eye development Flashcards
At what stage does eye development begin?
17-19 days after fertilisation in early embryogenesis
how is eye development induced?
by interactions between the neuroectoderm and the ectoderm
when is a miniature eye formed?
by the end of the embryonic period at around 8 weeks
what are cells derived from the mesoderm layer responsible for?
-sclera
-bony orbit
-blood vessels
-ciliary muscle
-connective tissue
what are the 3 parts of the ectoderm and what do each of these parts contain?
- surface ectoderm:
-lens
-corneal epithelium
-conjunctival epithelium
-lacrimal glands
-tarsal glands
-epithelium of eyelids and cilia - neural ectoderm
-retinal pigment epithelium
-neural retina
-optic nerve fibres
-neuroglia
-epithelium of ciliary body
-epithelium of the iris - neural crest
-corneal stroma
-corneal endothelium
-uveal melanocytes
-uveal connective tissue
where do cells destined to form the eyes reside?
the field in the anterior neural plate
give an eye defect as a result of early developmental disruption
cyclopia which is where the eye fields fail to separate during week 3 of development leading to the formation of a single eye
what happens in eye development when the embryo is 4 weeks old?
formation of the optic vesicle, optic stalk and lens placode, early development of brain and spinal cord also begin
when and what is the infolding of the neural plate?
at day 24 where the edges of the cranial neural folds fold inwards and fuse at the same time, forcing the optic groove into close contact with the surface ectoderm
what are the first definitive structures of the developing eye to form and when do they form
the optic vesicles which grow outwards from the forebrain, they form between day 25-28
what happens with the eye at week 5?
-invagination of the optic vesicle to form the optic cup
-invagination of the lens placode to form the lens vesicle
-formation of the choroidal fissure
-hyaloid vessels form network to nourish the lens
how do hyaloid vessels grow through the choroidal fissure?
- primitive blood vessels enter the closing fissure along the underside of the optic stalk growing forward to the rim of the cup and lens
- the hyaloid vessels form a capillary network surrounding the developing lens
hyaloid vessels are not present in adults
what happens to the eye development at week 6?
-closure of the lens vesicle as it pinches off of the surface ectoderm to form the hollow primitive lens
-closure of the choroidal fissure completes formation of the optic cup
-retina starts to differentiate
why does the lens pinch off of a patch of surface ectoderm at the end of it’s formation?
due to contraction of the sub membranous actin cytoskeleton linked to zonulae adherents
what is coloboma? what is it caused by? what does it lead to?
a gap in part of structures of the eye due to incomplete closure of the choroidal fissure which leads to failure of induction of multiple structures such as iris, ciliary body, choroid, retina and lens
How is the retina from the optic cup formed?
- as the choroidal fissure closes, the two layers of the retina start to differentiate
- the pigmented layer remains one cell thick while the neural portion thickens
- the neural part of the retina differentiates into numerous distinct cell layers
what happens in eye development at week 7?
-lens develops many primary fibres
-formation of cornea and anterior chamber
-formation of iridopupillary membrane
-the neural retina starts differentiating into different cell types
what are the steps in the further development of the lens?
- hyaloid vasculature surrounds the back of the developing lens to support lens growth
- lens fibres begin elongation
how does the cornea further develop?
- cornea starts to differentiate
- outer epithelial layer gets derived from surface ectoderm
- inner layers get derived from the neural crest
how do the anterior chamber and iridopupillary membrane form?
-anterior chamber develops between the lens and the iridopupilliary membrane and cornea
-iridopupilliary membrane is initially attached to the lens and influences lens presence as it develops
how does the retina develop?
- pigmented layer becomes relatively thinner while the neural portion thickens
- neural portion differentiates into several layers
what happens in eye development at week 8 and on?
-regression of iridopupillary membrane
-formation of the corneal epithelium and primary stroma
-development of iris and cilliary body
-development of secondary fibres
-formation of eyelids
what does the cornea consist of at week 8?
an outer epithelial layer derived from the surface ectoderm and inner layers derived from neural crest
how does the lens further develop in week 8?
