Lecture 5: Tear Film Structure & Function PDF
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This document is a lecture on the structure and function of the tear film, covering lipid, aqueous and mucin layers, their characteristics, as well as highlighting the role of important chemical components in maintaining eye health. It also includes functions and effects of different chemical components and proteins.
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Learning Objectives Sketch and name the layers of the tear film Outer to inner: lipid layer, aqueous layer, mucin layer Functions: ○ Lubricates ocular surface ○ Refractive surface ○ Smooth optical surface...
Learning Objectives Sketch and name the layers of the tear film Outer to inner: lipid layer, aqueous layer, mucin layer Functions: ○ Lubricates ocular surface ○ Refractive surface ○ Smooth optical surface ○ Must be stable ○ Mechanically removes foreign bodies ○ Supplies nutrients and antimicrobial compounds ○ Complex multi-layered ○ Short-lived ○ Produced in minute amounts Briefly describe the glycocalyx Binds tear film (mucin layer) to ocular surface, provides wettability Network of polysaccharides apart of glycoproteins Describe the characteristics of membrane-bound mucins Molecules attached to corneal cells Transmembrane glycoprotein → heavily glycocylated Keeps eye wet during inter-blink time List the functions of the mucous layer of the tear film Mucous layer = INNERMOST layer; secretes mucin; goblet cells Hydrophilic coating over corneal epithelium Helps aqueous layer adhere to ocular surface Lubricates and protects ocular surface Briefly describe and differentiate small, soluble mucins from large gel-forming mucins Small, soluble mucins ○ Secretory mucin ○ Extremely HYDROPHILIC (attracts water) ○ Antimicrobial activity ○ Synthesized by goblet cells and epithelial cells Large gel-forming mucins ○ Secretory mucin ○ Causes pockets of mucous layer to become gel-like ○ Provides structure to mucous layer List the functions of the aqueous layer of the tear film Protects against infection, hydration Lower concentration of soluble mucins Higher levels of ions and soluble non-mucin proteins Outline the mechanism for maintaining the appropriate electrolyte concentrations in the tear film Driving force: Na+/K+ pump ○ Establishes concentration gradient Membrane transport If not tight junctions → paracellular diffusion occurs (between cells) ⇒ Ions cannot go through phospholipid bilayer requires protein for transport Water moves towards areas of higher solute concentration (isotonic solution) Briefly describe the roles of: Lysozyme ○ Antibacterial ○ Catalyzes bond breakage in peptidoglycan of cell walls ○ 20-30% - basal and reflex tears Lactoferrin ○ Antibacterial Binds free iron; reduces availability necessary for microbial growth and survival; inhibits biofilm formation ○ Antiviral Inhibits entry of viruses ○ Anti-inflammatory ○ 20-30% - basal and reflex tears Lipocalin ○ Transport of cargo into corneal epithelial cells by endocytosis ○ Antimicrobial Inhibits uptake of iron by microbes ○ 25% - reflex tears Lacritin ○ Stimulates tear flow ○ Autophagy (clears damaged proteins and organelles) ○ Anti-microbial Disrupts microbial cell membranes Immunoglobulin A ○ Secreted antibody Primary antibody lining mucous tracts ○ Inactivates microbes ○ Promotes phagocytosis Growth factors ○ Affects cellular proliferation, migration, differentiation, survival List the functions of the lipid layer Reduces surface tension of tear film Smooths film surface Retards water evaporation (prevents evaporation) Prevents spillover of tear fluid at lid margins List the major components of the lipid layer 1. Completely hydrophobic a. Does not interact with water 2. Amphipathic → hydrophilic end (forms H bonds) and hydrophobic end Briefly describe the organization of lipids in the lipid layer Amphipathic lipid layer interacts with aqueous layer ○ Stable interaction Non-polar lipid layer (more outer) Briefly explain the role of fatty acid saturation in tear film stability Rapidly reorganizes and spreads as continuous layer over ocular surface following a blink Maintains integrity during inter-blink time
