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In Depth Tutorials and Information
is essential to the integrity of the globe and is resistant
to most trauma, helping to protect the vital eye compo-
nents used for vision. Along with proteoglycans, elastin
and glycoproteins, the sclera is composed of collagen
fibrils - with heterotypic structures of types I and III
collagen (but including small amounts of types V and
VI) - arranged in discontinuous fibers of variable diam-
eters in interlacing fiber bundles or defined lamellar
patterns.
1
The anterior of the sclera is open and merges
with the clear cornea at a border zone called the limbus.
Along with the choroid and retinal pigment epithelium,
the sclera helps prevent stray light from entering the
eye. Therefore, the sclera helps create the “dark box”
environment of the eye that only receives meaningful
visual images through light focused by the refractive
structures of the eye's anterior segment.
2
The sclera pro-
vides mechanical structure, integrity and rigidity for
the eye. The sclera has superficial tendon insertions of
six extraocular muscles that cause eye movement. Small
openings in the sclera are present for the blood vessels
of the anterior and posterior ciliary arteries and vortex
veins. Posteriorly, the sclera merges externally with the
dura and arachnoid layers of the optic nerve. At the
optic nerve head, the sclera has a sieve-like configura-
tion called the lamina cribosa through which retinal
ganglion cell axons pass as they exit from the eye to
form the optic nerve.
The sclera is thinnest (0.3 mm) posterior to the rectus
muscle insertions and thickest (1.0 mm) at the poste-
rior pole near the optic nerve head. It is 0.4 to 0.5 mm
thick at the equator and 0.6 mm thick anterior to the
muscle insertions. The sclera may rupture open from
blunt trauma at structural weak points, especially the
corneal limbus bordering the sclera and at the rectus
muscle insertions. At the anterior border of the sclera is
the trabecular meshwork within the anterior chamber
angle. The trabecular meshwork is the site where aque-
ous humor drains from the eye into Schlemm's canal
and ultimately into the systemic circulation. Another
pathway for fluid low from the anterior chamber to the
blood circulation is through the uveoscleral pathway
not involving the trabecular meshwork. As type I colla-
gen is a major component of the sclera, the sclera is also
prone to alterations in osteogenesis imperfecta (OI).
Many patients with OI have abnormally thin sclera
through which the dark, pigmented choroid within the
eye provides a bluish hue from altered light scattering.
Some patients with OI may have alterations in the tra-
becular meshwork, the uveoscleral pathway or the lam-
ina cribosa in a manner that can stress axons passing
through the optic nerve head leading to a progressive
and sometimes blinding condition known as glaucoma.
The decreased rigidity of the sclera in many OI eyes
places them at higher risk for rupture of the globe due
to trauma.
Cornea
The cornea is the anterior, optically transparent, avascu-
lar tissue that is 11-12 mm horizontally and 9-11 mm verti-
cally.
2
It is about 0.5 mm thick at the center and increases
gradually to about 0.7 mm thick in the periphery.
3
It is the
source of most of the refractive power of the eye as well
as the main cause of astigmatism (irregular refractive
power as an aberration from spherical curvature) in the
eye's optical system. From anterior to posterior, the cornea
histologically is composed of many layers: the outer epi-
thelium, the Bowman layer (basement membrane for the
epithelium containing types I and III collagen), the corneal
stroma, Descemet's membrane and the corneal endothe-
lium. The clarity of the cornea is due to the tight packing
of epithelial cells on the anterior surface with a nearly a
uniform refractive index and also the regular, organized
arrangement of stromal cells, collagen fibers and other
macromolecules.
4
Among other molecules, the extracellu-
lar matrix of the corneal stroma is made of an organized
lattice of type I and type V fibrillar collagens intertwined
with filaments of type VI collagen. The clarity of the cor-
nea is also maintained by regulating the hydration level
of the stroma by intact epithelial and endothelial barriers
and the endothelial pump function. Type I collagen is an
important structural component of the cornea and makes
up most of the corneal stroma. Consequently, the cornea
is prone to structural alterations that affect the refractive
properties of the eye as a result of altered type I collagen
synthesis in OI.
Lens
The lens is a transparent ovoid structure that is sus-
pended behind the iris. The lens' primary function, in
conjunction with the cornea, is to refract light and pro-
vide a clear image onto the retina. The lens has a refrac-
tive power of approximately 15 diopters. It is composed
of a capsule and lens epithelial cells that divide
throughout life and produce lens fiber cells. The lens is
suspended in place at the center of the pupil posterior
to the iris by thin zonular microfibrils made of fibrillin.
Zonules connect the outer lens capsule equator to the
ciliary body for 360 degrees, similar to spokes radiating
from the center of a wheel. The lens capsule contains
collagen type IV in youth while aged capsules contain
collagen types I, III and IV, suggesting the ratio of col-
lagen types may vary over time.
5
With aging, the lens
becomes more translucent or opaque, a process called
cataract formation. As lens fibers accumulate and with
other aging-related changes, the lens becomes less flexi-
ble and slowly loses the ability to change shape for near
vision, a process known as accommodation. This aging
process that results in loss of near vision is called pres-
byopia and is the cause for requiring reading glasses.
