Ocular Phys: Pre-Corneal Tear Film Part 1

Functions of precorneal tearfilm

1. provide a smooth refractive surface on the cornea

2. Allow diffusion of oxygen and other nutrients on the cornea

3. Flush material from the surface of the eye

4. maintain health of cornea and conjunctiva epithelium

5. lubrication of lids and globe

6. medium for antimicrobial agents

2 phase model of the tear film

surface lipid layer

mucoaqueous layer

Sources of the lipid layer

meibomian glands

glands of zeis

two phases of the lipid layer

1. Outer non-polar lipids

2. inner polar lipids

Outer non polar lipids

prevent evaporation and barrier to pathogen

inner polar lipid

acts as a surfactant to facilitate interaction with mucoaqueous layer

helps mucoaqueous layer spread over cornea

Lipid layer functions

1. reduce evaporation (main function)

2. lubrication

3. barrier to pathogens

4. increase spread-ability of tear film by lowering the surface tension of tears

Secretion by Meibomian Gland

meibum

Meibum has a melting range which

allows for good viscosity of meibomian oil and spreadability

secretion is a ____ process

continuous process over 24 hrs

Evidence of ----- influence

neural, hormonal and vascular

Secretion by Meibomian Gland increases with

age and androgen production

Secretion by Meibomian Gland decreases with

dry eye disease and menopause

Sources of aqueous of the Mucoaqueous layer

main lacrimal gland

accessory lacrimal gland

4 main components of the mucoaqueous layer

1. 98% H2O

2. Tear film solutes - Glucose, Vitamin A, Vitamin C, Amino Acids, Lactate(GACAL)

3. Aqueous Electrolytes - Na+, Cl-, K+, HCO3-

4. Aqueous proteins - many

Eyes open- tear film has _____ oxygen

high

Eyes closed - tear film has

low

Aqueous solutes of the Mucoaqueous layer

Glucose

Vitamin A

Vitamin C

Amino Acids

Lactate

(GACAL)

Glucose

low levels to prevent bacterial growth

growth. Cornea gets most of its glucose from ant chamber aqueous.

Vitamin A

need for gobelt cells and epithelial maintenance

Vitamin C (ascorbate)

important antioxidant in tears

Lactate

byproduct of metabolism

Electrolytes of the Mucoaqueous layer

HCO3-

NACL and KCL

HCO3-

buffers tears

regulates tear pH

NaCL and KCL or K+ Na+ Cl- n

Found in higher levels as a part of active secretion

regulate osmotic flow of fluids between corneal epithelial cells and tear film

Important to have normal tear film electrolyte balance to prevent corneal edema.

treat corneal edema with

hypertonic saline to dehydrate the cornea

Chronic hypertonic tears cause

damage to ocular surface

triggers inflammation and dry eye

Other tear electrolytes Fe2+, Cu2+ Mg2+, Ca2+ are enzyme cofactors for controlling membrane permeability

Major Aqueous proteins

- lipocalin

- sIgA

- lysozynme

- lactoferrin

- IgG

- IgE

- Transferin

-Phosolipase A2

Tear lipocalin

- important for tear stability by binding to meibomian lipids

- prevents infections by inhibiting siderophores (transport iron across cell membranes),

- increases viscosity of tear film

Secretory IgA

found on mucus membranes

prevents adhesion and colonization of bacteria

prevents attachment of viruses

IgG

low levels

stimualtes phagocytosis

IgE

allergic reations

Lysozyme

degrades cell wall of GM+ bacteria

most abundant

Lacteroferrin and Transferrin

- Binds Iron - prevents bacteria growth

Phospholipidase A2

degrades phospholipids

antibacterial vs GM+ bacteria

Cytokines and growth factors control wound healing by

by regulating epithelial proliferation, mobility and differentiation

matrix metalloproteinases (MMPs)

breaks down collagen, ECM, and BM proteins for the breakdown and remodeling following a corneal wound

MMP9 levels are elevated in

dry eye disease in patients that have corneal damge

levels are measured clinically in DED and elevated levels often represent a more severe form of DED

Functions of the Mucins in the Mucoaqueous layer

- fills irregularities of corneal surface

- mixes with the aqueous to create a hydrated gel like structure

- reduces the surface tensin of the aqueous layer for spread ability of tear film

- removes contaminantes, debris, and cells from the tear film --> are rolled into threads to be drained via the nasolacrimal system

Gel-forming mucins from

large mucins from gobelt cells of conjunctiva

Gel forming mucins function

scavengers of tears

remove foreign materials - cleaning crew

make tear film more hydrophilic - hydrate cornea

reduce surface tensino

Soluble mucins from

lacrimal gland and stratified squamous cells of conjunctica

Soluble mucins function

make tear film more hydrophilic

hydrate the cornea

membrane anchored mucins in the muoaqueous layer are secreted by

corneal epithelial cells and conjunctiva epithelial cells

membrane anchored mucins

attatch to glyocalyx

function to adhere tear film tp corneal surface

protect and hydrate corneal epithelium

Ocular defense mechanisms that prevent eye infections include

- rapid reflex blink

- flushing effect of tears

- water soluble components of tears

- lysozyme - gram postive degrades cell walls

- Tear lipocalin - sequesters microbial siderphores - reduces iron

- lactoferrin - binds iron

- phosopholipase A2 - degrades phospholipids in gram postive

SIgA - antiseptic paint

- IgG - promotes phagocytosis

- epithelium of cornea is a physical barrier

Water

98%

Thickness

3.0 micrometers

Volume

5-10 microliters

Osmolarity

275-300 mOsm/L

Temperatures

35 centrigrade at central cornea

pH

6.5 - 7.5

basal tears

open eye tears

Constitutively or continuously produces and continuously coat the ocular surface

reflex tears

produced in response to stimulus of V1

sometimes V2 or V3

emotional tears

stimulation like sadness or joy

Closed eye tears

produced continuously when you are asleep or when eyes are closed

like basal tears but with proteins enter from conjunctival blood vessels