Anatomy and Physiology of cornea

Dr.Lhacha Wangdi

1st year Resident

Department of Ophthalmology

JDWNRH/KGUMSB

Outline

Gross anatomy of cornea -Surface anatomy -clinical applicationMicroanatomy of cornea -anatomy of ultra structurePhysiology of corneal tissue

Introduction

Word cornea originated from Latin word-

Cornu (horn) Cornea tela(horny tissue)

Transparent avascular tissue with a convex anterior surface & concave posterior surface.

Main function is OPTICAL

Other functions are: -STRUCTURAL INTEGRITTY -PROTECTION FOR THE EYE

Gross anatomy of corneaCovers the anterior scleral foramenLimbus is transition zone between

cornea and scleraThe cornea protrudes slightly beyond

the scleral globe because of the different curvatures of the two structures

Radius of curvature (cornea)–6.7-9.4mm whereas scleral-11.5mm

Cornea appears elliptical in shape measuring 11-12mm horizontally and 10-11mm vertically

Surface area: About 1.3 cm² (one-sixth of the

globe)

11-12mm

10-11mm

limbus

Corneal thicknessPosterior surface of cornea is

curved more than anterior surface

Central zone- 0.52mm Paracentral zone- 0.52-

0.57mm(outer diameter-7-8mm)Periphery zone- 0.63-0.67mmThe thinnest zone is 1.50mm

temporal to the geographic center about 0.505-0.51mm

In endothelial/ epithelial cell dysfunction, corneal becomes unusually thick due to stromal edma

Thick cornea will give false IOP reading

Surface anatomy- corneaCornea is not a true sphereIt has a central spherical

(optical zone-4mm)with uniform flattening towards the periphery –prolate shape

Flattening is more extensive nasally and superiorlySurface cornea is divided into three zones-

1. Central zone of 1-2mm-spherical(red area)2. Paracentral zone of 3-4mm around the

central zone with progressive flattening3. Peripheral zone which is flattened more.

Clinical application-

1. Topographic information of

cornea is important in contact lens

fitting2. Flattening of

cornea at periphery helps to

reduce the spherical

aberration of optical system

3. Alteration of the uniform curvature of cornea will induce astimatism

Spherical aberrationIn total spherical object

peripheral parallel rays of light refract more and focus in front of ideal image point.

Effects- blurred vision

Spherical aberration is minimized by aspheric (prolate) shapes of eye surfaces- due to peripheral flattening

Cornea as a optical system

Main function of cornea- optical( refraction of light for clear vision)

Contributed by its special characteristics; 1. Transparency 2.Avascularity 3. Controlled hydration 4. High refractive power The optical power of the cornea=42.0 D Is equal to 2/3 of the total optical power of the human eye (57 – 62

D)

Refractive power of cornea=(refractive index of cornea-refractive index of air)

(radius of curvature of cornea)Refractive index of cornea is more -Cornea- 1.33765 -Air- 1.000Radius of curvature is inversely proportional to curvature: - Anterior surface – about 7.8 mm -Post. Surface – about 6.5 mm

Microanatomy of corneaCornea has five defined layers

1. Epithelium and basal lamina-5% of thickness

2. Bowman’s layer3. Stroma-90% of total

thickness4. Descemet’s Membrane5. Endothelium –single

cellular layer (germinal layer)

Composition of corneaWater: 78 %Collagen: 15 % of which: Type-I : 50-55 % Type-III : 1 % Type-IV : 8-10 % Type-VI : 25-30 %Other protein: 5 %Ground substances -Keratan sulphate: 0.7 % -Condroitin/dermatan sulphate: 0.3 %Hyaluronic acid: +Salts: 1 %

Embryonic origin of cornea

Clinical significance-cell line originating from surface ectoderm has regenerative capacity whereas those from neural crest has little regenerative capacity

Disease affecting other organ such as in atopic dermatitis may cause keratitis due to similar embryonic origin

Epithelium-Derived from surface ectoderm

Bowmans layer-mesenchyme(neural

crest cell)

Endothelium-mesenchyme(neural crest cell) 1st wave

Stroma -mesenchyme(neural crest cell) 2nd wave

descemet’s – synthesised by endothelium

Epithelium Derived from surface ectodermConstitutes of 5-6 layers of cell accounting for

about 5% of corneal thickness-(0.05mm/50um)

Its has three cell layers1. Apical cells-

nonkeratinised Squamous epithelium

2. Wing cells- 2-3 layers of polygonal cell

3. Basal columnar cells (germinative layer)

Nonepithelial cells- histocytes, macrophages, lymphocytes, antigen presenting langerhans cells are also present which becomes more numeruous during keratitis