- proliferation of the epithelial cells at the equator
- anterior cells at pole of lens elongate and surround primary fibres forming a complete ring
- formation of secondary fibres
give a development anomaly of the lens and explain what are its 2 causes
congenital cataract
-misalignment of the primary lens fibres due to failure of proper induction from the optic cup/retina leads to cataract in the embryonic nucleus
-viral infection: rubella can persist in the lens for up to 3 years and disturbs formation of primary lens fibres so can only tale affect during weeks 4-7
name an anomaly of the vitreous and it’s cause
persistent hyperplastic primary vitreous
caused by remnants of the hyaloid vessels that persist in the vitreous
when do eyelids form, fuse and reopen?
form in week 8, fuse at the start if the 2nd trimester and reopen at the start of the 3rd trimester
what is anophthalmos?
complete absence of the eye where the opticle vesicle forms but the eye fails to develops
what is microphthalmos? why? what could it be caused by?
a small eye possibly due to failure of optic cup expansion and correct development of retina and vitreous
could be caused by chromosome abnormalities like trisomy 13, infections during pregnancy like rubella or foetal alcohol syndrome
how does the presence of the retina affect the developing lens?
it causes formation of primary fibres as it induces the elongation of primary fibres from the posterior wall of the lens vesicle so the lens vesicle separates from the surface ectoderm
what does an early embryonic lens look like?
-lens is filled with primary fibres
-hyaloid vascular net encloses the developing lens
the hyaloid vessels regress from week 11
what are the steps in lens formation from secondary fibre formation?
- secondary fibres are derived from anterior cells at the poles of the lens
- these cells elongate and surround the primary fibres forming a complete ring
- majority of lens growth occurs during the foetal period however the lens continues to grow throughout life
- density of secondary fibres increases with some compression of the primary nucleus
- secondary fibres form sutures
how does retinal development start?
from a hollow pouch of the neural tube which becomes indented to form a stalked cup which forms the retina
when do the layers of the retina begin to differentiate?
at 7 weeks as the optic cup closes at 6 weeks
for the RPE, what is it formed from? what layer of the retina has to be present for it to correctly form?
-it is formed from the outer, thinner layer of the optic cup
-the neural layer of the retina
what is the first retinal layer to differentiate?
the RPE
what is the earliest pigmentation anywhere in an embryo?
melanin pigment that develops at 4-5 weeks in the RPE
where are melanocytes derived from?
The neural crest
what causes albinism?
a genetic defect in melanin synthesis due to either melanocytes or specialised epithelial cells or both being affected
how is oculocutaneous albinism different to ocular albinism?
oculocutaneous albinism affects both skin and eyes while ocular albinism affects eyes only
what does ocular albinism cause?
-absence of pigmentation in RPE
-underdeveloped macula, no macula pigment
-fovea may be absent, lack of foveal pit
-number of rods is decreased
-abnormal projecti0ons of RGCs axons to the lateral geniculate nucleus
-poor visual acuity, photophobia, nystagmus
what does the anterior part of the optic cup do?
forms the inner layer of the iris and contributes to the formation of the ciliary body
give the features of retinal formation at 1.5 months
- intraretinal space separating the RPE from neural retina is lost during week 7
- as the choroidal fissure closes, proliferating cells in the inner layer of the optic cup migrate to form tow layers being the proliferative zone and anuclear marginal zone
3.thin lamina separates the marginal zone from the vitreal cavity precursor to the internal limiting membrane
what happens to retinal formation at 2.5 moths
cells migrate to form the inner and outer neuroblastic layers. formation of 2 neuroblastic layers is completed during the 3rd month of gestation
what retinal layers exist in 2.5 months?
RPE
outer neuroblastic layer
transient layer of cheivitz
inner neuroblastic layer
nerve fibres
what happens to the outer neuroblastic layer of the retina at 2.5 months
proginetor cells will form rod and cone cells and bipolar and horizontal cells
what does the transient layer of chievitz the retina at 2.5 months do?
it separates the layers in the primitive retina but is later destroyed by cell migration from outer neuroblastic layer
what are the layers of the retina at 4.5 months
-rpe
-photoreceptors
-outer plexiform layer
-inner nuclear layer
-inner plexiform layer
-ganglion cell layer
-nerve fibres
what happens in the retina at 4.5 months?
retinal lamination is essentially complete
-large cone nuclei are aligned adjacent to the rpe, photoreceptor outer segments not yet formed
-outer plexiform layer established primitive lamellar synapses between bipolar cell dendrites and cone pedicles
-INL differentiating
-IPL has started to establish sites of primitive synapses
-ganglion cells are multi-layered
what retinal layers are present at 5.5 months
-RPE
-outer segments
-outer nuclear layer
-outer plexiform layer
-inner nuclear layer
-inner plexiform layer
-ganglion cell layer
-nerve fibres
what happens in the retinal layers at 5.5 months
-growing photoreceptor outer segments project between RPE and external limiting membrane
-ONL has 6-7 layers of nuclei, outermost layers are cones aligned to external limiting membrane
-OPL has a linear arrangement of synapses between bipolar cells and rod spherules
-cells in the INL differentiate to form muller cell nuclei and amacrine cells. INL also contains bipolar and horizontal cells
-ganglion cells are thinned out to one or two layers except in the macula
give the steps in the formation of the sensory retina
- retina has a centre to periphery developmental gradient starting from the side of the future fovea and extends outward
2.differentiation begins at about week 9 with formation of the ganglion cells - formation of cones and horizontal cells
- at week 10, amacrine cells, rods, muller cells and bipolar cells form
- the same waves of cell development are apparent in the periphery but begin later
- the rest if the rods, muller cells and bipolar cells form postnatally
- synapse formation starts in the fovea a week or so after the first cells in a given region are formed and then spread outwards to the periphery
what are the three major phases of synapse formation between the major neuronal classes of the retina?
