2. Anatomy of Cornea
• Cornea is a transparent, avascular, watch-glass
like structure.
• It forms anterior one-sixth of the outer fibrous
coat of the eyeball.
3.
4. Dimensions
• Anterior surface of cornea is elliptical with an average horizontal
diameter of 11.7 mm and vertical diameter of 11 mm.
• Posterior surface of cornea is circular with an average diameter
of 11.5 mm.
• Thickness of cornea in the centre varies from 0.5 to 0.6 mm
while at the periphery it varies from 1 to 1.2 mm.
• Radius of curvature. The central 5 mm area of the cornea forms
the powerful refracting surface of the eye. The anterior and
posterior radii of curvature of this central part of cornea are 7.8
mm and 6.5 mm, respectively.
• Refractive index of the cornea is 1.376.
• Refractive power of the cornea is about 45 dioptres, which is
roughly three-fourth of the total refractive power of the eye (60
dioptres).
5. Histology
Histologically, the cornea consists of six layers.
From anterior to posterior these are:
–Epithelium,
–Bowman’s membrane,
–Substantia propria (corneal stroma),
–Pre-Descemet’s membrane (Dua’s layer),
–Descemet’s membrane and endothelium
7. Epithelium
• It is of stratified squamous type and becomes continuous
with epithelium of bulbar conjunctiva at the limbus.
• It consists of 5–6 layers of cells. The deepest (basal) layer
is made up of columnar cells, next 2–3 layers of wing or
umbrella cells and the most superficial two layers are made
of flattened cells.
• Tight junctions between superficial epithelial cells prevent
penetration of tear fluid into the stroma.
• Limbal epithelium. The basal epithelial cells of the limbal
area constitute the limbal stem cells which amplify,
proliferate, and differentiate into corneal epithelium.
• Damage to this area results in the invasion of conjunctival
epithelium on to the cornea
8. Bowmans membrane
• This layer consists of acellular mass of condensed
collagen fibrils.
• It is about 12 mm in thickness and binds the
corneal stroma anteriorly with basement
membrane of the epithelium.
• It is not a true elastic membrane but simply a
condensed superficial part of the stroma.
• It shows considerable resistance to infection. But
once destroyed, it is unable to regenerate and,
therefore, heals by scarring.
9. Stroma – Substantia Propria
• This layer is about 0.5 mm in thickness and constitutes most of
the cornea (90% of total thickness).
• It consists of collagen fibrils embedded in hydrated matrix of
proteoglycans (chondroitin sulphate and keratan sulphate).
• The lamellae are arranged in many layers. In each layer they
are not only parallel to each other but also to the corneal plane
and become continuous with scleral lamellae at the limbus.
• The alternating layers of lamellae are at right angle to each
other.
• Among the lamellae are present keratocytes (modified
fibroblasts), wandering macrophages, histiocytes and a few
leucocytes
10. Pre- Descement Membrane (Dua
Layer)
• Pre–Descemet’s membrane, also known as
Dua’s layer has been discovered in 2013 by Dr
Harminder Dua, an Indian ophthalmologist
working in Great Britain.
• Located anterior to the Descemet’s membrane,
it is about 15 micrometer thick acellular
structure which is very strong and imprevious
to air
11. Descemet Membrane
• Descemet’s membrane (posterior elastic lamina). The
Descemet’s membrane is a strong homogenous
basement membrane of the corneal endothelium
which is separated from the stroma by pre-Descemet’s
membrane. It is very resistant to chemical agents,
trauma and pathological processes. Therefore,
‘Descemetocele’ can maintain the integrity of eyeball
for long. Descemet’s membrane consists of collagen
and glycoproteins. Unlike Bowman’s membrane it can
regenerate. Normally, it remains in a state of tension
and when torn, it curls inwards on itself. In the
periphery it appears to end at the anterior limit of
trabecular meshwork as Schwalbe’s line (ring).
12. Endothelium
• Endothelium. It consists of a single layer of flat polygonal
(mainly hexagonal) epithelial cells (misnamed as
endothelium) which on slit-lamp biomicroscopy appear as a
mosaic. The cell density of endothelium is around 3000
cells/mm2 in young adults, which decreases with the
advancing age. The human endothelium cells do not
proliferate in vivo and the cell loss with age is compensated
by enlargement (polymegathism) and migration of
neighbouring cells. There is a considerable functional
reserve for the endothelium. Therefore, corneal
decompensation occurs only after more than 75% of the
cells are lost. The endothelial cells contain ‘activepump’
mechanism which keeps cornea dehydrated.
13. Physiology of cornea
The two primary physiological functions of the cornea are
(i) To act as a major refracting medium
(ii) To protect the intraocular contents.
Cornea performs these functions by maintaining its
transparency and regular replacement of its tissues
14. Corneal Transparency
• Peculiar arrangement of corneal lamellae
• Avascularity of cornea.
• Relative state of dehydration (78% water content),
which is maintained by the barrier effects of
epithelium and endothelium and the active Na+ K+
ATP ase pump of the endothelium.
• Swelling pressure (SP) of the stroma which counters
the imbibition effect of intraocular pressure (IOP).
• Corneal crystallins, i.e, water soluble proteins of
keratocytes also contribute to corneal transparency at
the cellular level. For these processes, cornea needs
some energy.
15. Metabolism of cornea
• Most actively metabolizing layers of the
cornea are epithelium and endothelium, the
former being 10 times thicker than the latter
requires a proportionately larger supply of
metabolic substrates.
16. Source of nutrients for cornea
• 1. Solutes (glucose and others) enter the cornea by
either simple diffusion or active transport through
aqueous humour and by diffusion from the perilimbal
capillaries.
• 2. Oxygen is derived directly from air through the tear
film. This is an active process undertaken by the
epithelium. Therefore, corneal hypoxia may occur with
over wear of contact lenses.
• Some oxygen may reach the superficial layers of cornea
from the perilimbal capillaries (especially when eyelids
are closed) and deeper corneal layers through the
aqueous humour
17. • Endothelium requires oxygen and glucose for
metabolic activities and proper functioning of
Na+ - K+ ATPase pump. Like other tissues, the
epithelium can metabolize glucose both
aerobically and anaerobically into carbon
dioxide and water and lactic acid, respectively.
Thus, under anaerobic conditions lactic acid
accumulates in the cornea