Ultrastructure of epithelium

Apical cells layers consists of 2-3 layers of flattened hexagonal cells

Surface cells contain microvilli & microplicae coated with 300-nm thick glycocalyx/glycoprotein (buffy cell coat)The mucin layer of tear binds with glycocalyx

and helps in uniform spreading of tear film,

Ctn…Epithelial cells are adhered together by tight junctions –

1. Tight junctions & desmosomes – surface cells

2. Desmosomes – wings & superficial cells

3. Desmosomes & Hemidesmosomes – in basal cells

4. Cells are anchored to deeper tissue by anchoring proteins

Functions- 1. Maintains corneal homeostasis(impermeable to Na ions & confer

semipermeable membrane properties to the epithelium)2. Mechanical barrier – protective function against infection/toxins3. Tight junction ensure corneal transparency 4. Anchor epithelial cells to basal lamina and bowmans layer

Anchoring protein

desmosome

hemidesmosomes

Basal lamina Fibrous layer consisting of 1V collagen and glycoprotein.

Secreted by the basal cells 0.5 - 1 μm wide Ultra structurally it is

distinguished in to two parts1. Lamina lucida (superficial)-

electron lucen zone 2. Lamina densa (deep electron

dense zone)3. Anchored to bowman’s layer with

numerous anchoring fillaments

Lipid solvent, stromal oedema and inflamation may loosened the cohesion between Bowman’s zone,lamina and epithelial cells-– eg mucus filaments due to epithelial instability

With old age, in diabetes and in some corneal disorders it becomes thickened and multilamellar

Epithelial regeneration

The epithelium is constantly in a state of turn-over with exfoliating apical cells being replaced by underlying wing cells-weekly

Basals cells are only epithelial cells capable of mitosis

During normal apical cell exfoliation basal cells proliferates and replace lost cells in 7-14 days

Loss of basal cells and defective regeneration will lead to corneal scar formation

Apical cells loss

Wound healing

During epithelial defect either due to infection/trauma/inflamation there are extended proliferation and differentiation of basal cells

If Boman’s layer is intact the epithelium is regenerated in 7-14 days

Cellular events

Repair of corneal epithelial injury like abration/infection follows a distinctive sequence of events-1. Cells at wound edge retract, thicken

and lose attachment, produce various growth factors (egTGF-Bs)

2. Basal cells travel in an amoeboid movement to cover the defect

3. Migration process is halted by contact inhibition

4. They then anchor by secreting basal lamena

5. Mitosis resumes to re-establish epithelial thickness

6. Surface tight junctions re-establised7. Adhesion with Bowman’s layer

within 7 days (if basal lamina intact)8. The healing process occurs rapidly,

rate of cell migration is 60 – 80 μm/hr

Toxin, trauma,

infection, inflammatio

n

Spreading and dedifferentiati

on

Cell migration

Cell proliferation

Regeneration

redifferentiation

Germinative cellsIt is now recognized that

the germinative region lies at the limbus

Limbal stem cell migrate centrally to replace corneal epithelial basal cells

The stem cells migrate at a very slower rate (123 μm/week) to the center of the cornea which may be as long as a year

Loss of limbal stem cells will result in corneal scar

Limbal stem cells

The XYZ hypothesisRichard A. Thoft & Judith Friend(1983) proposed on

the basis of experimental evidence that both limbal basal and corneal basal cells are the source for corneal epithelial cells. The corneal epithelium is maintained by a balance among-

(Z)Sloughing of cells from the corneal surface is = (X)cell division in the basal layer + (Y) Migration of basal cells originating from the limbal stem cells

Ctn.. In normal healthy cornea there is a constant balance between; (cell turn over=regeneration)Regeneration= balanced basal cell proliferation +

migration+maturation+secretion of basement membrane+regeneration.

Abnormality of epithelial Cell turn over and regeneration causes epithelial opacity and haziness.

Example, Corneal Epithelial Basement membrane Dystrophy(EBMD) aslo called ‘map dot finger print’/’cogan microcystic dystrophy’

chateractised by; -dots/epithelail microcysts

(due to abnormal epithelium) -fingerprint/geographic map lines (due to thicken basement membrane)

Bowman’s layerModified region of anterior stroma8 – 14 μm thickAcellular homogeneous zone It is perforated by many nerve

axons which courses through toward the epithelium

Ant. surface is smooth & parallel with corneal surface

Posteriorly it becomes blended & interweaved with fibrils of ant. stroma

Functions- 1. Anchoring site for epithelial cells to ensure its stability2. Tough acellular layer provide mechanical supports3. Prevents stromal keratocytes from exposure to epithelial

growth factors- prevents keratocytes metaplasia to fibroblast and scar formation

Ultrastructural featuresUltrastructurally it is a

meshwork of fine collagen fibrils of uniform size in a ground substance (glycoprotein &proteoglecan)

Compact arrangement of collagen types I, III, V, and VI

it has great strength and relatively resistant to trauma both mechanical and infective

It is acellular and lacks fibroblast therefore after injury it is unable to regenerate- replaced by course scar tissue

Stroma About 450- 500 μm thick (about 90% of corneal thickness) Transperant and rich in collagen-predominantly of type I

collagen with types III, V, and VI also in evidence.

