Development of the Ear & Eye

A set of practice questions (Question and Answer) covering the development of the ear and eye, including anatomy, timelines, embryologic origins, and common congenital abnormalities.

What are the three distinctly different parts the ear develops from, and what is the function of each part?

External ear (sound-collecting), middle ear (sound conduction from external to inner ear), inner ear (converts sound waves into nerve impulses and registers changes in equilibrium).

From what structure do otic placodes invaginate, and what do they form?

Otic placodes invaginate to form otic vesicles. The vesicles divide into ventral components (saccule and cochlear duct) and dorsal components (utricle, semicircular canals, and endolymphatic ducts), forming the membranous labyrinth.

What do the ventral and dorsal components of the otic vesicle become?

Ventral component forms the saccule and cochlear duct; dorsal component forms the utricle, semicircular canals, and endolymphatic ducts.

How is the cochlear duct separated from the scala vestibuli and scala tympani?

Separated from scala vestibuli by the vestibular membrane and from scala tympani by the basilar membrane.

What is the spiral ligament and what is its role in ear development?

The spiral ligament is the lateral wall of the cochlear duct attached to surrounding cartilage; it helps support the cochlear duct and is associated with the modiolus, the future axis of the bony cochlea.

What are the key components of the organ of Corti as described in development?

Neuroepithelial cells including inner hair cells, outer hair cells; tectorial membrane; spiral tunnels; spiral limbus (inner ridge); auditory nerve fibers.

When do the utricle and semicircular canals appear, and what happens to their central walls?

Appear in the 6th week; central portions of the walls of their outpocketings appose each other and disappear, giving rise to three semicircular canals.

From which pharyngeal arches do the auditory ossicles derive, and which ossicle comes from which arch?

Malleus and incus derive from the first pharyngeal arch; stapes derives from the second pharyngeal arch.

What is the tubotympanic recess and what does it give rise to?

The tubotympanic recess originates from the endodermal tympanic cavity region; the proximal part forms the auditory tube, and the distal part widens to form the primitive tympanic cavity.

What forms the external auditory meatus and what happens to the meatal plug?

External auditory meatus is formed from the first pharyngeal cleft; the meatal plug dissolves by the seventh month; persistent plugs can cause congenital deafness.

How many auricular hillocks form the auricle and which arches do they come from, and what is a common developmental issue?

Six auricular hillocks form from the first and second pharyngeal arches; fusion abnormalities of these hillocks are common and can lead to auricular malformations.

What are preauricular appendages and pits and what might they indicate?

Skin tags (appendages) and shallow depressions (pits) anterior to the ear; pits may indicate abnormal development of the auricular hillocks and can be associated with other malformations.

When does the optic vesicle form and how does it interact with surface ectoderm to influence lens formation?

The optic vesicle forms around 22 days as outpocketings of the forebrain; it comes into contact with surface ectoderm and induces lens formation (lens placode).

What is the lens placode and what happens to it by about the 5th week?

The lens placode forms from surface ectoderm; it invaginates to form the lens vesicle and loses contact with the surface ectoderm by about the 5th week, lying at the mouth of the optic cup.

What is the optic cup and what is the role of the choroid fissure?

The optic cup is a double-walled structure that forms as the optic vesicle invaginates; the choroid fissure on the ventral surface allows the hyaloid artery to reach the inner eye, and its lips fuse by about the 7th week.

What happens to the lens vesicle during development?

The lens vesicle detaches from the surface ectoderm; by about the 6th week it has detached, with the lens lying at the mouth of the optic cup.

What is the hyaloid artery and how does it relate to the eye’s arterial system?

The hyaloid artery traverses the choroid fissure within the optic stalk; as development progresses, it contributes to the formation of the central retinal artery.

Which structures originate from surface ectoderm in the eye, and what are they called?

Lens (from lens placode) and corneal epithelium, as well as eyelid structures, originate from surface ectoderm.

What happens to the lens by 6–6.5 weeks in development?

The lens vesicle detaches from the surface ectoderm and the lens becomes fully detached around 6.5 weeks.

What is the fate of the choroid fissure and how does it relate to the optic stalk and optic nerve development?

The choroid fissure closes by around the 7th week, converting the optic stalk into the optic nerve as nerve fibers increase.

What are coloboma and coloboma iridis, and what causes them?

Coloboma results from failure of the choroid fissure to close, producing a gap that can involve the iris (coloboma iridis). Persistence of the iridopupillary membrane is another related defect.

What are congenital cataracts and their common causes?

Congenital cataracts can be genetic or due to rubella infection during the 4th–7th weeks of pregnancy; they may also relate to persistence of the hyaloid artery.

What conditions reflect failure of midline development in the eye, and what are their basic descriptions?

Microphthalmia (eye too small), anophthalmia (absence of eye), and cyclopia/synophthalmia (single or fused eyes) due to loss or disturbance of midline tissue during early gestation (around days 19–21 or later during facial development).