Embryology

Question 1

What age does myelination of the optic nerve start and when is it complete?

Answer 1

Myelination starts at 7 months gestation and is usually complete by 3 months of age.

It starts at the lamina cribrosa and progresses posteriorly.

Question 2

Closure of the chorioidal fissure - when and how?

Answer 2

Closes between weeks 5-7 of gestation
Starts in the middle and soreads anteriorly and posteriorly
Defects in closure —> optic disc coloboma (usually in inferotemporal quadrant)

Question 3

Most common location of:

• iris coloboma
• OD coloboma / pits

Answer 3

Iris: inferonasal
Disc: Inferotemporal

Question 4

Characteristics of central disc coloboma

Answer 4

Morning glory disc

• Central mass of glial tissue
• Spoke-like radiation of retinal vessels
• Peripapillary pigmentation and atrophy

Question 5

What are the causes of visual loss with OD pits?

Answer 5

Serous retinal detachment due to transmission of fluid from the vitreous through the pit
Visual loss due to disruption of nerve fibres

Question 6

What do the dorsal and ventral primitive ophthalmic arteries become?

Answer 6

Dorsal: ophthalmic artery
Ventral: nasal long posterior ciliary artery

Question 7

Each of the pharyngeal arches give rise to a nerve - name them.

Answer 7

Arch 1: Maxillary and mandibular branches of CN5
Arch 2: CN7
Arch 3: CN9
Arch 4: CN10
Arch 6: Recurrent laryngeal

(Arch 5 completely regresses in humans)