point wise description of cornea and limbus anatomy

1. Anatomy of cornea
Dr Nayana V Gowda
DNB Ophthalmology

2. Contents
 Brief introduction
 Topography
 Embryology
 Function
 Histology
 Transparency
 Blood and nerve supply
 Limbus

3. INTRODUCTION
Defintion: Transparent, avascular, anterior coat of the eye that
covers the iris, pupil and anterior chamber being continuous with the
sclera at the periphery.
Forms anterior one-sixth of fibrous coat of the eyeball
Refractive power: Anterior surface of cornea has a refractory
power of +48D and it’s posterior surface of
about -5D. Hence the net refractory power of cornea is
+43D which is 3/4th of the total refractory power of the eye (+60D).
• Refractive index is 1.37

4. DIMENSIONS
• Anterior surface of cornea is horizontally
elliptical 11.75 mm. by 11 mm.
• Posterior surface of cornea is circular with
diameter of 11.5 mm.

5. DIMENSIONS ctd…….
• Cornea is 0.52 mm. thick at the center which
gradually increases to 0.67mm at the
periphery.
• The radius of curvature of the central part of
cornea is 7.8 mm anteriorly and 6.5 mm.
posteriorly.

6. TOPOGRAPHY
• Steeper in infants.
• Greater flattening nasally than temporally.
• Near the limbus, the curvature increases before entering the trough
like contour of limbal zone.
• Flattens on convergence.
• Knowledge of topography important for CL fitting

7. EMBRYOLOGY
EPITHELIUM: Surface ectoderm
BOWMAN’S LAYER: Laid down by anterior stromal keratocytes
STROMA: Mesenchymal cells
ENDOTHELIUM AND DESCEMET’S MEMBRANE: neural crest cells

8. EMBROLOGY OF CORNEA
• Mesenchymal mass of neural crest origin is now considered to give rise to cornea , iris
and anterior chamber angle.
• Three waves of tissue come forward between surface ectoderm and developing lens,
from the developing undifferentiated mesenchymal mass of NCC origin to contribute to
structures of anterior segment origin

10. EMBRYOLOGY: EPITHELIUM
• 40 days POG: Superficial squamous layer & a
basal cuboidal layer.
• 2-3rd month POG: Superficial cells increase in
size & are covered by microvilli & microplicae.
• 5-6 months POG: corneal epithelium attains Adult morphology

12. EMBRYOLOGY: BOWMAN’S ZONE
• Not formed until 4th month POG.
• Always acellular.
• Superficial keratocytes synthesize & lay down the
ground substance & collagen.
• At this stage the hemidesmosomes & anchoring
filaments in the basal lamina are partially developed.
•By 26 wks they have an adult appearance

13. EMBRYOLOGY: STROMA
• 7 weeks POG: Anterior extension of mesenchymal cells
migrates btw epithelium and endothelium & contributes to
growth of stroma.
• Initially acellular but ingrowing cells differentiate to
keratocytes that secrete type 1 collagen & stromal matrix.
• Fibrils are organized as lamellae and successive lamellae are
formed.
• Posterior layers of stroma are confluent with the condensed
tissue of the future sclera.

14. EMBRYOLOGY: DM & ENDOTHELIUM
Formed from mesenchymal cells derived from neural crest , which are
situated at margins of rim of optic cup. These cells migrate into
developing eye beneath the basal lamina of corneal epithelium and
form primordial corneal endothelium

15. EMBRYOLOGY: DM & ENDOTHELIUM ctd…..
• At about 40 days of POG – corneal endothelium consist of 2 layered
flattened cells
• By 3rd month endothelium in the centre of cornea becomes a single
layer of flattened cells that rest on the interrupted basal lamina the
future DM
• Apices of endothelial cells are joined by zona occludens in the middle
of 4th month
• 6th months of gestation DM demarcated clearly

16. FUNCTIONS
Refraction - 70% of total refractory power of eye is due to
smooth surface tear film & regular anterior
Curvature (Power = +43.1D , RI = 1.376)
Contains IOP - Function of collagen fibres in stroma
Transparency - Mainly due to Corneal endothelium which
continuously pumps out excess water
Protection - Acts as mechanical barrier to external
noxious stimuli.

