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Cornea anatomy and physiology PowerPoint presentation
1. ANATOMY AND PHYSIOLOGY
OF CORNEA.
Dr. Ritu Chaturvedi
HOD Department of Ophthalmology
SRVS Medical College
2. INTRODUCTION
•Dome shaped, fibrous,
transparent, avascular, watch
glass like structure.
•Forms anterior 1/6th of the
surface area of the globe.
•Convex outer surface
•Concave inner surface
•Greek name - Kerato
3. DIMENSIONS OF CORNEA
ANTERIOR
Elliptical
11.7mm (Horizontal)
10.6mm (Vertical)
7.8 (Radius of curvature)
POSTERIOR
Circular
11.7mm (Horizontal)
11.7mm (Vertical)
6.5mm (Radius of
curvature)
4. Thickness
centre : 0.5-0.6 mm
periphery : 1.2 mm
Refractive index
1.376 (~1.38)
Refractive Power
Anterior surface : +48 D , Posterior:
surface: -5 D ,
Thus the net refractive power is +43 D.
(3/4thof total R.P. of eye)
7. • The basal cells are present in the deepest layer,
columnar in shape, attached to the underlying
basal lamina by hemidesmosomes.
• Overlying basal cell layer are 2 or 3 layers of
polygonal “wing” cells or umbrella cells.
• Superficial to these layers , 1–2 layers of corneal
epithelial “surface” cells is present. extremely thin
and attached to one another by tight junctions.
8.
9. LAYER OF SURFACE CELLS
•Most superficial layer of corneal
epithelium.
•2-3 cell layer thick.
•Polyhedral cells, becomes wider &
increasingly flattened towards the
surface.
•As wide as 45µm & 4µm in depth.
B= basal cells; S = surface cells; W = wing
cells.
10. • Flattened nuclei project
backwards, leaving surface
perfectly smooth.
• Membrane has extensive
microvilli which are covered
with glycocalyx layer.
• Microvilli play an imp role in
tear film stability
11. • Beneath the superficial
cells there is umbrella or
wing cells.
• 2-3 cell layered thick.
• Consists of polyhedral
cells with flattened
nuclei, convex anteriorly.
B= basal cells; S = surface cells; W = wing
cells.
LAYER OF WING CELLS
12. LAYER OF BASAL CELLS
•Single layer of tall columnar
cells arranged in pallisading
manner
•Cells contain oval nuclei.
•10 µm wide, 15 µm tall with
rounded heads & flat bases.
B= basal cells; S = surface cells; W = wing
cells.
13. • Laterally attached to other basal cells by
desmosomes and zonula occludens.
• Attached to the basement membrane by
extensive hemidesmosomal system
• Prevent the detachment of the multilayer
epithelial sheet from the cornea
14. BASEMENT MEMBRANE
• Basement membrane is the extracellular matrix
secreted by the basal cells.
• Under normal conditions consists of type 4
collagen and laminin.
• In acute injury, fibronectin production increases in
the basement membrane .
15. BOWMAN’S LAYER
• Modified region of anterior
Stroma
• Narrow, Acellular, homogenous
zone.
• 8-14 µm thick.
• Sharply defined from overlying
epithelium, infiltrated by lamina
densa & merges into stroma
behind.
• Once destroyed, does not
regenerate.
Bowman’s
layer
16. STROMAL LAYER
• Substantia Propria
• Provides structural integrity to the cornea.
• 500 µm thick
• Accounts for 90% of the corneal thickness.
17.
18.
19.
20. COLLAGEN FIBERS IN STROMA
Composed of type -1 collagen surrounded by specialized
proteoglycans consisting of :
a) keratan sulfate
b) chondroitin sulfate
c) dermatan sulfate
21. CELLS IN STROMA
Cells in stromal layer consist of-
• KERATOCYES- fibroblasts with flattened
cell body, a large eccentric nucleus.
Produce ground substance and collagen
fibrils.
• WANDERING MACROPHAGES
• HISTOCYTES
• FEW LYMPHOCYTES
22. DUA’S LAYER
It is the newly discovered 4th layer also
called
the Pre-Descemet’s layer
• It is located anterior to Descemet’s
membrane.
• It is 15 µm thick acellular structure.
• It is strong and impervious to air
23. DESCEMET’S MEMBRANE
• Strong, resistant, homogenous,
elastic layer which binds the stroma
posteriorly.
• Made up of collagen and
glycoprotein.
• Ends at the anterior limit of
trabecular meshwork as Schwalbe’s
line.
DESCEMET’S
MEMBRANE
Stroma
Endothelium
24.
25. • Posterior Surface of
DM at periphery
shows rounded wart
like excrescences
called Hassel Henle
Bodies which increase
with age.
• Similar central
excrescences called
guttatae are seen in
Fuch’s dystrophy.
26. ENDOTHELIUM
• Single layer of hexagonal,
cuboidal cells. Appears as
honey comb.
• Endothelial cell density
6000 cells/mm sq at birth.
26% lost in Ist yr.
Another 26% lost over next
11yrs.
Reduced to 2400-3000 cells/mm
sq in adults.
