Lacrimal system and Tear film - Week 2 Flashcards
Define lacrimal
Correlated with weeping or tears
Define lacrimal gland
An almond shaped gland that secretes tears into ducts that empty on the surface of the conjunctiva of the eye
Define lacrimal apparatus
A network of structures of the eye that secrete tears and drain them from the surface of the eyeball
List the structures of the lacrimal apparatus [7]
- lacrimal gland
- accessory lacrimal glands of Krause & wolfring
- goblet cells
- meibomian glands
- tear film
- drainage system
- eyelid surface
*Tm dagel
Or * M’ Lag Ted – it’s like you’re a nice guy telling ted about the lag you’re getting
List the components of the drainage system [5] [lacrimal lecture]
- plica semilunaris/caruncle
- Puncta
- canaliculae
- lacrimal sac
- nasolacrimal duct
List the functions of the lacrimal apparatus [8]
- normal tear production
- tear distribution
- tear stability
- tear drainage
- keep optical surface patent
- nutrition (O2 in, CO2 out)
- lubrication
- protection (antimicrobial, debris)
** way to remember
Normal tears keep protecting nutrients from lubrication
How many layers does the tear film have?
- Tri-laminar. Though recently it’s suggested that aqueous and mucous may be more like a gradient and not 2 separate layers.
What structure/s contribute to the aqueous component of the tear film?
Lacrimal gland and accessory lacrimal glands
What structures contribute to the mucous component of the tear film? [4]
- Accessory lacrimal glands
- Stratified squamous epithelia of Cornea …
- and Conjunctiva
- Goblet cells of conjunctiva
What structure contributes to the lipid component of the tear flim?
Meibomian Glands
What type of mucin is produced by:
- stratified squamous epithelia of cornea + conjunctiva
- goblet cells of conjunctiva
epithelia: special type of mucin that’s anchored to tissue
goblet: secretory mucin
These two types of mucins interact with each other and then interact with the aqueous to make a charged gel
How thick is the tear flim?
Around 10 microns - relatively thin
What percentage of the tear flim is water? What is the rest?
98% water
The rest is made from: oil, mucins, salts, metabolites, immunoglobulins, antibacterials
What is the baseline osmolarity of the tear flim vs with stimulation? How does it change in a patient with dry eye disease?
Basal: 0.90% NaCl, 285-295 mOsm/kg
Reflex: 0.97% NaCl, 310-334 mOsm/kg
In dry eye disease, osmolarity can get as high as 350 mOsm/kg
What are the 3 layers of the tear flim? What are they each important for?
- Basal Mucin Layer (mucous layer) = attachment of tear flim to cornea
- Central Aqeuous Layer = hydration of ocular surface
- Superficial Lipid Layer = creates a seal to stop evaporation
Out of the 3 tear film layers, which one would Phil Swift, of “flex seal” fame, approve of?
The Superficial lipid layer. It creates a “seal” to stop evaporation
What is the purpose of blinking in regards to tear film?
- distributes tears
- expels excess tears
What is the purpose of lid opening in regards to tear film?
- smooths out tear film to appropriate thickness
- creates surface conditions conducive to tear spreading
What is the purpose of lid closure in regards to tear film?
- wipes clean corneal surface
- assists drainage of tear lake
- aids expression of fresh secretions of: goblet cells (by mucins), glands of Krause & Wolfring (aqueous), Meibomian glands (oily layer)
What is the total volume of the tear film (including meniscus)?
5-9ul
How is the volume of tear film affected in dry eye disease?
Decreases
How does pH differ between open and closed eyes? Why is this?
Open eye: pH = 7.45
Closed eye: pH = 7.25 – more acidic possibly due to CO2 and lactic acid build up
How does crying affect the level of pH? What about infection?
Crying: increases pH
Infection: decreases pH
Where do the secretory ducts empty their aqueous secretions?
into the superior-temporal part of your orbit
Describe the pathway of tear drainage across your general anatomy
- Secretory ducts empty aqueous secretions into superior-temporal orbit
- tears then spread across eyelid and drain into ‘puncta’
- the 2 puncta then drain into the nose
When blinking, how do the eyelids close? (describe the movement). What is the purpose of this?
