Embryology of the eye Flashcards
describe embryology from the beginning
- sperm fertalises egg - they each have hjalf the number of chromosones required - they start dividing as it goes along the fallopian tube - all the way up to 16 cells when it becomes a blastocyst
describe the formation of the bilminar disc
fertilsation - egg and sperm
lots of cells divide
you have a blastocyst - comprised of two cell types
the cells of the blastocyst divide and you get a two layered structure called the bilaminar disc
describe the trilaminar disc
in week 3
cells of the bilaminar disc (epiblast and hypoblast) undergo a specalised process called gastrulation
the two cell layers become 3
(this process is called gastrulation)
the 3 germ layers = between the amniotic cavity and the yolk sac
ectoderm (outer)
mesoderm (middle)
endoderm (inner)
what are the three germinal cell layers
ectoderm - neural tissue, skin , nails and hair
mesoderm- muscle , bone, connective tissue, circulatory system
endoderm - internal organs
what happens to form the neural tube
their is a thickening in the ectoderm (the top layer) - which differentiates into neuroectoderm and we call that thickening the neural plate
-invaginates to form nerual tube
neural crest cells migrate in
in steps
1- neuroectodermal tissues differentiate from the ectoderm and thicken into the neural plate - the neural plate border seperates the ectoderm from the neural plate
the nerual plate invahinates with the two ends joining the neural plate borders
why is the neural tube important
- the neural tube is running down the length of the trilaminar disc
- it has vesicles
these go on to form the forebrain the midbrain and the hindbrain
the eyes develop from the diencephalon- the optic vesicel devlops from the diencephalon which itself is a part of the prosencephalon (which is the forebrain)
how many germinal cell layers are their
there are 3 main germinal cell layers
ectoderm
endoderm
esoderm
what is the inner layer of the germinal cell layers
the endoderm is the inner layer (forms the gut and does not contribute to the devlopment of the eye)
what is the middle layer of the germinal cell layers
the mesoderm is the middle layer
what is the outer layer of the germinal cell layers
- ectoderm is the outer layer which forms the neuroectoderm (neural tube) and neural crest cells
how many vesciles does the neural tube have
the neural tube has 3 primary and 5 secondary vesicles
what part of the neural tube are the eyes derived from
the eyes are derived from the diencephalon
what does the ectoderm from
ectoderm - contributes to ocular devlopment
forms the surface ecotderm , neruoectoderm and neural crest cells
neuroectoderm goes on to from the nerual tube
neural crest cells are left over as part of the invagination of the neural tube
neural crest cells and mesoderm are called mesenchyme
when is the first stage of eye devlopment
day 22 is the first event in devlopment of the eye
optic primordium appears in the neural fold
optic grooves form the optic vesicle
how is the lens vesicle and optic cup formed
optic stalk and optic vesicels form
they meet the surface ectoderm
induces surface ectoderm thickening called the lens placode
the lens placode and optic vesicle invaginate to form the lens vesicle and the optic cup
What are the consttiutents of the eye formed from
corena - ectoderm/mesenchyme
lens - ectoderm
choroid , cillary body, iris - mesenchyme
virteous body- mesenchyme
retina - ectoderm
sclera- mesenchyme
macula
central retinal artery
optic nerve
what does the ectoderm give off
surface ectoderm, neuroectoderm and neural crest cells
what are neural crest cells and mesoderm together called
mesenchyme
what contributes to eye development
surface ectoderm, neuroectoderm and mesenchyme
when is the first event in ocular devlopment
the first event in ocular devlopment is 22 days- formation of the optic grooves
when the optic vesicle meets the surface ectoderm what does it form
the optic vesicle meets the surface ectoderm and induces a thickening and forms a lens placode
what does the lens placode invaginate to form
the lens placode invaginates to form lens vesicle - invagination of the optic vesicle forms the optic cup
what is the choroidal fissure and why is it important
if you look at the optic cup inferiorly it is not closed because their = the choroidal fissure
it is an important structure because the hyaloid artery runs along side it
in the fetus the lens is still developing and needs a blood supply to develop
what is the cornea formed from
surface ectoderm
- when the optic cup moves towards the ectoderm it induced a thickening called the lens placode
