IV. NUTRITION

S.C.C.O.
A. Major nutrients
are glucose and
oxygen in: tears,
limbal blood vessels,
aqueous humor
4/13/2015
1
IV. NUTRITION
– 1. Glucose is not permeable to the
superficial epithelium. Glucose can diffuse
from limbal vessels, however the rate of
consumption is too great for this to provide
for any more than just the most peripheral
cornea.
S.C.C.O.
4/13/2015
2
IV. NUTRITION

S.C.C.O.
B. Experiments
• 1. ingested fluorescein will diffuse out of limbal
blood vessels, but never get to the central
cornea.
• 2. Deep limbal cuts, intended to destroy the
limbal blood supply, have no effect on the
cornea.
• 3. implantation of impermeable plastic discs
into the stroma, will eventually lead to
degeneration of the epithelium and anterior
stroma. CONCLUSION----Glucose must come
from the aqueous4/13/2015
humor.
3
IV. NUTRITION

S.C.C.O.
C. Endothelial glucose permeability is
10 times that for diffusion of a molecule
its size. Therefore glucose uptake
across the endothelium must be
facilitated.
4/13/2015
4
IV. NUTRITION

S.C.C.O.
D. Between 65-85% of glucose used by
the corneal epithelium is metabolized to
lactate even under normal conditions.
Much more lactate is produced when
the epithelium is made hypoxic, i.e.
during contact lens wear.
4/13/2015
5
S.C.C.O.
4/13/2015
6
IV. NUTRITION
– E. Oxygen is very permeable to all cell
layers. Removing oxygen from the tears by
using a nitrogen goggle or tight fitting
contact lens leads to corneal edema and
clouding. When the eye is closed oxygen
will diffuse out of the palpebral capillaries
to the tears. Oxygen flux from the aqueous
to the cornea is especially important when
the eye is closed. Limbal oxygen supply is
negligible.
S.C.C.O.
4/13/2015
7
V. STEADY-STATE O2
DISTRIBUTION

S.C.C.O.
A. First, carbon
dioxide distribution
4/13/2015
8
S.C.C.O.
4/13/2015
9
S.C.C.O.
4/13/2015
10
V. STEADY-STATE O2
DISTRIBUTION

B. The steady-state O2 curves are
derived from a complex, non-linear
function. If Q = dj/dx and j = Dk(dp/dx)
then:
– Q = (d/dx) Dk(dP / dx) = Dk (d^2P / dx^2)
S.C.C.O.
• Q = O2 consumption rate
• d^2P/dx^2 = concentration gradient for O2 across
the tissue
• D = diffusion coefficient
• k = solubility coefficient
4/13/2015
11
V. STEADY-STATE O2
DISTRIBUTION


S.C.C.O.
Dk = oxygen
permeability
Dk/L = oxygen
transmissibility of a
membrane (e.g.,
contact lens)
4/13/2015
12
V.
STEADY-STATE
DISTRIBUTION

C. In the open eye at sea level, pO2 =
155 mm Hg (132 mm Hg in Denver) at
the tears and 55 mm Hg in the aqueous
humor.
•
•
•
•
S.C.C.O.
O2
Layer = Q
Epi
Stroma
Endo
ml O2/ ml x sec
26.5 x 10-5
2.85 x 10-5
140 x 10-5
4/13/2015
%
40
39
21
13
V. STEADY-STATE O2
DISTRIBUTION

S.C.C.O.
Oxygen Tension = P = (QX^2 / 2Dk) + BX
+ C; Master Equation, where B and C are
constants of integration and are determined
by the boundary conditions.
4/13/2015
14
V. STEADY-STATE O2
DISTRIBUTION

1. Open Eye Boundary Conditions
– at x = 0 (aqueous), P = 55 i.e. C = 55
– at x = L (tears), P = 155
– 155 = (QL^2 / 2Dk) + BL + 55, solve for B:
– P = (QX^2/2Dk) + (X/L)[100- (QL^2/2Dk)] +
55
• = Oxygen tension
S.C.C.O.
4/13/2015
15
V. STEADY-STATE O2
DISTRIBUTION

