3. Dimensions
⢠Transparent, avascular
⢠Anterior one â sixth of outer fibrous coat
⢠Refractive Index : 1.37
⢠Power : +43.00D
⢠Horizontal Visible Iris Diameter
â 11.75mm
⢠Horizontal Visible Iris Diameter
â 11.00mm
4. ⢠Central thickness- 0.52mm
⢠Peripheral thickness- 0.67-1mm
⢠Anterior central curvature- 7.8mm
⢠Posterior central curvature- 6.5mm
⢠Refractive index
1.354 ground substance 1.47 - Collagen
5.
6. Epithelium
⢠Stratified squamous type
⢠50-90 microns thick
⢠5-6 layers of cells
â Deepest columnar cells
â 2-3 layers of wing cells
â Superficial 2 layers of flattened cells
⢠Sheds and is replaced entirely in 6-8 days
7.
8. Basal cells
⢠Firmly joined laterally to each other â
desmosomes
⢠Joined anteriorly to wing cells - maculae
occuludentes
⢠Account for barrier function and thus
transparency
⢠BM of basal cells blends with Bowmanâs
membrane
â adhesion formed due to filaments -
hemidesmosomes
10. Junctional complexes
⢠There are four main types of cell-cell junctions:
⢠Three are different types of connecting
junctions, that bind
the cells together
â occluding junctions (zonula occludens or tight
junctions)
â adhering junctions (zonula adherens).
â desmosomes (macula adherens).
11. There are also
'hemidesmosomes' that lie on the basal membrane,
to help
stick the cells to the underlying basal lamina.
â Gap junctions. These are communicating
junctions. (also known
as nexus, septate junction)
⢠These types of cell junctions are found between
epithelial
cells, but can also between other types of cells.
12.
13. Bowmanâs layer
⢠Condensed collagen fibrils
⢠8-14 microns
⢠Not true elastic membrane
⢠Resistance against infection and injury
⢠Does not degenerate
14. Stroma
⢠0.5mm thick â 90% total thickness
⢠Collagen fibrils (lamellae) and cells embedded in
proteoglycans (ground substance)
⢠Lamellae arranged parallel
⢠Cells â keratocytes - fibroblasts
⢠Keratin sulphate
⢠Chondroitin sulphate
⢠Chondroitin
⢠Produce GAGs and fibrils
15. Duaâs layer
⢠Injected tiny bubbles of air into the cornea
⢠15 micrometres thick
⢠Fourth layer from the front
⢠Lies between stroma and Descemetâs membrane
⢠Strong and impervious to air
⢠May help in improving corneal graft surgery
outcomes and understanding of acute hydrops
etc
16. Descemetâs layer
⢠Strong homogenous elastic layer
⢠Basement membrane of endothelium
⢠Thickness varies
⢠Highly resistant to chemical agents, trauma,
infection and pathological processes
⢠Regenerates when destroyed
⢠Maintains integrity even when stroma is
sloughed off
17. When torn, it curls
⢠In periphery, it ends anterior to trabecular
meshwork
as Schwalbeâs line
⢠Posterior peripheral surface shows rounded warts
like excrescences with age â Hassle Henle bodies
⢠Central excrescences â Guttatae are seen in Fuchsâ
dystrophy
18. Endothelium
⢠Single layer of flat polygonal cells â mosaic
appearance
⢠Density
â At birth : 6000cells/ mm2
â Young adults : 2400 â 3000 cells/mm2
â Corneal decompensation <500 cells/mm2
⢠Best evaluated with specular microscopy
⢠Polymegathism
⢠Pleomorphism
19.
