Embryology

Question 1

Reminder: define placode.

Answer 1

Thickened ectodermal patch on developing head

Question 2

Reminder: what is the pharyngeal apparatus?

Answer 2

Series of external ridges and furrows with corresponding internal pouches

Question 3

How do the tympanic cavity and Eustachian tube form?

Answer 3

1st pharyngeal pouch expands distally

Question 4

How does the external auditory meatus develop?

Answer 4

1st pharyngeal cleft

Question 5

How do the auricles form?

Answer 5

Proliferations within the 1st and 2nd pharyngeal arches surrounding the meatus

Question 6

What is the innervation of tensor tympani and stapedius?

Answer 6

Tensor tympani = mandibular nerve (CNV3)

Stapedius = facial nerve (CNVII)

Question 7

How does congenital inner ear deafness occur?

Answer 7

Maldevelopment of organ of Corti (mechanosensory hair cells in cochlea) due to teratogenic agents e.g. rubella (congenital rubella syndrome)

Question 8

What are the symptoms of congenital rubella syndrome?

Answer 8

Sensorineural deafness

Eye abnormalities e.g. cataracts

Congenital heart disease e.g. patent ductus arteriosus

Question 9

How does congenital middle ear deafness occur?

Answer 9

1st and 2nd pharyngeal arch problems

Question 10

How do the eyes form?

Answer 10

Optic vesicles are outpocketings of forebrain

Optic vesicle grows towards surface to make contact with lens placode

Lens placode invaginates and pinches off, so it becomes surrounded by the optic vesicle (forming the optic cup: retina, iris, and ciliary body)

Optic stalk (outgrowth of forebrain) forms (will become the optic nerve)
- has choroid fissure (should close once the hyaloid artery has supplied the lens ---> degenerates distally, proximal portion becomes the central artery of the retina) 

Intraretinal space is obliterated (pigment and sensory layers of retina fuse)

Ectoderm covering becomes the cornea

Extraocular muscles form from preotic myotomes

Question 11

What is coloboma?

Answer 11

Failure of the choroid fissure to close —> iris does not completely surround the pupil

Question 12

What are the components of the optic cup?

Answer 12

Retina = neural (inner) and pigmented (outer) layers

RIM OF OPTIC CUP:
Iris = contractile diaphragm with central aperture
Ciliary body = muscular and vascular structure connecting choroid to lens

Question 13

What is a detached retina? What is the presentation?

Answer 13

Separation of neural and pigment layers of the retina

e.g. retinal tear —> space between layers filled with blood

S&S:

• photopsia (flashing lights) induced by eye movements
• floaters
• “curtain over vision”

Question 14

Reminder: what are the cartilage bars responsible for the formation of the ossicles of the ear?

Answer 14

Meckel’s cartilage (1st pharyngeal arch)

• malleus
• incus
• mandible

Reichert’s cartilage (2nd pharyngeal arch)

• stapes
• styloid process
• hyoid bone