Tear Film 2

• Sebaceous glands are microscopic exocrine glands in the skin that secrete sebum to lubricate hair and skin
• Functions of the tear film include optical benefits like filling corneal irregularities, mechanical functions like lubrication and debris removal, nutritional support with oxygen and nutrients, and defense with antibacterial agents
• Lacrimal system dynamics involve tear production/volume, tear turnover rate, tear flow rate, and reflex tearing due to peripheral sensory and central sensory stimulation
• Tear dynamics and elimination involve the excretory system, evaporation, and conjunctival absorption
• An alternative model for tear stability involves mucus thinning, where mucus thins and exposes hydrophobic epithelium, leading to aqueous layer break up
• ·      Tear dynamics and elimination.

  1.    Excretory System

  –      Gravity: Tears fall by gravity into lower meniscus

  –      Capillary attraction: Conducts tears to the puncta

  –      Lid movements.

  2.    Evaporation

  –      4.1 x 10-7 g/cm2/s

  –      Lipid deficiency 7.0 x 10-7 g/cm2/s

  3.    Conjunctival absorption

  –      up to 2.0 microl/min.
• Reflex Tearing because of:

  1.    Peripheral sensory

  -       Stimulation of cornea or cilia

  2.    Central sensory

  -       Stimulation of the retina (light)

  -       Psychogenic (emotional) factors
• ·      Tear dynamics and elimination.

  1.    Excretory System

  –      Gravity: Tears fall by gravity into lower meniscus

  –      Capillary attraction: Conducts tears to the puncta

  –      Lid movements.

  2.    Evaporation

  –      4.1 x 10-7 g/cm2/s

  –      Lipid deficiency 7.0 x 10-7 g/cm2/s

  3.    Conjunctival absorption

  –      up to 2.0 microl/min.
• Lacrimal System Dynamics

  ·      Tear production/volume.

  -       Total volume 4-10 microlitres.

  -       Mean volume 7.0microlitres.

  -       Max capacity 25microlitres.

   

  ·      Tear Turnover Rate (%/min)

  –      Calculated from fluorescein decay rate.

  –      Typically, 16%/min.

  ·      Tear Flow Rate (microl/min)

  –      Requires knowledge of tear volume.

  –      Typically, 1.1microl/min.
•    Mucous Layer

  ·      Composition

  –      High molecular weight mucins (protein with many carbohydrate chains attached)

  ·      Properties

  –      Allows the aqueous to spread over the non- wettable corneal epithelium.

  ·      Function

  –      Essential in making the tear film spread over cornea.

  –      Important in maintaining tear stability.
• Aqueous Layer

  ·      Composition 

  Electrolytes, Hydrogen ions

  -       Na+, K+ & Ca2+ and others, maintain tonicity, important for epithelial integrity.  pH maintained at 7.4.

  ·      Proteins / Enzymes

  -       Lysozyme (20% of total protein) (digests bacterial walls)

  -       Lactoferrin (sequesters Fe is bacteriostatic)

  -       Lipocalin (helps lipid layer spread over tears)

  -       IgA (essential part of the immune system)

  -       60+ other proteins have been detected.

  ·      Metabolites

  -       Glucose 
• 1.    Lipid layer

  ·      Composition

  -       Polar and non-polar lipids

  ·      Properties

  -       Spreads well over the tear film, does not spread on saline, aq layer contains special proteins (lipocalin)

  ·      Function

  -       Retards evaporation of aqueous (4-20-fold).

  -       Prevents skin lipid contamination.

  -       Prevents over-spilling onto the eyelids.
• Functions of the Tear Film

   

  ·      Optical

  –      Fills in corneal epithelial irregularities. 

  –      Provides a perfect smooth optical surface.

  ·      Mechanical

  –      Lubrication

  –      Flushes debris

  –      Removes water from the cornea.

  ·      Nutrition

  –      O2 and Nutrients

  ·      Defence

  –      Antibacterial Agents
• Tear Film-Origin (Lipid)

  –      Meibomian glands

  –      Glands of Zeis                                     

  –      Glands of Moll
• Mucus Layer (Origin)

  ·      Mucus Layer

  -       Goblet Cells

  §  Glands of Manz (limbal ring)

  §  Crypts of Henle

  –      Non-goblet epithelial cells

  –      Lacrimal gland (recent evidence that soluble mucins produced here also).