Development of the Eye, Ear, and Olfactory Mucosa Flashcards
When does development of the eye begin, and in what region?
Begins in the 4th week, during sagittal head folding. Anterior neural plate (prosencephalon) region
Why does cyclopia occur?
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
What are optic grooves?
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
What are optic vesicles?
An invagination on the lateral surface of the diencephalon which after fusion (due to folding) becomes the optic vesicles.
What is anophthalmia?
Failure of optic vesicle formation so no eyes develop
What is the lens placode?
Surface ectoderm that is induced into a lens placode (specialized cells) outside of the optic vesicles. This is the earliest evidence of induction
What happens if induction of lens placode fails?
Congenital aphakia
What is the fate of the lens placode in folding?
It pinches off the surface ectoderm to form the lens vesicle
How is the optic cup formed?
Infolding of optic vesicle around the lens vesicle. It remains connected to the forebrain via the optic stalk
What forms the choroid fissure?
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)
What does the inner layer of the optic cup form?
The neural retina. Ganglion cell axons will grow towards the diencephalon
What does the inner layer of the optic stalk form?
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
What does the outer layer of the optic stalk + optic cup form?
They fuse to produce radial symmetry of the eye.
Outer layer of optic cuup becomes retinal pigment epithelium
How do the hyaloid vessels become trapped?
When the inner layer of the optic cup fuses to become central retinal artery and vein
What happens if the optic cup fails to seal the choroid fissure?
A coloboma, or hole, in any of the tissue derived from it. Most common from inner layer fusion failure leading to iris coloboma
How does the iris / ciliary body develop?
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
What forms the smooth muscle of the ciliary process for lens accommodation?
Neural crest cells
What happens if the optic cup margins failure to grow and fold inward? Why does this typically happen?
The iris becomes absent, called congenital an-iridia. No iris = impossible to control inlet of light. Result of a PAX6 mutation
What is PAX6?
A conserved transcription factor among vertebrates, expressed in surface ectoderm near optic vesicle. Probably the master regulator gene for eye formation
What induces the formation of the anterior chamber?
The lens vesicle sends signals to induce it, via causing regression of the pupillary membrane and forming the cornea (from surface ectoderm)
What is the space between the iris and lens?
Posterior chamber
What is the pupillary membrane? What is a possible complication
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
What is the lens? What are its primary fibers?
A bubble of epithelial tissue from the lens placode. Cells of posterior wall elongate towards anterior, forming long, transparent fibers (primary fibers)
What are secondary lens fibers?
Fibers continuously forming in central core of the lens
What can lead to aphakia?
Failure of induction of surface ectoderm by optic vesicle
What are congenital cataracts and what can cause them?
Infection (rubella, syphilis) or genetic (i.e. galactosemia)
How is the cornea derived?
Outer epithelial layer - surface ectoderm
Inner layer - neural crest cells
What causes the transparency of the cornea?
Hydration + orientation of collagen bundles which is stimulated by thyroid hormone
What is the otocyst?
The otic vesicle, produced by the infolding of the otic placode (originally the otic pit) in close proximity to the hindbrain neural tube (rhombencephalon)
What does the otocyst become?
Three major subdivisions of inner ear, endolymphatic sac + duct, utricular, saccular potions. (All the membranous labyrinth)
What forms from the outgrowth of the saccule? The utricle?
Saccule - The cochlear duct, via ductus reuniens
Utricle - The semicircular ducts
What forms the middle ear cavity?
Endoderm of first pharyngeal pouch lines the middle ear cavity and auditory tube
What are the mesodermal mounds called and what forms them? What do they become?
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
What can agenesis of cochlear duct cause?
congenital deafness. This generally is a result of a maternal rubella infection
Where is the olfactory placode? What does it make?
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.
What does the nasal pit become?
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
