cornea and layers
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The cornea is a transparent, avascular structure that forms part of the outer fibrous coat of the eye and serves as the major refracting surface. It consists of multiple layers, including epithelium, Bowman's membrane, stroma, Dua's layer, Descemet's membrane, and endothelium, each with distinct functions and characteristics. Blood supply is peripheral and minimal, while the cornea has a rich nerve supply that enhances sensitivity.
cornea and layers
- 3. Introduction Transparent,avascular,watch-glass like structure. Forms 1/6th part of outer fibrous coat (Sclera) It is the major refracting surface of the eye
- 4. Dimensions + Avg horizontal dia =11.75 mm (ant surface) + Avg vertical dia = 11 mm (ant surface ) + Avg dia (post surface)= 11.5 mm + Thickness(centre) =0.52mm + Thickness(peripheral) = 0.67mm + Radius of curvature (ant surface) = 7.8mm + Radius of curvature (post surface)= 6.5mm + Refractive power (ant surface) = +48D + Refractive power(post surface)= - 5D + R.I = 1.376
- 5. Histology + Epithelium + Bowman’s membrane + Stroma + Dua’s layer + Descemet’s membrane + Endothelium
- 6. Epithelium + Made up of stratified squamous epithelium + Thickness - 50-90 um + 5-6 layers of cells + Regenerative, entire epithelial layer is replaced every 6-8 days + Made up of 3 types of cells - basal,wing, flattened cells + Cells are attached by to each other by means of desmosomes & maculi occludents
- 7. Bowman’s membrane + Acellular,Non regenerative + Made up of condensed collagen fibrils. + Thickness - 12um + Resistant to infection & injury.
- 8. Stroma + Thickness: 0.5mm (90% of corneal thickness) + Consists of collagen fibrils ( type I & type V fibrillae interwined with filaments of type VI collagen ) embedded in hydrated matrix of proteoglycans + Lamallae are arranged in 200-250 layers + Oblique arrangement of lamallae - Ant. 1/3rd part of stroma + Right angle arrangement of lamallae - Post. 2/3rd part of stroma + Parallel arrangement of lamallae - periphery + Consists keratocytes, macrophages,histiocytes, leucocytes
- 9. Dua’s layer + It is present between stroma & descemet’s membrane + Tough, well - defined ,acellular layer + Thickness - 10 -15 um
- 10. Descemet’ membrane + Also known as posterior elastic lamallae + Regenerative,elastic,strong homogenous basement membrane of corneal Endothelium , bounds stroma posteriorly. + Very resistant to chemical agents,trauma & pathological processes. + Made up of collagen & glycoproteins + Thickness - 3um (birth) + Thickness- 10-12 um (young adults) + Maintains integrity of eyeball for long, even in absence of stroma
- 11. Endothelium + Consists of single layer of hexagonal cells fitting like mosaic tiles. + Cell density - 3000 cells/mm sq in young adults,which decreases with advancing age + It is involved in active secretion & energy production + Contains large number of mitochondria + In eye ,next to photoreceptors ,endothelial cells contain highest no of mitochondria + The cells have ‘active pump ‘mechanism + Corneal decomposition occurs only after more than 75% cells of endothelium are lost.
- 12. Blood supply + Cornea is avascular + Small loops derived from the anterior ciliary vessels invade its periphery for about 1 mm. + Loops of vessels are present in the sub - conjunctival tissues.
- 13. Nerve supply + Anterior ciliary nerves (branch of opthalmic nerve ) + After going 2 mm in cornea, the nerves lose their myelin sheath and divide dichotomously & form 3 plexuses - stromal,sub epithelial & intraepithelial