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Flashcards in CorneaSclera 1 Deck (51):
1

What is the size of the cornea?

H:12.6
V: 11.7

2

Which surface is the major refractive component of eye? How many Diopters?

Anterior surface; 48D

3

Which part of the cornea is spherical or toroidal?

central third

4

What is the radius curvature of the cornea?

7.8mm

5

True or false; peripheral part of cornea is thicker than center?

True: central 0.52 mm
peripheral 0.65 mm

6

What percentage of incidence light over 400nm is transferred through the cornea?

99%

7

How much radiation is passed through cornea?

from approximately 310 nm in the ultraviolet to 2500 nm in the infrared.

8

What are the layers of cornea?

Epithelium
Bowman's Layer
Stroma
Descemet's Membrane
Endothelium

9

Describe epithelium?

straftified nonkertanized, nonsecretory squamous layers
5-7 layers

10

What are the 3 cell layers that make up epithelium or cornea?

1. Surface cells; 3-4 layersdifferentiated squamous cells that are sloughed from surface
2. Wing cells: 1-3 layers, intermediate differentiated layer
3. Basal cells:l layer; where mitosis occurs and adhere to basement membrane As cell division occurs, the daughter cells move toward the surface of the cornea and begin to differentiate

As cell division occurs, the daughter cells move toward the surface of the cornea and begin to differentiate, forming one to three layers of wing cells.

11

What is the turnover for epithelium?

7 days ; however most trauma to cornea heals in 24 hours

12

Which layer does cell division occur in?

basal cells layer; Basal cells have high metabolic activity:
More prominent mitochondria, endoplasmic reticulum, and golgi apparatus
High glycogen storage

13

Where do corneal basal cells originate from?

originate from stem cells of the limbal epithelium

14

What is the mitotic rate of corneal epithelium?

10-15% a day

15

How is the cornea epithelium maintained?

1. a constant cycle of shedding of superficial cells and proliferation of cells in the basal layer.
2. a slow migration of basal cells toward the center of the cornea.
The limbus contains stem cells, which differentiate into basal cells and migrate onto the cornea, constantly renewing the supply of basal cells.



The corneal epithelium is maintained by a balance among sloughing of cells from the corneal surface, cell division in the basal layer, and centripetal migration of cells from the limbus.

16

Electron Microscopy of Corneal Epithelium

Surface of cornea is seen as irregular array of polygonal cells
Smaller, light cells = younger cells
Only recently have reached the cornea
Larger, dark cells = mature cells
Will be sloughed
Exfoliation holes = breaks in epithelium
cells in process of peeling off the surface

17

How do basal cells adhere to the basement membrane and stroma?

Hemidesmosomes that link to fibrils
are integral membrane protein complexes in the basal cell plasma membrane.

18

Movement of epithelium and adhesion of epi is due to?

Anchoring Fibrils

The anchoring fibrils end in structures known as anchoring plaques.

19

Adhesion complex is destroyed during?

photorefractive keratectomy
must be reassembled during the healing process, as discussed subsequently.


The adhesion is completely destroyed after surgery

New adhesion is formed post surgery
CL bandage helps

20

What is the greatest importance to the barrier function?

Zonula Occludens: tight junctions
ONLY found between the superficial cells of the epithelium
Completely encircle the cells

21

Other type of cells that connect in corneal epithelium

gap junctions; more numerous in basal than in superficial....but they are found all over

22

Epithelial Ion Transport

Na+ is pumped from tears to stroma
CL- is transported from stroma into tears
Balanced

23

EBMD

Painful recurrent epithelial erosions
Corneal susceptible to edema and infection
Basal cells have decreased number of hemidesmosomes
Abnormal adhesion

24

What is EBMD caused by?

abnormal adhesions to epithelium

25

which cranial nerve is exposed as a result of ebmd

cn 5

26

why does reduplication of basement membrane occur and what is it related to?

this occurs in diabetic and older pts. associated with an increased incidence of epithelial erosions.

This abnormality of epithelial adhesion may be a result of a reduced depth of penetration of anchoring fibrils through the thickened basement membrane into the stroma.