2. Closed Eye Boundary Conditions
– at x = 0, P= 55
– at x = L, P = 55
S.C.C.O.
4/13/2015
16
VI. OXYGEN TENSION UNDER
A CONTACT LENS
S.C.C.O.
4/13/2015
17
VI.
OXYGEN
TENSION
UNDER A CONTACT LENS

A.Mathematical determination: Diffusion
through the lens (Fick's Law)
– 1. J = dm / dt, J is the mass of substance
moving/unit time across a boundary or
layer, m is mass (grams), t is time or J = D
a (dc / dx), where D is the diffusion
coefficient (cm^2 / sec), a is the area, dc is
the concentration (grams/cc) gradient, x is
the thickness of the layer.
S.C.C.O.
4/13/2015
18
VI. OXYGEN TENSION
UNDER A CONTACT LENS
– 2. j = D (dc / dx), small j is the flux per unit
area.
– 3. remember that for gases c = kP (Henry's
Law of gas partial pressure)(c=dissolved
O2 conc., k = O2 solubility, P = O2 tension
in solution) therefore, J = dm / dt = Dka (dP
/ dx), if L is the thickness of a contact lens
then
• j = Dk (dP / dL) and Dk/L represents the oxygen
transmissibility of the lens.
S.C.C.O.
4/13/2015
19
S.C.C.O.
4/13/2015
20
S.C.C.O.
4/13/2015
21
VI. OXYGEN TENSION
UNDER A CONTACT LENS

B. Experimental Determination of
Oxygen Uptake
– Background
– 1. We can measure the oxygen tension of
a solution with an oxygen sensitive
electrode. Over the electrode is placed a
thin plastic membrane that protects the
electrode & acts as a reservoir for oxygen.
S.C.C.O.
4/13/2015
22
VI. OXYGEN TENSION
UNDER A CONTACT LENS

S.C.C.O.
The oxygen in the solution equilibrates
with the membrane and then diffuses to
the electrode to produce a steady
current, proportional to the oxygen
tension in the solution.
4/13/2015
23
S.C.C.O.
4/13/2015
24
S.C.C.O.
4/13/2015
25
– 2. Good for measuring O2 in large volumes
where the electrode is not affecting the
solution pO2. Want to know the O2 tension
under a contact lens. The problem is that
the tear volume is similar to the membrane
reservoir volume & the tear pO2 cannot be
maintained if the electrode is pressed to
the eye. If an electrode is pressed against
the eye the oxygen in the membrane will
diffuse out into the cornea.
S.C.C.O.
4/13/2015
26
– 3. The cornea will consume the O2 and
since the electrode is blocking O2 coming
from the air, the O2 in the membrane will
be depleted within a minute or so.
S.C.C.O.
4/13/2015
27
S.C.C.O.
4/13/2015
28
– 4. Suppose a contact lens is worn for a time (5 -10
min) so that the oxygen tension of the cornea is
reduced. When the lens is removed and the
electrode is placed on the eye, the oxygen in the
membrane may come out faster than if a lens
hadn't been worn since the difference in O2
tension between the membrane and the cornea is
greater. This in fact occurs, and the absolute O2
flux from the membrane can be calculated.
S.C.C.O.
4/13/2015
29
VI. OXYGEN TENSION
UNDER A CONTACT LENS

C. Experimental Methods
– 1. Equivalent Oxygen Percentage (R. Hill)
• a. An empirical method for estimating tear pO2
in humans wearing contact lenses.
• b. Bathe the cornea of a subject with various
levels of oxygen. Whip off the goggle and
measure the oxygen uptake rate. Create a
calibration curve showing tear pO2 vs. O2
uptake.
S.C.C.O.
4/13/2015
30
VI. OXYGEN TENSION
UNDER A CONTACT LENS
– 1. Equivalent
Oxygen Percentage
(R. Hill)
• Next place a contact
lens on the eye, allow
the O2 to equilibrate
for about 10 minutes,
whip off the lens and
measure the O2
uptake, find the
equivalent pO2 on the
calibration curve.
S.C.C.O.
4/13/2015
31
VI. OXYGEN TENSION UNDER
A CONTACT LENS

