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CORNEA

Cornea - 1

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CORNEA

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CORNEACORNEA

Transparent,Transparent,Avascular,Avascular,Principle optical surface Principle optical surface of the eyeof the eyeForms anterior one-sixth Forms anterior one-sixth of the outer fibrous coat of the outer fibrous coat of the eyeball.of the eyeball.

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Dimensions Dimensions

Anterior surfaceAnterior surface

Elliptical shapeElliptical shape

Horizontal diameter:11.75 mmHorizontal diameter:11.75 mmVertical diameter :10.6 mmVertical diameter :10.6 mm

Posterior surfacePosterior surface circular shapecircular shape average diameter : 11.75 mmaverage diameter : 11.75 mm

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Thickness Thickness

at centre : 0.52 mmat centre : 0.52 mm

at periphery : 1.2 mmat periphery : 1.2 mm

Radius of CurvatureRadius of Curvature central 5 mm of the cornea forms powerful refracting central 5 mm of the cornea forms powerful refracting

surface of the eye.surface of the eye.

Anterior : 7.8 mmAnterior : 7.8 mm

Posterior : 6.5 mmPosterior : 6.5 mm

Refractive powerRefractive power

45 diopters45 diopters

3/43/4thth of the total refractive power (60 D) of the total refractive power (60 D)

Refractive IndexRefractive Index 1.3761.376

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HistologyHistology

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HistologyHistology

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EpitheliumEpithelium

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Bowman's membraneBowman's membrane

Acellular mass of condensed collagen Acellular mass of condensed collagen fibrilsfibrils

Thickness : 8 – 14 microns Thickness : 8 – 14 microns

Shows considerable resistance to infection Shows considerable resistance to infection and injuryand injury

But once destroyed, But once destroyed, does not regeneratedoes not regenerate

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Stroma Stroma

Thickness : 0.5 mmThickness : 0.5 mm

90 % of total corneal 90 % of total corneal thicknessthickness

Consists of collagen fibrils Consists of collagen fibrils (lamellae) and cells (lamellae) and cells embedded in hydrated matrix embedded in hydrated matrix of proteoglycans (ground of proteoglycans (ground substance).substance).

Lamellae arranged parallelly Lamellae arranged parallelly (200 – 250 layers)(200 – 250 layers)

Cells : keratocytesCells : keratocytes

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Descemet`s membraneDescemet`s membraneRepresents the basement Represents the basement membrane of the endotheliummembrane of the endotheliumMade up of collagen and Made up of collagen and glycoproteinglycoproteinThickness :10-12 Thickness :10-12 µmµmVery resistant to chemical Very resistant to chemical agents, trauma, infection and agents, trauma, infection and pathological processes.pathological processes.Maintains integrity of the Maintains integrity of the eyeball eyeball It can regenerate, when It can regenerate, when destroyed.destroyed.

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EndotheliumEndothelium

Single layer of flat Single layer of flat hexagonal cells.hexagonal cells.

Mosaic patternMosaic pattern

Cell density : 2400 Cell density : 2400 3000 cells/mm2 3000 cells/mm2

Contains active pump Contains active pump mechanismmechanism

Best evaluated by Best evaluated by specular microscopyspecular microscopy

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Limbus Limbus

Anatomically the limbus Anatomically the limbus refers to circumcorneal refers to circumcorneal transitional zone of the transitional zone of the conjunctivo-corneal and conjunctivo-corneal and corneo-lscleral junction.corneo-lscleral junction.

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Blood & Nerve supply Blood & Nerve supply

Blood supplyBlood supplyAvascular structureAvascular structureSmall loops derived from the superficial and Small loops derived from the superficial and deep plexus, invade its periphery for about 1mm deep plexus, invade its periphery for about 1mm and provide nourishmentand provide nourishment

Nerve supplyNerve supplyHas rich sensory supplyHas rich sensory supplyLong ciliary nerves branches of nasociliary Long ciliary nerves branches of nasociliary nerve (a branch of ophthalmic division of nerve (a branch of ophthalmic division of trigeminal nerve)trigeminal nerve)Non myelinated Non myelinated

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Development of the corneaDevelopment of the cornea

Epithelium : surface ectodermEpithelium : surface ectoderm

Bowman`s layer, stroma , Endothelium & Bowman`s layer, stroma , Endothelium & descemet`s membrane : mesenchymal descemet`s membrane : mesenchymal cells derived from neural crest.cells derived from neural crest.

