The cornea is a transparent, nonvascular tissue which forms the anterior surface of the eye.

Light enters the eye through the major refractive structure of the eye, focusing light onto the retina

The cornea protects the fragile intraocular contents by its tough, yet pliable collagen structure.

Horizontally 11 – 12 mm

Vertically 10 – 11 mm

Radius of curvature 7.8mm (6.7 - 9.4 )

Power 43.25 D

It helps to shield the rest of the eye from germs, dust, and other harmful matter. The cornea shares this protective task with the eyelids, the eye socket, tears, and the sclera.

The cornea acts as the eye’s outermost lens.

Epithelium Bowman’s Layer Stroma Decrement's

membrane Endothelium

Epithelium, 5-7

cells layers

Apical cells3-4 cell layers.

Wing cells 1-3 cell layers.

Basal layer1 cell layers

Basal cells

Wing cells

Apical cells

These cells finally degenerate andare sloughed from the corneal surface

This process resultsIn turnover of the entireEpithelium every

7 days

There’s no cell devision.

Polygonal cells:• Small light cells

(young cells) reached the surface of the

cornea• Dark cells (mature

cells) will be sloughed

The surface cells are covered with a dense coat of microplicae, and dark cells have fewer microplicae than do light cells. The darker cells have fewer microvilli and microplicae near the cell margins. Adhering to this microvilli is a glycocalyx.

Apical membrane of the surface epithelium and intercellular junctions along the lateral membranes. Glycocalyx emerging from the surface microvilli.A tight junction and densely staining desmosomes are seen along the lateral membranes. Glycogen is evident as well as numerous vesicles point to microvilli from two adjacent surface cells.

There’s no cell division.A prominent characteristic of these cells is an abundance of intracellular keratin tonofilaments. Wing cells are distinguished by a variety of polygonal shapes and by their large ovoid nuclei.

The corneal epithelium is refered to as a nonkeratinized epithelium because it normally does not express the cornified cytoskeleton typical of epidermal cells.The situation changes in Vit A deficiency when the corneal epithelium express keratins normally found in the cornified epithelium, or epidermis, of the skin.

A single layer of cuboidal cells Originate from the stem cells in the basal layer of the limbal epithelium, which is peripheral to the cornea. They also contain glycogen.

Major keratin pair K3 & K12

Minor keratin pair K14 & K5

The basal cell are distinguished from limbal stem cells by the expression of major Keratin pair

Basement membrane is secreted extracellularly by the epithelial cells and formsone of several structural components associated with cell adhesion to Bawman’sLayer or stroma. Made up primarily of type IV, VII, XII collagen, laminin, Fibronectin and heparan sulfat proteoglycan.

• Lamina Lucida (anteriorly, 23nm)

• Lamina densa (apposed to Bawmen’s layer)

• Reticular lamina (within to Bawmen’s layer)

Basal cells and entire epithelium adhere to the basement membrane and stroma via hemidesmosomes (integral plazma membrane protein)The basal,wing, and superficial cells are linked to one another by desmosomes, which are more numerous among superficial cells than between basal cells.

Bullos pemfigoidantigen &

Integrinheterodimer

Intracellular domains linked

Keratin filamentsL.R

In the basement membrane linked Anchoring fibrils (VII collagen) Pass through B.L. into

The stroma, penetrates

Anchoring plaques (which composed laminin)

Cell adhesion complex

L.R.- laminin receptor B.L.- Bawman’s layer

Junctional complexes of the cornea

Gap junction Tight junctionMore numerous in the more basal

Layers than in the superficial cells.This junction is between basal andwing cells

Gap junction forms a functional Syncytium, which may be importantIn coordinating functions such as cell differentiation and migration.

Only between superficial cells!!!!Tight junction or zonula occludens (ZO), which form a highly effective Semipermeable membrane on the surface of the cornea.

Tight junction is important to the barrier function.

Y YXX + Y = Z

Z

X - proliferation of basal cellsY - centripetal movement of cellsZ - cell loss from the surface.

X, Y, Z Hypothesis of corneal epithelial maintenance.

II. Bowman’s layer Bowman' s layer appears as a feltlike, composite of randomly oriented, striated collagen fibrils dispersed throughout an amorphous matrix . In the adult this layer is approximately 8 to 12μm thick, being slightly thicker in the corneal periphery. Bowman's layer is acellular, except for nerve axons coursing toward the epithelium. The lack of keratocyte (fibroblast) cells within Bowman's layer may indicate its inability to regenerate after injury, or it may imply that keratocytes cannot migrate into Bowman's layer.

III. StromaStroma, a fibrous tissue layer approximately 450μm thick in the central cornea. There are type I collagen with types III, V, and VI.The stroma is a predominantly an ECM comprised of a lamellar arrrangement of collagen fibrils running parallel to the corneal surface, with the individual collagen fibrils separated by a matrix of proteoglycans. The collagen fibrils and ECM are maintained by cells called keratocytes . That lie between the collagen lamellas.

Central corneaCentral cornea

Peripheral cornea

Annulus of collagen fibers1.5-2.0 mm wide around the cornea

The collagen fibers refractive index is 1.4

The extrafibrillar material sometimes called the Ground Substance of the stroma, is largely made up

of proteoglycans

CD/DSDecorin (type IV) is the only CD/DS proteoglycan in the cornea. More abundant in the anterior cornea thanin the posterior.

KSMore abundant in the posterior stroma.

lumican keratocan mimecan

All in keratotocytessynthesizecollagen.

CD/DS - chondroitin sulfat/dermatan sulfat KS - keratan sulfat

Lumican,keratocan, and mimecan are also found in other tissues. Only in the cornea they are present in a highly sulfated form.

This sulfation of the cornea chains increases the wateretentive properties of the proteoglycans, which affects hydration of the stroma and hence corneal transparency

IV. Descemet’s layerThe cornel endothelium rests on a basement membrane, known as a descemet’s membrane, which in the adult eye 10-15 micron thick. This membrane is secreted by endothelial cells and increases in thickness throughout life.

The basement membrane components :

Type IV collagen Laminin Fibronectin (functions in adhesion endothelial cells to the membrane)

Descemet’s membrane may remain intact in cases of severe corneal ulceration, forming a descemetocele after destruction of the epithelium and stroma.this shows that the membrane is highly resistant to proteolytic enzymes.

V. EndotheliumSingle layer of cells covers the posterior surface of the cornea .Polygonal cells most of which arehexagonal (d- 20µm, thick – 4-6µm)

Newborns – 5500 cells/mm2

Adults -- 2500-3000 cells/mm2

Min level -- 400-700 cells/mm2

Endothelial cells are interconnected by tight and gap junction . The endotheliumforming a “leaky” barrier between the aqueous humor &the stroma (by tight junction).The gap junction do not contribute to the endothelial barrier, but the funtion in intercellular communication

Polymegathizm – the abnormal size of the endothelial cells. Polymorphizm – a decrease in hexagons with a concomitant increase in number of cells

Polymegatizm and pleomorphizmare increasing with aging.

Cornea’s innervation.

nerves In the middle third of the stroma

corneaanteriorly

forwardradially

Toward the central areaGiving branch and innervate

Anterior stromaPlexus subepithelial

Bowman’s layer

Subbasal epithelial

nerve plexus

Basal epithelial cell layer And terminating

Superficial epithelial layers.

Nutrition of the cornea Aqueous Humour

Perilimbal vessels

And in the end the funny cornea’s pictures for you!!!

I hope it was interesting,

Thank you for your attention.