S.C.C.O.
2. O2 Sensitive
Phosphorescence
Dye.
4/13/2015
32
S.C.C.O.
4/13/2015
33
S.C.C.O.
4/13/2015
34
S.C.C.O.
4/13/2015
35
VI. OXYGEN TENSION UNDER A
CONTACT LENS

S.C.C.O.
D. Effect of Blinking
4/13/2015
36
S.C.C.O.
4/13/2015
37
S.C.C.O.
4/13/2015
38
S.C.C.O.
4/13/2015
39
VI. OXYGEN TENSION
UNDER A CONTACT LENS

E. Hypoxia
– 1. When cells are
made hypoxic, ATP
production from
mitochondria slows
and glycolysis
speeds up to try to
maintain the ATP
supply. This is called
the Pasteur Effect.
S.C.C.O.
4/13/2015
40
VI. OXYGEN TENSION UNDER
A CONTACT LENS

S.C.C.O.
As a consequence,
lactate and proton
production will
increase and
glucose
consumption will
increase.
4/13/2015
41
VI. OXYGEN TENSION
UNDER A CONTACT LENS

E. Hypoxia
– 2. factors limiting the rate of glycolysis
under hypoxic conditions; glucose supply,
diffusion, glycogen stores
– 3. Epithelial cells store glycogen, which will
last about 2 hours during severe hypoxia.
S.C.C.O.
4/13/2015
42
VI. OXYGEN TENSION
UNDER A CONTACT LENS

F. Corneal Edema
– 1. Epithelium
• a. Haze seen in area covered by contact lens.
• b. Haze is eliminated by removing epithelium or
reduced by rolling a Q-tip across the cornea.
• c. Epithelial thickness does not change, so
water is not added between the layers of
epithelium.
• d. Haze is seen subjectively as colored fringes
around bright lights. This is called Sattler's Veil.
S.C.C.O.
4/13/2015
43
VI. OXYGEN TENSION
UNDER A CONTACT LENS
– 1. Epithelium
• e. Haze is due to light scatter at the level of the
basal cells. Photographs of basal layer with the
specular microscope reveal a matrix of light
scattering sites which act as a diffraction
grating. This grating produces a diffraction ring
that subtends a half angle of 3 degrees using
red light, 632.8 nm.
• f. Thus, increase in water between the basal
cells.
S.C.C.O.
4/13/2015
44
S.C.C.O.
4/13/2015
45
VI. OXYGEN TENSION
UNDER A CONTACT LENS
– 2. Stromal edema
• a. Cornea will swell, i.e. increase in thickness,
during hypoxia or contact lens wear.
Possibilities:
– i. Hypoxia reduces epi- or endothelial barrier
function.
– ii. Endothelial pump is slowed directly by hypoxia.
– iii. Corneal pH drop slows endothelial pump.
– iv. Lactate accumulation causes osmotic swelling.
• b. Cornea will swell about 4% during sleep, i.e.
eye closure.
S.C.C.O.
4/13/2015
46
VI. OXYGEN TENSION
UNDER A CONTACT LENS
– 2. Stromal edema
• c. Acute hypoxia does not compromise the
epithelial barrier, however chronic severe
hypoxia will. In addition, chronic mild hypoxia
can slow the epithelial mitotic and healing
rates. All of these problems increase the
chance for corneal infection. The effect of
reduced pH on these processes is not known,
but it could be a contributing factor.
S.C.C.O.
4/13/2015
47
VI. OXYGEN TENSION
UNDER A CONTACT LENS
– 3. Endothelial Changes from contact lens
wear
• a. Blebs are transitory dark spots appearing in
the specular reflection of the endothelium.
Occur about 30 min. after insertion of a contact
lens and last about 30-60 min. Most likely due
to pH changes.
• b. Polymegathism, increased variation in cell
size seen in long-term lens wearers:
– coefficient of variation = S.D. of cell size / mean cell
size
S.C.C.O.
• c. Pleomorphism 4/13/2015
– increased variation in cell
48
S.C.C.O.
4/13/2015
49
S.C.C.O.
4/13/2015
50
VI. OXYGEN TENSION UNDER
A CONTACT LENS
• d. Reduced function due to hypoxic dose
– Corneal deswelling
S.C.C.O.
4/13/2015
51
VI. OXYGEN TENSION
UNDER A CONTACT LENS
– 4. Critical O2
Tension
• a. Oxygen levels
below this result in
adverse tissue
changes.
S.C.C.O.
4/13/2015
52
VII. TISSUE MECHANICS
A. Cornea has a high tensile strength
and can resist rupture up to 30 Atm.
 B. Cornea cannot support lateral
shearing forces. Lamellae tend to slide
past each other & its relatively easy to
slide a probe into the stroma between
lamellae.
 C. Sclera is weaker, especially at EOM
insertions.
S.C.C.O.
4/13/2015

