Anatomy and physiology of cornea

1. Cornea

3. Dimensions
• Transparent, avascular
• Anterior one – sixth of outer fibrous coat
• Refractive Index : 1.37
• Power : +43.00D
• Horizontal Visible Iris Diameter
– 11.75mm
• Horizontal Visible Iris Diameter
– 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

6. Epithelium
• Stratified squamous type
• 50-90 microns thick
• 5-6 layers of cells
– Deepest columnar cells
– 2-3 layers of wing cells
– Superficial 2 layers of flattened cells
• Sheds and is replaced entirely in 6-8 days

8. Basal cells
• Firmly joined laterally to each other –
desmosomes
• Joined anteriorly to wing cells - maculae
occuludentes
• Account for barrier function and thus
transparency
• BM of basal cells blends with Bowman’s
membrane
– adhesion formed due to filaments -
hemidesmosomes

9. Wing cells
• 2-3 layers polyhedral
Flattened cells
• 2 superficial layers
• Numerous desmosomes and maculae
occludentes
• Zonulae occludentes exclusive here
• Anterior cell wall has Microvilli (0.5mm)

10. Junctional complexes
• There are four main types of cell-cell junctions:
• Three are different types of connecting
junctions, that bind
the cells together
– occluding junctions (zonula occludens or tight
junctions)
– adhering junctions (zonula adherens).
– desmosomes (macula adherens).

11. There are also
'hemidesmosomes' that lie on the basal membrane,
to help
stick the cells to the underlying basal lamina.
– 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.

13. Bowman’s layer
• Condensed collagen fibrils
• 8-14 microns
• Not true elastic membrane
• Resistance against infection and injury
• Does not degenerate

14. Stroma
• 0.5mm thick – 90% total thickness
• Collagen fibrils (lamellae) and cells embedded in
proteoglycans (ground substance)
• Lamellae arranged parallel
• Cells – keratocytes - fibroblasts
• Keratin sulphate
• Chondroitin sulphate
• Chondroitin
• Produce GAGs and fibrils

15. Dua’s layer
• Injected tiny bubbles of air into the cornea
• 15 micrometres thick
• Fourth layer from the front
• Lies between stroma and Descemet’s membrane
• Strong and impervious to air
• May help in improving corneal graft surgery
outcomes and understanding of acute hydrops
etc

16. Descemet’s 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 Schwalbe’s line
• Posterior peripheral surface shows rounded warts
like excrescences with age – Hassle Henle bodies
• Central excrescences – Guttatae are seen in Fuchs’
dystrophy

18. Endothelium
• Single layer of flat polygonal cells – mosaic
appearance
• Density
– At birth : 6000cells/ mm2
– Young adults : 2400 – 3000 cells/mm2
– Corneal decompensation <500 cells/mm2
• Best evaluated with specular microscopy
• Polymegathism
• Pleomorphism

20. • Endothelium to Descemet’s membrane –
• hemidesmosomes
• • Desmosomal linkage and zonulae occludents
• continues around the entire cell
• • Calcium dependent linkage – 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 Conjuctival epithelium
• Bowman’s membrane ends as a rounded
border
• Substantia propria Sclera (irregular lamellae)
• Descemet’s membrane Trabecular
meshwork
• Endothelium 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

26. PERIPHERAL CORNEAL
VASCULATURE
• Peripheral cornea (and sclera adjacent to
Schlemm’s 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
– Epithelium (21 %)
• Capillaries of limbal region
• Aqueous humor via corneal endothelium
– 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
ºC/Closed eye 36 ºC

31. Corneal Swelling
• Lactate and metabolite accumulation –
osmotic gradient causes water imbibition
• Hydrophilicity of GAGs causes a natural
water imbibition
• Swelling during sleep is due to
– Hypoxia
– Lower tear osmolarity

32. Corneal physiology
• Transmits light
• Powerful refracting medium
• Protect intra ocular content

33. How does it maintain normal
physiology?
• Bio-chemical composition
• Metabolism of cornea
• Corneal transparency
• Wound healing and repair

34. Biochemical composition
• Stromal :
– 78 % water
– 15 % collagen
– 5 % other proteins
– 1 % GAGs (Glycos Amino Glycans)
• Other lipids, salts and electrolytes in other
layers

35. Corneal metabolism
• Energy for dehydration and transparency
• 4 main pathways:
– Glycolysis (88%) = lactic acid + 2 ATP
– Kreb’s cycle/ TCA (12%) = 36 ATP + H2O + CO2
– 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 – 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
– Draw water in cornea (stromal swelling pressure,
IOP)
– Prevent flow of water in cornea (barrier function)
– Draw water out of cornea (endothelial pump)
• Evaporation of water from corneal surface
• IOP

38. Thank you