anatomy of cornea
TRANSCRIPT
ANATOMY OF CORNEA
Dr Nithin Keshav
Introduction
CORNEA – Medieval Latin “ cornea tela “ HORNY WEB (latin ,cornu = horn)
Transparent avascular tissue with a convex anterior surface & concave posterior surface.
Main function is OPTICAL Accounts for 70% of the total refractive
power of the eye (+ 43D) Other functions are: -STRUCTURAL
INTEGRITTY -PROTECTION FOR THE
EYE
DIMENSIONS
Anterior Surface : Vertical – 11.7 mm Horizontal – 10.6 mm Posterior Surface : Both 11.7 mm Thickness : Central 0.52 mm Peripheral 0.67 mm Surface Area: 1.3 cm2
Radius of Curvature Anterior – 7.8 mm Central 1/3 Posterior – 6.5 mm - Peripheral cornea is more flattened
Topography Anterior curvature is spherical in 2-4 mm zone
decentered upwards & outwards relative to visual axis but centered to the pupillary aperture( lies 0.4 mm temporally) -- CORNEAL CAP or APEX
Curvature varies from apex to limbus , greater flattening seen nasally & in upper part of cornea
STRUCTURES
5 LAYERS
A nterior EpitheliumB owman’s layerC entral stromaD escemets membraneE ndothelium
EPITHELIUM
Stratified , Squamous & Non Keratinized Continuous with conjunctiva , but no
goblet cells 50-90 u 5-6 layers
Posterior to anterior
1. BASAL CELLS Arranged in pallisade manner Germinative layer Columnar with an oval nucleus
2. WING or UMBRELLA cells Polyhedral Convex anteriorly
3. SURFACE CELLS 2-3 layers Polyhedral
Ultrastructural features
Abundant mitochondria in wing & middle cell layers
High glycogen content (Wing & Superficial layers)
Tonofibrils ( Intermediate filaments)
Desmosomes- lateral adhesion b/w cells, mainly at the basal level.
Zona Occludens- Tight jn seen at surface cells in addition to desmosomes.
Tight jn are impermeable to Na ions & confer semipermeable membrane properties to the epithelium
Surface cells contain MICROVILLI & MICROPLICAE– Helps in stabilizing precorneal tearfilm
Dendritic cells ( langerhans cells )- present in fetal epithelium but disappears in mature cornea.
BASAL LAMINA
2 LAYERS Superficial LAMINA LUCIDA Deep LAMINA DENSA
Thicker peripherally Thickened in Diabetes , Corneal pathology, Old
age Integrated with the underlying Bowmans layer
through ANCHORING FILAMENTS & ANCHORING PLAQUES
Cohesion between Basal Lamina & Bowman’s loosened by Lipid solvents Stromal edema Inflammation
Physiology of Epithelium
Rich in glycogen , serves as energy store in aerobic conditions
Glycogen levels Hypoxia Corneal sensitivity
Turn Over
Limbal stem cells migrate towards centre
XYZ Hypothesis : Limbal & Corneal basal epithelial
cells are source for CORNEAL EPITHELIAL CELLS
TRANSIENT AMPLIFYING CELLS : Daughter cells of limbal stem cells
TRANSITIONAL CELLS: Basal cells lying between limbus & peripheral cornea commonly seen at Superior Cornea
Markers
Epithelial cells – CK3
Cells of regenerative regions (limbal, transient amplifying cells & transitional cells)- CK19 , VIMENTIN
Hemidesmosome – a6b4 integrin
Repair
Mitosis inhibited by
1. Injury
2. Adrenergic agents
3. Surface anesthetics
Repair occurs by CENTRIPETAL SLIDE Rearrangement of Actin fibrils
Amoeboid migration
Halted by CONTACT INHIBITION
Anchor
MITOSIS resumes until epithelial thickness is
re-established
TOTAL EPITHELIAL LOSS
Adjacent Conjunctival epithelium
resurfaces Cornea
Vascularised conjunctival type of epithelium containing GOBLET CELLS
BOWMAN’S
Aka Anterior Limiting Lamina 8-14 u Modified region of anterior stroma Acellular homogenous zone Normally attached to Basal Lamina In pathological conditions
Corneal edema , Dystrophy After death
-Epithelium readily seperates from this layer
Ultrastructural features
Fine collagen fibrils of uniform size in ground substance
Relatively resistant to trauma (mechanical & infective)
Convex ridges can be seen when relaxed – POLYGONAL / CHICKEN WIRE PATTERN Responsible for Anterior Corneal Mosaic
In Prolonged Hypotony & Atrophic Bulbi degenerative changes in the ridges contributes to Secondary Anterior Crocodile Shagreen
STROMA
500u
Regularly arranged lamellae of collagen bundles
Contains keratocytes between lamellae
Keratocytes – production of COLLAGEN & PROTEOGLYCANS during development
Stromal repair
Keratocyte Activation
Migration
Transformation into Fibroblasts
Requires presence of overlying epithelium
DESCEMET’S Aka Posterior Limiting Lamina 2.2- 4.5 u It is Basal Lamina of Endothelium Appears at 2nd month of gestation Strong resistant sheet Sharply defined & the plane of seperation
is used in LAMELLAR KERATOPLASTY Thickens with age , endothelial
degenerations Type 4 collagen
Anterior 1/3 : Oldest Irregular banded pattern in cross section Banding develops at 5th month IUL
POSTERIOR 2/3: Formed after birth Homogenous fibrillogranular material
In Endothelial diseases where morphology & thickness of Descemets is altered , presence of normal anterior banded layer can be used to signify onset of disorder after birth.
