Eye Development Flashcards
What are the major placoderms and what do they form?
- hypophyseal placode: primordium for Ratheke’s pouch -> adenohypophysis
- olfactory placode: primordium for olfactory epithelium
- lens placode: primordium for lens
- trigeminal placode: primordium for CN V
- otic placode: primordium for inner ear
- epibrachial placode:primordium for sensory neurons supplying visceral structures (CN VII, IX, X)
What is the preplacodal region?
- encircles cranial plate
- induced by cranial mesoderm and neural tube
- involves activation of FGF pathway and inhibition of Wnt and BMP
What is the optic stalk?
-lateral evaginations of the diencephalon which enlarges distally to form the optic vesicle
What does the optic vesicle do?
- optic vesicle asymmetrically invaginate a to form a cup-like structure with a groove along one side of it
- the groove is the choroid fissure and it is continuous with the groove in the optic stalk
- the groove in the optic stalk is continuous with the posterior chamber of the eye
- the hyaloid artery uses the choroid fissure and optic stalk to pass into the posterior chamber of the eye
What does the initial expression of Pax6 produce?
- lens
- nasal placodes
- diencephalon
What does Pax6 do in Drosophilia?
- turns of gene sequence needed for eye development
- absence of gene leads to eyeless mutant
What does Pax6 do in mammals?
- absences of gene results in early optic vesicle formation, but the rest of eye formation does not occur
- prospective lens ectoderm does not respond to induction by optic vesicle
How does Pax6 affect the optic stalk?
- high conc of Shh inhibit expression of Pax6
- low conc of Shh in distal optic stalk permits expression of Pax6 and development of optic vesicle -> leads to formation of retina
- high conc of Shh in proximal optic stalk decreases expression of Pax6 and induces expression of Pax2 -> this will provide guidance of axons of ganglion cells from retina
Explain how the lens is form.
- formed by lens. Fibers derived from inner pole of lens vesicle
- lens fiber cells are postmitotic, elongated cells made up mostly of crystalline proteins alpha, beta, and gamma
- rest of lens fiber cells are derived from mitotic activity of cuboidal cells of anterior lens epithelium
- retinal secretions, esp FGF, accumulate in the vitreous humor and stimulate formation of lens fibers
Explain how the cornea is formed.
- Pax6 is necessary for corneal induction
- underlying lens vesicle induces overlying ectoderm to transform from bilayered epithelium (simple cuboidal basal layer +flattened periderm) to transport multilayered structure
The invaginate optic cup forms both the _________ and ________ ____________.
-pigmented and neural retina
What is the pigmented retina?
-outer thin layer of the optic cup after invagination occurs
What is necessary for the differentiation of the pigmented retina?
-Otx2 stimulated by Shh
In terms of the fixation of retinal polarity, what is formed first?
-nasotemoral (anterior-posterior) axis is fixed first
How is the nasotemoral axis produced?
- established by gradients composed of ephrins and receptors
- ganglion cells differentiate first and bipolar neurons and cones differentiate last
- the spread of the gradient is from the center to the periphery
- Notch gene expression keeps cells from differentiating too early