30. Annular plexus
Branches travel radially entering the
stroma losing their myelin sheath & unite
to form Subepithelial corneal plexus
Fibers move anteriorly, lose their schwann
cells & pierce bowman’s membrane to form
Intraepithelial nerve plexus
31.
32. APPLIED PHYSIOLOGY
The two primary physiological functions of the cornea
are:
1. To act as a major refracting medium;
2. To protect the intraocular contents.
33. COMPOSITION OF CORNEA
• 78% - Water
• 15% - Collagen (mainly type I & VI,
with small contribution of type III, IV)
• 5% - Other protein
• 2% - Keratan sulfate, Chondroitin
sulfate & Dermatan
sulfate(mucopolysaccharides)
34. METABOLISM OF CORNEA
Most actively metabolizing layers of the cornea are
epithelium and endothelium
Source of nutrients
• Oxygen is derived directly from air through the tear film
by the epithelium.
• Endothelium derives most of its required oxygen from
aqueous humor.
• Solutes (glucose and others) enter the cornea by either
simple diffusion or active transport.
35. METABOLIC PATHWAYS OF
CORNEA
Glucose is the primary metabolic substrate for the
epithelial cells, stromal keratocytes and endothelial
cells.
Glucose is metabolized in the cornea by all 3
metabolic pathways:
• Hexose monophosphate (HMP) shunt
• Tricarboxylic acid (TCA) cycle
• Glycolysis
46. Cornea is an avascular structure which adds
to the corneal transparency.
Corneal vascularization is seen in various
disease processes.
Pathogenesis of Corneal Vascularization
• Chemical factors
• Mechanical factors
• Combined factors (by Maurice et al)
•Inflammatory response due to Leucocytes
47. DRUG PERMEABILITY ACROSS THE CORNEA
Non-polar (lipid soluble) drugs readily cross corneal
epithelium and endothelium.
Polar (water soluble) drugs readily cross stroma.
Lipid soluble molecules cross the epithelium
irrespective of their molecular wt. while water soluble
molecules < 4Å can cross.
Also mol.wt. ,100 can pass and > 500 cannot pass.
Substances with large molecular size used in high
conc. cross cornea due to laws of mass action.
48.
49.
50. REFERENCES
• American Academy
• John Ferris-Basic sciences in
ophthalmology
• Parsons’ Diseases of the eye
• Kanski’s clinical ophthalmology
• A. K. Khurana - Anatomy & Physiology Of
Eye
• Internet
Editor's Notes
A. Anterior and posterior diagram of the corneal edges showing the elliptic shape anteriorly and the round shape posteriorly. Also noted are the vertical and horizontal diameters of the anterior and the posterior cornea.
Lateral wall of these cells are interconnected by tight barrier junctional complexes which restricts the entry of fluids into the intercellular spaces.
Similar to superficial layer it posseses tight lateral intercellular junctions.
Forms Germinative layer of epithelium, continuous peripherally with that of limbus.
Bonding anomaly leads to recurrent corneal erosion syndrome or persistent non-healing epithelial defect.
It is PAS positive.
The basement membrane is attached to the underlying bowman’s layer by anchoring fibrils and plaques.
Consists of regularly arranged lamellae of collagen bundles in proteoglycan ground substance along with few Keratocytes.
discovered in 2013 Dr. Harminder Dua at the University of Nottingham.
(First point) It represents the basement membrane of endothelial cells
(After 3rd point) Regenerative, synthesis continues throughout life. 3-4 µm at birth, 5 µm in childhood & 10-12 µm in adult
Does not proliferate, normal cells increase in size (3x) (polymegathism) to fill in the gap left behind by dead cells.
Vary in diameter from 18 µm in early life to 40 µm in aged.
The cornea has a rich supply of sensory nerve endings derived mainly from the long ciliary nerves which are branches of the nasociliary nerve ( a branch of ophthalmic division of the trigeminal nerve) . The long ciliary nerves after arising from the nasociliary nerve enter the eyeball around optic nerve.
Pierce the sclera to leave the eyball , divide and connect with each other and the conjunctival nerves to form a pericorneal plexus of nerves.
Cornea performs these functions by maintaining its transparency and
regular replacement of its tissues.
In addition to these, the cornea also plays an important role in:
Absorption of topically applied drugs and
Wound repair after anterior segment surgery or trauma
through aqueous humour and by diffusion from the perilimbal capillaries.
Swelling pressure of the stromal matrix and intraocular pressure
Mechanical barrier action of the epithelium which constitutes a relatively impermeable membrane
Active pumping action of the corneal epithelium.
Disturbance of any these factors lead to corneal edema
GAG’s act like a sponge
IOP : intraocular pressure SP : swelling pressure
Chemical factors
sulfate esters of hyaluronic acid acts as vasoinhibitory factors which prevent vascular invasion.
release of Vasostimulatory factors at the site of the lesion stimulates new vessel growth from the limbal plexus.
Mechanical factors – inability of blood vessels to invade the stroma due to its compactness.
So the drug has to be amphipathic to readily cross the cornea.
