ANATOMY OF CORNEA

The transparent portion of the eye that covers the pupil and iris and lets light in is called the cornea. Saying that the cornea is the black portion of the eye is misleading because, in reality, it is transparent, giving the appearance of being black or brownish due to iris pigmentation beneath the surface.

also read about surgical anatomy of limbus.

Now, let’s attempt to comprehend the anatomy of the cornea. As everyone is aware, the eyeball is primarily covered by three coats

  1. The innermost coat, also known as the retina.This is  also nervous coat)
  2. The middle vascular coat is known as the choroid.
  3. The outermost coat is the fibrous coat. The sclera forms the posterior 5/6th portion of the outer coat. The cornea forms the anterior 1/6th of the outer coat.
image showing cornea and sclera.cornea forms anterior 1/6th of outer coat of eye andn sclera forms posterior5/6th of outer coat of eye

Measurements of Cornea : Important Topographical Anatomy

  • Anterior surface of the cornea is elliptical whereas posterior surface is spherical .
  • Average diameter 11.5 mm.
  • Horizontal diameter of the anterior surface (HCD) is 11.7 mm and vertical diameter (VCD) is 11mm.
  • The radius of curvature of cornea is 8 (7.8 mm).
  • The radius of curvature of sclera is 12mm.
  • Power of cornea is 45D( average) and lens is 15 D .
  • Refractive index of cornea is 1.37

To understand better about radius of curvature, check detailed explanation in the video.

WHY IS CORNEA ELLIPTICAL?

  • The horizontal diameter of the anterior surface of the cornea is greater than the vertical diameter.
  • This as a result of this continuous vertical force applied by the eyelids on the corneal making the cornea  horizontally oval or elliptical.
  • the same vertical force also makes the vertical meridian of cornea more steeper than the horizontal meridian leading to WITH THE RULE astigmatism.
image of important corneal dimensions. The image depicts the radius of curvature of cornea and radius of curvature of sclera. It also depicst the elliptical anterior surface of cornea and spherical posterior surface of cornea.

CLINICAL NUGGET

  • MICROCORNEA= when horizontal corneal diameter (HCD) is less than 10mm
  • MACROCORNEA = when horizontal corneal diameter (HCD) is more than 13 mm

Central Corneal thickness

  • Central corneal thickness is 0.52mm (520 micrometers).
  • PERIPHERAL CORNEAL THICKNESS is 0.67mm (670 micrometers).

Therefore, the corona is thicker at the periphery than at the center, as can be seen in the picture. The central corneal thickness has some important clinical implications: –

  • Central 5 mm of cornea is the most powerful refracting surface.
  • It forms ¾ of the total power of the eyeball.
  • Central corneal thickness has an effect on IOP measurement.
  • Thicker corneas lead to overestimation of IOP, and thinner corneas lead to underestimation of IOP.
image depicting the variation in corneal thickness. the central corneal thickness average and peripheral corneal thcikness

Histology of Cornea

There are 6 layers in cornea namely:-

  1. Epithelium
  2. Bowman’s layer
  3. Stroma
  4. Pre-descemet’s layer / Duas layer
  5. Descemet’s membrane, and
  6. Endothelium
imagee showing the layers of the cornea.it shows epithelium, stroma, bowmans membrane, descemets ,duas layer and endothelium

Epithelium

The outermost layer of the cornea is the epithelium. It forms 10% of the total corneal thickness. It is made up of stratified, squamous, non-keratinized epithelium. Stratified epithelium means they have multiple layers of cells—about 5–6 layers tacked on top of each other. These cells are nucleated and non-keratinized.

Different-shaped cells are present at different layers in the cornea, like basal cells, wing cells, and superficial cells. The memberanes of the superficial cells are thrown into multiple foldings called the microvilli. It is important to note that the microvilli are covered with glycocalyx. This imparts a sticky nature to the microvilli and epithelium, leading to better tear film adhesion and stability.

