Cornea Anatomy and Physiology

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Anatomy and Physiology
DEEPAYAN KAR

ARIJIT MAITY

Anatomy

Physiology

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• • •

Embryology Gross Anatomy Optical Properties Microscopic Structure o Epithelium o Bowman's Membrane o stroma o Dua's Layer o Descemet's Membrane o Endothelium Anchoring Structures Nerve supply / Innervation and Sensitivity Blood Supply

• • • • • • • •

Nutrition Metabolism Pre-Corneal Tear Film Physiology of Epithelium Corneal Hydration Corneal Transparency Anatomical and Physiological Factors o Corneal Oedema Regenerative Mechanisms o Epithelium and Stromal Interaction During Wound Healing Physiology of Endothelium

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)

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DEEPAYAN KAR

Sclera External landmarks of the eye highlighting the cornea

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Anterior Pole of CORNEA

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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

DEEPAYAN KAR

Dimensions
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.)

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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 mm Posterior 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.)

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ENDOTHELIUM

DESCEMET's MEMBRANE
SOURCE SPECIMEN

STROMA

EPITHELIUM

BOWMAN's LAYER

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

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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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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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SUPERFICIAL CELLS WING/UMBRELLA CELLS BASAL CELLS

Microscopic Structure
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Acellular mass of condensed collagen fibres No elastic tissue Considerable resistance to infection and injury Does not regenerate if damaged

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Substantia propria /
• •

Modified connective tissue layer 3 components

Cornea Proper





• 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

 

Maintains Integrity(although if the entire stroma is sloughed off) Can regenerate if damaged.
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 Anterior 1/3rd-vertically banded  Posterior 2/3rd-granular

DESCEMET"s MEMBRANE

Stripping of Descemet’s membrane

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•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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•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.

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Innervation and Sensitivity

Long and Short PS Ciiary Nerves

Nasociiary nerves

Ophthalmic Division

Trigeminal Nerve

2013 Image Copyrights Reserved.

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●Limbal Vessels
small branches from anterior ciliary vessels supplies its periphery and provides nourishment This branches are not present in the cornea,but in the sub conjunctival tissue,which overlaps the cornea The cornea is generally avascular.

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

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 The optical interface between the anterior surface of cornea and air is formed by the Tearfilm 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 layers1. 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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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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 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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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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• 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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Water 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 swelling Epithelium and endothelium acts as a barrier to rapid fluid passage Stromal swelling pressure Endothelial pump activity Evaporation of water from corneal tear film, which is balanced by the aqueous humour.

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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

Arrangement of Stromal Lamellae
•Maurice’s Theory:
•Goldman’s Theory

•Except small loops which invade periphery for about 1 mm
•Deviations of engorged vessels in diseased conditions

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Maurice’s Theory

Goldman’s Theory

Transparency Deviations

Fluids percolate into the corneal collagen network
Usually Quantitative accompani Easily seen ly, it can be es eye diseases, or with a slitassessed with the contact lens lamp using retroaddition of a wear with low oxygen illumination pachomete transmissibil or sclerotic r that measures ity. scatter corneal Endothelial illumination. disbalance swelling. etc.
Beyond about 4% swelling, there appear striae (wispy greyishwhite 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.
DEEPAYAN KAR

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

Decline in central endothelial cell density with age that typical involves two phases: a rapid and a slow component (Due to limited regenerative properties)

Primary function of the corneal endothelium is to maintain the health, deturgescence, and clarity of the cornea through a pumpleak mechanism .
DEEPAYAN KAR

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.

DEEPAYAN KAR

DEEPAYAN KAR

ARIJIT MAITY

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