Proteoglycan(glycosaminoglycan)ground substance between the collagen fibers

5% of stromal volume occupied by keratocytes which synthesizes both collagen and proteoglycan

stroma

Stromal lamellaeStroma ensure transparency

of cornea by lamellar arrangement of collagen bundles

Stroma has about 200 layers of lamellae

Lamelae are arranged regularly almost right angle to each other

Each lamellae consists of bundle of collagen-

1. 200 – 300 bundles – centrally 2. 500 bundles – peripherally3. Width about 9 – 260 μm4. Thickness about 1.15 – 2 μm

Ultrastructural featuresEach lamellae comprises of a

band of collgen fibrils arranged in parallel with each other

Fibrils are regularly placed

each other with center-to-center distance of 55-60nm.

There is a unique uniformity of fibril diameter of 22 (±1) nm from ant. to post.

Regularly arranged lamellae with uniform diameter and seperation of collagen fibers makes cornea transparent

Ground substances of stroma The ground substance of cornea consists of

proteoglycan that run between the collagen fibers

It constitutes approximately 10% of corneal weight

Proteoglycan are glycosylated with glycosaminoglycan(GAGs)-disaccharides

GAGs include- 1. Keratin salphate 2.Chondroitin sulphate 3.Dermatan sulphateFunction- 1. Confer hydrophilic properties of stroma2. Maintains corneal transparency by controlled stromal

hydration by contributing fixed negative charge of stroma (normally stroma is 78% hydrated)

3. Helps in regular spacing of collagen fibers to ensure transparency

Cellular components of stromaKeratocytes: Long, thin, flattened cells (maximally 2μm

thick) running parallel to corneal surface Position – between the lamellae Having long flattened nuclei, sparse

cytoplasm but contains full component of organells

Function 1. responsible for synthesis and maintaining

of collagen & proteoglycan substance of stoma

2. helps in corneal regeneration after injury3. Part of corneal anti-oxidant

defense(proteinase inhibiter, inhibitors of metalloproteinases e.t.c) Other cells-

Lymphocytes, macrophages and polymorphonuclear leucocytes (very rarely) also found in stroma ocationally- becomes numerous in corneal ulcer/stromal abscess

Stromal transparency theoryThe cornea transmits nearly

100% of the light that enters it. Transparency achieved by –

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

Ctn…

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(55-60nm) and diameter(22nm)is less than a third of the wavelength of incident light,(400-700nm) then almost perfect transparency will ensue.

This is the situation which obtains in normal cornea.

Other factors of corneal transparency

1. Epithelial non-keratinization 2. Regular & uniform arrangement of

corneal epithelium 3. Junctions between cells & its

compactness and also tear film maintain a homogenicity of its refractive index

4. Relative controlled hydraton of normal cornea

5. Corneal avascularity 6. Non myelenated nerve fibres

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) adult (10 – 12 μm)It has two zones-Anterior 1/3 zone - developed in utero -irregular banded zone Posterior 2/3 zone -developed after birth - Homogenous fibrillogranular material It is a strong resistant sheet -Major protein of DM is Type IV collagen

Ctn…. Due to aging ther can be

focal overproduction of basal lamina- peripheral excrescence called Hassal-Henle warts

No clinical abnormality in corneal function

In extensive stromal thinning eg. in corneal ulcer descemet’s membrane may bulge forward to form desmatocele

Ctn…The peripheral rim of DM is

the internal landmark of corneal limbus

It is the anterior limit of drainage angle, is called Schwalbe’ line

Schwalbe’s line may hypertrophied in congenital anomalies and appears as visible shelf on gonioscopy, is called posterior embryotoxon

Endothelium It is a single layer of hexagonal,

cuboidal cells attached to posterior aspect of DM

It is nuroectodermal in origin Corneal endothelial cells

production is relatively fixedIt is about 500000

(2500cells/mm2)

Endothelial cells density – -At birth-About 6000 cells/mm² -26% lost in 1st year -Further 26% lost over next 11 years -Rate of cell loss slows and stabilizes around middle age and then it is about 2500 cells/mm²

