Tear Film

Tear film functions

1. Provide the first refractive surface of the eye

2. Protect and maintain outermost tissues of eye and adnexa

How does the tear film protect the eye?

• Lubrication for blinks and movement

• Seals lid margin during prolonged closure

• Defend against foreign particles and microbes

• Deliver oxygen and other nutrients to cornea/conjunctiva

• Wound repair

Types of tears

Basal, reflexive, psycho-emotional

Basal tears

Continuous lacrimal secretion

Decreases with age → Associated with dry eye

Reflexive tears

Increase in basal secretion in response to external stimuli (temp, chemical, pressure, light)

Not found in newborns

Psycho-emotional tears

Only in humans

Tear film producers

Ocular surface epithelia, meibomian, lacrimal, accessory glands

Tear film characteristics

• Thicker in periphery

• Isotonic (similar to plasma, doesn’t push or pull hydration into cornea)

• 7.0-7.4 pH

• 34-36°C (body temp), increases when eyes are closed and decreases in dry eye

• 5-10µL volume and 1-2 µL/min viscosity, decreases during blinks and increases between blinks

• Refractive index of 1.33

Glycocalyx

Membrane bound mucins that extend from microvilli of cornea and conjunctiva cells

Membrane bound mucins

Protein core and carbohydrate side chains

MUC1, MUC4, MUC7, MUC16

Deficient in Sjogren’s

Glycocalyx functions

• Carbohydrate side chains hold water at surface (hydrophobic → hydrophilic)

• Pathogen barrier

• Facilitates movement of mucins in mucoaqueous layer

Mucoaqueous layer

2 components that intermix

1. Mucus covering the glycocalyx

2. Watery aqueous

Mucous aspect functions

• Maintain hydration of cornea and conjunctiva

• Provide lubrication

• Prevent cornea and conjunctiva adhesion

Mucous aspect producers

Conjunctiva and goblet cells

Mucous aspect components

Ectodomains shed from membrane spanning mucins, soluble membrane spanning mucins, proteins, electrolytes, water

Main: MUC5AC

Goblet cells

Dispersed among conjunctiva and concentrated in medial canthus

Merocrine secretion

MUC5AC

Produced in goblet cell ER and modified in golgi

MUC5AC production Ach pathway

Ach from parasympathetic bind to M2 and M3 muscarinic receptors → Production of MUC5AC

Goblet cell stimulation

Afferent sensory nerves in cornea stimulate efferent parasympathetic nerves that surround goblet cells

MUC5AC production VIP pathway

Vasoactive intestinal peptide from parasympathetic bind VIPAC2 receptors near M3 muscarinic receptors → Production of MUC5AC

Allergic conjunctivitis

Histamine and proinflammatory mediators from mast cells bind to receptors on goblet cells and overstimulate mucus production

Dry eye disease

Hyperosmolarity of tears activates signaling pathways in goblet cells to increase mucus production as coping mechanism

Overstimulation causes goblet cell death

Epidermal Growth Factor

Binds goblet cells and induces mitosis

Helps corneal abrasions heal

Cyclosporin

Tx for dry eye

Cyclosporin primary mechanism of action

Decrease inflammation by modulating the activity of T cells on ocular surface

Cyclosporin secondary mechanism of action

Increase density of goblet cells

Vitamin A deficiency

Causes loss of goblet cells and keratinization of conjunctiva and cornea

Aqueous aspect functions

• Maintain pH and osmolarity of tears through ion regulation

• Protect from infection

• Wash away noxious substances

• Deliver O2 to cornea

Aqueous aspect components

Water (99%), lysozyme, lactoferrin, lipocalin, IgA, lactrin, ions, glucose

Lysozyme

Enzyme that breaks down cell wall of bacteria

Lactoferrin

Protein that binds to iron and inhibits pathogen growth

Lipocalin

Group of proteins that have antioxidant properties, regulate inflammation, and decrease surface tension of tears

IgA

Ab produced by plasma cells and acinar cells in the lacrimal gland that bind to pathogens and prevent them from entering tissues

Lactrin

Protein that induces basal tear secretion and promotes corneal wound healing

Ions

Key mediators of water transportation in tears

Buffer for tear pH, especially HCO3-

Glucose

7.4 mg/dL (correlated with serum concentration)

Increase plasma glucose → Increase tear glucose

Aqueous aspect secretion mechanism

Electrochemical gradient

Tight junctions

Near apical surface, restrict paracellular water movement

Adherens and desmosomes

Glue cells together, associated with cytoskeletons

Hemidesmosomes

Between the epithelial cell and basement membrane

Important corneal abrasion healing

Gap junctions

Ion channels for communication

Passive diffusion

Movement follows concentration gradient

Small gasses, hydrophobic, polar, uncharged molecules can cross

Transcellular transport

Use channel or carrier protein to move substances across membrane

Uniport, symport, antiport

Paracellular transport

Transport of substances between cells

Mostly water driven by osmolarity generated by transport of ions

Aqueous aspect conjunctival secretion

Cl- secreted into tears and Na+ is absorbed from tears at a ratio of 1.5 to 1

Ionic imbalance creates hyperosmotic environment causing water to flow into tears

