CorneaAnatomy and Physiology

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THE

Topics Covered

Anatomy

• Embryology

• Gross Anatomy

• Optical Properties

• Microscopic Structureo Epitheliumo Bowman's Membraneo stromao Dua's Layero Descemet's Membraneo Endothelium

• Anchoring Structures

• Nerve supply / Innervation and Sensitivity

• Blood Supply

Physiology

• Nutrition

• Metabolism

• Pre-Corneal Tear Film

• Physiology of Epithelium

• Corneal Hydration

• Corneal Transparency - Anatomical and Physiological Factorso Corneal Oedema

• Regenerative Mechanismso Epithelium and Stromal

Interaction During Wound Healing

• Physiology of Endothelium

Anatomy of Cornea

Embryology

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Cornea is developed at the 9th week of gestation

Corneal epithelium is formed by the surface ectoderm

The other layers such as,• Bowman’s membrane• Stroma/substantia propria• Descemet’s membrane• Endothelium,Are derived from the outer layer of mesoderm(Dura mater)

Gross Anatomy

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Anterior watch-glass like structure 1st Refractive Media of the Eyeball Forms the outer fibrous 1/6th part of

the eyeball Transparent Joins with the sclera at the corneo-

scleral junction or the limbus

Sclera

Anterior Pole of CORNEA

External landmarks of the eye highlighting the cornea

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Gross Anatomy

Optical Properties

Coronal views show the elliptical shape of the right cornea when viewed anteriorly (upper left) and the circular shape when viewed posteriorly (lower left). Superior axial view (right) illustrates how the right globe deviates from a perfect sphere. Dashed lines = theoretical spherical globe;solid lines = actual contour of the globe.

(Modified from Bron AJ, Tripathi R, Tripathi B. In: Wolff’s anatomy of the eye and Orbit, 8th edn. London, UK: Chapman & Hall, 1997.)

Dimensions

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Parameters of Cornea

Absolute Refractive Index of Cornea = 1.376

Thickness of:Axial Cornea = 0.5mm approx.Peripheral Cornea = 1.0 mm approx

Radius of Curvature of:Anterior Surface = 7.7 mmPosterior Surface = 6.8 mm

The cornea can be nomenclatured as a convexo-concave lens.

It accounts for 43 - 44 dioptres of total refractive power of the eye (60D).

This is high due to the air-corneal interface in which light refracts the most.

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Corneal Astigmatism

Changes in corneal shape associated with aging showing the shift from with-the-rule astigmatism to against-the-rule astigmatism.

(From Hayashi K, Hayashi H, Hayashi F. Cornea 1995; 14:527–32.)DEEPAYAN

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Microscopic Structure

EPITHELIUM

BOWMAN's LAYER

STROMA

DESCEMET's MEMBRANEENDOTHELIUM

Histological Structure of the CORNEA showing its 5 distinct layers. Stained using Eosin-based staining technique.

SOURCE SPECIMEN

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Layers of cornea

Epithelium◦ Flattened cell layer-superficial layer◦ Wing/Umbrella cell layer-middle layer◦ Basal Layer(columnar cells)-deepest layer

Bowman’s membrane Stroma Descemet’s membrane Endothelium

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Epithelium

Stratified Squamous Epithelium Continuous with the bulbar conjunctiva at

the limbus 3 Layers

◦ Superficial layer (Flattened Cells – 2 layers)◦ Middle layer (Wing or Umbrella Cells – 2-3 layers)◦ Deepest layer (Basal/Columnar Cells – 1 layer)

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Epithelium

Microscopic Structure

WING/UMBRELLA CELLS

SUPERFICIAL CELLS

BASAL CELLS

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Bowman’s Membrane

• Acellular mass of condensed collagen fibres• No elastic tissue• Considerable resistance to infection and

injury• Does not regenerate if damaged

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StromaSubstantia propria / Cornea Proper• Modified connective tissue layer• 3 components

• Collagen fibres• Cells

• Embedded in the hydrated matrix of proteoglycans (ground substance)

• lamellae (200-250 layers)• Anterior 1/3rd || to each other• Posterior 2/3rd right angled to each other• Results in corneal transparency

• Cells – 2 types• Fixed keratocytes• Wandering histocytes• Few lymphocytes

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Strong homogenous layer(strongest layer) Bounds the stroma posteriorly ELASTICITY (main physical property) Collagen and glycoproteins Resistant to chemical agents,trauma,infections,

and pathoogical processes Divided into

Anterior 1/3rd-vertically banded Posterior 2/3rd-granular

Maintains Integrity(although if the entire stroma is sloughed off)

Can regenerate if damaged.

Descemet’s Membrane

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Descemet's Membrane

DESCEMET"s MEMBRANE

Stripping of Descemet’s membrane

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Endothelium

•Polygonal Cells (1 layer)•Attached to Descemet’s membrane by HEMIDESMOSOMES and ZONULA OCCLUDENTS

•Barrier function•Presence of Bicarbonate and Hydrogen ion pumps which pumps out excess water,thus maintaining corneal hydration and transparency

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Microscopic Structure

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Anchoring Structures

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• DESMOSOMES• HEMIDESMOSOMES• ZONULA

OCCLUDENTES• ZONULAE ADHERENS

Transmission electron micrograph of epithelial basal lamina in the human cornea, Bowman's layer is seen below, with basal epithelial cells above.