- when the lens placode invaginates to form the lens vesicle the remaining surface ectoderm goes on to form the cornea - corneal epithelium
describe the degeneration of the hyaloid artery
in the adult eye their is no blood vessels - there are no arteries travelling through the virteous anymore
- at a certain point in fetal devlopment when the eye is already formed- the hyaloid arteries begin to regress and your left with remincents (like a canal) in the virteous in the adult eye
the hyaloid arteries give rise to the central retinal artery- however the part that goes to the virteous through the lens that goes away and what is left behind is purley the retinal vessels
what is cloquets canal
in the adult eye in the middle of the virteous you have a empty space - and that is where you hyaloid arteries used to be and that is called the hyaloid canal
if their is a incomplete breakdown of the hyaloid artery blood vessels you have remincents of the arteries - and that is what forms the mittendorf dot - you can see that in a newborn where their is a incomplete regression of the hyaloid arteries behind the lens and you have an opacity behind the lens
what is mittendorfs dot and bergmeister papilla
- in the fetus blood is supplied via the hyaloid artery
- during 30 weeks blood supply is reabsorped and forms cloquets canal
- incomplete resporption of these structures during devlopment can result in persistent fetal vasculature such as the mittendorf dot and the bergmeister papilla- remincents of the hyaloid artery around the optic disc itself
how does the retina develop
there are two layers of the optic cup - you have a inner layer and a thin outer layer (these are developing from neuroectoderm )
the outer layer will go on to form the retinal pigmented epithelium
the thicker inner layer will go on to form the neurosensory retina
neurosensory retina contains photoreceptors - and this is the part that transduces the light and transduces it into electrical activity and transmits it towards the brain
what does the inner and outer layer of the retina go on to form
the thin outer layer goes on to form the retinal pigmented epithelium and the thick inner layer goes on to form the neurosensory retina - in between these two layers you have the subretinal space- this closes up as the adult eye develops
what is retinal dettachment
separation from the retinal pigmented epithelium and the neurosensory retina-when you have a retinal dettachment you have the opening of the subretinal space again
which part of the inner layer of the optic cup forms the neurosensory retina
the anterior 1/5 that overlaps the lens does not devlop into nervous tissue
the posterior 4/5 devlops into neurosensory retina- and contains the photoreceptors
where is the demarcation between the anterior 1/5 and posterior 4/5 visible in the adult eye
the demarcation between the anterior 1/5 and the posterior 1/5 is visible in the adult eye as the ora serrata
what is your maccula and fovea responsible for
the macular and the fovea are responsible for your central vision
describe macular and foveal devlopment
at the macular you have a high density of photoreceptors - which are called cone photoreceptors- you dont have rod photoreceptors
the fovea in the adult eye is very thin
macular devlopment starts at 4.5 months
ganglion cell increases temporal to the optic disc
foveal depression seen at 7 months
by 4 months postpartum, the fovea is only 1 layer cell thick (cone nuclei)
the foveal devlopment is not complete when the baby is born- it keeps happening as the baby gets older - that is why a new born baby has very poor vision
- you only have photoreceptors in your fovea
how does the optic nerve develop
the optic nerve is formed from the optic cup
the outer layer of the optic stalk closes aorund the hyalid fissure and it encloses the hyaloid artery and vein - hyaloid artery and vein forms the central retinal artery and central retinal vein
outer and inner layer of neuroecoderm begins to close
optic nerve is formed from retinal ganglion cell axons
hyloid arterys become enclosed within the structure of the optic stalk
hollow optic stalk connects the third ventricle of the forebrain and the cavity of the optic vesicles
as the choroidal fissure closes a double layer of neuroectoderm is produced
ganglion cell axons from the retina pass along the inner layer of the stalk
eventually the cavity between the inner and outer layer closes completley forming the optic nerve
how does the hyaloid artery become enclosed within the choroidal fissure
between the choroidal fissure and the inner layer - the retinal ganglion cells begin to fill that space and it traps the hyaloid artery and vein inside of it
how is the lens vesicle formed
