Development of the Eye Ear and Olfactory Mucosa Flashcards
what is the origin of the optic vesicle?
it comes from an invagination of the lateral surface of the diencephalon (division of the proencephalon);
formed as the fused optic grooves
how does the optic vesicle transform into the optic cup?
once the lens placode is formed (via induction of surface ectoderm by the optic vesicle) it begins to infold and eventually it pinches off the surface ectoderm. But the infolding of the optic vesicle forms the double walled (bilayer) optic cup
what is the inductive influence that the optic vesicle has on the overlying ectoderm?
the surface ectoderm lies in close proximity to the optic vesicle aka neuroectoderm (really apparent as the optic grooves start to move towards the midline for fusion into the optic vesicle); when the optic vesicle contacts the overlying ectoderm, it induces it to form the LENS PLACODE
what are all of the fates of the layers of the optic cup?
layers form due to optic vesicle infolding (with the lens placode and then the lens placode pinches off):
(imagine a semi-moon double lined):
- Outer layer of optic stalk: pigment epithelium layer, which lies outside the sensitive portion ie: iris
- Inner layer of optic stalk:forms into retinic tunic because it has ganglion cell axons from the neural retina.. eventually turns into optic nerve
- Hyaloid artery: gets entrapped by inner and outer layer fusions, gonna be the central retinal artery/vein
- Choroid Fissure: gonna be the choroid (vasculature between retina and sclera)
what structures lie in the optic fissure?
blood vessels en route to enclosed mesenchyme
what transcription factor controls eye development?
PAX6; it is expressed in the sruface ectoderm near optic vesicles (if you inject mRNA of PAX6 into a fly leg/wing you will make an eye there!!!)
what results from failure of lens induction?
congenital aphakia
what does the prosencephalon divide into?
the telencephalon and diencephalon
what does the rhombencephalon divide into?
the metencephalon and myelencephalon
from what population of cells do the eyes develop?
anterior neural plate
what is cyclopia?
failure of development of the ventral midline portion of the prosencephalon, which allows the eye fields to form in close proximity
what does failure of optic vesicle formation result in?
anophthalmia
what is the lens placode? what is it derived from?
it is a thickening of specialized ectodermal cells that gives rise to sensory organs or parts of sensory organs that allow us to interact with our environment; derived from optic vesicle induction of surface ectoderm
what is the function of the hyaloid vessels?
course through the vitreal chamber and cover the surface of the lens
what does the inner layer of the optic stalk give rise to?
optic nerve (because it has ganglion cell axons from the neural retina)
Describe the differentiation of the retina
As the retina develops, the pigmented layer becomes thinner and the neural portion becomes thicker; the neural portion differentiates into distinct cell layers
describe the differentiation of the ciliary body epithelium
foldings of the nerual and pigmented layers of the eye forms the ciliary processes which are smooth muscles derived from neural ectoderm; the ciliary body controls lens accomodation while the ciliary epithelium produce aqueous humor
Describe the differentiation of the iris epithelium
derived from the outer rim of the optic cup; the iris constrictor and dilation muscles are derived from neural ectoderm
Describe the growth of the retinal ganglion cell axons
connect to other nuclear layers and connect to the rods and cones (in the posterior of the eye)… that is the dendritic side receiving sensory info and then it travels to the optic nerves (ganglion = optic nerves); start comprehending light before you reach the ganglion though!
what part of the retina is most prone to retinal detachment? Why?
intraretnal space; detachment occurs by opening of this space
what is coloboma and where can it occur?
due to failure of the optic cup to seal the choroid fissure; produces cleft like defect = coloboma (ie; defect in iris = coloboma iridis) .. can happen in any tissue derived from the choroid fissue.. looks like a “hole” in the eye
what is the cause of congenital aniridia?
failure of the margins of the optic cupe to continue to growing resulting in the absence of an iris ; thus it is IMPOSSIBLE to control the inlet of light; results from PAX 6 mutation
what is persistent pupillary membrane?
Normally: a pupillary membrane is formed before the iris is formed; it functions as a vascular membrane anterior to the lens. it is supposed to regress, if it doesnt it is called “pupillary persistant membrane”
what happens once the lens separates from the surface?
the posterior lens fibers elongate to fill the lens cavity and the cornea begins to differentiate. Once the corner is formed, the lens induces the formation of the anterior chamber between the lens, cornea, and pupillary membrane
Describe the formation of the lens including: the fate of remaining ectodermal tissue, the formation of the lens vesicle, the lens epithelial and primary lens fiber cells, secondary lens fiber cells and lens capsule
lens = a bubble of epithelial tissue.
Formed by the lens placode
the primary lens fiber cells are formed by the posterior fibers of the lens elongating toward the anterior wall and forming long, transparent fibers filling lumen of the lens vesicle by the end of the 7th week;
The seconardy lens fiber cells are continously forming in the central core of the lens
what is the shape of the lends (and hence its focal length) determined by?
tension exerted through the suspensor fibers, controlled by smooth muscles of the ciliary body
what is unique to the adult lens vs. fetal lens?
