The Cornea and Sclera

OPTOMETRY · SEMESTER 2

Ocular Anatomy and Physiology

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The Cornea and Sclera

CHAPTER 7. THE CORNEA AND THE SCLERA

BY THE END OF THIS CHAPTER YOU WILL HAVE COVERED THE

FOLLOWING ASPECTS OF THE TOPIC:

1. DEFINITION AND CORNEAL MEASUREMENTS
2. THE STRUCTURE OF THE CORNEA

3. CORNEAL PROPERTIES, FUNCTIONS, METABOLISM AND

INNERVATION

4. THE ANATOMY AND FUNCTIONS OF THE SCLERA
5. SOME CLINICAL CONSIDERATIONS
1. DEFINITION AND CORNEAL MEASUREMENTS.
The cornea isthe transparent avascular tissue with a smooth , convex
surface , that
It measures 11-12mm horizontally and 9-11mm vertically.
It is 0.52mm thick centrally and 0.67mm thick at the periphery.
The radius of curvature of the central cornea is 7.8mm ; the peripheral
corneal curvature is less marked.
2. THE STRUCTURE OF THE CORNEA.
The cornea consists of five layers :
The epithelium;
Bowman’s layer;
The stroma;
Descemet’s membrane;
The endothelium.

———7~ Basement

membrane
Epithelium———
Basement. —
membrane
Bowman's
layer
-_ —.
Stroma-—-—— Pp ees moms |
Descemet’s _
membrane -.
Endothelium-~
Fig. 7.1. Diagram representing the 5 layers of the human cornea.
2.1. The epithelium.
It accounts for 10% (0.05mm) of the total corneal thickness. five to seven cells
thick. The epithelium thickens in the periphery and is continuous with the conjunctival
epithelium at the limbus.
It is a stratified (made up by 5-6 layers ), squamous nonkeratinised
epithelium each of which contains a flattened nucleus and fewer cellular organelles
than deeper cells
There is a continual loss of epithelium
Maintenance of the smooth corneal surface depends on replacement of the surface cells
that constantly are being shed into the tear film.
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Cell proliferation occurs in the basal layer, basal cells move up to become
wing
cells, and wing cells move up to become surface cells. A slow migration of
basal cells occurs from the periphery toward the center of the cornea. This
replacement and differenciation process takes about 7-14 days. Repair to
corneal epithelial tissue
proceeds quickly; minor abrasions heal within hours, and larger ones often
heal overnight.
New cells also derive from the mitotic activity of the limbal cells.
Due to this continuous renewal potential, the corneal epithelium responds
rapidly to repair disruptions.
Because of its excellent ability to regenerate, the epithelium does not scar.
Despite cells constantly being sloughed, the barrier function is maintained as
the cell below moves into position to replace the one that has been shed.
2.2. Bowman's layer.
Bowman’s layer is a dense, fibrous sheet of interwoven collagen fibrils
randomly arranged in a mucoprotein ground substance. The second layer of
the cornea is seit onic 8 to 14 um thick. It isa road acl Foion
There is strong adherence between Bowman's and the basal lamina of the
epithelium.
It does not regenerate when damaged . Therefore, if injured, the layer usually
is replaced by epithelial cells or stromal scar tissue. Bowman's layer
sometimes is referred to as a “membrane,” but it is more correctly a
transition layer to the stroma rather than a true membrane. It differs from
the stroma in that it is acellular and contains collagen fibrils of a smaller
diameter.
2.3. The stroma/ substantia propia
The middle layer of the cornea is approximately 500 pm thick,It makes up
90% of the corneal thickness. When the stroma is 75% to 80% water, the
negatively charged molecules located around each collagen fibril maintain
this precise arrangement
by their bonds with the water molecules, and corneal transparency is optimal.
This
regularity is partly responsible for cornea.
It is composed
Keratocytes (corneal fibroblasts) are flattened cells that lie between and
occasionally within the lamellae
Ground substance fills the areas between fibrils, lamellae, and cells.
When stroma is damaged, keratocytesiincreaselininumberandisynthesize
the connective tissue components.
2.4. Descemet’s membrane.
Descemet’s membrane is considered the basement membrane of the
endothelium. It is produced constantly and therefore thickens throughout
life, such that it has doubled by age 40 years. This is the basal lamina of the
sotiteal endothelium afi consists of Gollagehifbtls. Descemet's membrane
consists of two laminae
If the basement membrane is intact, this healing takes only days, but if the
basement membrane is damaged, several weeks will be needed
for recovery.1 Descemet’s membrane is a strong, resistant membrane and, if
damaged, can be regenerated by the endothelial cells that secrete it.
2.5.The endothelium.
Consists of a single layer of hexagonal cells.
It contains metabolic aa eae for the maintenance of corneal
With age, the number of endothelial cells gradually decreases.
The endothelium is incapable of regeneration;
. the endothelium, lies adjacent to the anterior chamber
and is composed of a single layer of flattened cells. The very regular
arrangement of these cells is described as the endothelial mosaic.
Endothelial cells are not replaced when damaged but migrate to cover the
afealicalsingithelcellsinvolveditorenlargefandiflatteny)esulting in a decrease
in cell density.
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3. CORNEAL PROPERTIES, FUNCTIONS , METABOLISM AND