Due to thickness..fibrils can't get through cornea

27

Why is it worse for diabetic pts

if easy to dislocate, epi can have erosions you have pain etc
Increases chances because epi is exposed and you have no sensation

Diabetic have greater chance of infection; blood flow is not good in diabetics
Epi is protection of cornea and prevent infection

Peripheral nerves will die
Loss of innervation in cornea

Epithelium is not good/ you’ll experience pain but if nerves arent functioning
Don’t feel senses as normal ppl would do…they don’t feel it…increases chance if infection

28

Epithelial Basement Membrane

aka Basal Lamina
Basal cells of epithelium rests on basal lamina
≈ 40-60 nm thick
Contains:
type IV collagen, laminin, the proteoglycan perlecan, fibronectin, and fibrin

29

Epithelial Wound Response

Abrasion of the corneal epithelium demands a prompt healing response
Must recover the exposed basement membrane with cells
After the abrasion, mitosis ceases and the attachment to the basement membrane is lost
Cells enlarge, the epithelial sheet migrates by ameboid movement to cover the defect (When you have injury…first step is to close the wound)

30

Primary function of epithelium?

to form a barrier to invasion of the eye by pathogens and to uptake of excess fluid by the stroma.

31

What happens when the wound closes?

Mitosis resumes

An experimental epithelial wound 6mm in diameter is closed with 48 hours and the rate of epithelial cell migration is 60 to 80 µm/hr.
An early observation in studies of the cell biology of corneal epithelial wound healing was that protein synthesis by epithelial cells increases during cell migration.

32

During an epithelial wound response, what type of glycolysis is used and why?

Cell Migration requires ENERGY
increase in glycolytic activity during cell migration

decrease in glycogen levels in the migrating cells

Cells are dependent on anaerobic glycolysis (quick!)

33

GLUT1 and mRNA and protein levels during wound response

Following wounding of rat corneas, increased expression of glucose transporter GLUT1 mRNA is detectable by 2 hours after wounding, and GLUT1 protein levels increase by 4 hours, peaking at 24 hours after wounding and remaining elevated for at least 2 weeks. By this mechanism, an adequate supply for support of the healing response is maintained.

34

increased synthesis of growth factors during epith. wound response

Synthesis of the growth factors and their receptors increases after corneal wounding, again accounting for the increase in protein synthesis after wounding.
Epidermal growth factor (EGF) is present in tear fluid and EGF mRNA is expressed by corneal epithelium, whereas keratocyte growth factor (KGF) and hepatocyte growth factor (HGF) are synthesized by stromal keratocytes.

35

Epithelial Wound response in patients with EBMD, Diabetes, Persistent Epithelial Defects, Severe Injuries (eg Alkali Burns)

healing is delayed or normal adhesion is not established

36

limbus and peripheral epithelium wound response

length of cell cycle decreases
replication increases

When wounding occurs, the length of the cell cycle in the peripheral epithelium and limbus decreases and the number of rounds of replication of transient amplifying cells in the limbus and peripheral corneal increases.
After wound healing, the adhesion of the epithelium is reestablished by formation of new hemidesmosomes in the basal cell layer.

Periphery of cornea has more nutrition and blood supply through diffusion

37

Reestablishment of adhesion and basement membrane of wounded epith.

the adhesion of the epithelium is reestablished by formation of new hemidesmosomes
If the basement membrane is NOT damaged, normal epithelium with adhesion complexes is formed quickly
If the basement membrane is damaged, the formation of adhesion complexes is delayed

38

Healing of wounded corneal epithelium of a keratectomy pt.

basement membrane is removed. a new basement membrane must be formed by epitheleial

development of normal adhesion delayed for more than 2 months

39

Bowman's Layer

Beneath the Basement Membrane
“modified superficial layer of stroma”
≈ 12 µm thick
Made of randomly arranged collagen fibrils
Type I collagen
Acellular

40

Stroma

An extracellular matrix (ECM) comprised of a lamellar arrangement of collagen fibrils
Parallel to corneal surface
Individual collagen fibrils separated by a matrix of proteoglycans
Keratocytes and fibroblasts – produce and maintain the ECM

41

Collagen Fibers of the Stroma

200-250 lamellas of collagen fibers
Run limbus to limbus
Anterior stroma = lie obliquely
Posterior stroma = lie orthogonally
At the limbus the fibers run circumferentially forming an annulus 1.5 to 2.0 mm wide around the cornea

42

What are collagen fibers made up of? and what is the refractive index?

