Flashcards in The Eye in Health and Disease Deck (37):
Where do the first processes which are going to form the eye grow from?
What are they called?
The optic vesicles grow outwards from the diencephalic part of the neural tube towards surface ectoderm.
What happens as the optic vesicles grow outwards towards the surface ectoderm?
As they reach the surface ectoderm, they cause some of the cells on this surface ectoderm to thicken forming the lens placode.
The lens placode starts to push inwards (invaginate), which causes the optic vesicle too, to form a 2 layered optic cup.
The lens vesicle ultimately loses its connection with the surface ectoderm
What happens to the blood vessels as the optic cup is forming?
Blood vessels that pass through form a fissure in its inferior wall called the choroid fissure.
Over time the choroid fissure disappears as the two sides of it fuse.
What happens if the choroid fissure does not fuse?
It usually disappears but if it persists it will leave a gap in the iris, retina or the optic disc.
This gap is called a Coloboma:
-Coloboma of iris
-Coloboma of retina
-Coloboma of optic disc
What does the surface ectoderm from in the eye?
eyelids, conjunctiva, corneal epithelium
What does the mesenchyme of the eye form?
choroid, storm of cornea, sclera and extra-ocular muscles
Mesenchyme derived from mesoderm and neural crest cells
What do the lens fibres form from?
Epithelium of lens vesicle and later loose their nuclei
The orbit has very thin walls and is therefore easily fractured.
What structures of the bony orbit are not thin?
What can happen in a fracture of the thin bones?
Except the orbital rims and lateral and superior walls, the rest of the orbit has very thin walls, is easily fractured leading to herniation of contents into surrounding sinuses.
This is called a blow out fracture
What signs are there in a blow out fracture?
Quick onset double vision due to trauma.
Double vision doesn't clear.
Muscles get tethered to fracture so one eye will struggle to move.
What is the function of fat in the orbit?
Very important for support.
One of the last sources of fat you can lose when you go on an extreme diet.
What is the staring appearance?
Orbital fat hypertrophies in certain conditions like thyroid diseases leading to the staring appearance.
Eyelid should cover the upper sclera.
When you can see it its called "scleral show"
Describe the fibrous coat of the eyeball
-Is the anterior 1/6 which is transparent and is a window to allow light rays to enter the eyeball
-Is the opaque posterior 5/6 which gives attachment to muscles moving eyeball
Describe the histological layers of the cornea
1. Epithelium: stratified squamous non-keratinised
2. Bowman's membrane: basement membrane of corneal epithelium
3. Stroma: regularly arranged collagen, NO BLOOD VESSELS
4. Descemet's layer
5. Endothelium: single layer
What is the clinical significance of bowman's membrane?
Trauma of the cornea:
-Epithelium grown back very easily
-Bowman's will scar easily
How come the cornea is transparent but sclera is not?
Collagen fibres are regularly arranged in the cornea so light can pass through
In the sclera they are irregular so light cannot pass through
How is the aqueous humour prevented from entering the cornea?
Endothelium cell layer has a pump that actively keeps the aqueous humour out.
Describe inflammatory and non-inflammatory pathology or the cornea
Inflammatory- e.g. corneal ulcers
Non-inflammatory- e.g. dystrophies
Corneal pathologies frequently lead to opacification of the cornea.
This might need to be treated by corneal transplant- Keratoplasty
Describe why the avascularity of the cornea is quite useful
The vascularity of the cornea is of benefit to surgeons when performing a graft surgery as it means there is a lesser chance of foreign antigens from a corneal graft being recognised by the recipient, so a lesser chance of a graft rejection.
This has lead researchers to believe that the cornea is an "immune-privilaged" site
What makes up the vascular layer of the eye?
What are their functions?
Iris- Controls the diameter of the pupil and thereby controls the amount of light rays entering the eyeball.
Ciliary body- Suspends the lens and produces aqueous humor.
Choroid- Supplies blood to outer layers of retina
What makes up the sensory layer of the eyeball?
Retina- has light sensitive rods and cones which enable us to see
How many layers are there in the retina?
Which ones do i have to know?
Where do they layers come from?
Nerve fibre layer
Layer of rods and cones
Pigment epithelial layer
Retina pigment layer (layer no. 10) is the only layer derived from the outer layer of optic cup.
Describe the choroid
Made up of fenestrated blood vessels of varying diameter. these supply the outer layer of the retina with blood by diffusion.
(The inner layers of the retina get blood from branches of the central retinal artery.)
Where does the central retinal artery come from?
Branch of the ophthalmic artery
What is the optic disk?
The start of the optic nerve.
The physiological blind spot
What is the fovea centralis?
Area packed with cones, so has maximum visual acuity.
It is the area of the eye you focus your vision.
Only has cones, no rods
Describe the lens of the eye
Transparent, crystalline biconvex structure which is suspended by zonules from the ciliary body. It can change shape (i.e. become less or more convex)
For it to be transparent, it needs to be avascular. This vascularity is one of the factors that predisposes it to become opaque later in life.
Lens opacification is called cataract
How is the eye divided?
Anterior segment (in front of the lens):
-Anterior chamber (in front of iris)
-Posterior chamber (behind iris)
Posterior segment (behind the lens)
What is the aqueous humor?
Watery fluid anterior to the lens.
It helps to maintain intraocular pressure (pressure within the eyeball).
It is a constantly circulating fluid that also nourishes the lens and cornea
What is the vitreous humor?
Transparent gel posterior to the lens.
It helps cushion the retina
How is aqueous humour produced?
Produced by the ciliary body, in the posterior chamber.
It flows through the pupil to the anterior chamber
It then reaches the angle of the anterior chamber where it gets filtered out through a meshwork called the trabecular meshwork.
This takes the AH to Schlemm's canal where it feeds into veins and so back into circulation
What is a Glaucoma?
An obstruction anywhere along the pathway of circulation of aqueous humor would lead to damming up of AH -> increased intraocular pressure -> glaucoma
What are the conjunctiva?
Thin vascular membrane that covers inner surface of eyelids and loops back over sclera.
DOES NOT COVER THE CORNEA
What is the lacrimal gland?
What nerve supplies it?
Where is it located?
Situated in the orbit laterally.
Parasympathetic from facial nerve.
Its ducts open into the conjunctival sac.
During each blink, eyelids spread tears evenly on the surface of the cornea.
Where do tears drain?
Tears drain through puncture on the medial side of each eyelid.
Drains into lacrimal sac which sits over the lacrimal bone.
Then drains through nasolacrimal duct into the inferior meatus of the nasal cavity.
What are the functions of the tear film?
1. Keeps cornea moist, prevents drying
2. Washes away any particulate foreign bodies
3. Has antibodies and lysozyme to kill microbes
4. Smooths outer surface of cornea providing smooth surface for refraction
What are the 3 layers of the typical tear film?
-Overlying corneal epithelium
-Lies over the mucinous layer
-Is the most superficial