18. EPITHELIUM
 Epithelium type-Stratified squamous
 non keratinized
 Thickness-50-90 microns,
 5-6 layers of nucleated cells.
 Continuous with conjunctiva at limbus but devoid of goblet cells.
 Consists of 5-6 layers of nucleated cells resting on a basal lamina, namely –
basal cells , wing cells , surface cells

19. BASAL LAYER
 Deepest layer
 Cells are arranged in palisade manner
 Germinativelayer of epithelium
 Cells are columnar with rounded heads & flat bases
 Nucleus is oval & oriented parallel to long axis of the cell

20. WING CELLS
 Second layer of epithelium (1-2 layers) Polyhedral cells
 Convex anteriorly forming cap over basal cells & send processes
between them
 Nucleus is oval & oriented parallel to corneal surface

21. SURFACE CELLS
 Most superficial layer (2-3 layers)
 Cells are polyhedral & become wider & flatter towards the surface
 Flattened nuclei project backwards leaving the surface perfectly smooth
 Most superficial cells are mostly hexagonal & exhibit surface microvilli /
microplicae

22. BASAL LAMINA
Secreted by basal cells.
•Made up of collagen (*Type7 & glycoprotein
•Structurally attached with the underlying Bowman’s layer by short anchoring
filaments forming anchoring plaques
•Cohesion between basal lamina & Bowman’s zone maybe loosened by lipid
solvents, edema or inflammation but it remains attached to basal cells.
•Thicker in periphery and in diabetes & certain corneal disorders
Thickened with old age.

23. EPITHELIAL TURNOVER
 The germinative region lies at the limbus, the stem cells, and cells migrate at a very
slower rate (123 μm/week) to the centre of cornea.
 The XYZ hypothesis:ThoftR. and Friend J. (1983) proposed on the basis of experimental
evidence ,that both limbal basal and corneal basal cells are the source for corneal
epithelial cells,and there is a balance among division, migration & shedding.
 The corneal epithelium is maintained by a balance among sloughing (Z) of cells from
the corneal surface, cell division (X) in the basal layer and renewal of basal cells by
centripetal migration (Y) of new basal cells originating from the limbalstem cells.

25. BOWMANS MEMBRANE
 Modified region of anterior stroma
 Acellular homogeneous zone
 8 –14 μm thick
 Ant. surface is smooth & parallel with corneal surface
 It delineates the anterior junction between cornea and limbus
 Perimeter delineates anterior junction btw the cornea & the limbus &
is clinically marked by summits of marginal arcades of
limbalcapillaries.

26.  Ultrastructurallyit is a felted meshwork of fine collagen fibrils of
uniform size in a ground substance
 Posteriorly it becomes blended & interweaving with fibrils of ant.
Stroma
 Compact arrangement of collagen gives it great strength and relatively
resistant to trauma both mechanical and infective
 Perforated in many places by corneal nerves.
 Convex ridges may generate over surface if its tension is relaxed during
indentation, hypotonyor manipulation causes ant.cornealmosaic,
polygonal or chicken-wire pattern over surface
 No regeneration and replaced by coarse scar tissue

27. STROMA
 About 500 μm thick (about 90% of corneal thickness)
 Two components : 1) Lamellae 2) Cells
 stroma consists of regularly arranged lamellae of collagen bundles, lie in
proteoglycan ground substance.
 In the anterior 1/3rd there is more interweave & some lamellae pass
forward to be inserted into Bowman’s layer
 In the deep stroma lamellae form strap like ribbons which run
approxright angle to consecutive layers. Approx300 lamellae.

28.  Interfibrillar separation is equal to the fibril diameter. This separation
decreases with age.
 Precise ordering is responsible for transparency of the stroma.
 CELLS-
1)Keratocytesmake up 2.5 –5% of the stromal volume and are responsible
for synthesis of collagen & proteoglycan during development &
maintaining it thereafter.
2) Other cells found in stroma occasionally: lymphocytes,macrophages and
very rarely polymorphonuclearlymphocytes.

