396
UNIT 3
Organ Systems
Tunica Fibrosa (Tunica Externa)
A
Pavement
Pavement
epithelium
epithelium
Posterior
Anterior
Stroma
Corneal
endothelium
Corneal
endothelium
Descemet
membrane
Pavement epithelium
(anterior corneal epithelium)
Pavement
epithelium
Bowman
membrane
Bowman
membrane
Descemet
membrane
Figure 20-5A.
Cornea.
H&E,
3
77.5; insets
3
173
The
cornea
is a transparent and avascular tissue,
which is composed of F ve layers: three cellular
layers (epithelial layers and stroma) and two non-
cellular layers (Bowman membrane and Descemet
membrane). These include (1)
pavement epithe-
lium
(
anterior corneal epithelium
); (2) the
Bowman
membrane
(basement membrane of the pavement
epithelium); (3)
stroma
, consisting of F broblasts (also
called
keratocytes
in the cornea) and alternating lamel-
lae of collagen F bers; (4) the
Descemet membrane
(basement membrane of the
posterior corneal epithe-
lium
(
endothelium)
; and (5) the
corneal endothelium
(posterior corneal epithelium). The
cornea has a rich
nerve supply from CN V. The superF cial corneal
layer contains numerous sensory nerve F bers, and
irritation can cause severe eye pain. CN V also car-
ries the afferent limb of the corneal refl ex.
B
Descemet membrane
(noncellular layer)
Stroma (cellular layer)
Endothelium
(cellular layer)
Pavement epithelium
(cellular layer)
Bowman membrane
(noncellular layer)
D. Cui
Figure 20-5B.
A representation of the cornea.
Pavement epithelium
is formed by stratiF ed
squamous
epithelium consisting of four to six layers and is about 50 mm in
thickness; it has a high capacity for regeneration. The transpar-
ency of the cornea is due to its avascularity and due to its state
of relative dehydration.
Corneal endothelium
is a single layer of
squamous cells. This layer functions in the regulation of water
in the stroma. The primary task of corneal endothelium is to
pump the excess fl uid out of the stroma and, therefore, is criti-
cal for keeping the cornea clear. If this endothelium is damaged,
the result may be corneal swelling due to fl uid retention within
the stroma and loss of its transparency.
CLINICAL CORRELATION
Figure 20-5C.
LASEK.
Laser refractive surgery
reshapes the cornea in order to focus
images more accurately onto the retina. The most common
procedures include
laser in-situ keratomileusis
(
LASIK
), and
laser-assisted epithelial keratomileusis
(
LASEK
). Laser tech-
niques have been continuously modiF
ed to reduce complica-
tions and enhance surgical outcomes. Unlike with LASIK,
the most recent LASEK surgery saves the epithelium by using
an alcohol solution to weaken epithelial cell adhesion so the
epithelial layer can be lifted as a fl
ap. After the
epithelial
fl ap
is moved out of the way, excimer laser energy is applied
through the
Bowman membrane layer
and into the upper
stroma to reshape the cornea. The epithelial fl
ap is then
returned to its original position. The advantages of LASEK
are a reduction of postoperative discomfort, a decreased risk
of infection, and an increase in the overall thickness of the
untouched area of the cornea.
C
Cornea flap
Laser beam
Exposed Bowman membrane
D. Cui
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