The Eye in Health and Disease

Question 1

Where do the first processes which are going to form the eye grow from?
What are they called?

Answer 1

The optic vesicles grow outwards from the diencephalic part of the neural tube towards surface ectoderm.

Question 2

What happens as the optic vesicles grow outwards towards the surface ectoderm?

Answer 2

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

Question 3

What happens to the blood vessels as the optic cup is forming?

Answer 3

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.

Question 4

What happens if the choroid fissure does not fuse?

Answer 4

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

Question 5

What does the surface ectoderm from in the eye?

Answer 5

eyelids, conjunctiva, corneal epithelium

Question 6

What does the mesenchyme of the eye form?

Answer 6

choroid, storm of cornea, sclera and extra-ocular muscles

Mesenchyme derived from mesoderm and neural crest cells

Question 7

What do the lens fibres form from?

Answer 7

Epithelium of lens vesicle and later loose their nuclei

Question 8

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?

Answer 8

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

Question 9

What signs are there in a blow out fracture?

Answer 9

Quick onset double vision due to trauma.
Double vision doesn’t clear.
Muscles get tethered to fracture so one eye will struggle to move.

Question 10

What is the function of fat in the orbit?

Answer 10

Cushions globe
Very important for support.

One of the last sources of fat you can lose when you go on an extreme diet.

Question 11

What is the staring appearance?

Answer 11

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”

Question 12

Describe the fibrous coat of the eyeball

Answer 12

Cornea:
-Is the anterior 1/6 which is transparent and is a window to allow light rays to enter the eyeball

Sclera:
-Is the opaque posterior 5/6 which gives attachment to muscles moving eyeball

Question 13

Describe the histological layers of the cornea

Answer 13

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

Question 14

What is the clinical significance of bowman’s membrane?

Answer 14

Trauma of the cornea:

• Epithelium grown back very easily
• Bowman’s will scar easily

Question 15

How come the cornea is transparent but sclera is not?

Answer 15

Collagen fibres are regularly arranged in the cornea so light can pass through

In the sclera they are irregular so light cannot pass through

Question 16

How is the aqueous humour prevented from entering the cornea?

Answer 16

Endothelium cell layer has a pump that actively keeps the aqueous humour out.

Question 17

Describe inflammatory and non-inflammatory pathology or the cornea

Answer 17

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

Question 18

Describe why the avascularity of the cornea is quite useful

Answer 18

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

Question 19

What makes up the vascular layer of the eye?

What are their functions?

Answer 19

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

Question 20

What makes up the sensory layer of the eyeball?

Answer 20

Retina- has light sensitive rods and cones which enable us to see

Question 21

How many layers are there in the retina?
Which ones do i have to know?
Where do they layers come from?

Answer 21

10 layers

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.

Question 22

Describe the choroid

Answer 22

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.)

Question 23

Where does the central retinal artery come from?

Answer 23

Branch of the ophthalmic artery

Question 24

What is the optic disk?

Answer 24

The start of the optic nerve.

The physiological blind spot

Question 25

What is the fovea centralis?

Answer 25

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

Question 26

Describe the lens of the eye

Answer 26

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

Question 27

How is the eye divided?

Answer 27

Anterior segment (in front of the lens):

• Anterior chamber (in front of iris)
• Posterior chamber (behind iris)

Posterior segment (behind the lens)

Question 28

What is the aqueous humor?

Answer 28

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

Question 29

What is the vitreous humor?

Answer 29

Transparent gel posterior to the lens.

It helps cushion the retina

Question 30

How is aqueous humour produced?

Answer 30

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

Question 31

What is a Glaucoma?

Answer 31

An obstruction anywhere along the pathway of circulation of aqueous humor would lead to damming up of AH -> increased intraocular pressure -> glaucoma

Question 32

What are the conjunctiva?

Answer 32

Thin vascular membrane that covers inner surface of eyelids and loops back over sclera.

DOES NOT COVER THE CORNEA

Question 33

What is the lacrimal gland?
What nerve supplies it?
Where is it located?

Answer 33

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.

Question 34

Where do tears drain?

Answer 34

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.

Question 35

What are the functions of the tear film?

Answer 35

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

Question 36

What are the 3 layers of the typical tear film?

Answer 36

Layer 1:

• Mucous layer
• Overlying corneal epithelium

Layer 2:

• Aqueous layer
• Lies over the mucinous layer

Layer 3:

• Oily layer
• Is the most superficial

Question 37

Describe the dynamics of the tear film

Answer 37

Facial nerve -> secretions of tears into conjunctival sac

During a blink, the sharp lower border of the upper eyelid distributes the tear film evenly.

When eyelids are open, aqueous component of tears begin to evaporate, and oily layer comes close to the mucin layer.

When these 2 layers touch each other, the tear film breaks up and stimulates further blinking