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Embryology of the eye Flashcards

1
Q

describe embryology from the beginning

A
  • 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
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2
Q

describe the formation of the bilminar disc

A

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

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3
Q

describe the trilaminar disc

A

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)

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4
Q

what are the three germinal cell layers

A

ectoderm - neural tissue, skin , nails and hair

mesoderm- muscle , bone, connective tissue, circulatory system

endoderm - internal organs

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5
Q

what happens to form the neural tube

A

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

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6
Q

why is the neural tube important

A
  • 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)

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7
Q

how many germinal cell layers are their

A

there are 3 main germinal cell layers

ectoderm

endoderm

esoderm

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8
Q

what is the inner layer of the germinal cell layers

A

the endoderm is the inner layer (forms the gut and does not contribute to the devlopment of the eye)

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9
Q

what is the middle layer of the germinal cell layers

A

the mesoderm is the middle layer

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10
Q

what is the outer layer of the germinal cell layers

A
  • ectoderm is the outer layer which forms the neuroectoderm (neural tube) and neural crest cells
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11
Q

how many vesciles does the neural tube have

A

the neural tube has 3 primary and 5 secondary vesicles

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12
Q

what part of the neural tube are the eyes derived from

A

the eyes are derived from the diencephalon

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13
Q

what does the ectoderm from

A

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

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14
Q

when is the first stage of eye devlopment

A

day 22 is the first event in devlopment of the eye

optic primordium appears in the neural fold

optic grooves form the optic vesicle

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15
Q

how is the lens vesicle and optic cup formed

A

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

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16
Q

What are the consttiutents of the eye formed from

A

corena - ectoderm/mesenchyme

lens - ectoderm

choroid , cillary body, iris - mesenchyme

virteous body- mesenchyme

retina - ectoderm

sclera- mesenchyme

macula

central retinal artery

optic nerve

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17
Q

what does the ectoderm give off

A

surface ectoderm, neuroectoderm and neural crest cells

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18
Q

what are neural crest cells and mesoderm together called

A

mesenchyme

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19
Q

what contributes to eye development

A

surface ectoderm, neuroectoderm and mesenchyme

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20
Q

when is the first event in ocular devlopment

A

the first event in ocular devlopment is 22 days- formation of the optic grooves

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21
Q

when the optic vesicle meets the surface ectoderm what does it form

A

the optic vesicle meets the surface ectoderm and induces a thickening and forms a lens placode

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22
Q

what does the lens placode invaginate to form

A

the lens placode invaginates to form lens vesicle - invagination of the optic vesicle forms the optic cup

23
Q

what is the choroidal fissure and why is it important

A

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

24
Q

what is the cornea formed from

A

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
25
Q

describe the degeneration of the hyaloid artery

A

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

26
Q

what is cloquets canal

A

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

27
Q

what is mittendorfs dot and bergmeister papilla

A
  • 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
28
Q

how does the retina develop

A

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

29
Q

what does the inner and outer layer of the retina go on to form

A

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

30
Q

what is retinal dettachment

A

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

31
Q

which part of the inner layer of the optic cup forms the neurosensory retina

A

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

32
Q

where is the demarcation between the anterior 1/5 and posterior 4/5 visible in the adult eye

A

the demarcation between the anterior 1/5 and the posterior 1/5 is visible in the adult eye as the ora serrata

33
Q

what is your maccula and fovea responsible for

A

the macular and the fovea are responsible for your central vision

34
Q

describe macular and foveal devlopment

A

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
35
Q

how does the optic nerve develop

A

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

36
Q

how does the hyaloid artery become enclosed within the choroidal fissure

A

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

37
Q

how is the lens vesicle formed

A

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

38
Q

describe conception to neurlation

A
  • 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

39
Q

what does your ectoderm differentiate into

A
  • 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

40
Q

how does the optic cup and the lens vesicle form

A
  • 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
41
Q

Describe the devlopment of the eye in steps

A
  • 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
42
Q

what is the importance of the choroidal fissure and its relation to colobomas

A
  • 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
43
Q

what is a colombomas

A

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)

44
Q

what area of the eye does a colomboma usually affect

A

usually the inferonasal quadrant

45
Q

how can you tell on a eye scan weather it is a right eye or a left eye

A
  • 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

46
Q

how does the retina develop

A
  • 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

47
Q

what is the optic nerve formed from

A

retinal ganglion cell axons - in the adult eye the ganglion cell axons then fill that space nd that is what makes the optic nerve

48
Q

what was the central artery and central retinal vein in the fetus

A
  • the central retinal artery and vein in the fetus was the hyaloid artery and vein
49
Q

what is the pax6 gene

A
    • 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

50
Q

what happens after birth

A

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

51
Q

what is amblyopia

A
  • 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

52
Q

what are the causes of amblyopia

A
  • 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

53
Q

what treatment is their for amblyopia

A

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

54
Q

decribe the formation of the lens placode and the lens vesicle

A

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

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