Anatomy Of Cornea

Anatomy Of

CorneaBy-

Husain J. PatanwalaFinal year B.optom

Origin

The word cornea has come from “Kerato”. The term “Kerato” in Greek means horn or shield like. Ancient Greek used to believe that cornea is derived from same material like that of thinly sliced horn of animal.

Introduction

• Avascular, Transparent• Accounts for 70% of the total refractive power of the

eye(+43D).• Principle optical surface of the eye.

Refractive indices of different media through which light enters the eye

Air 1.00Tear fluid 1.33Cornea 1.37

Aqueous humor 1.33Lens 1.42

Vitreous 1.33

Dimensions• Anterior Surface : Elliptical Diameter: horizontal:11.75mm, vertical:11mm• Posterior Surface: Circular Diameter:11.5 mm

Microcornea when HCD is less than 10mm

Macrocornea when HCD is more than 13 mm

• Radius Of Curvature : The anterior and posterior radii of curvature are 7.8mm

and 6.5 mm .

• Thickness :The central corneal thickness is 0.52 mm, whereas periphery is 0.67 mm thick. Central corneal thickness has direct influence in IOP measurement. The central 5 mm of the cornea forms the most powerful refracting surface

Histology:6 layersAnterior epitheliumBowman’s membraneCentral StromaDuas layerDescemet’s membraneEndothelium

1.Epithelium• 50 -90µm thick.

Stratified squamous type

• Consist of 5-6 layers

• Corneal epithelium sheds at regular interval. It takes normally 7 days for replacement entire corneal epithelium

• Tight junctions in basal cells account for epithelium’s transparency.

Layers of

epithelium

Description Mitotic activity

Basal Cell

Single layer of columnar cells found adjacent to basal layer

-

Wing cell

2-3 layers of cells with elongated process

-

Surface cells

3 layers of flattened

epithelial cells with microvilli in contact with the

tear film.

+

2.Bowman’s membrane• Binds the corneal stroma anteriorly with basement

membrane of epithelium.

• Not a true elastic membrane.

• Once destroyed , does not regenerate.

• Shows considerable resistance to infection and injury.

3.Stroma• 0.5mm thick.

• The stroma or substantia propria, occupies about 90% of the total corneal thickness,

• Composed of collagen fibrils, keratocytes and extracellular ground substances.

• The lamellae are arranged in many layers(200-250) and are parallel to each other.

• Anterior 1/3 stroma : lamellae are thin( 0.5-30μm wide ,0.2-1.2 μm thick).

• Posterior 2/3 stroma : lamallae are larger ( 100- 200 μm wide, 1 -2.5 μm thick).

Duas layer• Was discovered by Dr.Harminder singh dua at

the university of nottingham in 2013• It is the 4th layer 15 microns thick• Strong layer and impervious to air

5.Descemet’s membrane• A strong homogenous layer.

• Basement membrane of endothelium.

• Made up of collagen and glycoprotien.

• Elastic.

• It can regenerate . Resistant to chemical agents, infection and pathological processes.

• On posterior surface it shows Hassel Henle bodies,which increase with advancing age.

6.Endothelium• Single layer of flat polygonal cells

• At birth cell density is around 6000 cells/sqmm

• With increasing age the no. of cells is reduced

• Involved in active secretion and protein synthesis.

• Contains highest no. of mitochondria next to photoreceptors.

Blood supply • Cornea is an avascular structure.

• Small loops derived from the anterior ciliary vessels invade its periphery for about 1mm and provide nourishment.

• These loops are not in cornea but in the subconjunctival tissue which overlaps the cornea.

Nerve Supply• The cornea is one of the most sensitive tissues of the body

and has one of the richest sensory nerve supplies in the body.

• It is supplied by long and short ciliary nerves from the ophthalmic division of the trigeminal nerve.

• Most are concentrated in the anterior stroma beneath the Bowman zone and send branches forward into epithelium

• Descemets membrane and endothelium are not innervated

At limbus,•Corneal epithelium becomes bulbar conjunctival epithelium•Bowman's membrane becomes continuous with the lamina propria of the conjunctiva and tenon's capsule.•Stroma becomes sclera•Descemets membrane becomes schwalbe's line.•Endothelium lines the trabecular meshwork and becomes continuous with the anterior surface of the epithelium

Junction of Cornea and Sclera

Contains 1.Trabecular Meshwork

2.Canal of Schlemn

Transparency• Tight junctions of the epithelial cells.

• Endothelial pump mechanism.

• Absence of blood vessels.

• Absence of pigments.

• Scarcity of cell nuclei in stroma.

• Uniform structure of stroma.

Corneal analysis• Keratometer• Confocal microscopy• Specular microscopy• AS-OCT• Orbscan• Aberrometer• Pentacam• Keratoscope• Video keratoscope

• Oculus topographer• Ocular response analyser

(OCA)• 3netra• Galilie

References

• Anatomy and Physiology of the Eye : Khurana• Clinical Anatomy of the visual System : Remmington• www.eophtha.com

Thank You