Anatomy of cornea Physiology of cornea Transparency of cornea

1. CORNEA
Anatomy & Corneal
Transperency
Dr Fabna mahmood

2. The cornea is a transparent avascular
tissue with convex outer surface and
concave inner surface which resembles
a small watch glass.

4. DIMENSIONS
• Anterior surface:elliptical
horizontal diamter:11.75mm
and verticaldiameter:11mm
Posterior surface:circular
Avg diamter:11.5mm

8. EPITHELIUM

9. • Basal layer :tall columnar
polygonal shaped cells in palisade
manner on basement membrane.
• It forms the germinal layer and
undrgoes mitosis to form
daughtercells which migrate
anterorly into wing cell layer
• Oval nucleus and organelles like
RER,Free ribosomeand occasional
golgi apparatus seen
• Mitochondria are small and few
s/o low aerobic oxidation and
more dependent on pentose
shunt for metabolism

10. • Basal cells:joined laterally to other cells and anteriorly to
wing cells by DESMOSOMES and zonula ocludens.
• It accounts for the epithelium’s transparency and barrier
function
•

12. • Wing cells----- 2to 3 Layers of polyhedral cells
------attached with wing cells and basal cells via tight
junctions
• Flattened cells---cells are long(45mm)and thin (4mm) with
flattened nuclei
• desmosomes,zonula occludens are seen in this layer
Microvilli present---helps in tear film stability

13. Functions of epithelium
• Major refractive surface of the eye
• Major surface to respond to wound healing
• Provide barrier to fluid loss and pathological
entrance to the organisms

14. Bowmanns membrane
 Contains collagen fibrils(type1 and 5)
 8-14micrometer thickness
 Binds corneal stroma with basement
membrane of the epithelium
 It does not regenerate
 smooth base for epithelium
uniformity
Helps in refraction

18. Predescement’s layer(duas layer)
• Discovered by harminder dua
• 15mm thick acellular structure
• Composed of collagen type 1
• Contains proteoglycan like lumican,nimecan and decorin

19. Descement’s membrane(posterior elastic lamina)
• Strong homogenous layer
• Made up of collagen and glycoproteins
• Thickness:
• birth:3 micrometer
• young adults:10-12 micrometer
• It can regenerate.resistance to chemical
agents,infection and pathological process
• On posterior surface shows rounded wart like
excrescences called Hassal henle bodies

21. • Endothelial cells are
best evaluated by
specular microscopy
• Attached to descements
membarne by
hemidesmosomes
• Abundant
mitochondria,freee
ribosomes,RER,golgi
apparatus

26. CORNEAL TRANSPARENCY
• Main physiologic function of cornea is to act as a
major refracting medium,so clear retinal image is
formed.
• Normal corneal transparency is the result of
• ANATOMICAL FACTORS
• such as uniform and regular arrangement of the corneal
epithelium,a peculiar arrangement of the corneal lamellae
and corneal avascularity
PHYSIOLOGICAL FACTORS
• i e realtive state of corneal dehydration

27. FACTORS AFFECTING CORNEAL TRANSPARENCY
• 1.Corneal epithelium and tear film
• 2.arrangement of stromal lamellae
• 3.corneal vascularisation
• 4.corneal hydration
• 5.cellular facctors affecting transparency

28. Corneal epithelium and tear film
• Normal epithelium is transparent
due to homogenicity of its
refractive index.
• Basal cells are firmly joined to other
basal cells and anteriorly to
• the wing cells by desomosomes and
zonula occludens.
• These tight intercellular junctions
account for corneal transparency
• As well as resistance to flow of
water,electrolytes and
glucose(BARRIER FUNCTION)
• Normal precorneal tear film plays
an important role in maintaining
the transparency.

29. Arrangement of stromal lamellae
• Two theories have been put forward to explain the
role of a peculiar arrangement of stromal lamella in
corneal transparency
o MAURICE THEORY
o THEORY OF GOLDMAN ET AL

30. Maurice theory
• Corneal transparency is
because of the uniform
collagen fibrils which are
arranged in regular lattice so
that scattered light is
destroyed by mutual
interfernce.
• He stated that as long as the
fibrils are arranged in
regular lattice, seperated by
less than the wavelentgh of
light(4000 to 7000A),the
cornea will remain
transparent

31. • Loss of transparency will result if this
arrangement is altered by stromal edema or
mechanical stress.

32. Theory of goldman et al
• Goldman et al after applying diffraction thoery concluded
that lattice arrangement is not necessary condition for
stromal transparency.
• Rather he postulated that cornea is transparent because the
fibrils are small in relationship to light and donot interfere
with light transmission unless they are larger than one half a
wavelength of light.
• ‘Lakes’---areas devoid of collagen, were found in human
cornea(2000A)

33. • However theory of maurice as well as goldman fail to explain
the occurrence of rapid cloouding of cornea assosiated with
acute rise of intraocular pressure and rapid clearing of cornea
with reduction of intraocular pressure

34. CORNEAL VASCULARISATION
• Cornea is avascular expect for small loops which invade the
periphery for about 1mm.
• It facilitates nutrition,transport of systemic antibiotics and
drugs.
• Progressive vascularisation,however is a harmful process—as
it interferes with the functional properties of
cornea,especially its transparency.

35. Pathogenesis of corneal vascularisation
• Chemical theory
 Presence of VSF(Vasostimulatory factor)
 Role of VIF(vasoinhibitory factor)
• Mechanical theory
 Loosening of tissue by corneal edema---
Neovascularisation
• Combined mechanical and chemical theory
 VSF+ edema –necessary for neovascularisarion
• Role of luekocytes
 Leucocytes inflammatory resposnse
Neovasularisation

36. PHYSIOLOGICAL
FACTORS

37. Corneal hydration
• Normal cornea maintains itself in a state of relative
dehydration which 8is essential for corneal transparency
• It is kept constant by---
1. Factor which draw water in the cornea,like
 Stromal swelling pressure(SP)
 Intraocular pressure(IOP)
2.Factor which prevent flow of water in the cornea
 Active pumbimg action of endothelium

39. • In vitro,
 IP=SP
• In vivo,IP is reduced by values
equivalent to IOP
 That is, IP=IOP-SP
 That is ,IP=17-60=-43mmhg
• Negative imbibition pressure
draws out water from the
stroma

41. Hydration control by active pump mechansim
1.Na+/k+ATPase pump system
• Pumbs are located in the
basolateral membrane system.
• Enzyme Na+/K+ ATPase
mediated pump causes
extrusion of Na+and water
from the stroma and thus
maintain the stromal
transparency.
• Corneal hydration depend on
extend to which endothelial
barrier and pump function can
be reestablished

42. 2.Bicarbonate dependent ATPase are also reported to
have fluid or ion balance in the cornea
3.Carbonic anhydrase
Co2+H2O carbonic anhydrase HCO3-and H+
Thus provides imp source for endothelial pumps
4.Na+/H+ pump has also been postulated
5.passive ion movement,like K+,cl,and HCO3-diffusion
in contralateral direction

43. Cellular factors affecting transparency
• Corneal fibroblasts (keratocytes) are important in
maintaing transparency,as they are the source of
stromal collagen and proteoglycans.
• Enzymes required for the matrix assembly are
encoded in the genes of keratocytes for psot
transitional effect.
• Any defect in the gene causes corneal opacification.