Nerve Supply

Innervation and Sensitivity

2013 Image Copyrights Reserved.

Long and Short PS Ciiary Nerves

Nasociiary nerves

Ophthalmic Division

Trigeminal Nerve

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Blood Supply

●Limbal Vesselssmall branches from anterior ciliary vessels supplies its periphery and provides nourishmentThis branches are not present in the cornea,but in the sub conjunctival tissue,which overlaps the corneaThe cornea is generally avascular.

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Physiology of Cornea

NutritionThe nutrition of cornea is derived from mainly 3 sources- Aqueous humour Exudation from the prelimbal vessels Pre-corneal tear film

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Pre-Corneal Tear Film

The optical interface between the anterior surface of cornea and air is formed by the Tear-film

The tear film is the product of Lacrimal gland, Meibomian gland, unicellular mucous gland of the conjunctiva and the cornea itself

It consists of 3 layers-1. The external oily layer(0.1µ thick)2. Middle aqueous layer(7µ thick)3. Innermost mucous layer(0.02-0.05µ thick)

External layer- wax and cholesterol esters Middle layer- aqueous solution Innermost layer- hydrated mucoprotein layer

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Metabolism

The metabolism of glucose is the chief source of energy, required for transparency,cellular activity and growth of the cornea.

Glucose is stored in the epithelium as glycogen, which in state of emergency such as wound healing, breaks into glucose

In the cornea, glucose derived mainly from the aqueous humour is utilised by anaerobic process(65%) and partly by aerobic process(35%)

There are 2 processes in glucose breakdown, which results in generation of energy, in the form of ATP

1. Glycolysis- breakdown of glucose into lactic and pyruvic acid. This may be an aerobic process. It occurs mainly in corneal epithelium and stroma

2. Respiration- oxidation of carbon dioxide into lactic acid and water. This process is always aerobic. It occurs in the epithelium.

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Role of Epithelium in Metabolism

The epithelium derives oxygen mainly from the atmosphere, tear film, and from the limbal capillaries.

The oxygen required by the epithelium is 1/10th of the available atmospheric oxygen, when the eyes are open, and

1/4th from the palpebral conjunctiva, when the eyes are closed.

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Protein Synthesis in Metabolism

A continuous supply of amino acid is required to allow synthesis of proteins needed for the constant shedding and replacement(by mitosis) of the corneal epithelial cells

Amino acids are also supplied by the aqueous humour, principally by passive diffusion.

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Physiology of Epithelium

• Together with the tear film, it is the major refractive surface of the eye.• Tight junctions provide barrier function.• Limbal basal epithelium contains the reservoir of stem cells.• Metabolism

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Corneal HydrationWater content of cornea is about 80%, which is higher than any other connective tissue in the human body.There are some maintenance factors of corneal hydration:-Structural rigidity of corneal layers and scleral

restriction of swellingEpithelium and endothelium acts as a barrier to

rapid fluid passageStromal swelling pressureEndothelial pump activityEvaporation of water from corneal tear film,

which is balanced by the aqueous humour.ARIJIT

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Corneal Transparency

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Corneal Epithelium• Normal

epithelium transparent due to homogeneity of RI

• Tightly adhered with tight junctions

• Barrier function

Tear Film• Supportive to

the pre-corneal aqueous film in helping maintain epithelium transparency

• Acts as windshield

• Removes persistent dust particles

Avascularity• Generally

avascular• Except small

loops which invade periphery for about 1 mm

• Deviations of engorged vessels in diseased conditions

Arrangement of Stromal Lamellae• Maurice’s

Theory:• Goldman’s

Theory

Maurice’s Theory Goldman’s Theory

Corneal Transparency based on Arrangement of Stromal Lamellae

Corneal Œdema

Transparency Deviations

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Fluids percolate into the corneal collagen network

Usually accompanies eye diseases, or contact lens wear with low oxygen

transmissibility.

Endothelial

disbalance etc.

Easily seen with

a slit-lamp using retro-

illumination or

sclerotic scatter

illumination.

Quantitatively, it can

be assessed with the

addition of a

pachometer that

measures corneal

swelling.

Beyond about 4% swelling, there appear striae (wispy greyish-white lines usually

vertical) in the stroma. Beyond

about 8% swelling, there appear folds (dark lines) believed to represent physical

buckling of the posterior

corneal layers.

Gives rise to the

appearance of haloes around

lights, photophobia,

spectacle blur, losses in

corneal transparency

and sometimes

stinging

Management depends on the cause and tissue involved.

Physiology of Endothelium

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Single layer of 400,000 to 500,000 cells (neural crest-derived cells)

At birth, the central endothelial cell density of the cornea is around 5,000 cells/mm2

Primary function of the corneal endothelium is to maintain the health, deturgescence, and clarity of the cornea through a pump-leak mechanism .

Diagram illustrating the central, paracentral, and peripheral corneal endothelial cell densities in healthy, normal subjects .

Diagram illustrating the opposing forces of the corneal endothelial barrier and metabolic pump.

When the leak rate equals the metabolic pump rate, the corneal stroma is 78% hydrated and

the corneal thickness is maintained.

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Physiology of Endothelium

Thank You!

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