BS - Lacrimal Glands and Tear Flim - Week 2 Flashcards by Davud Akgun

What 7 structures contribute to the lacrimal apparatus?

Lacrimal gland
Accessory lacrimal glands
Goblet cells
Meibomian glands
Tear film
Drainage system
Eyelid surfaces

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How many layers does the tear film have?

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What two structures contribute to the aqueous component of the tear film?

Lacrimal and accessory lacrimal glands

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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

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What structure contributes to the lipid component of the tear film?

Meibomian glands

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How t h i c c is the tear film?

~10 microns

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What percentage of the tear film is water? What about the rest?

95% water, rest are oil, mucins, salts, metabolites, immunoglobins, and antibachterials.

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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

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What two purposes does blinking have regarding the tear film?

Distributes the tears, and expels excess tears

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How does lid opening affect the tear film?

Smooths out the tear film to an appropriate thickness, creating surface conditions conducive to tear spreading.

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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)

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Whaqt is the total volume of the tear film?

5-9 microlitres

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What is the rate of tear film secretion?

1.2-1.3uL/min

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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.

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Does the pH increase, decrease, or remain the same with crying?

Increases

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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

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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

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What kind of gland is the lacrimal gland?

Adenoid/glandular

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Is the lacrimal gland richly or poorly vascularised?

Richly

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Where in the orbit is the lacrimal gland situated?

Upper temporal orbit, under the rim

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Where does the lacrimal gland extend to, and in which direction?

Extends anteriorly to the septum

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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.

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For what percentage of aqueous production is the lacrimal gland responsible for?

90%

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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

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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