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Corneal physiologyCorneal physiology

Two primary physiologic functionsTwo primary physiologic functions

1.1. Act as a powerful refracting medium, Act as a powerful refracting medium, transmits light in an orderly fashion for transmits light in an orderly fashion for proper image formation.proper image formation.

2.2. Protects intraocular contents.Protects intraocular contents.

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Corneal physiologyCorneal physiology

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Corneal transparencyCorneal transparencyAnatomic / physical factorsAnatomic / physical factorsUniform and regular arrangement of corneal epitheliumUniform and regular arrangement of corneal epithelium

Regular arrangement of stromaRegular arrangement of stroma

Corneal avascularity, non myelinated nerve fibresCorneal avascularity, non myelinated nerve fibres

Normal tear filmNormal tear film

Physiological factors Physiological factors Relative state of corneal dehydrationRelative state of corneal dehydration

Stromal swelling pressureStromal swelling pressure

Normal intraocular pressureNormal intraocular pressure

Endothelial pump mechanismEndothelial pump mechanism

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Tear filmTear filmThree layersThree layers

1.1. Mucus layerMucus layer : innermost & : innermost & thinnestthinnest

0.02 -0.05 microns0.02 -0.05 microns

secreted by conjunctival secreted by conjunctival goblet cellsgoblet cells

2.2. Aqueous layerAqueous layer : thickest : thickest

6.5 – 7.5 microns6.5 – 7.5 microns

secreted by main & secreted by main & accessory lacrimal glandsaccessory lacrimal glands

Alkaline & antibacterialAlkaline & antibacterial

3.3. Lipid or oily layerLipid or oily layer : outermost : outermost

0.1 microns0.1 microns secreted by meibomian, secreted by meibomian,

zeis & moll glands.zeis & moll glands.

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Functions of the tear filmFunctions of the tear film

1.1. Keeps the cornea & conjunctiva moistKeeps the cornea & conjunctiva moist

2.2. Provides oxygen to the corneaProvides oxygen to the cornea

3.3. Washes away debris & noxious irritantsWashes away debris & noxious irritants

4.4. Prevents infection due to presence of Prevents infection due to presence of antibacterial substancesantibacterial substances

5.5. Facilitates smooth movements of the lids over Facilitates smooth movements of the lids over the globethe globe

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CONGENITAL ANOMALIESCONGENITAL ANOMALIES

Microcornea : Microcornea : horizontal diameterhorizontal diameter

< 10 mm since birth.< 10 mm since birth.

Megalocornea : Megalocornea : horizontal diameter of horizontal diameter of the cornea is of adult the cornea is of adult size or 13 mm .size or 13 mm .

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CONGENITAL ANOMALIESCONGENITAL ANOMALIES

Cornea plana : Cornea plana : cornea is cornea is comparatively flatcomparatively flat

since birth.since birth.

Congenital cloudy Congenital cloudy corneacornea

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KeratitisKeratitis

Any type of corneal inflammation –KeratitisAny type of corneal inflammation –Keratitis

Types –Types –

superficial –ant to Bowman’s membrane superficial –ant to Bowman’s membrane

Deep –stromal or interstitial or endothelitisDeep –stromal or interstitial or endothelitis

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Few terminologiesFew terminologies

Abrasion-defect in the epitheliumAbrasion-defect in the epithelium

Corneal ulcer-defect in the Corneal ulcer-defect in the epithelium+inflammation+surrounding epithelium+inflammation+surrounding tissue necrosis.tissue necrosis.

Corneal scar or opacity-healing of any Corneal scar or opacity-healing of any corneal injury beyond the level of corneal injury beyond the level of bowman’s membrane bowman’s membrane

Types-nebula, macula and leucomaTypes-nebula, macula and leucoma

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Corneal edemaCorneal edema

Keratic Keratic precipitates-precipitates-deposition of deposition of leucocytes on the leucocytes on the back of corneaback of cornea

Hypopyon Hypopyon