just for knowledge

1. R. HASYA ARIANDA

2. Transparent
Avascular tissue
11–12 mm horizontally and 10–11 mm vertically
Refractive index 1.376
Curvature of central cornea --> 7.8 mm
The power of cornea is 43.25 D, its contributes 74% of the
total power of a normal human eye

3. 1
4 mm
11-12 mm
10-11 mm

4.  Clinically, cornea maybe divide into zones:
 The central zone: 1-2mm and closely fits a spherical surface.
 Paracentral zone: 3-4mm doughnut with an outer diameter of 7-8 mm, which is an area of
progressive flattening from the center
 Together, the paracentral and central zones constitute the apical zone, which is used in
contact lens fitting and also are primarily responsible for the refractive power of the
cornea
 Peripheral zone (transitional zone): 11 mm. This is the area of greatest flattening and
asphericity in the normal cornea.
 Limbal zone, the outer diameter average 12mm
 The cornea becomes flatter in the periphery, but the rate of flattening is not symmetric.
Flattening is more extensive nasally and superiorly than temporally and inferiorly. This
topography is important in contact lens fitting.

6.  For its nutrition, the cornea depends on glucose diffusing from the aqueous humor and
oxygen diffusing through the tear film
 peripheral cornea is supplied with oxygen from the limbal circulation
 The cornea has one of the body’s highest densities of nerve endings, and the sensitivity of
the cornea is 100 times that of the conjunctiva
 Sensory nerve fibers extend from the long ciliary nerves and form a subepithelial plexus
 Neurotransmitter : acetylcholine, catecholamines, substance P, calcitonin gene–related
peptide, neuropeptide Y, intestinal peptide, galanin, and methionine-enkephalin.

8.  It makes up approximately 5-10% of total cornea thickness.
 The anterior surface of the cornea is derived from surface ectoderm and is
covered by a nonkeratinized, stratified squamous epithelium whose
columnar layer is attached to a basal lamina by hemidesmosomes
 Basal epithelial cells serete a continuous, 50-nm-thick basement
membrane, composed of type IV collagen, laminin and other protein.
 The basal cells have a width of 12 μm and a density of approximately 6000
cells/mm2
 The clarity of cornea depends on the tight packing of epithelial cells to
produce a layer with a nearly uniform refractive index and minimal light
scattering.

9. Overlying the basal cell layer are 2 or 3 layers of
polygonal “wing” cells
The superficial corneal epithelial cells are extremely thin
(30 μm) and are attached to one another by occlusion of
the zonular fibers (sometimes referred to as zonules),
confer the properties of a semipermeable membrane to
the epithelium
Microplicae and microvilli make the apical surfaces of the
wing cells highly irregular ; the precorneal tear film
renders the surfaces optically smooth

10. Although the deeper epithelial cells are firmly attached to one
another by desmosomes, they migrate continuously from the
basal region toward the tear film, into which they are shed
They also migrate centripetally from their stem cell source at
the limbus
Division of the slow-cycling stem cells rise to a progeny of
daughter cells (transient amplifying cells), whose division
serves to maintain the corneal epithelium
Diffuse damage to the limbal stem cells (eg, by chemical burns
or trachoma) leads to chronic epithelial surface defects

11.  Nonepithelial cells may appear within the corneal epithelial layer
 Wandering histiocytes, macrophages, lymphocytes, and pigmented
melanocytes are usually components of the peripheral cornea.
 Antigen presenting Langerhans cells are found peripherally and move
centrally with age or in response to keratitis.

13.  A tough layer consisting of randomly dispersed collagen fibrils.
 It is a modified region of the anterior stroma that is 8–12 μm thick.
 Unlike the Descemet membrane, it is not restored after injury but rather
is replaced by scar tissue.

14.  The stroma constitutes approximately 90% of the total corneal thickness in humans
 Composed of:
1. collagen-producing keratocytes
2. ground substance
3. collagen lamellae
 The collagen fibrils form obliquely oriented lamellae in the anterior third of the stroma (with
some interlacing) and parallel lamellae in the posterior two-thirds
 The corneal collagen fibrils extend across the entire diameter of the cornea, finally winding
circumferentially around the limbus
 The fibrils are remarkably uniform in size and separation, and this regularity helps determine
the transparency of the cornea
 Separation of the collagen fibrils by edema fluid leads to stromal clouding
 The stroma’s collagen types are I, III, V, and VI.

15.  The ground substance of the cornea consists of proteoglycans that run along and between the
collagen fibrils
 Their glycosaminoglycan components (eg, keratan sulfate) are highly charged and account for
the swelling property of the stroma
 The keratocytes lie between the corneal lamellae and synthesize both collagen and
proteoglycans.
 The cornea has approximately 2.4 million keratocytes, which occupy about 5% of the stromal
volume; the density is higher anteriorly (1058 cells/mm2) than posteriorly (771 cells/mm2).
 Keratocytes are highly active cells rich in mitochondria, rough endoplasmic reticula, and Golgi
apparatuses, attachment structures, communicate by gap junctions, and have unusual
penetrations in their plasma membranes

16.  The basal lamina of the corneal endothelium, the Descemet membrane, is
periodic acid–Schiff (PAS) positive
 At birth, the Descemet membrane is 3–4 μm thick; its thickness increases
to 10–12 μm at adulthood
 rich in type IV collagen
 Peripheral excrescences = Hassal Henle warts
 Central excrescences = cornea guttae

18.  The corneal endothelium is composed of a single layer of mostly hexagonal cells derived
from the neural crest
 the corneal endothelium is of neuroectodermal origin
 Approximately 500,000 cells are present, at a density of about 3000/mm2.
 The apical surfaces of these cells face the anterior chamber; their basal surfaces abut the
Descemet membrane.
 young endothelial cells have large nuclei and abundant mitochondria.
 The active transport of ions by these cells leads to the transfer of water from the corneal
stroma and the maintenance of stromal deturgescence and transparency
 Mitosis of the endothelium is rare in humans, and the overall number of endothelial
cells decreases with age.

19.  Adjacent endothelial cells interdigitate in a complex way and form a variety of
adherent junctions, but desmosomes are never observed between normal cells
 Junctional complexes are present at the overlapping apicolateral boundaries of
contiguous cells
 They form a significant but lesser barrier to ion and water flow than do the tight
junctions of the epithelium
 Endothelial cell dysfunction and loss—through surgical injury,inflammation, or
inherited disease (eg, Fuchs endothelial dystrophy)—may cause endothelial
decompensation, stromal edema, and vision failure
 endothelial mitosis is limited, and destruction of cells causes cell density to decrease
and residual cells to spread and enlarge.

20.  American Academy of Ophthalmology. External Disease and Cornea. Section 8. Chapter 1.
Basic and Clinical Science Course; 2016-2017.
 American Academy of Ophthalmology. Fundamentals and Principles of Ophthalmology.
Section 2. Chapter 1. Basic and Clinical Science Course; 2016-2017.