Cornea

The cornea is the transparent window of the eye. It is
about 12 mm (0.5 inch) in diameter and, except at its margins,
contains no blood vessels. However, it does contain
many nerves and is very sensitive to pain or touch. It is
nourished and provided with oxygen anteriorly by tears
and is bathed posteriorly by aqueous humour. It protects
the pupil, the iris, and the inside of the eye from penetration
by foreign bodies and is the first and most powerful
element in the eye’s focusing system. As light passes
through the cornea, it is partially refracted before reaching
the lens. The curvature of the cornea, which is spherical
in infancy but changes with age, gives it its focusing power;
when the curve becomes irregular, it causes a focusing
defect called astigmatism, in which images appear elongated
or distorted.
The cornea contains five distinguishable layers: the
epithelium, or outer covering; Bowman’s membrane;
the stroma, or supporting structure; Descemet’s membrane;
and the endothelium, or inner lining. Up to 90
percent of the thickness of the cornea is made up of
the stroma. The epithelium, which is a continuation of the
epithelium of the conjunctiva, is itself made up of about
six layers of cells. The superficial layer is continuously
being shed, and the layers are renewed by multiplication
of the cells in the innermost, or basal, layer.
The stroma appears as a set of lamellae, or plates, running
parallel with the surface and superimposed on each
other like the leaves of a book; between the lamellae lie
the corneal corpuscles, cells that synthesize new collagen
(connective tissue protein) essential for the repair and
maintenance of this layer. The lamellae are made up of
microscopically visible fibres that run parallel to form
sheets; in successive lamellae the fibres make a large angle
with each other. The lamellae in humans are about 1.5 to
2.5 micrometres (1 micrometre = 0.001 millimetre) thick,
so that there are about 200 lamellae in the human cornea.
The fibrous basis of the stroma is collagen.
Immediately above the stroma, adjacent to the epithelium,
is Bowman’s membrane, about 8 to 14 micrometres
thick; with the electron microscope it is evident that it is
really stroma, but with the collagen fibrils not arranged in
the orderly fashion seen in the rest of the stroma.
Beneath the stroma are Descemet’s membrane and
the endothelium. The former is about 5 to 10 micrometres
thick and is made up of a different type of collagen from
that in the stroma; it is secreted by the cells of the endothelium,
which is a single layer of flattened cells. There is
apparently no continuous renewal of these cells as with
the epithelium, so that damage to this layer is a more serious
matter.
The most obvious difference between the opaque
sclera and the transparent cornea is the irregularity in the
sizes and arrangement of the collagen fibrils in the sclera
by contrast with the almost uniform thickness and strictly
parallel array in the cornea; in addition, the cornea has a
much higher percentage of mucopolysaccharide (a carbohydrate
that has among its repeating units a nitrogenous
sugar, hexosamine) as embedding material for the collagen
fibrils. It has been shown that the regular arrangement of
the fibrils is, in fact, the essential factor leading to the
transparency of the cornea.