CorneaSclera 1 Flashcards
What is the size of the cornea?
H:12.6
V: 11.7
Which surface is the major refractive component of eye? How many Diopters?
Anterior surface; 48D
Which part of the cornea is spherical or toroidal?
central third
What is the radius curvature of the cornea?
7.8mm
True or false; peripheral part of cornea is thicker than center?
True: central 0.52 mm
peripheral 0.65 mm
What percentage of incidence light over 400nm is transferred through the cornea?
99%
How much radiation is passed through cornea?
from approximately 310 nm in the ultraviolet to 2500 nm in the infrared.
What are the layers of cornea?
Epithelium Bowman's Layer Stroma Descemet's Membrane Endothelium
Describe epithelium?
straftified nonkertanized, nonsecretory squamous layers
5-7 layers
What are the 3 cell layers that make up epithelium or cornea?
- Surface cells; 3-4 layersdifferentiated squamous cells that are sloughed from surface
- Wing cells: 1-3 layers, intermediate differentiated layer
- 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.
What is the turnover for epithelium?
7 days ; however most trauma to cornea heals in 24 hours
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
Where do corneal basal cells originate from?
originate from stem cells of the limbal epithelium
What is the mitotic rate of corneal epithelium?
10-15% a day
How is the cornea epithelium maintained?
- a constant cycle of shedding of superficial cells and proliferation of cells in the basal layer.
- 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.
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
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.
Movement of epithelium and adhesion of epi is due to?
Anchoring Fibrils
The anchoring fibrils end in structures known as anchoring plaques.
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
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
Other type of cells that connect in corneal epithelium
gap junctions; more numerous in basal than in superficial….but they are found all over
Epithelial Ion Transport
Na+ is pumped from tears to stroma
CL- is transported from stroma into tears
Balanced
EBMD
Painful recurrent epithelial erosions
Corneal susceptible to edema and infection
Basal cells have decreased number of hemidesmosomes
Abnormal adhesion
What is EBMD caused by?
abnormal adhesions to epithelium
which cranial nerve is exposed as a result of ebmd
cn 5
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
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
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
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)
Primary function of epithelium?
to form a barrier to invasion of the eye by pathogens and to uptake of excess fluid by the stroma.
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.
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!)
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.
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.
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
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
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
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
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
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
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
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!
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.
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
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
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.
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.
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
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
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
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.
