CHAPTER 20
Eye
397
Figure 20-6A.
Pavement
epithe-
lium (anterior epithelium).
EM,
×
6,000
The
anterior corneal epithelium
is a
stratif
ed squamous epithelium
that
covers the external surface of the
cornea. The lack of keratinization
and the uniform thickness of the
epithelium contribute to the essen-
tial transparency of the cornea. The
epithelial cells have a fairly fast
turnover rate of about one week.
The surface cells are not perfectly
smooth, but rather have small
microvilli
(only a few are preserved
here) that serve to anchor the vitally
important tear F
lm. Not shown in
this F
eld are free nociceptive nerve
endings that penetrate the epithe-
lium and provide the afferent limb
of the corneal blink refl ex.
Bowman
membrane
is the commonly used
term for the thick basement mem-
brane at the interface between the
anterior corneal epithelium and the
corneal stroma
.
Corneal
Corneal
stroma
stroma
Bowman
Bowman
membrane
membrane
Collagen
Collagen
fibers
fibers
Basal
Basal
epithelial cells
epithelial cells
Nuclei of the
Nuclei of the
epithelial cells
epithelial cells
Nuclei of the
epithelial cells
Basal
epithelial cells
Collagen
fibers
Bowman
membrane
Corneal
stroma
Anterior (Fig. 20-6A)
Anterior (Fig. 20-6A)
Posterior (Fig. 20-6B)
Posterior (Fig. 20-6B)
Anterior (Fig. 20-6A)
Posterior (Fig. 20-6B)
A
Figure 20-6B.
Corneal endothe-
lium (posterior corneal epithelium).
EM,
3
4,500
The
posterior corneal epithelium
is
sometimes called the
corneal
endothe-
lium
. It is a simple squamous epithe-
lium that faces the aqueous humor
of the anterior chamber.
Descemet
membrane
is the commonly used
term for the sharply deF ned
base-
ment membrane
of the posterior
epithelium. Descemet membrane is
unusual in that it consists largely of
an orderly network of
type VIII col-
lagen
, a relatively rare collagen type.
The epithelial cells are joined by
tight junctions
, and they control the
movement of water, ions, and metab-
olites between the stroma and the
aqueous humor, the source of nutri-
tion for the
corneal stroma
. Orderly
layers of
type I collagen
F brils form
the bulk of the corneal stroma. The
extremely fl attened
f broblasts
that
produce and maintain the stroma
are called
keratocytes
.
Corneal stroma
Corneal stroma
Nuclei of
Nuclei of
endothelial cells
endothelial cells
Keratocytes
Keratocytes
(fibroblasts)
(fibroblasts)
Descemet
Descemet
membrane
membrane
Collagen
Collagen
fibers
fibers
Keratocytes
(fibroblasts)
Nuclei of
endothelial cells
Collagen
fibers
Descemet
membrane
Corneal stroma
B
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