Tear Film Layers and Functions

The tear film has three layers: the outer lipid layer, the middle aqueous layer, and the inner mucin layer.

The tear film lubricates, provides a smooth refractive surface, removes foreign objects, and delivers nutrients and antimicrobial compounds.

A network of polysaccharides attached to the corneal surface that helps bind the tear film, particularly the mucin layer, and ensures wettability.

Glycoproteins attached to corneal cells that help keep the eye moist between blinks.

The mucous layer, composed of mucin secreted by goblet cells, provides a hydrophilic coating for the corneal epithelium, helps the tear film adhere, and lubricates and protects the eye.

These mucins are highly hydrophilic, have antimicrobial properties, and are synthesized by goblet and epithelial cells.

These mucins create gel-like pockets within the mucous layer, providing structure.

The aqueous layer provides hydration and protection against infection, containing low concentrations of soluble mucins and high levels of ions and soluble non-mucin proteins.

Lysozyme is an antibacterial enzyme found in tears. It breaks down the peptidoglycan in bacterial cell walls, inhibiting bacterial growth.

Lactoferrin is an antimicrobial protein in tears. It binds to free iron, making it unavailable for bacterial growth and survival.

Lipocalin helps transport molecules into corneal epithelial cells and also inhibits microbial iron uptake.

Lacritin stimulates tear flow to keep the eyes moist and healthy. It also aids in clearing damaged cellular components.

IgA is an antibody found in tears and other secretions of the mucous tract. It neutralizes microbes and enhances phagocytosis.

The lipid layer reduces surface tension, smooths the tear film surface, prevents evaporation, and prevents tear spillover at the eyelid margins.

The lipid layer contains completely hydrophobic lipids, which repel water, and amphipathic lipids, which have both hydrophobic and hydrophilic ends.

The tear film is organized in layers: the amphipathic lipid layer interacts with the aqueous layer, while the non-polar lipid layer forms the outermost layer.

The glycocalyx is a sticky network of polysaccharides attached to the corneal surface, acting as a bridge between the tear film and the cornea.

These mucins are attached to corneal cells, acting as a barrier to maintain constant hydration between blinks.

The innermost layer of the tear film is composed of mucin, secreted by goblet cells, creating a hydrophilic coating over the cornea.

Small, soluble mucins are highly hydrophilic, attracting water and possessing antimicrobial properties. Large, gel-forming mucins create gel-like pockets within the mucin layer, providing structural integrity.

The aqueous layer is rich in ions and proteins, providing hydration and protecting against infection.

The Na+/K+ pump maintains the electrolyte balance by establishing a concentration gradient, while membrane transport facilitates ion movement. If the junctions are not tight, paracellular diffusion occurs.

Water moves towards areas with higher solute concentration (like an isotonic solution), ensuring the proper balance of fluids in the tear film.

Lysozyme is an antibacterial enzyme in tears that breaks down peptidoglycan, a key component of bacterial cell walls, hindering bacterial growth.

Lactoferrin is a protein in tears that binds to free iron, making it unavailable for bacterial growth and survival, effectively starving microbes.

Lipocalin is a tear component that transports vital cargo into corneal cells and inhibits microbes from stealing iron, a vital nutrient.

Lacritin is a powerful signaling molecule in tears that triggers the production of more tears to keep the eyes moist and fight infections.

IgA is an antibody in tears that neutralizes microbes and enhances phagocytosis (engulfment) by immune cells, protecting the eye from infection.

The lipid layer on the tear film surface reduces surface tension, smooths the tear film, prevents water evaporation reducing dryness, and prevents tears from overflowing.

The lipid layer is composed of two types of lipids: Completely hydrophobic lipids that repel water and amphipathic lipids that have both a water-loving (hydrophilic) and water-repelling (hydrophobic) end.

A network of polysaccharides attached to the corneal surface that helps bind the tear film, particularly the mucin layer, and ensures wettability.

Water in the tear film moves towards areas where there is a higher concentration of solutes, like an isotonic solution.

Lysozyme is an enzyme in tears that breaks down the peptidoglycan in bacterial cell walls, preventing bacterial growth.

Lactoferrin is a protein in tears that grabs onto free iron, making it unavailable for bacteria to use, effectively starving them.

Lipocalin is a protein in tears that acts as a transporter for molecules into corneal cells and also prevents bacteria from using iron.

IgA is an antibody in tears that neutralizes microbes and helps immune cells engulf them, protecting the eye from infection.

The lipid layer on the tear film surface reduces surface tension, smooths the tear film, prevents water evaporation, and stops tears from overflowing.

The lipid layer is made of two types of lipids: completely hydrophobic lipids, which repel water, and amphipathic lipids, which have both hydrophobic and hydrophilic ends.