surface ectoderm forms the lens placode which has then pinched off to form the lens vesicle - the lens vesicle gives rise to the lens in your eye
posterior cells of the lens (these are called primary lens fibres ) vesicles elongate and you have a cavity called the cavity of the lens vesicle which eventually closes off
then the secondary lens fibres overlap it
as you get older you keep adding layers - the lens devlops throughout life - foetal lens is always present
describe conception to neurlation
- you have 2 haploid cells which have 23 chromosomes - which combine to form what the normal human cells have - i.e. 46 chromsones
23 chromosones in egg and 23 in sperm
cell divides until it attaches to the wall of the uterus - once it attaches to the wall of the uterus you have bilaminar and then trilaminar disc - development of 3 different layers - ectoderm, mesoderm, endoderm and in the middle you have the notochord which goes on to form the spinal cord
mesoderm and ectoderm contribute to ocular devlopment - two types of ectoderm neuroectoderm and surface ectoderm - the mesoderm gives rise to mesencymal
mesencyhmal cells begin to form the uveal tract which is the choroid , iris and cillary body and virtteous and eom
what does your ectoderm differentiate into
- differentiates into surface ectoderm
- neuroectoderm (neural plate) - it invaginates to form the neural tube
- and surface ectoderm lies on top
there are several different layers of neural tube - the main parts are the porsencephalon , mesaocephalon and the rhombencephalon - the forebrain itself can be divided into the telencephalon and the diencephalon — the eyes develop from the diecephalon which is a part of your forebrain
how does the optic cup and the lens vesicle form
- if you take a section through the diencephalon
- when the optic vesicle invaginates to form the optic cup
- their = a process called induction - the optic vesicle induces the surface ectoderm to become the lens placode
- the lens placode itself then begins to invaginate itslelf with the optic cup - thier is an area of invagination of the lens placode as you go along their is a seperation of the two layers which starts to invaginate - you have a remaining layer of surface ectoderm and the lens vesicle forms
- the remaining surface ectoderm goes on to form the corneal epithelium
Describe the devlopment of the eye in steps
- if you work to take a cross section through the diencephalon
- the ectoderm differentiates to form neuroectoderm (neural plate) with surface ectoderm on the sides
- it starts to form optic grooves (day 22) at it begins to invaginate it then pinches off and you have an optic vesicle
- and then you have a process of inudction wherby the optic vesicle induces a thickening of the surface ectoderm forming the lens placode
- optic vesicle then begins to form more of a cup- like structure and is then referred to as the optic cup
- the optic cup invaginates and pulls the lens placode forming the lens vesicle - you then have an overlying area of surface ectoderm which will eventually become your conreal epithelium and your lids
- optic cup goes on to form your retina
what is the importance of the choroidal fissure and its relation to colobomas
- the hyaloid artery travels through the choroidal fissure and supplies the lens in the fetal eye
- travels through the virteous
- in the adult eye the hyaloid artery has regressed
- the hyaloid atery gives of the central retinal artery in the adult eye -the part that goes through the virteous regresses - and you end up with a empty structure called the hyaloid canal or cloquets canal
- a colomboma arises when your choroidal fissure does not fully close during development of the eye
what is a colombomas
results from failure of the closure of the choroidal fissure - as the eye devlops the choroidal fissure closes and encloses the hyaloid artery and vein inside of it - eventually their is a complete closure of the choroidal fissure - it encloses the vessels which have now become your central retinal artery and central retinal vein
- usually inferonasal quadrant
can affect the iris, cillary body, optic disc and retina
association with the chd7 gene
may be present as a part of CHARGE syndrome ( colomboma, heart disease, atresia chpoane , retardation , growht , genital hypoplasia and ear malformation)
what area of the eye does a colomboma usually affect
usually the inferonasal quadrant
how can you tell on a eye scan weather it is a right eye or a left eye
- look at the maccula and the optic disc - if the disc is on the right it is a right eye - if the disc is on the left it is a left eye
the optic disc lies nasal in a adult eye - ( and the maccula is the centre of your vision_ - so in a coloboma you can tell if its inferotemporal or nasal using optic disc
