Lecture 4 - Tear Film & Lacrimal System

What are the three main layers of the tear film?

Mucin (innermost), aqueous (middle), and lipid (outermost)

Approximately what percentage of tear volume is contributed by the aqueous layer?

About 90%

Do the tear film layers always remain completely separate?

No, the tear film components do not always remain separate

What is the mixed aqueous and mucin layer sometimes called?

The mucoaqueous layer

What is the normal pH of tears?

7.45

Which buffer system maintains tear pH?

The bicarbonate buffer system (H₂CO₃ / HCO₃⁻)

How does prolonged eyelid closure (such as during sleep) affect tear pH?

Increased CO₂ leads to decreased pH

How are tears affected during chemical injuries?

Tear pH is impacted during chemical injuries

Why are basic ocular chemical injuries worse than acidic injuries?

Due to their level and rate of penetration of ocular tissues

How does blinking affect tear temperature?

Tears warm with blinking

How does exposure to the atmosphere affect tear temperature?

Tears cool with exposure to the atmosphere

What is the viscosity of tears compared to water?

Tears are about 50% more viscous than water. This can vary due to blinking or environmental factors.

What is the ideal tear osmolarity?

308 mOsm/L

What does osmolarity mean?

The concentration of solute particles in a solution

Which ions are the main contributors to tear osmolarity?

Sodium (Na⁺) and chloride (Cl⁻)

Which other ions are present in the aqueous layer of tears?

Calcium (Ca²⁺) and potassium (K⁺)

What is hyperosmolarity?

Tear osmolarity higher than 308 mOsm/L

What causes hyperosmolarity in tears?

Reduced aqueous tear content

In which condition is tear hyperosmolarity commonly seen?

Dry eye syndrome

How does hyperosmolarity affect epithelial cells?

It stimulates epithelial cell damage and apoptosis, leading to inflammation

How does hyperosmolarity affect corneal nerves?

It damages corneal nerves, affecting tearing and reflexes

Why is a smooth anterior tear film surface important?

It is necessary for proper optics

Where does the largest change in refractive index in the eye occur?

From air (1.00) to tear film (1.337)

What is the average thickness of the tear film?

Approximately 5 µm

What is the average rate of tear production?

1–2 microliters per minute

What is the average total tear volume?

7–9 microliters

What is the maximum tear volume including the fornices?

20–30 microliters

(1 eyedrop is usually too much volume for the eye to hold!)

What are basal tears?

Constant, baseline tear production that lubricates and nourishes the ocular surface

How do basal tears differ in composition?

They have higher protein and lipid content than reflex tears. (Makes them more stable on the eye/higher quality.)

What are reflex tears?

Tears produced in response to external stimuli or irritants

How much can reflex tear production increase?

Up to 100 times normal production

What is the purpose of reflex tearing?

To clear and flush the ocular surface

What are mucins?

Glycoproteins

What is mucus composed of?

Mucins, proteins, electrolytes, and water

What additional substances are found in the mucin layer?

Immunoglobulins, urea, glucose, and protein

What is the primary function of the mucin layer?

To adhere the aqueous layer to the ocular surface

What is the glycocalyx?

A layer at the apical surface of the corneal epithelium

What is the glycocalyx made of?

Glycoproteins, glycolipids, proteoglycans, and related molecules

Why is the glycocalyx important for the corneal epithelium?

Because the corneal epithelium is hydrophobic; helps maintain tear film

How does the mucin layer protect against pathogens?

It traps pathogens and prevents their invasion

How do mucins block microbial attachment?

By blocking binding sites on microbes

What are examples of membrane-associated mucins on the ocular surface?

MUC1, MUC4, and MUC16

(Don’t need to memorize this)

Which cells produce mucins?

Goblet cells

Where are goblet cells located?

In the stratified squamous epithelium of the conjunctiva

What type of glands are goblet cells?

Exocrine glands with apocrine secretions

Where is goblet cell density highest?

In the conjunctival fornices

What factors affect mucin production and secretion?

Autonomic innervation and goblet cell density

Which autonomic receptors are present on goblet cells?

Parasympathetic and sympathetic receptors

Which autonomic system primarily initiates mucin secretion?

Parasympathetic stimulation

How does goblet cell density affect mucin secretion?

Fewer goblet cells decrease secretion; more goblet cells increase secretion

What conditions decrease goblet cell density?

