BS - Lacrimal Glands and Tear Flim - Week 2 Flashcards
What 7 structures contribute to the lacrimal apparatus?
Lacrimal gland Accessory lacrimal glands Goblet cells Meibomian glands Tear film Drainage system Eyelid surfaces
How many layers does the tear film have?
3
What two structures contribute to the aqueous component of the tear film?
Lacrimal and accessory lacrimal glands
What four structures contribute to the mucous component of the tear film?
Stratified squamous epithelia of the conjunctiva, cornea, the accessory lacrimal glands, and goblet cells within the conjunctiva
What structure contributes to the lipid component of the tear film?
Meibomian glands
How t h i c c is the tear film?
~10 microns
What percentage of the tear film is water? What about the rest?
95% water, rest are oil, mucins, salts, metabolites, immunoglobins, and antibachterials.
What kind of layer does the tear film form? Name each layer, and its purpose.
Trilaminar layer of:
Basal mucin - attachment
Central aqueous - hydration
Superficial lipid - anti-evaporation
What two purposes does blinking have regarding the tear film?
Distributes the tears, and expels excess tears
How does lid opening affect the tear film?
Smooths out the tear film to an appropriate thickness, creating surface conditions conducive to tear spreading.
What three functions does lid closure have on the tear film, and what does it aid in the secretion of (3, cell name and secretion)?
Wipes the corneal surface clean Assists in tear film drainage Aids in fresh secretions by: Goblet cells (mucins) Krause/Wolfring glands (aqueous) Meibomian (lipids)
Whaqt is the total volume of the tear film?
5-9 microlitres
What is the rate of tear film secretion?
1.2-1.3uL/min
What is the pH of the open eye vs closed eye, and why the difference (2)?
Oen - 7.45
Closed - 7.25
This is due to increased CO2 and lactic acid when closed, resulting in more acidic conditions.
Does the pH increase, decrease, or remain the same with crying?
Increases
Does the pH increase, decrease, or remain the same with infection? Give a possible reason for this.
Decreases, possibly due to corneal antibacterial response resulting in H+ release
Describe the structure of the lacrimal gland, which part is larger/smaller (name them), and what structure it can be found on either side of.
It has two portions, but is continuous:
Orbital (larger)
Palpebral (smaller)
They are found on either side of the aponeurosis of the levator palpebrae superioris
What kind of gland is the lacrimal gland?
Adenoid/glandular
Is the lacrimal gland richly or poorly vascularised?
Richly
Where in the orbit is the lacrimal gland situated?
Upper temporal orbit, under the rim
Where does the lacrimal gland extend to, and in which direction?
Extends anteriorly to the septum
Hoe mny secretory ducts does the lacrimal gland have and where do they empty into?
6-12 secretory ducts emptying into the superior fornix just above the tarsal plate.
For what percentage of aqueous production is the lacrimal gland responsible for?
90%
Where does the lacrimal gland receive its sympathetic innervation from? What effect does it have, including secretory roles?
From the superior cervical ganglion
Causes vasoconstriction
No apparent direct secretory role
Where does the lacrimal gland receive its parasympathetic innervation from? What effect does it have, including secretory roles?
From CNVII
-secretomotor and vasodilator
Explains reflex tearing with noxious smells
Where does the lacrimal gland receive its sensory innervation from? What effect does it have, including secretory roles?
Branches of CNV ophthalmic division
What are the smallest ducts within the lacrimal gland called, and what do they form when one or more merge?
Acinus
They merge to form intercalated ducts.
These merge to form larger ducts, eventually forming one of 6-12 large secretory ducts
What is the structure of a lacrimal gland, and what are these structures separated by?
It is lobulated, tubulo-acinar
Lobules are separated by loose connective tissue
Define acini
Round tube-shaped masses of columnar secretory cells that drain into small ducts
Consider the epithelial cells of lacrimal gland ducts. Do they have tight or loose junctions?
Tight
Which cells drive secretion, and where are they found?
Innervated myoepithelial cells around the duct
What kind of immune cells can be found in the lacrimal glands, where are they exactly, and what do they produce?
Found in the loose connective tissue of the lacrimal glands, producing mostly IgA.
On which eyelid are glands of krause mainly found? Give approximate numbers per eyelid.
Mostly superior, at 42
Inferior has 6-8
On which eyelid are glands of wolfring mainly found? Give approximate numbers per eyelid.
Likely superior, 2-5
2 inferiorly
What are accessory lacrimal glands like structurally?
Like a miniature-sized regular lacrimal gland
What is basal tear secretion primarily mediated by?
Na+K+ATPase
What is reflex tear secretion primarily mediated by?
