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CORNEA
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.
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
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
HistologyHistology
HistologyHistology
EpitheliumEpithelium
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
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
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.
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
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.
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
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.
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.
Corneal physiologyCorneal physiology
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
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.
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
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 .
CONGENITAL ANOMALIESCONGENITAL ANOMALIES
Cornea plana : Cornea plana : cornea is cornea is comparatively flatcomparatively flat
since birth.since birth.
Congenital cloudy Congenital cloudy corneacornea
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
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
Corneal edemaCorneal edema
Keratic Keratic precipitates-precipitates-deposition of deposition of leucocytes on the leucocytes on the back of corneaback of cornea
Hypopyon Hypopyon