20. ⢠Endothelium to Descemetâs membrane â
⢠hemidesmosomes
⢠⢠Desmosomal linkage and zonulae occludents
⢠continues around the entire cell
⢠⢠Calcium dependent linkage â important for
barrier
⢠function
⢠⢠Active endothelial pump, high metabolic
activity and
⢠energy production
21. ⢠Limbus
⢠⢠Transition zone between cornea & sclera
⢠⢠Circumferential strip of tissue around cornea
⢠and sclera junction
⢠⢠Width is about 1.5 mm horizontally & 2.0
⢠mm vertically
⢠⢠Functions - Nourishment to Cornea
22. And forms
Corneal epithelium ď Conjuctival epithelium
⢠Bowmanâs membrane ends as a rounded
border
⢠Substantia propria ď Sclera (irregular lamellae)
⢠Descemetâs membrane ď Trabecular
meshwork
⢠Endothelium ď Endothelium of the angle of
AC
23. Nerve Supply
⢠Richly innervated
⢠Long ciliary nerves of naso ciliary branch of
ophthalmic division of trigeminal nerve
⢠Enter at level of anterior stroma
⢠Become demyelinated 2-3 mm from limbus
⢠Fibres become more visible in oedema
24.
25.
26. PERIPHERAL CORNEAL
VASCULATURE
⢠Peripheral cornea (and sclera adjacent to
Schlemmâs canal) supplied by circumcorneal
vessels of anterior ciliary artery
⢠Minor role in corneal nutrition
⢠Remainder of cornea is avascular
27. Oxygen supply to Cornea
⢠Atmosphere via Tear film
â Epithelium (21 %)
⢠Capillaries of limbal region
⢠Aqueous humor via corneal endothelium
â Endothelium (7.4%)
⢠Capillaries of palpebral conjunctiva
⢠Co 2 is thrown out
28. Corneal nutrition
Cornea is avascular
Nutrition by diffusion
⢠Tear Film
⢠Aqueous humour
⢠Limbal capillaries
29. Corneal Transparency
⢠Stroma transmits 90% of incident light
⢠Potentially a non transparent layer
⢠Fibrils: n = 1.47
⢠Ground substance: n = 1.354
⢠Regular fibril spacing: 60 nm
30. Corneal Transparency
⢠Scattering effect increases as swelling increases
(even during sleep)
⢠Corneal integrity requires 15 % to 21 % Oxygen
for
regular function.
⢠Increase in corneal temperature under a soft lens
is
due to insulating effect of the lens. Open eye 34
ÂşC/Closed eye 36 ÂşC
31. Corneal Swelling
⢠Lactate and metabolite accumulation â
osmotic gradient causes water imbibition
⢠Hydrophilicity of GAGs causes a natural
water imbibition
⢠Swelling during sleep is due to
â Hypoxia
â Lower tear osmolarity
33. How does it maintain normal
physiology?
⢠Bio-chemical composition
⢠Metabolism of cornea
⢠Corneal transparency
⢠Wound healing and repair
34. Biochemical composition
⢠Stromal :
â 78 % water
â 15 % collagen
â 5 % other proteins
â 1 % GAGs (Glycos Amino Glycans)
⢠Other lipids, salts and electrolytes in other
layers
35. Corneal metabolism
⢠Energy for dehydration and transparency
⢠4 main pathways:
â Glycolysis (88%) = lactic acid + 2 ATP
â Krebâs cycle/ TCA (12%) = 36 ATP + H2O + CO2
â HMP shunt = NAPDH (biosynthesis of lipids,
building nucleic acids)
⢠90% energy consumed by epithelium
36. Corneal transparency
Anatomical factors:
⢠Homogenous refractive index of epithelium
⢠Tight intracellular junctions â barrier
⢠Cornea is avascular
⢠Epithelium is nonkeratinized
⢠Stromal lamellae are regular
⢠Nerves are nonmyelinated
⢠Precorneal tear film
37. Corneal hydration
⢠Maintained by balance of factors that
â Draw water in cornea (stromal swelling pressure,
IOP)
â Prevent flow of water in cornea (barrier function)
â Draw water out of cornea (endothelial pump)
⢠Evaporation of water from corneal surface
⢠IOP