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Anatomy of Cornea.pptx

  1. Anatomy of Cornea Optomety Fans
  2. Anatomy of Cornea • Cornea is a transparent, avascular, watch-glass like structure. • It forms anterior one-sixth of the outer fibrous coat of the eyeball.
  3. Dimensions • 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 varies from 0.5 to 0.6 mm while at the periphery it varies from 1 to 1.2 mm. • Radius of curvature. The central 5 mm area of the cornea forms the powerful refracting surface of the eye. The anterior and posterior radii of curvature of this 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 dioptres, which is roughly three-fourth of the total refractive power of the eye (60 dioptres).
  4. Histology Histologically, the cornea consists of six layers. From 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
  5. Cornea Epithelium Bowmans membrane Stroma Dua Layer Descemet Membrane Endothelium
  6. Epithelium • It is of stratified squamous type and becomes continuous with epithelium 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
  7. Bowmans membrane • This layer consists of acellular mass of condensed collagen fibrils. • It is about 12 mm 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.
  8. 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 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 fibroblasts), wandering macrophages, histiocytes and a few leucocytes
  9. Pre- Descement Membrane (Dua Layer) • Pre–Descemet’s membrane, also known as Dua’s layer has been discovered in 2013 by Dr Harminder Dua, an Indian ophthalmologist working in Great Britain. • Located anterior to the Descemet’s membrane, it is about 15 micrometer thick acellular structure which is very strong and imprevious to air
  10. Descemet Membrane • 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 periphery it appears to end at the anterior limit of trabecular meshwork as Schwalbe’s line (ring).
  11. Endothelium • Endothelium. It consists of a single layer of flat polygonal (mainly hexagonal) epithelial cells (misnamed as endothelium) which on slit-lamp biomicroscopy appear as a mosaic. The cell density of endothelium is around 3000 cells/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 endothelium. Therefore, corneal decompensation occurs only after more than 75% of the cells are lost. The endothelial cells contain ‘activepump’ mechanism which keeps cornea dehydrated.
  12. Physiology of cornea The two primary physiological functions of the cornea are (i) To act as a major refracting medium (ii) To protect the intraocular contents. Cornea performs these functions by maintaining its transparency and regular replacement of its tissues
  13. Corneal Transparency • Peculiar arrangement of corneal lamellae • Avascularity of cornea. • Relative state of dehydration (78% water content), which is maintained by the barrier effects of epithelium and endothelium and the active Na+ K+ ATP ase pump of the endothelium. • Swelling pressure (SP) of the stroma which counters the imbibition effect of intraocular pressure (IOP). • Corneal crystallins, i.e, water soluble proteins of keratocytes also contribute to corneal transparency at the cellular level. For these processes, cornea needs some energy.
  14. Metabolism of cornea • Most actively metabolizing layers of the cornea are epithelium and endothelium, the former being 10 times thicker than the latter requires a proportionately larger supply of metabolic substrates.
  15. Source of nutrients for cornea • 1. Solutes (glucose and others) enter the cornea by either simple diffusion or active transport through aqueous humour and by diffusion from the perilimbal capillaries. • 2. Oxygen is derived directly from air through the tear film. This is an active process undertaken by the epithelium. Therefore, corneal hypoxia may occur with over wear of contact lenses. • Some oxygen may reach the superficial layers of cornea from the perilimbal capillaries (especially when eyelids are closed) and deeper corneal layers through the aqueous humour
  16. • Endothelium requires oxygen and glucose for metabolic activities and proper functioning of Na+ - K+ ATPase pump. Like other tissues, the epithelium can metabolize glucose both aerobically and anaerobically into carbon dioxide and water and lactic acid, respectively. Thus, under anaerobic conditions lactic acid accumulates in the cornea
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