Blink starts from the lateral side of your eyelids and then slowly closes towards the nasal side.
- this assists in pushing away old tear fluid towards the caruncle region (where your puncta are)
Is the lacrimal gland continuous or discontinuous?
Continuous but in 2 portions that sit either side of the LPS (levator muscle)
Name the 2 portions of the Lacrimal Gland. Which is larger?
Orbital portion - larger
Palpebral portion - smaller
How much of the aqueous production is the lacrimal gland responsible for?
90% of aqueous production
What kind of nerve fibres does the lacrimal gland receive? (innervation)
Receives both autonomic and sensory nerve fibres
Where does the lacrimal gland’s parasympathetic supply come from?
CN VII nucleus (secretomotor and vasodilatory)
What is the lacrimal gland’s parasympathetic innervation important for?
Reflex tearing with noxious smells
Where does the lacrimal gland’s sympathetic supply come from?
Superior Cervical Ganglion
*(Sup Cerv Gang)
What is the lacrimal gland’s sympathetic innervation important for?
Vasoconstriction of the vessels
True or False: lacrimal gland sympathetic innervation has no apparent direct secretory role
True
Where is the lacrimal gland’s sensory innervation?
Branches of CN V1 (ophthalmic division of CNV)
Describe the appearance of Acinus
is like a grape with lots of cells in it
What is the smallest functional unit of the lacrimal gland structure?
Acinus
In the acinus secretory unit, where do the round nuclei usually sit?
towards the basal compartment of the cell
In the acinus secretory unit, what portion gets secreted into the lumen?
Apical portion
How many secretory ducts does the lacrimal gland have and where do they empty into?
6-12 secretory ducts. They empty into the superior fornix just above the tarsal plate
Is the lacrimal gland richly or poorly vascularised?
Richly
Where in the orbit is the lacrimal gland located?
Upper-temporal orbit, under the rim
Where does the lacrimal gland extend to, and in what direction?
Extends anteriorly to the septum
In lacrimal gland structure, how are intercalated ducts formed?
By the merger of acini lumens
Define acini
round tube-shaped masses of columnar secretory cells that drain into small ducts
Describe the structure of the lacrimal gland. What separates these structures?
Lobulated, tubulo-acinar
- the lobules are separated by loose CT
How does the structure of the lacrimal gland ensure that secreted fluid doesn’t get re-absorbed by the ducts?
Tight junctions between epithelial cells of duct
What method of secretion is used to secrete into acini?
Apocrine secretion
What type of cells sit around duct cells to help drive secretion?
Innervate myoepithelial cells
How many glands of Krause are there? How are they distributed?
42 superior, 6-8 inferior
How many glands of Wolfring are there? How are they distributed?
2-5 superior tarsus
2 inferior tarsus
How much do the accessory glands of Krause and wolfring contribute to aqueous secretion?
10%
What is the primary mediator for basal secretion of tears?
Na+/K+ ATPase
What is the primary mediator for reflex (stimulated) secretion of tears?
Parasympathetic control
Name the causes of reflex (stimulated) tears
- Conjunctival sensory stimulation (e.g dust/wind/cold)
- Retinal stimulation (light)
- psychological (emotions)
What creates polarisation of the acinar cell and what does this allow?
Tight junctions, which separate the apical from the basolateral plasma membranes
This enabled secretion of electrolytes, water, and proteins
What does the basolateral membrane contain to help unregulate tearing rate?
Receptors for neurotransmitters
What does the basolateral membrane contain to help initiate the secretory process?
Growth factors
What does the basolateral membrane contain for initiating electrolyte and water secretion?
Ion transport proteins and ion channels
Describe the biochemistry of electrolyte and water secretion
- driven by Na+/K+ ATPase-mediated efflux of Na and K across basolateral membrane
- net result: K+ and Cl- into lumen
- result in negative potential difference causing Na+ to move by paracellular pathway
- water secreted into lumen through aquaporins
