Tear Film Flashcards
What are functions of the precorneal tear film?
Provide a smooth optical surface at the air corneal interface
Allow diffusion of oxygen and other nutrients
Serve as a medium for removal of debris and protect the ocular surface
Provdies a medium for antimicrobial agents (lysozyme, lactoferrin and Iga)
What is the layers of the precorneal tear film? Thickness?
Lipid layer - 0.1um
Overlying
Mucoaqueous later
Aq (10um) , mucous (30um)
How thick is the human tear film?
3.4um
What are the functions of the lipid layer?
Retards evaporation of tear film
Contributes to the optical properties of the tear film because of its position at the air-tearfilm interface
Maintains hydrophobic barrier that prevents tear overflow by decreasing surface tension
Prevents damage tot he eyelid margin skin by tears
Where are the lipids secreted from? How may are there in each eyelid?
Meibomian glands
30-40 in the upper lid
20-30 in the lower lid
Sebaceous glands of Zeis - in the eyelid margin close to eyelash roots
Where do Meibomian glands open?
Eye lid margin between tarsal gray line and mucocutaneous junction
What are the functions of the mucoaqueous layer?
Transmits oxygen to the avascular corneal epithelium
Maintains a constant electrolyte composition over the ocular surface epithelium
Provides an antibacterial and antiviral defecse
Smooths minute irregularities of the anterior corneal surface
Modulates corneal and conjunctival epithelial cell function
Converts the corneal epitehlium from the hydrophobic to hydrophilic layer, which is essential for even and spontaneous distribution of the tear film
Interacts with the tear lipid layer to reduce sruface tension thereby stabilising the tear film
Lubcircates the eyelids as they pass over the globe
What are the two parts of the lacrimal gland
Orbital and palpebral lobes
By lateral horn of levator aponeurosis
What are glands of Krause? Where are they located?
2/3 of the accesory lacrimal glands
Located in the lateral part of upper fornix
Where are glands of Wolfring?
Along proximal margin of each tarsus
What does the aqueous component consist of? FUnctions?
Electrolytes:
- regulate osmotic flow of fluids, buffer tear pH, enzyme cofactors in controlling membrane permeability
Water
Proteins
What is the concentration of sodium and potassium in tear compare to blood?
Na same
K 5-7 times
What is used to dehydrate the cornea in corneal oedema?
Hypertonic saline
5%NaCl
What solutes are in tear film
Urea
Glucose
Lactate
Citrate
Ascorbate
Amino acids
What immunoglobulins are in tears?
IgA and secretory IgA
Where is IgA made?
Plasma cells in intersitital tissues of the main and accessory lacrimal galnds
Substantia propria of the conjunctiva
Secretory IgA in the lacrimal gland acini
What is he function of IgA?
Local host defense mechanism of external eye
What immunoglobulin is elecated in tear film in vernal conjunctivitis?
IgE
Increased IgE producing plasma cells in the giant papillae of the superior tarsal conjunctiva and elevated histamine levels
Where is the mucin component of the mucoaq layer?
Coats the microplicae of the superficial corneal epithelial cells and forms a fine networs over the conjunctival surfaceW
What does the mucin component contain?
Mucin
Proteins
Electrolytes
Water
Carbohydrates
Polar glycocalyx
What are the types of mucins?
Secreted mucins - gel-forming and soluble, released into extracellular environment, secreted by goblet cells of conj
MEmbrane spanning mucins
Embedded in the lipid bilayer of cells, expressed by the stratified squamous cells of the conj and corneal epithelia
What are mucins?
glycoproteins
What is mucus discharge in DES? Infection? vernal conjunctivitis?
DES - stringy thin translucent
Infection - globular, crusting
vernal - thick, stretchy strands
What are the stimuli of lacrimal secretion?
Neural tear secretion
Peptide hormones including alpha-MSH and ACRH stimulate protein secretion from main lacrimal gland
Steroid hormones - androgens, stimulate secretion of sIgA from the main lacrimal gland and lipid from the Meibomian glands