53
VII. TISSUE MECHANICS

S.C.C.O.
D. The total force due to the wall
tension is the product of the tension, T
(Force / cm), and the circumference, 2
Pi(r) of the hemisphere, or 2 Pi(r)T. At
equilibrium, the force to the left must
equal the force to the right. The force
vectors due to the IOP act
perpendicular to the surface of the wall.
4/13/2015
54
VII. TISSUE MECHANICS

S.C.C.O.
D. Each force vector can be
represented by a vertical & horizontal
component. All the vertical components
cancel, those to the right add up to the
IOP times the projected area, Pi(r)^2.
Therefore at equilibrium:
4/13/2015
55
VII. TISSUE MECHANICS
– Force on the wall = 2 Pi(r)T = (IOP) Pi(r)^2
and
– T = (IOP) (Pi(r)^2 / 2 Pi(r)) = (IOP) r / 2
• This result predicts that the tension on the
cornea and sclera for the human eye are
different.
S.C.C.O.
4/13/2015
56
VII. TISSUE MECHANICS

S.C.C.O.
E. The corneo-scleral junction is stable,
so there must be a compensating
tension at the limbus. This is provided
by the circular fibers.
4/13/2015
57
VII. TISSUE MECHANICS
F. Radial Keratotomy makes cuts 9095% through the cornea. This weakens
the cornea allowing the IOP to push out
the weakened area.
 G. PRK removes a portion of the
stroma.

S.C.C.O.
4/13/2015
58
VIII. DISEASE
S.C.C.O.
4/13/2015
59
VIII. DISEASE

S.C.C.O.
A. Epithelial wound healing
– 1. The goal is to re-establish the anterior
surface seal, i.e. tight junctions as quickly
as possible.
– 2. Following a wound, the superficial cells
at the wound margin will slough off, loss of
hemidesmosomes occurs, latent phase
approx. 2 hours.
– 3. The basal cells at the margin begin to
flatten and spread4/13/2015
into the wound.
60
S.C.C.O.
4/13/2015
61
S.C.C.O.
4/13/2015
62
S.C.C.O.
4/13/2015
63
S.C.C.O.
4/13/2015
64
VIII. DISEASE
– 4. While spreading continues, mitotic
activity is enhanced in an area surrounding
the wound, but removed from the edge.
(Mitotic activity is slowed by sensory
denervation or contact lens wear).
– 5. Cell attachment achieved via transitory
fibronectin & laminin.
– 6. Once contact inhibition is achieved
hemidesmosomes are established.
– 7. Therapy is Pressure patching
S.C.C.O.
4/13/2015
65
S.C.C.O.
4/13/2015
66
VIII. DISEASE

B. Recurrent Erosion
– 1. Damage to underlying basement
membrane prevents good adhesion of
epithelium even after wound is closed. May
take several weeks to months for BM to be
repaired. Therapies:
•
•
•
•
S.C.C.O.
a. hyperosmotic drops (high MW)
b. fibronectin drops
c. EGF
d. aprotinin, a plasmin blocker
4/13/2015
67
C. Endothelial Cell Damage
S.C.C.O.
4/13/2015
68
C. Endothelial Cell Damage
S.C.C.O.
4/13/2015
69