Also understand about how corneal epithelium differs from conjunctival epithelium from here!

image of different types of cells in corneal epithelium. image also depicts the superficial cells, wing cells, basal cells and basal lamina of epithelium of cornea

CORNEAL EPITHELIAL REGENERATION

The corneal Epithelial cells are connected to each other with connections called desmosomes. However, it is the hemi-desmosomes that connect the cells to their basal lamina. The basal lamina is secreted by basal cells.

Among the three types of cells discussed, basal cells have replicating capability. Basal cells undergo mitosis to produce new cells, which gradually move outward, transforming first into a wing cell and then into a superficial squamous cell. Finally, superficial cells lose their connection with the epithelium and are shed into the tear film. The entire epithelium takes about 7 days to regenerate completely, provided the basal lamina is not damaged.

Once the basal lamina is damaged, it takes about 6 weeks for the entire epithelium to be replaced.

IMPORTANCE OF PALISADES OF VOGT

The limbus is the junction where conjunctival epithelium changes into corneal epithelium, and the sclera becomes corneal stroma.

The limbal basal epithelium cells harbor pluripotent stem cells. This niche of stem cells is pigmented and is called the palisades of Vogt.

image depicting th location of stem cells of cornea in palisades of vogt in limbus

What is XYZ hypothesis of Corneal proliferation ? 

  • X represents the proliferation and stratification (layering) of the limbal basal cells.
  • Y represents the centripetal migration of the basal cells as the new cells form
  • Z represents the sloughing /desquamation of the superficial cells of the cornea.
image showing the X-Y-Z hypothesis of corneal regeneration

Bowman's Memberane

  • Acellular tough membrane like zone situated in between corneal epithelium and stroma.
  • It is 8-14 µm thick and composed of randomly arranged collagen fibres.
  • Some consider it to be an anterior condensation of the corneal stromal fibres.
  • Comparatively resistant to trauma.
  • However, once destroyed it cannot be regenerated.

Corneal Stroma/Substantia Propria

  • The corneal stroma forms about 90% of the total corneal thickness
  • It consists of three components.
  • These are the collagen fibrils + keratocytes + extracellular ground substances like KERATAN SULPHATE AND DERMATAN SULPHATE.

CORNEAL COLLAGEN FIBRILS 

  • All collagen fibrils have a  uniform diameter of 25- to 35-nm.
  • These are arranged to form flattened  bundles called the corneal lamellae.
  • There are about 300 lamellae in corneal stroma.

FACTORS RESPONSIBLE FOR MAINTAINING THE TRANSPARENCY OF CORNEA.

  • Highly uniform diameter of each corneal fibrils(25-35 nm)
  • The distance between two corneal fibers is also highly uniform (41.5nm)
  • Apart from the two points mentioned above, there are two theories explaining the corneal transparency These are the MAURICE THEORY and the GOLDMAN THEORY. 
  • Avascular nature of cornea, unmyelinated nerve endings .
  • Maintenance of a state of relative dehydration of 70%(more about it under endothelium section)
image depicting the composition of the corneal stroma: keratocytes with collagen fibrils

MAURICE THEORY

  • The Maurice theory says that there is a regular arrangement of the collagen fibrils. Each collagen fibril is surrounded by 6 other fibrils. This give them a regular arrangement known as the Lattice arrangement.
  • The arrangement places them in a way that the distance between them is less than the wavelength of light.
  • This prevents scattering of light by the process of destructive interference.
  • Since this theory explains the lattice arrangement and the phenomenon of destructive interference, this is also called the LATTICE THEORY 
  • This theory can explain the development of corneal edema. ( check video@ 20:40)

GOLDMANN THEORY

  • The Goldmann theory, contrary to the Maurice theory, says that the lattice arrangement is not very important for maintaining transparency.
  • It instead focuses on the size of the corneal lamellae.
  • The size of the fibrils should be less than half the wavelength of the light(200nm or 2000A).
image showing maurice/lattice theory and goldmann theory of corneal stromal fibres arrangement

