Development of the Ear Softchalk

Question 1

Name the main anatomical components of the ear and their embryonic origin.

Answer 1

The ear has three distinct anatomical sections.
The inner ear develops from the ectodermal otic placode that appears on either side of the neural tube at the level of the future hindbrain.
The external and middle ear arise from the first and second pharyngeal arches and the first pharyngeal cleft and pouch.

Question 2

Describe the embryological development of the otic vesicle.

Answer 2

A thickening of surface ectoderm, the otic placode, appears on each side of the future hindbrain. The otic placode gradually invaginates to first form an otic pit and then a closed hollow otic vesicle.

Question 3

Discuss the embryological development of the utricular part.

Answer 3

The utricular part of the internal ear forms the semicircular canals. The semicircular canals are oriented perpendicularly to each other. There are ampulla at the end of each semicircular canal. These are derivatives of the pars superior part of the otic vesicle.

Question 4

Describe the embryological development of the tympanic membrane.

Answer 4

The primordium of the tympanic membrane is the first pharyngeal membrane.
the tympanic membrane develops from three sources:
Ectoderm
Endoderm
Mesenchyme (connective tissue) Infiltrated neural crest cells may contribute to this layer

Question 5

Discuss the process of pneumatization and its clinical significance.

Answer 5

Pneumatization is the process by which the mastoid air cells in the mastoid portion of the temporal bone form. This is clinically important because after pneumatization the mastoid cells come into contact with the tympanic cavity. That explains why middle ear infections are frequently associated with mastoid cells inflammation.

Question 6

Describe the embryological development of the external ear.

Answer 6

The external auditory meatus develops by invagination of the first pharyngeal cleft during the 6th week. However, the ectodermal ling produces a solid core of tissue called the meatal plug. Recanalization produces the medial two thirds of the definitive meatus. The meatus does not achieve its final length until the age of 9 or 10 years.
The auricle develops from six auricular hillocks (mesenchymal proliferations) that arise during the 5th week on the first and second pharyngeal arches.

Question 7

Discuss the embryological development of the saccular part.

Answer 7

Saccular part (pars inferior) is the ventral portion of the Otic vesicle. Will become the cochlear duct, spiral organ of Corti, and the saccule.

[Develops into cochlear duct by elongating and coiling out into surrounding mesenchyme. Cells of the cochlear duct also differentiate to form the spiral organ of Corti.]

Question 8

Embryological origin of the semicircular canals

Answer 8

Utricular part (pars superior) of Otic vesicle

Question 9

Embryological origin of the cochlear duct

Answer 9

Saccular part (pars inferior) of Otic vesicle

Question 10

Embryological origin of spiral organ of Corti

Answer 10

Cells of the cochlear duct (from sacular part or pars inferior) differentiate to form the spiral organ of Corti

Question 11

Embryological origin of the ampulla

Answer 11

Ampulla is small expansion at one end of each semicircular canal housing sensory cells.

(semicircular canals develop from utricular part of pars superior of Otic vesicle)

Question 12

Discuss the embryological development and the role of the perilymphatic space. Describe the formation of the bony labyrinth.

Answer 12

The otic vesicle induces its surrounding mesenchyme to differentiate into a cartilaginous otic capsule. As this capsule enlarges, vacuoles appear and form the perilymphatic space. The perilymphatic space is important b/c it allows the membrnous labyrinth of the inner ear to float in perilymph fluid that communicates with CSF of subarachnoid space.

Cartilaginous otic capsule later ossifies into the bony labyrinth of the internal ear, which is part of the TEMPORAL bone.

Question 13

The inner layer derives from what germ layer?

Answer 13

Ectoderm

Question 14

Describe the embryological development of the auditory tube and tympanic cavity.

Answer 14

the auditory tube and tympanic (middle ear) cavity both form from the 1st pharyngeal POUCH.

[The 1st pharyng pouch elongates to form the tubotympanic recess. Prox part = auditory (pharyngotympanic) tube. Distal part = tympanic cavity. Distal part of the tube gradually expands and wraps around the middle ear ossicles (malleus, incus, and stapes)]

Question 15

Embryological origin of the middle ear ossicles. How does this explain the innervation of two muscles associated with the ossicles - the tensor tympani and stapedius.

Answer 15

• Malleus and Incus arise from FIRST pharyngeal ARCH
• Stapes arises from SECOND pharyngeal ARCH

The tensor tympani (assoc w/ malleus) is innervated by CN V, since derived from 1st arch. The stapedius (assoc w/ stapes) is innervated by CN VII, since derived from 2nd arch.

Question 16

Recognize main clinical characteristics of CHARGE syndrome.

Answer 16

C = Coloboma of the eye
H = Heart defects
A = Atresia of the choanae
R = Retarded growth and development
G = Genital and urinary anomalies
**E = Ear anomalies and hearing loss. External ear anomaly = cardinal feature of syndrome**

• *Hearing loss in CHARGE can result from defects in any of 3 ear divisions:
• Inner ear defects: ex. reduced # of cochlear turns (less than 2.5) or semicircular canal defects
• abnormal development of middle ear
• external ear abnormalities