Chapter 7: Tear Film Flashcards
What is the primary function of the tear film?
- provide a smooth optical surface @ air-cornea interface
- mediun for debris removal
- protect ocular surface
- contains antimicrobial agents
- lubricates cornea-eyelid interface
- prevents dessication of ocular surface
Where does the marginal tear strip cover? The precorneal tear film? The conjunctival tear film?
Marginal tear film= tear meniscus
Pre-Corneal tear film covers exposed bulbar conjunctiva and cornea, and is trilaminar
conjunctival film covers b/c eyelids and bulbar conjunctiva
What are the three layers of the pre-corneal tear film?
Outer lipid layer
Middle Aqueous layer
Inner mucin layer
What are the functions of the outer lipid layer?
- slow tear evaporation
- contribute to optical properties at air-cornea interface
- Increase surface tension via maintenance of a hydrophobic barrier, preventing tear overflow
- prevent damage to eyelid margin skin by tears
How thick is the outer lipid layer of precorneal tear film?
about 100 molecules thick
How many glands produce the lipid layer? Where are they located?
Glands are located in the tarsal plates of the upper and lower lid.
Upper lid= 30-40 orifices
Lower Lid= 20-30 orifices
Where do the meibomian glands of the tarsal plates open to?
they open onto the skin of the eyelid margin, b/w the tarsal grey line and mucocutaneous junction
What is the innervation of the meibomian glands?
Parasympathetics (main source; cholinesterase, vip)
Sympathetics
Sensory
Neuropeptide Y
Why are tear lipids not susceptible to lipid peroxidation?
They are not susceptible to lipid peroxidation because they contain extremely low amounts of polyunsaturated fatty acids
What are the functions of the aqueous layer of the pre corneal tear film?
- supply 02 to avascular corneal epithelium
- maintain constant electrolyte concentration
- Antibacterial/Antiviral defenses
- smooth minute irregularities
- wash away debris
- modulate corneal and conjunctival epithelial cell fx
What cells secrete the aqueous layer? What does the aqueous layer consist of?
The main and accessory lacrimal glands (Krause and wolfing)
Consists of electrolytes, H2O, proteins, and a variety of solutes
How much do the glands of Krause contribute to the aqueous layer? Where are they located?
They contribute 2/3 of the product of the accessory lacrimal glands. They are located in the lateral portion of the upper fornix (some also present in the lower fornix as well)
How much do the glands of Wolffring contribute to the aqueous later? Where are they located?
1/3 of total accessory lacrimal gland production. They are located on the proximal margin of each tarsus.
What is the innervation of the main lacrimal gland?
Parasympathetics (Main innervation; ACh and VIP)
Sympathetics (NE and NPY)
Sensory (Substance P and CGRP)
What are the electrolyte concentrations of the aqueous layer compared to serum?
Na+- equivalent to serum
K+- 5-7x greater than serum value
HCO3- regulates tear pH
What other solutes are located in the aqueous layer of the pre corneal tear film?
Urea, glucose, lactate, citrate, ascorbate, and amino acids
What proteins are found in the aqueous layer of the pre corneal tear film?
IgA, sIgA, IgM, IgD, IgE
IgA and sIgA are formed by plasma cells of interstitial tissues of the main/accessory lacrimal glands and substantia propria of conjunctiva
What antimicrobial agents are found in the aqueous layer of the pre corneal tear film?
Lysozymes, lactoferrin, Group II phospholipase A2, lipocalins, degensins, IFNs, cytokines, and GFs
What affect does vernal conjunctivitis have on the aqueous layer of the pre corneal tear film and why?
It increases tear and serum IgE, increases IgE-producing plasma cells in the giant papillae of the superior tarsal conjunctiva, and increases histamine.
What does the mucin layer of the pre corneal tear film contain?
Mucins, proteins, electrolytes, and H2O
What is the surface of the mucin layer of the pre corneal tear film?
- converts the corneal surface from hydrophobic to hydrophilic
- intersects with lipid layer to decrease surface tension (stabilizes tear film)
- trap exfoliated surface cells/foreign particles/bacteria
- lubricates eyelids as they pass over the globe
What cells secrete the mucin layer of the pre corneal tear film?
The conjunctival goblet cells and stratified squamous epithelium, minimally by lacrimal glands of Henle and Manz.
What is the daily production of the mucin layer of the pre corneal tear film?
2-3 microliters per day
How do parasympathetic and sympathetic nerves act of the tear glands?
via G-protein linked receptors, and activating one of two pathways:
- Ca2+/Protein dependent Kinase pathway
- cAMP pathway
Describe the Ca2+/Protein dependent kinase pathway of lacrimal gland activation
activation of muscarinic (via ACh) or alpha1 adrenergic (via NE–all except main lacrimal gland) receptors–> G Protein activation–> phospholipase C activation -> IP3 and Diacylglycerol–> Ca2+ release –> protein/h2O/electrolyte secretion
Describe the cAMP pathway of lacrimal gland activation
Activation by NE/VIP-> stimulation of VIP/beta adrenergic receptors-> activates G Proteins-> activates adenylyl cyclase-> cAMP production-> activates cAMP-dependent kinase-> stimulates secretion
What other types of stimulation can activate the tear glands?
Steroid and Peptide hormones can also activate tear gland secretion
Peptide- stimulate secretion from main lacrimal gland via cAMP pathway
Steroid-stimulate secretion of sIgA from main lacrimal gland and lipid secretion from meibomian glands via diffusing into nucleus, binding steroid receptors, and activating gene transcription and protein synthesis
How does eyelid movement affect tear secretion?
- As eyelids close, superior and inferior fornices are compressed by force of preseptal muscles and the eyelids moving toward each other.
- force from upper lid clears surface of debris and expresses secretions of meibomian glands
- lower lid moves horizontally toward the nose during blinking, which pushes tear fluid and debris toward the puncta.
- When eyelids open, the tear film is re-distributed.
What are diagnostic tests for tear dysfunction
- tear breakup time
- fluorescein staining
- lisomine green staining
- rose bengal staining
- osmolarity test
- schirmer test