Mostly parasympathetic innervation
Name 3 stimulated production means of tear secretion.
Conjunctival sensory (dust, wind, cold etc) Retinal stimulation (light) Psychological (emotions)
What creates polarisation of the acinar cell, and what does this allow for?
Tight junctions separate the apical from the basolateral plasma membranes, polarising the acinar cells, and allowing secretion of elecrolytes, water, and proteins.
Describe the biochemical basis of electrolyte and water secretion (4).
- Driven by Na+K+ATPase-mediated efflux of Na+ and influx of K+ across the basolateral membrane
- Net result is K+ and Cl- in the lumen
- Negative potential difference results, causing Na+ to move by paracellular oathway
- Water secreted into the lumen by aquaporins
What is electrolyte and water secretion regulated by? Which system activates this system, and what effect does it have?
By M3 muscarinic receptors
Parasympathetic activation causes K+ and Cl- channels to open, leading to increased secretion.
How t h i c c is the aqueous layer of the tear film? What percentage is water, and what is the rest composed of?
7um thick
98% water, the rest are inorganic cations and anions
Relative to plasma, compare the following in the aqueous layer of the tear flim: Lactate Potassium Bicarbonate Glucose Calcium Chloride sodium
Increased
Chloride
Potassium
Lactate
Same
Sodium
Bicarbonate
Decreased
Calcium
Glucose
What is the role of lysozyme, and in what percentage is it found within which tear film layer?
Antibacterial - 30-40%, in the aqueous layer
What is the role of lactoferrin, and in what percentage is it found within which tear film layer?
Bacteriostatic, UV protection, 25%, in the aqueous layer
What is the role of lactoferrin, and in which tear film layer is it found?
Antibacterial, in the aqueous layer
What is the role of substance P, and in which tear film layer is it found?
Neuropeptide responsible for pain perception within the aqueous layer.
Name 2 glycoproteins in the aqueous layer, and their roles.
Transferrin - iron transport
Caeruloplasmin - copper transport
Name the three main immunoglobins found in the aqueous layer of the tear flim, and their role. Name which is the predominant Ig.
IgA for infection, predominant Ig in tears.
IgG for post-trauma
IgE for allergic conjunctivitis
How t h i c c is the mucoid layer?
0.04um thick
Where does most of the mucoid layer originate from?
Goblet cell secretion
What 3 compounds contribute to form mucus?
Glycoprotein, protein. and lipoprotein
Describe mucin, what happens when it dissolves in water, how it associates with carbohydrates, and the major sugar groups (3).
Dissolves in water to form mucus.
Has many short carbohydrate chains
Major sugar groups are sialic acid, galactose, and n-acetylglucosamine
What are membrane mucins produced by? With what other structure do they form what?
Surface epithelial cells of the cornea and conjunctiva.
They form the glycocalyx with the microplicae
What is the role of secreted mucins?
They move freely over the glycocalyx and move throughout the tear film collecting debris
Describe the concentration gradient of mucins through the aqueous layer. What does this confer?
Gradually increases moving towards the cornea, but increases significantly very close to it.
Increases the wetting properties of the epithelium and increases aqueous adherence to the surface.
What would the epithelium be like if there were no mucins in the tear flim?
Very hydrophobic
What is an advantage of having the tear film as it is, in regards to the cornea’s surface?
Makes the corneal surface smooth by filling in irregularities.
What is the glycocalyx formed by?
Membrane-bound mucins attaching to microplicae of the epithelium.
What does the lipid layer bind to?
Lipocalin
If the lipid layer is lost, how much more quickly does evaporation occur?
10-20 fold increase
Describe the percentage composition of the lipid layer.
Non-polar lipids - 60-70%
Polar lipids - 30-40%
Which eyelid dominates blinking?
Upper
What is the normal tear break up time?
10-20 seconds
Name 3 reasons why the tear film stays in place.
Thinness of the film
Mucins provide stability
Lipids prevent evaporation
In what direction does eyelid closure occur?
Temporal to nasal
What two structures form the lacrimal lake?
Plica semilunaris and caruncle
What force draws tears into the lacrimal canaliculus?
Capillary action
Where do the two canaliculi drain into?
The nasolacrimal sac, then to the nasolacrimal duct, then through the valve of hasner, into the nasal sinuses.
When do the puncta in the eyes open?
Until up to 13 weeks of age
Name 3 possible causes of dry eyes, and the disease name if applicable.
Lacrimal gland dysfunction/inflammation/tumour
Meibomian gland dysfunction
Goblet cell disease - xerophthalmia
Name 3 possible causes of impaired tear drainage, and the disease name if applicable.
Eyelids - ectropian, causes epiphora
Nasolacrimal infection
Stenosis of puncta