FACTORS RESPONSIBLE FOR CORNEAL TRANSPARENCY 

  • Optically smooth tear film
  • Role of corneal epithelium
  • Arrangement of stromal fibres
  • Avascularity of cornea
  • Absence of myelination in corneal nerves

Pre-Descemet's Memberane / Dua's Layer

A newly discovered layer, Dua’s layer (named after Harminder Dua) is 15 microns in thickness Despite its thinness, the layer is very strong and impervious to air.

The discovery of this layer has led to description of three new surgical techniques :

  • PRE-DESCEMET’S ENDOTHELIAL KERATOPLASTY ( PDEK)
  • DALK with type 1 bubble 
  • DALK with type 2 bubble 

Descemet's Membrane

  • The basement membrane secreted by the endothelium cells is called the descemets memberane.
  • It is constantly produced so thickens throughout life (3 micrometer at birth-10 micrometer in adults)
  • It is weakly attached to the stroma
  • It is resistant to enzymatic degradation by phagocytes and toxins

 DESMETOCELE : remains intact in corneal ulcer and herniates due to raised IOP

DESCEMTOCELE

When a cornea becomes infected, the pathogen and the inflammation that results from the illness will melt the cornea all the way to the descemet’s memberane. It is unable to readily break down the descemet’s memberane, though. Because of intraocular pressure inside the eye, this intact descemet will expand and form a descmetocele.
Thus, the term “descmetocele” refers to the protrusion of the descemets memberane through the cornea that has thinned out. Glaucoma or even sporadic periods of elevated IOP, such as coughing, harsh laughter, straining at feces, etc., could be the cause of elevated IOP.

Corneal Endothelium

  • The corneal endothelial cells are Hexagonal in shape  which forms a continuous mosaic pattern, best seen on specular microscopy.
  • These endothelial cells are interconnected with each other with various junctional complexes like zonula occludans, macula occludans and macula adherens.
  • Its important to note that these barriers are not as water tight as the desmosomes and therefore may allow passage of water from the aqueous into the stroma . Therefore these junctions are called the LEAKY BARRIERS.
  • These cells possess ion transport system which is known as endothelial pump. These endothelial pumps regulate the water content of corneal stroma and maintain its state of relative hydration of  70 %
  • Therefore the cornea remains transparent 

Endothelial Pump

We know that the leaky barrier between the cells of the endothelium allow diffusion of some water into the stroma from the Aqueous humor. Therefore we need a mechanism to push out this excess water back into the aqueous humor from the stroma. As shown in the image , the endothelial pump does this job. The pump pushes out the sodium from the stroma towards the aqueous, creating an osmotic gradient that causes fluid to move from stroma to the aqueous humor. This maintains the state of relative hydration (70%).

Functioning of the endothelial cell pump

  • BASOLATERAL MEMBERANE consists of the sodium-potassium ATPase channel( Na-K-ATPase ). It uses energy from ATP to pump 3 molecules of sodium out of the endothelial cell from the basolateral membrane. In exchange for 3 molecules of sodium, 2 molecules of  potassium are brought into the cell via the  Na-K-ATPase.
  • The sodium ions  in the stroma enter the endothelial cells through an anti transporter that pushes a hydrogen ion from the endothelial cell into the stroma and takes in the sodium ion.
  • The movement of hydrogen ions into the stroma leads to acidification. This forces the carbon dioxide in the stroma to move into the endothelium cells.
  • Here carbon dioxide combines with water to form bicarbonic acid. This further splits into Hydrogen ion and bicarbonate ions inside the endothelial cell using the enzyme carbonic anhydrase.
  • Now the sodium ion that had reached the endothelial cell makes its way into the aqueous humor via a symporter that also transports a bicarbonate ion along with it.
  • The bicarbonate is made available inside the endothelial because of the activity of the carbonic anhydrase.
  • This leads to the net movement of the sodium into the aqueous creating an osmotic gradient( more osmotic pressure in aqueous humor than in stroma. 
  • Finally, water follows and excess water is removed from the cornea stroma and pushed into the aqueous humor.