INNERVATION.

The complex structure of the cornea gives some indication of the diverse
functional demands upon this tissue.
The cornea must be:
transparent ,avascular,
refract light ,
contain the intraocular pressure and
provide a protective interface with the environment( physical barrier to
trauma and infection ).
Each of these functions is provided by a highly specialised substructural
organisation , and in an absence of vessels.
3.1. Corneal transparency , its most important property, is due to anumber
of related factors:
regularity and smoothness of the covering epithelium;
avascularity;
e regular, homogenous, arrangement of the stromal components;
e state of relative dehydration ( deturgescence ).
The cornea is thus an
3.2. The optical function.
The cornea has two primary functions: to refract light and to transmit light.
Factors that affect the amount of corneal refraction include (1) the curvature
of the anterior
corneal surface, (2) the change in refractive index from air to cornea (actually
the tear film), (3) corneal thickness, (4) the curvature of the posterior corneal
surface, and (5) the change in refractive index from cornea to aqueous
humor
The cornea is the principal refractive surface of the eye. It accounts for
some 70% (43D ) of the total ( 58.6D) dioptric power of the normal human
eye.
The refractive index of the cornea is 1.376 , and changes little with age.
The cornea transmits light with wavelengths between 310 and 2500 nm,
and acts as an ultraviolet (UV) filter absorbing light in the UV range of
wavelengths between 240 and 310 nm.72,73 The epithelium and Bowman's
layer appear to be the site for this UV absorbance.72 Acute exposure to UV
radiation can result in a painful photokeratitis, such as occurs with
exposure to sunlamps, tanning beds, or a welder’s arc. Chronic exposure
can result in keratopathies affecting the epithelium and anterior stroma or
can cause endothelial pleomorphism.
3.3. Corneal nutrition and metabolism is dependent on glucose diffusing
from aqueous and oxygen diffusing through the tear film.
In addition , the peripheral cornea is supplied with oxygen from the limbal
circulation.
Water is constantly pumped out of the cornea by the metabolic pumps
located in the endothelium, thus maintaining corneal deturgescence anid
The cornea is metabolically active and depends on oxygen, derived primarily
from the atmosphere and absorbed through the tear film, with small
amounts also absorbed from the aqueous humor and limbal capillaries.
Nutrients are obtained from limbal capillaries and the aqueous humor
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3.4. Corneal innervation.
The cornea is densely innervated with sensory fibers. Seventy to 80 large
nerves, branches of the anterior ciliary and short ciliary nerves, enter the
peripheral
stroma.
The cornea has one of the body's h
a.
Sensory nerve fibers extend from the trigemen (cranial nerve V ), and form
4. THE ANATOMY AND FUNCTIONS OF THE SCLERA
The sclera forms the posterior five sixths of the connective
tissue coat of the globe
The sclera forms the larger part of the outer fibrous coat of the eye and
e protect the intraocular contents and
e maintain the shape of the eyeball.
Consists of dense, opaque, irregular connective tissue.
The white, opaque nature of the sclera, in contrast to the transparency of
the cornea, can be ascribed to:
the irregular arrangement of the collagen fibers;
the variable fiber diameter;
Episclera
The episclera is a loose, vascularized, connective tissue layer that lies just
outer to the sclera. The larger episcleral vessels are visible through the
conjunctiva
Sclera
The sclera is a thick, dense connective tissue layer that is continuous with
the corneal stroma at the limbus. collagen fibrils in this tissue varies from 25
to 230 nm. These fibrils are arranged in irregular bundles that branch and
interlace. Bundle widths and thicknesses vary, with the external bundles
narrower and thinner than the deeper bundles. The orientation of the
bundles is very irregular compared with bundles in the

