Ear

I. OVERVIEW- The ear is the organ of balance and hearing. It consists of an internal, a middle, and an external ear.

II. INTERNAL EAR (Figure 13-1). The internal ear develops in Week 4 from a thickening of the surface ectoderm called the otic placode.

A. Otic vesicle. The otic placode invaginates into the mesoderm adjacent to the rhombencephalon and becomes the otic vesicle. The otic vesicle divides into utricular and saccular portions.

1. Utricular portion. This portion of the otic vesicle gives rise to:

a. The utricle, which contains the sensory hair cells and otoliths of the macula utriculi. It responds to linear acceleration and the force of gravity.

b. The semicircular ducts, contain the sensory hair cells of the cristae am-pullaris. They respond to angular acceleration.

C. The vestibular ganglion of cranial nerve (CN) VIII, which lies at the base of the internal auditory meatus, d. The endolymphatic duct and sac, a membranous canal connects with the saccule and utricle and terminates in a blind dilatation beneath the dura. The endolymphatic sac absorbs endolymph.

2. Saccular portion. This portion of the otic vesicle gives rise to:

a. The saccule, which contains the sensory hair cells and otoliths of the macula sacculi. It responds to linear acceleration and the force of gravity.

b. The cochlear duct (organ of Corti). This duct has pitch (tonotopic) localization whereby high frequencies (20,000 Hz) are detected at the base and low frequencies (20 Hz) are detected at the apex.

C. The spiral ganglion of CN VIII, which lies in the modiolus of the bony labyrinth.

B. Membranous labyrinth and bony labyrinth

1. The membranous labyrinth consists of all the structures derived from the otic vesicle (Table 13-1).

2. The membranous labyrinth is initially surrounded by mesoderm, which later becomes cartilaginous and ossifies to become the bony labyrinth of the temporal bone.

3. The mesoderm closest to the membranous labyrinth degenerates, thus forming the perilymphatic space that contains perilymph.

Auricle

Pharyngeal membrane 1

Utricle

Semicircular ducts

Vestibular ganglion (CN VIII)

Cochlear duct (organ of Corti)

Spiral ganglion (CN VIII)

Corti And Viii

Auditory tube

External auditory meatus

Tympanic membrane

Figure 13-1. Schematic transverse sections show the formation of the otic placode and otic vesiclc from the surface ectoderm. (A) The otic placode invaginates into the mesoderm and becomes the otic vesicle. The mesoderm later develops into bone, that is, the bony labyrinth. (B) The vestibular and spiral ganglia of CN VIII are derived from the otic vesicle. (C) The adult ear. M = malleus; 1 = incus; St = stapes; Ut = utricle; Sac = saccule. (From Dudek RW, Fix JD: BRS Embryology, 2ed. Baltimore, Williams & Wilkins, 1998, p 106.)

Notochord

Otic Placode

Otic placode

Vestibular ganglion

1 Saccular i portion

Spiral ganglion

Spiral Ganglion

Pharyngeal arch 1 and 2

Pharyngeal groove 1

Auricle

Otic placode

1 Saccular i portion

Spiral ganglion

Vestibular ganglion

Pharynx Pharyngeal pouch 1

Pharyngeal arch 1 and 2

Pharyngeal groove 1

Pharyngeal membrane 1

Auditory tube

Notochord

Dorsal aorta

Otic vesicle

Utricle

Semicircular ducts

Vestibular ganglion (CN VIII)

Saccule

Cochlear duct (organ of Corti)

Spiral ganglion (CN VIII)

External auditory meatus

Tympanic membrane

Figure 13-1. Schematic transverse sections show the formation of the otic placode and otic vesiclc from the surface ectoderm. (A) The otic placode invaginates into the mesoderm and becomes the otic vesicle. The mesoderm later develops into bone, that is, the bony labyrinth. (B) The vestibular and spiral ganglia of CN VIII are derived from the otic vesicle. (C) The adult ear. M = malleus; 1 = incus; St = stapes; Ut = utricle; Sac = saccule. (From Dudek RW, Fix JD: BRS Embryology, 2ed. Baltimore, Williams & Wilkins, 1998, p 106.)

Table 13-1

Derivation of the Structures of the Ear

Table 13-1

Derivation of the Structures of the Ear

Embryonic Structure

Adult Derivative

Internal Ear

Otic Vesicle

Utricle, semicircular ducts, vestibular ganglion of CN VIII, endolymphatic

U:ricular portion

duct and sac

Saccular portion

Saccule, cochlear duct (organ of Corti), spiral ganglion of CN VIII

Middle Ear

Pharyngeal arch 1

Malleus, incus, tensor tympani muscle

Pharyngeal arch 2

Stapes, stapedius muscle

Pharyngeal pouch 1

Auditory tube and middle ear cavity

Pharyngeal membrane 1

Tympanic membrane

External Ear

Pharyngeal groove 1

External auditory meatus

Auricular hillocks

Auricle

CN = cranial nerve.

CN = cranial nerve.

4. The membranous labyrinth is suspended within the bony labyrinth by perilymph. Perilymph, which is similar in composition to cerebrospinal fluid, communicates with the subarachnoid space via the perilymphatic duct.

100 Hair Growth Tips

100 Hair Growth Tips

100 Hair Growth Tips EVERY Balding Person Should Know. This Report

Get My Free Ebook


Post a comment