how does the retina develop
- from the optic cup
as 2 distinct layers - inner layer is thicker than the outer layer and in between these two layers you have a space - subretinal space in between - this distinction is important because it closes in the adult eye forms a potential space - but in a retinal dettachment - their is a dettachement again from the neurosensory retina from the retinal pigmented epithelium
outer layer and inner layer
innerlayer froms - neurosensory retina - where photoreceptors are
outer layer forms - rpe
the anterior 1/5 of the inner layer does not become neurosensory retina - (does not contain photoreceptors)
the posterior 4/5 does
and this demarcation in the adult eye can be seen as the ora serrata
what is the optic nerve formed from
retinal ganglion cell axons - in the adult eye the ganglion cell axons then fill that space nd that is what makes the optic nerve
what was the central artery and central retinal vein in the fetus
- the central retinal artery and vein in the fetus was the hyaloid artery and vein
what is the pax6 gene
- essential gene in ocular development
mutations can lead to anophthalmia (no eye)
anterior segment dysgenersis - the angle has no developed - can lead to glaucoma because the aqueous humour is unable to drain
congential galucoma
aniridia - you can have failure of the development of the iris - - in a coloboma - you would have devlopment of the iris but not the inferonasal quadrant - in pax6 gene you have no iris at all
what happens after birth
eyeball grows rapidly
initally relativley short (myopic)
with rapid eyeball growth the refractive error is corrected
lens grows rapidly and continues grwoing through an adults life
pigmentation of the iris stroma occurs in the first few years
embroyonic membranes in the nasolacrimal system open at birth/ shortly after which allows tears to begin to flow
retinal ganglion cells overlying fovea migrate allowing for improved va
what is amblyopia
- essentially refers to poor development of the nerual circuits connecting the eye to the brain
aka lazy eye
as such the eye itself may be structurally normal
occurs if their is poor focusing in early childhood when neural circuits are still developing
e.g. you have a catract in your right eye during early childhood - you cant see anything out of the right eye - the babies brain will only devlop to consider imformation from the left eye -it will ignore imformation from the right eye because imformation from the right eye -will be very poor vision- if they then undergo surgery and the cataract is removed - however by this time period the brain as already began to ignore imformation from this eye - even if you take the cataract out at this stage it will not make a difference - because the brain has already learned to ignore the eye
what are the causes of amblyopia
- three main types of amblyopia , strabismic , refractive and deprivational
strabismus - one eye is dominant while the other eye is suppressed , e.g. tropias, meridional - this is when the two eyes are not aligned with one an other - this will always be one fixating eye- i.e. the domiant eye and the other eye which is intorted or exotorted - your brain will learn to supress imformation from that eye
refractive - large refractive difference between the two eyes - - i.e. one eye is very long sighted and the other is very near sighted - anisemtropia - your brain will learn to supress imformation from one of the eyes - astigmatism
deprivational - e.g. cataracts, tumours, capillary haemangiomas - i.e. their is something that is blocking the light
what treatment is their for amblyopia
e. .g strabismic amblyopia - i.e. eyes are not aligned correctly - you would have a right eye which would be dominant and a left eye would be supressed - you would patch the right eye - then the brain is forced to use the left eye - you force the brain to use the other eye - you patch the good eye and then you force the brain to use the other eye - you alternate the patching now and again. - to force the devlopment of the neural circuits from both sides
- atropine - dilator - put a drop of atropine in the right eye - that would dilate the right eye - which would make the vision blurry - and then you force the brain to use the left eye
decribe the formation of the lens placode and the lens vesicle
as the optic vesicle develops as an outpouching of the diencephalon the overlying surface ectoderm is induced and develops into the lens placode
the lens placode and the optic vesicle then start to simultaneously invaginate until the lens placode separates from the surface ectodermal layer to form the lens vesicle while the optic vesicle becomes the optic cup