Aqueous aspect conjunctival secretion mechanism

1. Na/K ATPase expels 3 Na+ and absorbs 2 K+ → Negative intracellular voltage

2. NKCC pulls Na+ back into cell with 2 Cl-

3. HCO3- pumped out and Cl- pumped in

4. Cl- builds up in cell

5. Cl- flows down concentration gradient on apical side

6. Water follows into tears

Accessory lacrimal glands

Embedded in conjunctiva (Krause) and tarsal plates (Wolfring)

Lacrimal gland

Produces 95% of tears and is the major producer of electrolytes, water, and proteins in the mucoaqueous layer of the tear film

Lacrimal gland structure

Myoepithelial cells, acini cells, duct cells

Myoepithelial cells

Surround and protein acinar cells

Acini cells

Columnar secretory cells

Directly feed into ducts

Duct cells

Modify initial secretion from acini cells

Lacrimal gland afferent innervation

Lacrimal branch off CNV carries sensory stimuli from the lacrimal gland

Lacrimal gland efferent innervation

Mainly parasympathetic nerves that surround the acini (some sympathetic)

Lacrimal gland secretion

Afferent nerves from cornea activate efferent parasympathetic nerves → lacrimal secretion

Lacrimal gland secretion afferent pathway

Cornea and conjunctiva sensory nerves → Trigeminal ganglion → Trigeminal brainstem complex nucleus → Superior salivatory nucleus → Reflexive tearing

What stimuli cause reflex tears?

Painful stimuli

What stimuli causes basal tears?

Interblink periods

Transient Receptor Potential Melastatin 8 (TRPM8)

Ion channel expressed on cold thermoreceptor and nociceptor nerve endings in cornea

Detect reduction in ocular surface temp and increase in tear osmolarity

Master regulatory of basal tear production

Acoltremon

Binds TRPM8 thermoreceptors to activate trigeminal parasympathetic pathway → Basal tear production

Lacrimal gland secretion efferent pathway

Parasympathetic nucleus of CNVII → Pterygopalatine ganglion → Post-ganglionic parasympathetic nerves → Lacrimal gland

Varenicline

Bines nicotinic Ach receptors to activate trigeminal parasympathetic pathway → Basal tear production

Efferent sympathetic nerves on lacrimal gland secretion

Descending sympathetic pathway travels through superior cervical ganglion → Post-ganglionic sympathetic nerves innervate the gland

Parasympathetic nerves in the lacrimal gland

Release Ach onto M3 receptors on myoepithelial, acinar, and duct cells → Lacrimal secretion

What receptor does VIP bind to on acinar cells?

VIPAC1

What receptor does VIP bind to on myoepithelial cells?

VIPAC2

Sympathetic innervation on the lacrimal gland

Sympathetic nerves release norepinephrine and neuropeptide Y onto alpha-1 and beta-1 adrenergic receptors on acinar cells

Lacrimal gland has primary what type of innervation?

Parasympathetic

Triggered primarily by M3 receptors

Lacrimal secretion mechanism pathway

1. Na+ pumped out through Na+/K+ ATPase

2. Na+ pulled in through NHE, AE, NKCC along with Cl-

3. Cl- leaves apically through channels building a neg charge outside → Pulls Na+ paracellularly

4. Water follows down concentration gradient

Exocytosis

Primary way proteins are secreted into tears

Granule vesicles fuse with apical membranes and release contents into lumen

Transcytosis

Secondary way proteins are secreted into tears

IgA is endocytosed at basal aspect and transported to be secreted at apical aspect

What stimulates secretion of proteins into tears?

• ACh binding M3 receptors

• VIP binding VIPAC1 and VIPAC2 receptors

• Norepinephrine binding alpha-1 receptors

What causes aqueous layer deficiencies?

• Age decreases lacrimal production

• Sjogren’s

• Trauma/scarring can block ducts

• CL abuse and diabetes can decrease corneal sensitivity

Lipid layer functions

• Prevent aqueous layer evaporation

• Acts as a surfactant to allow the tear film to spread

• Hydrophobic barrier preventing tear overflow and skin sebum from entering

• Water tight seal between lids

• Smooth optical surface

  • Long chain fatty acids disrupt bacteria integrity

Lipid layer contents

Non-polar lipids: Face outside world, wax esters, cholesterol, and cholesterol esters

Polar lipids: Face inside, phospholipids

Meibomian glands

Acini that secrete meibum into ducts that converge and dump near mucocutaneous junction

What surround the basal surface of acini in meibomian glands?

Nerves and blood vessels

Meibum production

Holocrine secretion

Outer cells move inward and synthesize lipids → Burst and becomes secretory product

Cells are replaced by basal cell proliferation