Lecture 4: Tear Film PDF
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This lecture discusses the tear film, its layers, function, and components. It covers the production and distribution of tears, focusing on the structures involved and the role of eyelids. The lecture explains the lipid, aqueous, and mucin layers. Keywords: tear film, eye anatomy, ocular surface.
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Learning Objectives Describe the functions of the tear film and identify the different layers Creates smooth refractive surface Provides O2 to cornea Lubricates surface of the globe and eyelids Traps debris, sloughed epithelial cells, microbes ...
Learning Objectives Describe the functions of the tear film and identify the different layers Creates smooth refractive surface Provides O2 to cornea Lubricates surface of the globe and eyelids Traps debris, sloughed epithelial cells, microbes Offers antimicrobial protection Encourages corneal wound healing with growth factors and peptides a. Describe the roles and various components of the different layers in the tear film i. Lipid layer (OUTER) ⇒ 1. Meibomian glands (stimulated by blink) within tarsal plate a. Produce meibum 2. Composed of lipids, cholesterol, fatty acids, waxy esters 3. Role in evaporation (prevents evaporation), smooths ocular surface ii. Aqueous layer 1. Lacrimal gland and accessory lacrimal glands a. Lacrimal gland: temporal side of frontal bone; separated by levator aponeurosis (palpebral and orbital) b. Accessory lacrimal glands: Krause (near fornix), Wolfring (near tarsal plate) 2. Composed of ions, glucose, enzymes, proteins, antibacterial substances 3. Protects from infections, hydration iii. Mucin/mucous layer (INNER) 1. Goblet cells 2. Composed of mucin a. Barrier function, hydration/lubrication b. Which structure anchors the tear film to the ocular surface? ⇒ i. Mucous layer binds tear film to the ocular surface via theglycocalyx ii. Glycocalyx: binds mucin to ocular surface, provides wettability Be able to describe the role of eyelids in the production and distribution of tears a. Which structures produce each layer of the tear film and where specifically, are the structures located? i. Lipid layer (OUTER) 1. Meibomian glands → located in the tarsal plate 2. Produce/secrete meibum; expressed by blinks 3. Composed of cholesterol, lipids, fatty acids, waxy esters ii. Aqueous layer . L acrimal glands → temporal side of frontal bone 1 2. Accessory lacrimal glands a. Glands of Krause → near fornix b. Glands of Wolfring → near tarsal plate 3. Composed of ions, glucose, enzymes, proteins, antibacterial substances iii. Mucous layer (INNER) ⇒ 1. Goblet cells → located in palpebral conjunctival epithelium Crypts of Henle (invaginations of conj) 2. Mucin types: a. Large: goblet cells b. Small: lacrimal and accessory lacrimal glands c. Transmembrane mucins: goblet cells b. Identify any structures that may help with expression of tear film components i. Lipid layer: meibum (produced by meibomian glands) expressed by blinks ii. Mucin layer: glycocalyx adheres tear film to ocular surface 1. Binds mucin/tear film to surface, provides wettability Describe step-by-step the drainage of tears from the ocular surface Structures: ○ Lacrimal lake Made up by plica semilunaris and caruncle Holds tears to be drained from ocular surface into each punctum ○ Puncta (where tears are drained) Located on lacrimal papilla (elevated area of epithelium) Drains tears from lacrimal lake to canaliculi ○ Canaliculus (extend from puncta) Fibers of muscle of Horner wrap around canaliculi Drain into nasolacrimal sac Pulls lid to move tear film towards nasal campus ○ Common canaliculus ○ Nasolacrimal sac (continuous with canaliculus) Continuous with nasolacrimal duct ○ Nasolacrimal duct (continuing to drain) Passes through valve of Hasner and drains into inferior nasal meatus ○ Valve of Hasner Prevents backflow of tears Tears move into inferior nasal meatus ○ Inferior nasal meatus (drainage from valve of Hasner) a. Which structures help to facilitate the flow of tears off the ocular surface? i. Lacrimal lake holds tears to be drained ii. Puncta drains tears into canaliculus iii. analiculus pulls lid to move tear film towards nasal campus C iv. Nasolacrimal duct → tears flow through the valve of Hasner and drains into inferior nasal meatus v. Valve of Hasner → when tears pass through here, the tears are unable to backflow vi. Orbicularis oculi: moves tears to medial canthus Identify factors that may decrease tear film production or induce tear film instability Age Contact lens wear (interrupts mucoaqueous layer, unable to bind to glycocalyx) Environment (quicker evaporation of tear film) Inflammation Hormones a. What is the importance of Vitamin A? i. Viability of epithelial conjunctival cells ii. Goblet cells DECREASE with Vitamin A deficiency 1. Make up mucin/mucous layer (INNERMOST LAYER) → hydration/lubrication hich cranial nerves are responsible for blinking and innervating other muscles and/or glands that W may help with tear film production CN III ○ Superior palpebral levator muscle Opens or elevates eyelids ○ Inferior tarsal plate of Muller Additional lid opening ○ Superior tarsal plate of Muller Additional lid elevation CN VII ○ Orbicularis oculi Closes eyelids Orbital portion of orbicularis oculi: reflex blink, forceful closure Palpebral portion of orbicularis oculi: gentle eye closure, spontaneous blink ○ Frontalis Surprised, raises eyebrows ○ Corrugator Concentration, brings eyebrows medially and down ○ Procerus Aggression, brings eyebrows down and medially Exocrine glands ○ Apocrine: Glands of Moll ○ Merocrine: Lacrimal glands ○ Holocrine: meibomian glands and glands of Zeis Lipid layer (outer) ○ Meibomian glands (palpebral conjunctiva epithelium) ○ Secrete meibum, expressed with blinks Aqueous layer ○ Lacrimal glands (temporal side of frontal bone) ○ Accessory lacrimal glands (Krause = fornix, Wolfring = tarsal plate) Mucin layer (inner) ○ Goblet cells (within tarsal plate; Crypts of Henle) ○ Secrete mucin Identify the difference between the lacrimal gland and lacrimal sac Lacrimal gland ○ Makes up aqueous layer (middle layer of tear film) Composed of enzymes, proteins, ions, glucose, antibacterial substances Protects from infection; hydration ○ Located on temporal side of frontal bone Lacrimal sac ○ Nasolacrimal sac → where tears continue to drain into inferior nasal meatus ○ Continuous with canaliculus and nasolacrimal duct ○ Involved in tear drainage Describe nasolacrimal duct obstruction, dacryocystitis, dacryoadenitis Nasolacrimal duct obstruction ○ Due to Valve of Hasner (prevent backflow of tears) Something blocking valve = allergies, infection, inflammation, aging Dacryocystitis ○ Inflammation of nasolacrimal sac Dacryoadenitis ○ Inflammation of lacrimal gland