Collagen fibers are made of:
Mostly type I collagen
Also type V and VI collagen

Refractive index
n= 1.411 for the collagen fibers
n= 1.365 for the extrafibrillar matrix
Despite the disparity, minimal light scattering occurs (only 10%): because of the highly uniform size and spacing of the collagen fibers!

43

Maurice's Proposal

Maurice proposed that corneal transparency is a consequence of a crystalline lattice arrangement of collagen fibrils within stromal lamellas and that light scattered by individual fibrils of uniform diameter is canceled by destructive interference with scattered light from adjacent fibers; therefore light is scattered only in the forward direction.
Such an arrangement requires that all collagen fibrils be of equal diameter and that all fibrils be equidistant from each other. Subsequent studies showed that these conditions are not satisfied in the cornea.

44

Difference between collagen in cornea and sclera

Compared to the cornea, in the SCLERA:
Fibers are large with greatly varying diameters
Not orderly or closely spaced

This is why the sclera has a great deal of light scatter and is nontransparent

45

to maintain transparency the distance bwteen fibrils must be?

must be less than1/2 the wavelength of visible light


The sizes of collagen fibers do vary, abeit within a relatively small range. Although the collagen fibers vary in diameter, they remain weak scatterers of light because their diameter is a small fraction of the wavelengths of visible light.

Edema would increase size between fibrils

46

What causes loss of corneal transparency?

With damage to epithelial or endothelial barrier, the cornea swells = loss of corneal transparency


This dependence of corneal transparency on the distribution and size of collagen fibrils is supported by observations of swollen corneas and by the structure of the opaque sclera. When the epithelial or endothelial barrier of the cornea is damaged, the stroma imbibes water and swells, leading to a loss of corneal transparency. This uptake of water causes formation of “lakes” devoid of collagen fibers within the stroma. This causes increased divergence of refractive index within the stroma, as well as an increase in distance between collagen fibrils, leading to a wavelength-dependent loss of light transmittance that increases with the amount of corneal swelling.

47

Diameter and Density of Cornea

Fibril diameter:
Anterior cornea > Posterior cornea
Density of fibrils:
Anterior cornea < Posterior cornea



It was found that fibril diameter is greater in the anterior cornea than in the posterior cornea, and that the density of fibrils is lower in the anterior cornea than in the posterior cornea in both rabbits and humans. This leads to a twofold (in humans) and threefold (in rabbits) increase in light scatter by the anterior cornea as compared with the posterior cornea.

48

Evaporation of Cornea

Water evaporates from the corneal surface at a rate of 2.5 µL/cm2/hr. Evaporation accounts for a 5% thinning of the cornea during the day, compared with the corneal thickness measured when the eyelids open in the morning after nighttime closure.

In patients with comprised endothelial metabolic pump function, such as in Fuch’s endothelial dystrophy, epithelial edema is worse in the morning when arising as a result of lack of evaporation at night when the lids are close.

Loss of water= 5% leaves cornea in morning

Thickness of cornea changes throughout the day

Epith not functioning properly
Takes in nutrients
As water goes out- takes nutrients
Water coming in and out of cornea should be same rate….fuchs nothing oushing out= fuchs
Edema worse in morning

49

What is Dellen?

Corneal drying may result in Dellen

Dellen
Localized area of corneal drying and evaporation

Persistance of dellen may reflect:
a decrease in stromal fluid flow when stroma hydration is abnormal or minimal lateral flow of water in the cornea

50

IOP and Corneal Swelling

Normal cornea maintains a constant thickness (where IOP is below 50 mm Hg)
Because stromal swelling pressure is in a similar range
If the IOP is higher than 50 mm Hg or if there is abnormal endothelial function: there is epithelial edema and increased stromal thickness

51

T/F Stromal pressure decreases with increased corneal thickness?

True


Thus mild corneal edema with elevated pressure can lead to high imbibition pressure and subsequent epithelial edema and bullae.