General corneal structure
• Cornea is a anteriormost part of eye, it is transparent, avascular ( No Blood supply ), watch-glass like structure.
• Cornea forms anterior one-sixth of the outer fibrous coat of the eyeball.
• The main work of cornea is optical power with protection of interior part of eye
Dimensions of cornea
• Anterior surface of cornea is elliptical with an average horizontal diameter of 11.7 mm and vertical diameter of 11 mm.
• Posterior surface of cornea is circular with an average diameter of 11.5 mm .
• Thickness of cornea in the centre various from 0.5 to 0.6 mm while at the periphery it varies from 1 to 1.2 mm.
• Corneal thickness is 540 micrometer and it is measured with the help of corneal pachemeter ( Pachemeter is instrument used for to mesure the thickness of Cornea).
Radius of curvature of Cornea
• The central 5 mm part of the cornea is highly powerful refracting surface of the eye.
• The anterior and posterior radious of curvature of central part of cornea are 7.8 mm and 6.5 mm, respectively.
• Refractive index of the cornea is 1.376.
• Refractive power of the cornea is about 45 dioptre
, which is roughly three-fourth of the total refractive power of the eye (60 dioptres).
• It is worth noting that refractive power of anterior surface of cornea is about +48 D but due to negative power (-5.80 D) of the posterior corneal surface, the overall corneal power is less, i.e. + 43-44 D.
• Most of the refraction in eye occurs at anterior surface of cornea (air-tear interface).
• Anterior surface of cornea is the most important refractive structure of eye.
Histology of Cornea
• Histologically the cornea consists of six layers.
• Anterior to posterior these are: epithelium, Bowman's membrane, substantia propria (corneal stroma), Pre- Descemet's membrane (Dua's layer), Descemet's membrane and endothelium
1. Epithelium.
• It is the outermost layer of cornea and is composed of stratified squamous non- keratinized epithelial cells.
• It becomes continuous with epith elium of bulbar conjunctiva at the limbus.
• It consists of 5-6 layers of cells. The deepest (basal) layer is made up of columnar cells, next 2-3 layers of wing or umbrella cells and the most superficial two layers are made of flattened cells.
• Tight junctions between superficial epithelial cells prevent penetration of tear fluid into the stroma.
Limbal epithelium.
• The basal epithelial cells of the limbal area constitute the limbal stem cells which amplify, proliferate, and differentiate into corneal epithelium.
• Damage to this area results in the invasion of conjunctival epithelium on to the cornea.
2. Bowman's membrane.
• This layer consists of acellular mass of condensed collagen fibrils.
• It is about 12 µm in thickness and binds the corneal stroma anteriorly with basement membrane of the epithelium.
• It is not a true elastic membrane but simply a condensed superficial part of the stroma.
• It shows considerable resistance to infection But once destroyed, it is unable to regenerate and therefore heals by scarring.
3. Stroma (substantia propria).
• This layer is about 0.5 mm in thickness and constitutes most of the cornea (90% of total thickness).
• It consists of collagen fibrils (type-I and type-V fibrillae interwined with filaments of type-VI collagen) embedded in hydrated matrix of proteoglycans (chondroitin sulphate and keratan sulphate).
•The lamellae are arranged in many layers.
• In each layer they are not only parallel to each other but also to the corneal plane and become continuous with scleral lamellae at the limbus.
The alternating layers of lamellae are at right angle to each other. Among the lamellae are present keratocytes (modified fibrob lasts), wandering macrophages, histiocytes and a few leucocytes.
4. Pre- Descemet's membrane
It is also known as Dua's layer has been discovered in 2013 by Dr Harminder Dua, a n Indian ophthalmologist working in Britain.
• Located anterior to the Descemec's membrnne, it is about 15 micrometer thick ocular structurie which is ve1y strong and imprevious to air.
5. Descemet's membrane (posterior elastic lamina).
The Descemet's membrane is a strong homogenous basement membrane of the corneal endothelium which is separated from the stroma by pre-Descemet's membrane.
• It is very resistant to chemical agents, trauma and pathological processes. Therefore, 'Descemetocele' can maintain the integrity of eyeball for long.
• Descemet's membrane consists of collagen and glycoproteins. Unlike Bowman's membrane it can regenerate.
• Normally, it remains in a state of tension and when torn, it curls inwards on itself.
• In the peri1Phery it appears to end at the anterior limit of trabecular meshwork as Schwalbe's line (ring).
6. Endothelium.
It consists of a single layer of fl at polygonal (mainly hexagonal) epith elial cells (misnamed as endothelium) which on slit-lamp biomicroscopy appear as a mosaic.
• Endothelium is best examined with the help of specular microscope.
• The cell density of endothelium is around 3000 cclls/ mm2 in young adults, which decreases with the advancing age.
• The human endothelium cells do not proliferate in vivo and the cell loss with age is compensated by enlargement (polymegathism) and migration of neighbouring cells.
• There is a considerable functional reserve for the endotheliwn.
• Therefore, corneal decompensation occurs only after more than 75% of the cells are lost.
• The endothelial cells contain 'active-pump' mechanism which keeps cornea dehydrated.
Blood supply
• Cornea 1is an avascular structure. Small loops derived from the anterior ciliary vessels invade its periphery for about 1 mm.
• Actually, these loops are not inthe cornea but in the subconjunctival tissue which overlaps the cornea.
Nerve supply
Cornea is su pplied by long ciliary nerves which are branches of nasocitiary nerve from ophthalmic division of the 5th cranial nerve.
• After going about 2 mm in cornea the nerves lose their mycli n sh eath and divide dichotomously, and form three plexuses: the stromal. subepithelial and intraepithelial.