If you wish to learn it in a visual form , check out our video on the anatomy of cornea 

schematic showing working of endothelium pump in cornea

Importance of Endothelial Cells

  • Endothelial cells cannot divide or replicate
  • With ageing, the cell density of the endothelium decreases which is compensated by an increase in cell size (Polymegathism) or shape (Pleomorphism).
  • As these endothelial cells are involved in corneal hydration (which helps in maintenance of corneal transparency), endothelial cell density below 800 cells/mm2 leads to corneal decompensation. the corneal endothelial pump fails leading to corneal edema .

APPORXIMATE ENDOTHELIAL DENSITY WITH AGE

  • Newborn and infants : 3000-4000/mm3
  • Adolescent :2500/mm3 
  • Old Age : 2000/mm3 

Corneal Response To Injury

  • Epithelium :Responds immediately to establish barrier action and protect underlying tissue .
  • Bowmans Membrane :no mechanism of repair.
  • Stroma : keratocytes produce reparative collagen and proteoglycan . This may lead to scar production. 
  • Descemets membrane  :Resecreted by corneal endothelium
  • Endothelium :no mechanism to repair. Might try to compensate by an increase in cell size (Polymegathism) or shape (Pleomorphism).

Corneal Scar types :-

  1. NEBULA :Involving the Bowman’s Membrane and superficial stroma 
  2. MACULA: When scar involves the superficial 1/3rd of the stroma 
  3. LEUCOMA : scar involving more than 1/2 of the stroma 
different types of corneal scars ,namely nebular scar of cornea, macular scar of cornea and leucomatous scar of cornea

Blood Supply of Cornea?

  • In normal condition, cornea does not contain any blood vessels( AVASCULAR)
  • Anterior ciliary artery, a branch ophthalmic artery forms a vascular arcade in the limbal region and helps in corneal metabolism and wound repair by providing nourishment.
  • Absence of blood vessel in cornea is one of the contributing factors for its transparency.

Nerve supply of cornea

  • One of the human body’s most sensitive tissues is the cornea.
  • The cornea’s nerve ending density is roughly 300 times higher than that of the skin.
  • As many as 100 nerve endings can be found in a 0.01 mm2 corneal region.
  • The trigeminal nerve’s ophthalmic branch provides the majority of the innervation for the cornea.
  • The frontal nerve, lacrimal nerve, and nasociliary nerve comprise the trigeminal nerve’s ophthalmic division.
  • The cornea receives sensory innervation from the nasociliary nerve.
  • The nasociliary nerve gives a branch called the long posterior ciliary nerves .
  • Important to note that these nerves are non myelinated , this helps in maintaining corneal transparency .
  • These long posterior ciliary nerves on reaching the limbus divide and form three plexus :-
  1. STROMAL PLEXUS : nerve network within the stroma
  2. SUBEPITHELIAL PLEXUS : nerve network below the epithelium 
  3. INTRAEPITHELIAL PLEXUS : nerve networks within the epithelium .
 
image showing the nerve supply of cornea. the image depicts the subepithelial, intraepithelial and stromal plexus in cornea

CLINICAL POINTS : 

  • Loss of corneal epithelium, cause severe pain due to exposure of corneal nerve ending.
  • While doing slit lamp examination, you may have seen corneal nerves as thin fibers in corneal periphery.
  • In normal condition, corneal nerves are mainly visible in periphery as the diameters of the corneal nerves are larger in periphery as compared to center.
  • Infection or reactivation of latent herpes simplex virus located in trigeminal ganglion affecting eye reduces corneal sensation due to damage to the nerve endings.

 

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