Introduction 1: Introduction and Occular Fluids

Biochemistry of the Eye, Lecture 1 , Quiz 1, and Test 1

Describe the bilayer composition.

Amphipathic phospholipids self-assemble into fluid bilayers with hydrophilic heads and hydrophobic acyl chains.

Describe membrane fluidity.

It is tightly regulated by cholesterol incorporation which prevents phase transitions under temperature fluctuations.

What are lipid rafts?

They are areas enriched with sphingolipids and cholesterol.

What do lipid rafts do?

They organize signaling cascades in retinal photoreceptors outer segments and corneal epithelial cells. They accommodate key G-protein coupled receptors such as rhodopsin.

What are common fluids in the non-ocular regions?

1. Cytoplasm

2. Interstital Fluid

3. Blood

What are the common fluids in ocular specific regions?

Aqueous humor, vitreous humor, and pre-corneal tears.

How much of the eye is fluid?

80%

How much water is present in adults?

60%

How much water is present in infants?

75%

How much water does the body need to function metabolic activity?

Greater than 65%

What are the properties of water due to?

It’s hydrogen bonds

What are the Properies of water?

1. High dielectric constant

2. High melting point and heat of vaporization

3. High surface tension

4. Liquid state denser than solid state

5. High overall strength of molecules with many H-bonds (ex. Proteins)

How much blood are in adults? How about newborns?

1. ~ 5L

2. ~ 270 mL

How much blood does a pregnant woman have as compared to a normal human?

~30-50% more in a pregnant woman

What is the connection between ocular fluids and blood?

Ocular fluids are filtrate of blood

What are the functions of blood?

1. Nourishment

2. Waste disposal

3. Generation of IOP

4. Homeostasis of retinal functions

What is the standardized pH for blood?

7.33 – 7.45

What does the blood SPECIFICALLY carry?

O₂, N₂, CO₂

What does the ciliary body supply?

Aqueous

What does the choroid supply?

Retina

What does Iris and Cornea supply?

Vascular beds

True or False: Lens have a direct blood supply.

False, Lens do NOT have a direct blood supply.

What is the production of the Aqueous humor?

1. Product of the Na/K ATPase (primarily) and bicarbonate ATPase

2. Pumps are in the basolateral borders of the non-pigmented epithelial cells of the ciliary body

3. Many Na/K pumps has large net effect on ion gradient

4. Bicarbonate pump function unknown

What is the composition of the aqueous humor?

99% water

Compare the Aqueous Humor to Blood.

1. Increased Vit C (ascorbic acid)

2. Unchanged bicarbonate & globulin

3. Other components decreased

What is the function of the Aqueous Humor?

1. Nourishment of most of the cornea and all of the lens

2. Carries oxygen

3. Antioxidant for lens

4. IOP protects eye from physical stress/shock

Describe the fluid and gel mixture composition in the Vitreous humor.

80% Gel / 20% Fluid —> 40% Gel / 60% Fluid with age

What happens when there is a decrease in gel composition?

Retina Detachment

What is the gel in the vitreous humor made up of?

It is primarily made up of Type II Collagen and hyaluronic acid. It is also made up of XI/ IX collagen.

What is the function of the Vitreous humor?

Provides viscoelasticity

Compare the vitreous humor to blood.

1. Increased Vit C, GAGs, and proteins

2. Unchanged bicarbonate

3. Slightly decreased glucose

4. Other components decreased

What are the functions of the precorneal tear film

1. Hydration of corneal epithelium

2. Creates smooth ocular surface

3. Supplies O2 to the cornea

4. Anti-bacterial

5. Flushes debris from eye

What are the layers of the Precorneal tear film?

1. Lipid

2. Aqueous

3. Mucous

4. Glycocalyx

Describe the lipid layer.

Produced by meibomian glands

Describe the aqueous layer.

1. ~60% of tear film

2. Contains proteins and electrolytes

3. Produced by main and accessory lacrimal glands

Describe the mucous layer.

1. Produced by goblet cells

2. Contains MUC5AC

Describe the glycocalyx.

Membrane spanning mucins on corneal epithelium cells

Contains MUC1, MUC4, MUC16

Describe the comparison of blood and vitreous humor.

1. K+ increased 7x

2. Na+ slightly decreased

3. Bicarbonate unchanged

4. Other components decreased