Ultrastructural featuresSingle oval nucleus located centrallyEndothelium is rich in subcellular

organeles – large number of mitochondria, both rough and smooth endoplasmic reticulum, free ribozomes, these reflects that endothelium is extremely active metabolically

The posterior cell membrane (Apical) facing Anterior chamber shows 20-30 microvilli- increases absorption areaCellular junction-1. The anterior cell membrane (Basal) is

attached with DM by modified hemidesmosomes

2. Ant. 2/3rd – maculae adharentes3. Post. 1/3rd & apicolateral edges –

macculae occludentes

Endothelial functions

a)Maintains corneal hydration(slightly dehydrated stae-78% hydration) by ‘pump-leak hypothesis’-1. Providing physiological barrier to salts and

metabolites to stroma. 2. Active transport transport of bicarbonate by

Na+/K+ATPase actively removes H2O from stroma by pump action.

NA+/K+ATPase is located at the endothelial cell membrane

Using ATP, the pump actively transport Na+, K+ and bicarbonate to the AC

Creats + osmotic gradient in the aqueous

H2O moves from stroma to AC

NA+/K+ATPase failure

Failure of NA+/K+ATPase to maintain corneal hydration will cause;

- stromal edema, -subepithelail fibrosis -epithelial bullae -corneal guttata

Example- Fuchs endothelial dystrophy

Endothelial function ctn..

b)Injury and repair-Endothelial cell regeneration is not possible by

mitosisHealing occurs by cell enlargement.Immediately after injury; 1.Descet’s membrane retracts and injured endo.cell detaches. 2. fibrin clots formed at wound 3.within hours adjacent endo.cell attenuate with cytoplasmic processes 4.migrate to wound site 5. cellular reorganization and enlargement- reconstitute monolayer (completes by 1-3 days)

Ctn….Endothelial decompensation will cause stromal

edema, reduced transparency and loss of visionEndothelial decompensation occurs when cells

density falls upto 500 cells/mm² .

With advancing age the endothelial cells become polymorphic in shape due to cell enlargement during repair

Corneal Nutrition & Metabolism

Glucose, amino acid, vitamins, and other nutrients supplied to cornea by aqueous humor, a lesser amounts from tears or limbal vessels

Glucose also derived from glycogen stores in corneal epithelium

Glucose is metabolized in cornea by three metabolic pathways;

1. Tricarboxylic acid cycle(TCA)- epithelium & endothelium

2. Anaerobic glycolysis- when there is lack of O2

3. Hexose monophosphate(HMT) shunt –mainly in endohelium

Ctn..During normal aerobic metabolism end product of

glucose- pyruvic acid is converted to H2O via TCA cycle.

During anaerobic state as in tight contact lens lactic acid is produced via anaerobic glycolysis which causes

1.Stomal acidosis, 2.Edothelial cell dysfunction 3.Corneal edema and visual impairment

Oxygen – mainly from atmosphere through tear film, with minor amounts supplied by the aqueous and limbal vasculature

Nerve supply of CorneaCornea is rich in

sensory nerve supply derived from ophthalmic division of trigeminal which give branch to;

- Nasociliary nerve and -Ciliary nerves (terminal branch)

Ciliary nerve enter the pericoroidal space a short distance behind the limbus.

60-80 myelinated branches pass into cornea

Ctn… 1-2 mm from the limbus nerves axon lose myelin sheaths and divide into;

- anterior branche -posterior banche Anterior nervs (40-50) pass through stroma and form plexus subjacent to Bowman’s layerNerve fibres then penetrate

Bowman’s layer and form subepithelial plexus

Fibres then divide dichotomously to form a parallel network which run for upto 2 mm

Free nerve terminals finally supplies superficial epithelial layers

The posterior groups of nerves (40-50) pass posteriorly to

innervate the posterior stroma excluding Descemet’s membrane

Subepithelial plexus

Anterior posterio

r

Ctn…

HSV infection of cornea spreads along the nerve axons

Involvement of terminal nerve causes;

-dendritic appearance - loss of corneal sensation

Nerve innervation is important to maintain balanced epithelial cell division

Lesion of fifth nerve will cause abnormal cell turn over and loss of reflex tearing and leads to Neurotrophic keratitis

References

Yanoff & Duker Ophthalmology- 4th edition

American Academy of Ophthalmology Jack J. Kanski Clinical ophthalmology- 7th

editionOxford textbook of ophthalmologyJournal- Association for Research in

Vision and OphthalmologyDuane’s clinical ophthalmologyImages and graphics- internet sources