Q: With the aid of a table , list the names, functions and production of the three
layers of the tear film.

[19/20 final( May), S IV, Q1]
Lipid layer
Function: Prevent the evaporation of tears, provide lubrication between the lids
(cornea) and eyeball, anti-bacterial effects by affecting the surface or the metabolism
of bacteria (inhibit bacteria’ metabolic activity)
Production: The meibomian glands in the upper and lower lids (major ), glands of zeis
and moll ( minor)

Aqueous layer
Function: Storage of electrolytes, water, proteins, small soluble mucins
Lubrication of eyelids and the eyeball
Wash away particles, prevents infection
Production: The main and accessory lacrimal glands of Krause and Wolfring (Acinar
cells)

Mucin layer
Function : Lower the surface tension of the eye and may improve the wettability of
the nonreal surface by binding to the glycocalyx ( glycoprotein secreted by corneal
epithelium), protect the underlying epithelium from sheering forces and damage of
blinking
Production: Secreted by the globlet cells of the conjunctiva by MUC5AC gene
Q: What is the definition of corneal stromal swelling pressure (SP)? Describe the role
of SP in corneal transparency. [19/20 final (May), S IV, Q4]
Definition: This is the tendency of stroma to swell due to interfibrillar proteoglycans
and other proteins.
How SP contribute to corneal transparency:
When the Imbibition pressure (Ip) is negative, this pressure will draw out water from
the cornea. While when Ip is positive, this pressure will draw in water from the
aqueous humor to the cornea.
IP=IOP-SP
From the equation above, When IOP>SP , Ip will be positive and vice versa.
In other words, swelling pressure (sp) is one of the factors determining the drawing
of water from or to the cornea.
So when elevated IOP combined with normal stromal Sp, IP will be positive and this
pressure will drives fluid across the endothelium, creating corneal edema by
increasing in the thickness of cornea due to the accumulation of extracellular fluid in
epithelium and stroma. The cornea is thus not dehydrated, and its optical property is
downgraded with the loss of corneal transparency.
Q: Describe the physiological properties that contribute to the transparency of the
cornea. [19/20 final (May) , S IV, Q6]

Avascularity (no blood vessels) : The blood supply to the cornea is originated
from the anterior ciliary arteries to the small lopes derived from the
subconjunctival tissue, not directly to the cornea. So, there is no blood vessels
blocking the light entering our eye, ensuring corneal transparency.

Lattice like arrangement of collagen fibrils in the corneal stroma:
Unique spacing, arrangement and composition of collagen fibers.
(i.e., collagen fibers are highly uniform in diameter, the distance between two
corneal fibers is also highly uniform, mean diameter of the individual collagen
fiber and mean distance between collagen fibers are almost the same

Tight barrier of epithelium and endothelium:
These barriers prevent the flow of water from tears film or aqueous humor into
the stroma, ensuring the state of dehydration of the cornea stroma as well as
the corneal transparency.
Q: Why is the healthy corneal endothelium important for maintaining corneal
transparency [19/20 final (Aug) , S IV, Q3]
The endothelium cells allow leakage of solutes and nutrients from the aqueous
humor to the more superficial layers of the cornea while actively pumping water in
the opposite direction, from the stroma to the aqueous .
Details of the mechanism: The barrier portion of the endothelium with a high density
of NA+ / K+ - ATPase pump is unique , permitting the ion flux necessary to establish
the osmotic gradient.
This dual function is described by the pump-leak hypothesis. This ensure stroma is
relatively dehydrated as well as the corneal transparency.
Q: Describe the morphological and physiological changes of the cornea in dry eye
disease [19/20 final (Aug) , S IV, Q7a]
Dry eye disease increases epithelial cell density and thickness, decreases epithelial
cell size and increases epithelial cell turnover, reducing corneal transparency and
senstitivity.
Dry eye disease is associated with the tear loss. So as there is the reduced tear, some
irregular surface of cornea is not smoothed. The optical property of cornea is
downgraded.
Reduced oxygen supply from the atmosphere with the tear , the cornea metabolism
is affected. The reduced oxygen supply may result in hypoxia of the cornea. This
decrease mitosis in epithelium and corneal thickness as well as the corneal
sensitivity.
This will also lead to production of lactate at epithelium that diffuses to the
endothelium and creates transient edema.
The endothelial layer is damaged, the neighbors will enlarge to fill the gap –
polymegethism
Blood vessels grow from the limbus into the cornea – neovascularization.
From the limbal vessels, providing extra oxygen and nutrients.