What are the layers of the eye?
Outer Fibrous Coat
- Cornea Anteriorly
- Sclera Posteriorly & Laterally
- Iris & Pupil
- Ciliary Body
What are the layers of the Cornea?
- Stratified Squamous non-keratinized Epithelium
- Bowman’s Membrane (basement membrane)
- Stroma (regular connective tissue - collagen)
- Descemet’s Layer (basement membrane)
- Endothelium (single layer)
What would be the reason for examining Bowman’s membrane in an ocular injury?
It the injury only affects the epithelium it will heal to normal.
If it breaks the bowmans membrane then it will heal with collagen and form a scar, affecting corneal transparancy and therfore sight
How do we maintain corneal transparency?
- No blood vessels in the stroma
- Regular arrangement of the stroma
- Pumps in the endothelium keep out the aqueous humor
How does nutrition reach the cornea?
- Tear Film
- Blood vessels in the sclera
- Aqueous Humor
How is the avascular nature of the cornea relevant to surgery?
Because its avascular theres a low chance of foreign antigens being noticed, this means theres a lower chance of corneal graft rejection than any other type of transplant.
This has led to the cornea being dubbed an “immune-privileged” site
How do we divide up the eye (segments/chambers)
Anterior Segment and Posterior Segment divided by the lens.
The anterior segment is split into the ant/post chambers divided by the iris.
Where is aqueous humor produced/
The ciliary body in the posterior chamber of the anterior segment
Explain the passage of aqueous humor
- Produced in the ciliary body
- Passes between lens and iris into anterior chamber
- Then drains through the trabecular meshwork and out schlemm’s canal
What is the choroid?
A part of the vascular coat.
Its a layer of fenestrated blood vessels of varying diameter below the retina
Explain the layers of the retina?
Split into 10 layers, these are not all separate cell layers, the organisation of the cells make them appear like 10 layers.
The important layers are:
10) Pigment Epithelial layer
9) Rods and cones layer (dendrites of the lowest neuron layer)
2) Nerve Fibre layer (Axons of the highest neuron layer)
Explain the blood supply of the retina?
- Outer layers are supplied by the choroid by diffusion
- Inner Layers are supplied by branches of the central retinal artery
What do you see with an opthalmoscope
The retina and its blood vessels (branches of the central retinal artery)
What is special about the central part of the retina?
It has the densest packing of cones/rods anywhere in the retina so its where the image is sharpest.
The outer layers of the retina are also pushed aside at the fovea centralis to expose the rods/cones.
What suspends the lens?
Suspensory Ligaments (Zonules)
Why do cataracts form?
Lens cant shed its old cells like skin or remove them through blood.
So over time it opacifies leading to cataracts
What shape is the lens?
What makes up the eyelid?
They are given shape by the tarsal plates (connective tissue).
They contain glands that contribute to tears.
The lids also contain the palpebral part of the orbicularis oculi
How many layers are there in the tear film?
1 - Mucinous Layer - 30%
2 - Aqueous Layer - 69%
3 - Oily Layer - 1%
how do we inspect the tear film?
With a dye called Fluorescein
What are the functions of the tear film?
- Prevents drying of cornea
- Washes away foreign bodies
- Contains lysozymes and antibodies to fight off microbes
- Maintains smooth corneal surface for refraction
Whats the function of the mucinous layer of tear film?
Overcomes surface tension of the eye to prevent the aqueous layer simply rolling off
Whats the function of the oily layer of the tear film?
Prevents the aqueous layer from evaporating too fast
How does tear secretion occur?
Parasympathetic fibres from cranial nerve VII (Facial) trigger the conjunctival sac to secrete tears
What triggers blinking?
When the aqueous layer evaporates enough for the oily layer to contact the mucinous layer it triggers reflex blinking
What does the eye develop from embryologically?
Optic Vesicles grow outwards from the diencephalic part of the neural tube
- Grows towards the surface ectoderm forming optic cup
- Surface Ectoderm invaginates forming lens vesicle
Together they form the eye