratified columnar epithelium. It functions to ventilate, clear secretions from, and protect the middle ear. At rest it is closed, and its primary dilator is the tensor veli palatini muscle (CN V).

Inner Ear Bony Labyrinth

The vestibule is the central chamber. Its medial wall has two depressions.

Posterosuperiorly is the elliptical recess for the macula of the utricle, and anteroinferiorly is the spherical recess for the macula of the saccule. The vestibular aqueduct contains the endolymphatic duct, and enters postero- inferiorly.

The cochlea lies anteriorly. It is a 32-mm canal that rotates 2½ turns around a bony modiolus. The cochlear aqueduct contains the periotic duct.

The semicircular canals lie posteriorly. They lie at a 90-degree angle to one another, and connect to the vestibule with an ampullated and a nonampullated end. They are the horizontal (or lateral), the superior (or anterior), and the posterior canal. All of the ampullated ends enter the vestibule separately, but the posterior and superior canals fuse at their nonampullated ends to join the vestibule together at the crus commune.

Superior semicircular canal dehiscence syndrome is a well-recognized entity consisting of symptoms of disequilibrium with motion, exacer- bated by loud sounds, and computed tomography (CT) evidence of a bony dehiscence.

Membranous Labyrinth

The membranous labyrinth is the fluid-filled chamber within the bony labyrinth; it is surrounded by perilymph, similar to extracellular fluid (high in sodium, low in potassium), and filled with endolymph, similar to intracellular fluid (high in potassium, low in sodium).

The vestibule is filled with perilymph and holds the saccule and the utricle.

Both contain gelatinous membranes coated with otoconia. They sense linear acceleration and gravity.

The cochlear duct mimics the bony cochlea in size and shape (Fig. 2.4).

It is also known as the scala media, and is filled with endolymph and contains the organ of Corti. Superiorly is the scala vestibuli, and inferiorly is the scala tympani, both filled with perilymph and joined at the helicotrema of the cochlear apex.

Semicircular Ducts

The semicircular ducts are within the semicircular canals. Each has an ampullated end with a crista that sends hair cells into a gelatinous cupula to sense angular acceleration. The horizontal canal lies 30 degrees from the true horizontal.

The endolymphatic duct lies medial to the crus commune and opens into the vestibule. It leads to the endolymphatic sac that lies within the dura of the posterior fossa adjacent to the temporal bone.

The cochlear aqueduct runs from the basal turn of the cochlea to the subarachnoid space.

Internal Auditory Canal

The internal auditory canal is the bony channel from the posterior fossa that houses the facial nerve, the cochlear nerve, and the vestibular nerves. It starts medially at the porous acusticus, and runs laterally to end at the fundus.

At the fundus, it is split horizontally by the falciform or transverse crest. The superior portion of the canal above the falciform crest is then split again into an anterior and posterior compartment by the vertical crest or Bill’s bar.

2.1 Otologic Emergencies

Dalam dokumen Handbook of Otolaryngology (Halaman 125-132)

2 Otology

2.0 Embryology and Anatomy of the Ear

Embryology

Anatomy

Further Reading

2.1 Otologic Emergencies