Biochemistry and Metabolism- Tear Film Flashcards
Function of precorneal tear film
- provide a smooth optical surface at the air–cornea interface
- allow diffusion of oxygen and other nutrients
- serve as a medium for removal of debris and protect the ocular surface
Components of precorneal tear film
Evidence continues to support a 2phase model of the tear film, in which a lipid layer overlies a mucoaqueous phase. Components of the tear film (lipids, mucins, proteins, and salts) may interact to prevent tear film evaporation and collapse; however, additional studies are needed to confirm this concept.
Normal tear lake
A normal tear lake is approximately 1 mm above the eyelid margin.
How thick is the tear film
3.4 μm thick.
Lipid layer 0.015–0.16 μm
Function of the lipid layer of tear film
- retards evaporation of the tear film
- contributes to the optical properties of the tear film because of its position at the
air–tear film interface - maintains a hydrophobic barrier (lipid strip) that prevents tear overflow by decreasing surface tension
- prevents damage to eyelid margin skin by tears
How many meibomian glands are in the upper and lower eyelids
There are approximately 30–40 meibomian glands in the upper eyelid and 20–30 in the lower eyelid
What is the melting point of meibomian gland secretions
32°C to 40°C.
Oral supplements for dry eye syndrome
omega3 essential fatty acids (eg, fish oil) has been dem onstrated to decrease symptoms associated with dry eye syndrome (DES) in women, pre sumably because of its direct effects on tear film fatty acids.
Functions of mucoaqueous layer
- transmits oxygen to the avascular corneal epithelium
- maintains a constant electrolyte composition over the ocular surface epithelium
- provides an antibacterial and antiviral defense
- smooths minute irregularities of the anterior corneal surface
- modulates corneal and conjunctival epithelial cell function
- converts the corneal epithelium from a hydrophobic to a hydrophilic layer, which is
essential for the even and spontaneous distribution of the tear film - interacts with the tear lipid layer to reduce surface tension, thereby stabilizing the
tear film - lubricates the eyelids as they pass over the globe
Which gland constitute 2/3 of the accessory lacrimal glands
The glands of Krause located in the lateral part of the upper fornix.
What separates the orbital and palpebral lacrimal lobes?
the lateral horn of the levator aponeurosis
Concentration of electrolytes in the cornea
The sodium (Na+) concentration of tears parallels that of serum; however, the concentration of potassium (K+) is 5–7 times that of serum. Na+, K+, and chloride (Cl–) regulate the osmotic flow of fluids from the cornea to the tear film and thereby contribute to corneal clarity. Bicarbonate (HCO–3) regulates tear pH. Other tear electrolytes (Fe2+, Cu2+, Mg2+, Ca2+, PO43–) are enzyme cofactors.
Tear film solutes
urea, glucose, lactate, citrate, ascorbate, and amino acids. All enter the mucoaqueous layer of the tear film via the systemic circulation, and their con centrations parallel those of serum levels
Protein in tear film
Proteins in the mucoaqueous layer of the tear film include immunoglobulin (Ig) A and secretory IgA (sIgA). IgA is formed by plasma cells in interstitial tissues of the main and accessory lacrimal glands and by the substantia propria of the conjunctiva
What condition is MMP-9 affected in?
Levels of matrix metalloproteinase 9 (MMP9) in the tear film have been shown to be elevated in patients with severe disorders affecting the ocular surface, including Sjögren syndrome and graftvshost disease, as well as in patients after laser in situ keratomileusis (LASIK).
Function of MMP-9
MMP9 cleaves epithelial basement membrane components and tightjunction proteins.
Antimicrobial components of the mucoaqueous layer
Lysozyme, lactoferrin, group II phospholipase A2, lipocalins, and defensins are important antimicrobial constituents of the mucoaqueous layer
Function of mucin component of mucoaqueous layer
The mucin component of the mucoaqueous layer coats the microplicae of the superficial corneal epithelial cells and forms a fine network over the conjunctival surface. In addition to mucin, it contains proteins, electrolytes, water, and carbohydrates in a polar glycocalyx.
Two types of mucin in mucoaqeous layer
Secreted mucins and Membrane spanning mucins (also called membrane-anchored, membrane-bound, membrane-tethered mucins)
Dry eye syndrome neural feedback mechanism
The cor nea and lacrimal gland are not directly connected; however, corneal damage profoundly affects the lacrimal gland, which, in turn, downregulates tear production. In the vicious circle theory of DES, this downregulation is due to the secretion of inflammatory cytokines that block neural signals for tear secretion. The feedback loop, initiated by inflammation on the surface of the eye, further suppresses or downgrades lacrimal gland function, creating a vicious circle that worsens DES.
Where are glands of Wolfring located
located along the proximal margin of each tarsus
Diagnostic tests for tear dysfunction
tear breakup time, fluorescein staining, lissamine green staining, rose bengal staining, osmolarity testing, Schirmer test, tear meniscus evaluation, and MMP9 testing.
Medications for Dry Eye Syndrome (DES)
a variety of antiinflammatory drugs (including corticosteroids, cyclosporine, lifitegrast, and doxycycline) have been used as therapy for DES and observed to improve the clinical symptoms of these patients
How does cyclosporine work in DES
Cyclosporine, a fungus derived peptide emulsion, has been shown to be effective in stimulating aqueous tear production in patients with DES
How does Lifitegrast work in DES
Lifitegrast, a lymphocyte function–associated antigen1 (LFA1) antagonist that inhibits binding of ICAM1 to LFA1, has been shown to reduce inferior corneal staining and provided greater symptom relief in treated patients with DES than in control groups