Dry eye and autoimmune disease

What conditions increase goblet cell density?

Allergies, chronic irritation, or injury

What is the primary component of the aqueous layer?

Water

What inorganic salts are found in the aqueous layer?

Sodium chloride (NaCl)

What electrolytes are present in the aqueous layer?

Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl⁻, and HCO₃⁻

What antioxidants are found in the aqueous layer?

Urea, ascorbate, glutathione, and vitamin D

What nutrients are present in the aqueous layer?

Glucose, amino acids, proteins, glycoproteins, and vitamin A

Why is vitamin A important in the tear film?

It is important for goblet cell development

How does oxygen reach the aqueous layer?

It dissolves from the atmosphere

What antibacterial substances are found in the aqueous layer?

Lysozyme, lactoferrin, and IgA

What is the role of the aqueous layer in ocular surface health?

It maintains ocular surface health

How does the aqueous layer support the cornea?

It provides oxygen to the avascular cornea

What role does the aqueous layer play in hydration?

It regulates hydration

What role does the aqueous layer play in pH balance?

It regulates pH

How does the aqueous layer protect the eye?

It protects from inflammation and infection

What is the function of lysozyme in tears?

It destroys bacterial cell walls

What is the function of lactoferrin in tears?

It binds free iron to prevent microbial growth and modulates cytokine production

What is the function of IgA in tears?

It prevents microbial adhesion

How does the aqueous layer contribute to wound repair?

It supports healing processes

How does the aqueous layer help maintain ocular clarity?

It clears debris

Which structure produces reflex tears?

The lacrimal gland

What type of gland is the lacrimal gland?

An exocrine gland with merocrine secretion

How do lacrimal gland acini drain?

Into tubules and ducts

What is the lacrimal gland’s contribution to tear volume?

It is the major contributor

What substances does the lacrimal gland secrete?

Electrolytes, water, and proteins

What immune cells are found in the lacrimal gland?

Lymphocytes, plasma cells, mast cells, and macrophages

Which immunoglobulin do plasma cells in the lacrimal gland express?

IgA

How does the lacrimal gland contribute to immunity?

Through immune cells and IgA secretion

What drives electrolyte and water secretion in the lacrimal gland?

Na⁺/K⁺ ATPase activity

What is the role of basolateral Na⁺/K⁺ ATPase?

It transports Na⁺ away from the lumen to create an ion gradient

What does the Na⁺ gradient created by basolateral pumps cause?

Other transporters bring Na⁺ back into the cell

What is the role of apical Na⁺/K⁺ ATPase?

It pumps Na⁺ into the lumen (out of the cell, on the lumen side)

How does chloride move during lacrimal secretion?

Cl⁻ follows the Na⁺ gradient into the lumen

How does water move during lacrimal secretion?

Water follows Cl⁻

What are the accessory lacrimal glands?

The glands of Krause and Wolfring

How are accessory lacrimal glands histologically classified?

They are identical to the main lacrimal gland

What is the function of accessory lacrimal glands?

Similar to the main lacrimal gland in secretion

What is the net result of conjunctival epithelial ion secretion?

Isotonic fluid secretion into the tear film

What ions are secreted and absorbed by the conjunctival epithelium?

Cl⁻ is secreted; Na⁺ is absorbed

What is the ratio of Cl⁻ secretion to Na⁺ absorption?

1.5:1

Why does fluid enter the tear film from the conjunctiva?

Because higher Cl⁻ secretion draws water

How does water move across conjunctival epithelium?

Through aquaporins along a concentration gradient

How does Na⁺ move in conjunctival epithelium?

Absorbed apically and secreted basolaterally

How does Cl⁻ move in conjunctival epithelium?

Absorbed basolaterally and secreted apically

How does Na⁺ move from tears into the corneal epithelium?

Via apical channels

How does Na⁺ move from epithelium to stroma?

Via Na⁺/K⁺ ATPase in the basolateral membrane

Which ions are transported from stroma into epithelium?

Na⁺, K⁺, and Cl⁻

Which transporter moves these ions?

Na⁺/K⁺/2Cl⁻ cotransport protein

What effect do high intracellular K⁺ levels have?

K⁺ leaks into the stroma

How does K⁺ leak affect Cl⁻ movement?

It forces Cl⁻ from epithelium into tears via apical channels