In AGEING CORNEA: Bands of long spacing collagen found Focal overproduction of basal lamina like
material produces peripheral exceresences
HASSAL HENLE WARTS
Physiological
Resemble Descemet’s warts of central cornea – CORNEA GUTTATA in Fuch’s
Peripheral rim of Descemets forms internal landmark of corneal limbus & marks anterior limit of angle – SCHWALBE’S LINE
Prominent in 15-20% of individuals
Hypertrophied in congenital anomalies –POSTERIOR EMBRYOTOXON
On stripping Descemet’s it ROLLS INTO STROMA
Lens capsule curls outwards
On injury endothelial cells resurfaces & deposits Basal Lamina identical to Descemets
ENDOTHELIUM
Single layer of hexagonal / cuboidal cells
Counts At birth : 6000/mm2 1 yr : falls by 26% 11yr : another 26%
Gradual decrease in density & increase in shape variation – POLYMEGATHISM
Ultrastructural features
Lateral borders convoluted forming marked interdigitation
Cell junction Ant 2/3 : Maculae adherentes Post 1/3 : maculae occludentes
Posterior surface shows Microvilli - Absorptive surface area
Abundant mitochondria Condensation of cytoplasm rich in actin
lies close to posterior membrane – TERMINAL WEB
PHYSIOLOGY
1. NUTRITION : Glucose & aa
2. FLUID REGULATION: Maintains relative deturgescence by 1. Provides barrier to prevent ingress of salt
& metabolites into stroma2. Decreases osmotic pressure of stroma by
active pumping out of bicarbonate.
3. INJURY & REPAIR: Physical & chemical (ouabain)
SLIDING PHENOMENA
STRUCTURAL PROTEINS OF CORNEA
COLLAGEN
Basal lamina - type 4Bowman’s - 5Stroma - 1 (90%)Descemets - 4
PROTEOGLYCANS Keratan sulphate – 50%Chondoritin sulphate Chondroitin
Peripheral cornea Dermatan sulphate & Keratan sulphate
Stromal edema
Altered biosynthesis of ground substance Dermatan sulphate present centrally
Scarring
Keratan sulphate & Heparan sulphate and Hyaluronate
CORNEAL TRANSPARENCY
MAURICE THEORY: LATTICE ARRANGEMENT of collagen
fibres is responsible for transparency.
Due to small diameter & regular seperation of collagen, back scattered light would be suppressed by DESTRUCTIVE INTERFERENCE
GOLDMAN THEORY : If fibril seperation & diameter is less than 1/3
of wavelength of incident light –TRANSPARENCY ensues
Other factors
Absence of blood vessels & pigments
Absence of myelinated nerve fibres
Uniform refractive index of all layers & uniform spacing of collagen fibrils
In ill fitting contact lenses & IOP, basal cells which are regularly arranged are seperated by edema fluid of differing refractive index to cells
DIFFRACTION GRATING EFFECT
HALOS AROUND LIGHT
NERVE SUPPLY
Ophthalmic division of Trigeminal via Anterior Ciliary Nerve
Supply also from Cervical Sympathetic
Anterior ciliary nerve enters sclera from perichoroidal space just behind the limbus & joins with the conjunctival nerve to form PERICORNEAL PLEXUS
Divides into 2 branches Anterior & Posterior
Anterior passes subjacent to the BOWMANS forming SUBEPITHELIAL PLEXUS
Posterior innervates posterior stroma , does not involve Descemets .