anatomy of cornea

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 .