- retinal ganglion cells and amacrine cells form the earliest functional circuits of the inner plexiform layer (IPL)
- later, horizontal cells and photoreceptors form contacts in the outer retina, giving rise to the outer plexiform layer OPL
- vertical networks in the inner and outer retina are later interconnected when bipolar cells form connections with ganglion cells
when does the fovea start to form?
at 22 weeks
how does the foveal pit form?
- the ganglion cells then inner nuclear layer cells migrate away from the centre of the future fovea
- cones mainly L and M migrate into the fovea increasing cone density
alot of foveal formation happens postnatally
what is foveal cell density due to?
migration and lack of expansion
what is peripheral cell density due to?
a combination of migration away from the fovea, greater retinal expansion and apoptosis of ganglion cells
how are the cone cells at birth immature?
-as the inner segments start off thick and outer segments short
-the cone inner segment reaches adult length at around 36 months and the cone outer segment around 6 years
-mature cones are thinner in the centre of the fovea than elsewhere in the retina allowing more to be packed into a small area hence increasing visual acuity
what is vasculogenesis? what does it spread through?
the formation of new vessels from precursor cells which appear near the optic nerve head following the first phase of differentiation in the central retina
-spreads through the nerve fibre layer from the optic nerve head to the periphery avoiding the fovea
what does the central retinal artery do?
it supplies the inner neural retina and divides to form arterioles which run through the different layers of the neural retina forming a capillary network
how do the central retinal artery and vein form?
by angiogenesis from regressing hyaloid vessels
where do capillary beds initially form and where in the retina do they penetrate?
initally form around the optic nerve head and ramify at several levels in the retina where shallow capillary beds lie on either side of the GC layer and deep capillaries on either side of the INL
describe the steps in retinopathy of prematurity
- when in the incubator under increased oxygen, immature retinal blood vessels respond to high o2 levels by constriction and stop developing
- after removal of the supplemental o2, rebound vascular proliferation occurs but new vessels are poorly formed and leaky
- new vessels may grow into vitrous or form vitreoretinal adhesions
- fundus appearance monitored to regulate o2 exposure and try to reduce neovascularisation
give the steps of optic nerve formation
- closure of the choroidal fissure traps the hyaloid artery creating a 2 layered optic stalk
- the outer layer forms the neuroglial sheath that surrounds the optic nerve and gives rise to glial components of the lamina cribrosa
- apoptosis of inner layer provides passage for developing ganglion cell axons growing to the CNS. Other cells of the inner wall become the glial cells of the optic nerve
- growth of GC axons into the optic nerve peaks at 4 months
- numbers of GC axons decline as they are pruned by apoptosis over the next 5 to 6 weeks and adult numbers are established by birth
when and where does myelination occur in the optic nerve?
myelination of the axons begins at the lateral geniculate nucleus during moNTH 5 and reaches the optic chiasma at 6 moths gestation and stops at the lamina cribrosa at 1-3 months after birth
how big is the eye of a newborn compared to an adult eye
70% of the length and 35% of the volume pf an adult eye
for each part of the eye, what age does it reach
axial length
anterior chamber depth
corneal area and thickness
-12-16 years
-8 to 12 years old
-approx. 3 years old
what does the immaturity of cones mean for postnatal development?
-adult visual acuity is achieved 5-7 years post birth due to immature fovea
-colour discrimination is relatively poor at birth however this improves rapidly as at 1 year colour vision is similar to adults
what is emmetropization
the process by which the eye adjusts itself to maintain normal vision
how does emmetropization occur?
the retina detects an image blur and distinguishes hyperopic from myopic blur to generate a signal that increases or decreases scleral growth to compensate for refractive error
what structrures originate from the optic cup?
-retina
-rpe
-neural retina
invagination of what part leads to formation of the lens vesicle in the developing embryo?
-the lens placode
-a dedicated part of the surface ectoderm
what structures are formed from the most anterior portion of the optic cup?
-the inner layer of the iris
-a part of the ciliary body
which part appears first during embryonic development of the eye?
the optic vesicles