Adult lens is devoid of blood and nerves
what is the function of suspensory fibers (zonule fibers)
connect the lens capsul to the ciliary body
is the cornea avascular? aneural?
avascular BUT NOT ANEURAL (has innervation)
how many layers make up the cornea? what are they? what are the layers derived from?
5: ONLY anterior epithelium derived from surface ectoderm; inner layers derived from neural crest cells
1. Anterior epithelium
2. Bowman’s layer
3. Corneal stroma
4. decemet’s layer
5. corneal endothelium
Describe the anterior epithelium of the cornea
turns over every week or so; it regenerates, but the other layers of the cornea do no; so deeper lesions form scar tissue
describe the Bowmans layer of the cornea
acellular fibrous layer; slows bacterial invasion
describe the cornal stroma layer of the cornea
thickest layer; it is made of dense CT
describe the Decemet’s layer of the cornea
acellular
describe the corneal endothelium
has tight cell-cell junctions; part of BAB, hydration and ion content homeostasis
Describe the path of the aqueous humor
products by ciliary processes –>posterior chamber –> anterior chamber via pupil –> out the trachecular meshwork in the limbus region–> flow into the canal of schlemm which is continous with the venous system
what are the origins of iris and cilary muscles?
neural ectoderm
Vascularization of the lens during development?
Hyaloid vessels course through thr vitreous chamber and cover the posterior surface of the lens; the vessels in the in vitreous from a transitory system that nourishes the internal structure. It degenerates later in development and by birth is almost completely disappears; it leaves a thin fluid filled canal within the vitreous that later becomes the central retinal artery and vein in the optic stalk
what is a congenital cataract and what are 3 causes of cataracts?
congenital cataract = opacity of the lens
causes:
1. present at birth
2. infection (rubella ,syphilis)
3. Genetic (galactosemia)
what contents of the eye are produced from neural ectoderm?
retina, eithelium + muscles of the iris, epithelium of ciliary body, (deeper layers of the cornea?)
Descibe the formation of the inner ear, beginning with the auditory placode
In the hindbrain (close to the neural tube), there is an otic placode (formed from specilaized ectoderm); as the placod infolds, it makes an otocyst. the otic pit/cyst deepens and pinches off from the surface and invaginates into the mesoderm
How do otocysts differentiate into the components of the membranous labyrinth ?
differentiation yields 3 subdivisions of the inner ear: 1. endolymphatic sac and duct 2. utricular portion 3. sacular portion
outgrowth of the saccule = cochlear duct
what fluid fills the membranous labyrinth?
endolymph
Describe the derivation of the middle and external ear from the pharyngeal apparatus
Middle : occurs while the inner ear differentiates; come from neural crest cells (neural ectroderm) of pharyngeal arch 1. the tympanic membrane is formed by ectodermal and endodermal mesenchyme (cleft and pouch);
External ear: forms from 6 mesodermal mounds (hillocks) that underlie the ectoderm
3 hillocks from first pharyngeal arch, 3 hillocks from 2nd pharyngeal arch
How does the olfactory placode differentiate into olfactory epithelium?
after the neuropore closes, the ectoderm that lines the nasal cavities = olfactory placodes (located lateral aspect of frontonasal prominence whcih is on the forebrain); the olfactory placeodes line the nasal pits and the medial and lateral nasal prominence form around the nasal pits/sacs; olfactory placode develops into bipolar olfactory neurons and supporting cells, the axons extending from the neurons pierce the developing bone to form the cribiform plate and olfactory nerves
what makes up the vestibular labyrinth
3 semicircular canals; utricle; saccule
what detects changes in head position?
neuroepithelium via crista ampullaris and macula
How do you hear?
outgrowth of the saccule forms the cocclear duct; neuroepithelium line the duct with mechanoreceptors aka hair cells aka organ of corti; vibrations in the surrounding endolymph in cochlear duct or perilymph between the bony labyrinth and cochlear duct stimulate hair cells and create signal transduction in the cochlear division of VIII
where is the external ear located initially with respect to the lower jaw?
initially ear is more caudal than lower jaw, but then growth of the lower jaw pulls the external ear higher and more vertical
what arches form the bones of the ear?
arch 1 = malleus + incus
arch 2 = stappes
what causes congenital conductive hearing loss?
anomalies of the external and/or middle lear
what causes congenital sensorineural hearing loss?
anomalies of the cochlear labyrinth of the inner ear;
CAN OCCUR WITH OR WITHOUT VESTIBULAR DYSFUNCTION
what is agenesis? what does it result in? what is it caused by?
= lack of cochlear duct; results in deafness; caused by maternal rubella
what regions of neuroepithelial are involved in equilibrium and balance? hearing?
balance = ampulla of semicircular ducts, macula of utricle, macula of saccule hearing = organ of corti
where is the posterior chamber of the eye located?
between the iris and the lens