cornea.

SCLERAL SPUR

The scleral spur is a region of circularly oriented collagen bundles that
extends from the inner aspect of the sclera.1 In its entirety the scleral spur is
actually a ring, although on cross section it appears wedge shaped,
resembling a spur
The posterior scleral spur is the origin of the longitudinal ciliary muscle
fibers, and most of the trabecular meshwork sheets attach to its anterior
aspect
The sclera is traversed by anumber of channels or emissaria which provide

passage for arteries, veins and nerves

. SCLERAL OPACITY

The opacity of the sclera depends on anumber of factors, including the
number of GAGs, the amount of water, and the size and distribution of the
collagen fibrils.

SCLERAL COLOR

The anterior sclera is visible through the conjunctiva and, if healthy, is white
‘but may appear colored as a result of age or disease. In the newborn the
sclera has a bluish tint because it is almost transparent and the underlying
pigmented vascular uvea shows through. The sclera also may appear blue in
connective tissue diseases that cause scleral thinning. The sclera might
appear yellow in the presence of fatty deposits, which can occur with age.
Likewise, in liver disease the sclera may appear yellow because of the buildup

of metabolic wastes.

SCLERAL BLOOD SUPPLY

Because it is relatively inactive metabolically, the sclera has a minimal blood
supply. A number of vessels pass through the sclera to other tissues, but the
sclera is considered avascular because it contains no capillary beds.
Nourishment is furnished by small branches from the episcleral and —

‘choroidal vessels and branches of the long posterior ciliary arteries

SCLERAL INNERVATION

Sensory innervation is supplied to the posterior sclera by branches of the
‘short ciliary nerves; the remainder the sclera is served by branches of the
long ciliary nerves:
5. SOME CLINICAL CONSIDERATIONS.
Inflammation of the cornea is referred to as keratitis.
Corneal ulcer represents a break in the corneal epithelium;may involve
deeper layers.
As the ulcer deepens to nearly full thickness , the endothelial layer bulges
out as a descematocele.
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The corneal ulcer appears as an opacity which stains with fluoresceine.
A corneal scar it’s a corneal opacity too, but is does not stain with
fluoresceine.
Arcus senilis , a common degenerative change, is aring shaped opacification
of the peripheral cornea.
Keratoconus is a disorder characterised by an abnormal , conical (and not
elliptical ) shape of the cornea.
Inflammation of the sclera is known as scleritis , while episcleritis represents
the inflammation of the outermost layer of the sclera.
Recurrent corneal erosion is a condition in which the corneal epithelium
sloughs off either continually or periodically. This condition may occur
because of either poor attachment between the epithelium and its basement
membrane or poor attachment between the basement membrane and the
underlying tissue. Recurrent corneal erosion can occur after incomplete
healing of an abrasion in which the hemidesmosomes are malformed or it
may be caused by an epithelial basement membrane dystrophy stemming
from defective nutrition or metabolism.5