29. DUA’S LAYER
 Named after Dr.Harminder Dua, who
discovered it
 Acellular layer between stroma & DM , 6-15 μ
thick
 Consists of about 5-8 lamellae of type-1
collagen bundles
 Bundle spacing is similar to stromal tissue but
devoid of keratocytes
 Very strong layer
 Can withstand upto200 kPa pressure

30. DESCEMET’S MEMBRANE
 It is the basal lamina of corneal endothelium
 First appears at 2nd month of gestation and synthesis continue
throughout adult life
 Thickness –at birth :-3 –4 μm at childhood :-about 5 μm
 at adult :-10 –12 μm
 Acts as a strong resistant sheet
 It thickens with age and in some corneal degenerative conditions
 Major protein of DM is Type IV collagen

31. ENDOTHELIUM
 It is a single layer of hexagonal, cuboidal cells attached posterior aspect of
DM
 It is neuroectodermalin origin
 Corneal endothelial cells production is relatively fixed
 Endothelial cells density –about 6000cells/mm² at birth
 26%lost in 1styear, Further 26% lost over next 11years
 cell loss slows stabilizes around middle age about 2500 cells/mm²
 If cells density falls upto500 cells/mm² corneal oedema develops and
transparency reduced

32. CORNEAL TRANSPARENCY
The cornea transmits nearly 100% of the light that enters it. Transparency achieved by –
 1. Arrangement of stromal lamellae
Two theories –
i)Maurice(1957): The transparency of the stroma is due to the lattice arrangement of
collagen fibrils. He explained, because of their small diameter and regularity of separation,
back scattered light would be almost completely suppressed by destructive interference
ii) Goldman et al. (1968): He suggest, a perfect crystalline lattice periodicity is not always
necessary for sufficient destructive interference. He explained, if fibril separation and
diameter is less than a third of the wavelength of incident light, then almost perfect
transparency will ensue. This is the situation which obtains in normal cornea.

33.  2.Corneal epithelium & tear film
•Epithelial non-keratinization
•Regular & uniform arrangement of corneal epithelium
•Junctions between cells & its compactness and also tear film
maintain a homogenicityof its refractive index
 3.Relative deturgescencestate of normal cornea
 4.Corneal avascularity
 5.Non myelenatednerve fibres

34. BLOOD SUPPLY
• Avascular
• Loops of anterior ciliary artery invade the
subconjunctival tissue at corneal periphery by 1mm.
• Many pathologic processes show corneal
vascularisation.
• Superficial vessels are bright red, arborescent and
may raise the epithelium
• Deep vessels are greyish red, run parallel and do not raise the
epithelium.

37. NERVE SUPPLY OF CORNEA

40. LIMBUS
Anatomically , the limbus refers to circumcorneal transition zone of
conjunctivo-corneal and corneoscleral
Junction
 1.5 mm wide in horizontal plane and 2mm in the vertical plane.
 Limbus is the weakest region in the corneoscleralenvelope.

42. CONJUNCTIVOCORNEAL JUNCTION
 At this point bulbar conjunctiva is firmly adherent to the underlying
structures
 Epithelium becomes several layers thick here and arranged
irregularly at limbus
 Limbal epithelial basal cells are arranged in a peculiar pattern of
palisade of vogt, containing stem cells

43. SCLEROCORNEAL JUNCTION
At this point transparent corneal lamellae becomes continuous with
the oblique , circular and opaque fibres of sclera

44. SURGICAL LIMBUS
Its 2mm wide circumcorneal transition zone between the clear cornea
on one side and the opaque sclera on the other side

45. BORDERS OF SURGICAL LIMBUS
 Anterior limbal border- marked by insertion of conjunctiva and tenon’s capsule into
cornea , forms anterior border of surgical limbus
 Mid limbal line-junction of blue and white zone overlies termination of Descemet’s
membrane
 Posterior limbal border-lies 1mm posterior to mid limbal line. It overlies the scleral spur
and can only be seen with the use of sclerotic scatter illumination

46. ZONES OF SURGICAL LIMBUS
 Blue limbal zone – translucent zone seen posterior to anterior limbal border
dissecting limbus free of conjunctiva and tenon’s capsule
 Extent - superior quadrant – 1mm
Inferior quadrant– 0.8mm
nasal and temporal – 0.4mm

47. WHITE LIMBAL ZONE
 Its 1mm wide whitish area lies between mid-limbal line and the posterior limbal
border
 Overlies trabecular meshwork
 Its width is constant in all quadrant 1mm , so surgical limbus is greatest in superior
quadrant where width of blue zone is 1mm