Development of the Eye, Ear, and Olfactory Mucosa

Question 1

When does development of the eye begin, and in what region?

Answer 1

Begins in the 4th week, during sagittal head folding. Anterior neural plate (prosencephalon) region

Question 2

Why does cyclopia occur?

Answer 2

Failure to dvelop the ventral midline portion of the prosencephalon, so the eyes develop in close proximity. It is due to a defect in sonic hedgehog

Question 3

What are optic grooves?

Answer 3

First noticeable in neural plate at about 3 weeks, they form when cells in eye field infold. When the two sides actually fuse, it makes the optic vesicles

Question 4

What are optic vesicles?

Answer 4

An invagination on the lateral surface of the diencephalon which after fusion (due to folding) becomes the optic vesicles.

Question 5

What is anophthalmia?

Answer 5

Failure of optic vesicle formation so no eyes develop

Question 6

What is the lens placode?

Answer 6

Surface ectoderm that is induced into a lens placode (specialized cells) outside of the optic vesicles. This is the earliest evidence of induction

Question 7

What happens if induction of lens placode fails?

Answer 7

Congenital aphakia

Question 8

What is the fate of the lens placode in folding?

Answer 8

It pinches off the surface ectoderm to form the lens vesicle

Question 9

How is the optic cup formed?

Answer 9

Infolding of optic vesicle around the lens vesicle. It remains connected to the forebrain via the optic stalk

Question 10

What forms the choroid fissure?

Answer 10

The ventral side of the optic stalk has an opening due to the fact that it infolds on itself. Inferiorly it contains the hyaloid vessels which nourish the developing lens. This fissure is an opening from the inner layer of the optic stalk infolding (I think the inner layer is the bottom layer when pictured in cross section from rostral to caudal)

Question 11

What does the inner layer of the optic cup form?

Answer 11

The neural retina. Ganglion cell axons will grow towards the diencephalon

Question 12

What does the inner layer of the optic stalk form?

Answer 12

It initially encircles the hyaloid vessels, and will thus form the optic nerve. This makes sense because the hyaloid vessels which nourish the lens are actually the precursor to the central retinal artery

Question 13

What does the outer layer of the optic stalk + optic cup form?

Answer 13

They fuse to produce radial symmetry of the eye.

Outer layer of optic cuup becomes retinal pigment epithelium

Question 14

How do the hyaloid vessels become trapped?

Answer 14

When the inner layer of the optic cup fuses to become central retinal artery and vein

Question 15

What happens if the optic cup fails to seal the choroid fissure?

Answer 15

A coloboma, or hole, in any of the tissue derived from it. Most common from inner layer fusion failure leading to iris coloboma

Question 16

How does the iris / ciliary body develop?

Answer 16

Via the outer rim of the optic cup, develop from the anterior rim of retina. Thus, they are derived from neural ectoderm. When the inner layer of the optic cup folds in with the outer layer, it forms the ciliary body / iris epithelium which is continuous with the retina

Question 17

What forms the smooth muscle of the ciliary process for lens accommodation?

Answer 17

Neural crest cells

Question 18

What happens if the optic cup margins failure to grow and fold inward? Why does this typically happen?

Answer 18

The iris becomes absent, called congenital an-iridia. No iris = impossible to control inlet of light. Result of a PAX6 mutation

Question 19

What is PAX6?

Answer 19

A conserved transcription factor among vertebrates, expressed in surface ectoderm near optic vesicle. Probably the master regulator gene for eye formation

Question 20

What induces the formation of the anterior chamber?

Answer 20

The lens vesicle sends signals to induce it, via causing regression of the pupillary membrane and forming the cornea (from surface ectoderm)

Question 21

What is the space between the iris and lens?

Answer 21

Posterior chamber

Question 22

What is the pupillary membrane? What is a possible complication

Answer 22

The structure present anterior to the lens which normally degenerates to leave the iris sides open. It should regress or you will have a “persistent pupillary membrane” in newborn

Question 23

What is the lens? What are its primary fibers?

Answer 23

A bubble of epithelial tissue from the lens placode. Cells of posterior wall elongate towards anterior, forming long, transparent fibers (primary fibers)

Question 24

What are secondary lens fibers?

Answer 24

Fibers continuously forming in central core of the lens

Question 25

What can lead to aphakia?

Answer 25

Failure of induction of surface ectoderm by optic vesicle

Question 26

What are congenital cataracts and what can cause them?

Answer 26

Infection (rubella, syphilis) or genetic (i.e. galactosemia)

Question 27

How is the cornea derived?

Answer 27

Outer epithelial layer - surface ectoderm | Inner layer - neural crest cells

Question 28

What causes the transparency of the cornea?

Answer 28

Hydration + orientation of collagen bundles which is stimulated by thyroid hormone

Question 29

What is the otocyst?

Answer 29

The otic vesicle, produced by the infolding of the otic placode (originally the otic pit) in close proximity to the hindbrain neural tube (rhombencephalon)

Question 30

What does the otocyst become?

Answer 30

Three major subdivisions of inner ear, endolymphatic sac + duct, utricular, saccular potions. (All the membranous labyrinth)

Question 31

What forms from the outgrowth of the saccule? The utricle?

Answer 31

Saccule - The cochlear duct, via ductus reuniens | Utricle - The semicircular ducts

Question 32

What forms the middle ear cavity?

Answer 32

Endoderm of first pharyngeal pouch lines the middle ear cavity and auditory tube

Question 33

What are the mesodermal mounds called and what forms them? What do they become?

Answer 33

The are called hillocks, and they underlie the surface ectoderm of the pharyngeal arches. They are made of mesoderm (neural crest cells), and there are 6 (3 from arch 1, 3 from arch 2). They become the auricle / pinna, around the 1st groove which becomes the EAM

Question 34

What can agenesis of cochlear duct cause?

Answer 34

congenital deafness. This generally is a result of a maternal rubella infection

Question 35

Where is the olfactory placode? What does it make?

Answer 35

Surface ectoderm induced by prosencephalon (frontonasal prominence). The margins of the placode are thickened by growth of mesenchyme to form medial and lateral nasal prominences, with center of placode forming nasal pit.

Question 36

What does the nasal pit become?

Answer 36

Nasal pit enlarges posteriorly to form nasal sac. Since it lies close to the brain it gives rise to bipolar olfactory neurons and supporting cells which will lie in the spheno-ethmoidal recess