anatomy of cornea powerpoint presentatation .pptx

ANATOMY OF CORNEA

1. ANATOMY OF CORNEA PRESENTED BY: DR.MEGHANA.M JR1 OPHTHALMOLGY

2. INTRODUCTION Transparent, avascular structure Forms anterior one sixth of outer fibrous coat of eyeball Principal refracting surface of the eye- forms ¾ th of the total refracting power of the eye Refracting power- Anterior surface +48D ,posterior surface -5D, Net refractive power +43D Refractive index:1.37 Anterior corneal surface- covered in tear film,posterior corneal surface- bathed in aqueous humor

3. DIMENSIONS Anterior surface – elliptical --11.7mm wide in horizontal meridian 10.6mm in vertical meridian Posterior surface- circular-11.7mm diameter Thickness in center 0.52mm and periphery 0.67mm

4. Central third- the optical zone -radius of curvature: anterior surface -7.8 mm posterior surface- 6.5 mm Adult males-Peripheral cornea is more flattened.

5. COMPOSITION OF CORNEA water : 78% Collagen : 15% - type I: 50-55% type III : 1% type IV :8-10% type VI :25-30% other protein : 5% keratan sulphate :0.7% Chondroitin/dermatan sulphate: 0.3% Hyaluronic acid + Salts 1%

6. HISTOLOGY Epithelium; Bowman's layer Stroma Dua’s Layer Descemet's membrane; Endothelium.

7. EPITHELIUM Stratified, squamous and non-keratinized. Continuous with that of the conjunctiva at the corneal limbus,but no goblet cells. 50-60 µm thick (10% of cornea) 5 to 6 layers of nucleated cells- basal cells(Deepest) Wing cells/umbrella cells Superficial cells

8. Basal layer Tall columnar polygonal shaped cells Arranged – palisading like manner on basement epithelium form the germinative layer of the epithelium, continuous peripherally with that of the limbus Basal cells- oval nucleus, cytoplasm contains few organelles Mitochondria small and few => low aerobic oxidation and more dependance on pentose shunt for metabolism

9. Cells joined laterally to other basal cells by desmosomes Attached to basal lamina by hemidesmosomes Anteriorly to wing cells by zona occludens Anchoring filaments pass through hemidesmosome and inserted to basal lamina

10. Epithelial stem cells : Basal cells of limbal area which are undifferentiated pluripotent cells stem cells found in limbal basal epithelium of palisades of vogt They are source of new corneal epithelium CLINICAL APPLICATION : Diffuse damage to limbal stem cells – chronic epithelial surface defects and invasion of conjunctival epithelium on to cornea

11. BASAL LAMINA Basement membrane of basal cells –PAS positive structure Extracellular secretory product of basal epithelial cells Irregular zone of granuloamorphous material Composed of type VII collagen and glycoprotein Ultrastructurally - deep lamina densa and superficial lamina lucida Becomes thick with age, DM, corneal pathology etc

12. WING CELLS 2-3 Layers of polyhedral shaped cells Flattened nuclei and parallel to surface Cytoplasmic organelles- lesser than basal cells Attached to basal cells posteriorly and other wing cells laterally and anteriorly- via tight junctions

13. FLATTENED CELLS 2 to 3 layers of polyhedral cells Wider and increasingly flattened towards surface Flattened nucleus projects backwards leaving surface smooth Most superficial cells exhibit microvilli or microplicae coated with glycocalyx (associated with tear film stability) Zonulae occludentes + Desmosomes and macula occludents numerous Juctional complexes formed with laterally adjacent cells maintain the barrier function of epithelium

14. Confocal biomicroscopy of the human cornea. (A–C) Superficial, wing, and basal cell layers of the corneal epithelium

15. Transmission electron microscopy of the human corneal epithelium. (A) The epithelium comprises five or six layers of epithelial cells. The electron-dense cell is about to undergo desquamation. (B) Basal cells. Note the numerous junctional complexes. (C) Basement membrane and anterior portion of Bowman's layer. Note hemidesmosomes at thebasal surface of the epithelial cells . (D) Interdigitation and junctional complexes at the lateralsurface of basal epithelial cells. (E) Gap junction at the lateral surface of basal cells.

16. FUNCTIONS OF EPITHELIUM It is extraordinary regular in thickness with smooth wet apical surface serving as major refractive surface of eye Major surface to respond to wound healing Provide barrier to fluid loss and pathological entrance to organisms

17. BOWMANS MEMBRANE Bowman's layer is a narrow, acellular homogeneous zone immediately subjacent to the basal lamina of the cornea epithelium 8-14 µm thick Condensed superficial part of the stroma Composed of - type I and type V collagen fibres in a matrix consisting of proteoglycans and glycoproteins. Relatively resistant to trauma and infection Once destroyed- replaced by coarse scar tissue

18. STROMA 500 µm thick and constitute 90% of total cornea consists of regularly arranged lamellae of collagen bundles (200-300centrally and 500 in the periphery)

19. CORNEAL LAMELLAE The lamellae consist of fibrils with a macroperiodicity (640A) typical of collagen. The stroma’s collagen types are I, III,V and VI - type I predominates Type VII forms the anchoring fibril of the epithelium. Arranged in many layers- each layer parallel to each other and also to corneal plane – becomes continuous with scleral lamellae at the limbus Oblique orientation at anterior one third of stroma Alternating layer of lamellae are right angles to each other – posterior 2/3rd

20. At periphery fibrils adopt concentric configuration to form a weave at the limbus. This imparts considerable strength to peripheral corneal and maintain its curvature and thus its optical property CLINICAL APPLICATION : Parallel arrangement of lamellae allows easy interlamellar dissection during superficial keratectomy and lamellar keratoplasty

21. Stromal cells Corneal keratocytes : fibroblasts which are found throughout the stroma Flattened cell body, large eccentric nucleus and long branching processes Produce ground substance and collagen fibrils during embryogenesis and after injury Wandering cells of stroma migrate from marginal loops of corneal blood vessels to site of injury

22. Ground substance of stroma Ground substance of cornea consists of hydrated matrix of proteoglycans. The primary glycosaminoglycans of stroma are keratin sulphate and chondroitin sulphate in the ratio of 3:1. Maximum concentration of keratin sulphate occurs in the centre and chondroitin sulphate in the periphery. The glycosaminoglycan components ( e.g.keratin sulphate) of the ground substance are highly charged - account for the swelling property of the stroma.

23. DUA’S LAYER/PRE DESCEMETS LAYER discovered in 2013 by Dr. Harminder Dua. Located anterior to Descemet membrane, it is about 15 mm thick acellular structure which is very strong and impervious to air. primarily composed of collagen type 1 Proteoglycans present are lumican , mimecan and decorin .

24. DESCEMET’S MEMBRANE/ POSTERIOR ELASTIC LAMINA It is a strong homogenous layer which is separated from the stroma by preDescemet’s membrane It is made up of collagen and glycoprotein thickness varies with age, being 3 μm at birth and 10–12 μm in young adults. very resistant to chemical agents, trauma, infection and pathological processes. it can regenerate. Normally, - 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 the trabecular meshwork as Schwalbe’s line (ring)

25. On electron microscopy, divided into two distinct regions: - An anterior one-third having a vertically banded pattern and the posterior two-thirds appearing amorphous and granular. The posterior surface of the Descemet’s membrane, at the periphery, shows rounded wart-like excrescences called Hassel Henle bodies , which increase with advancing age. Similar central excrescences, known as guttatae , are seen with advancing age in Fuch’s dystrophy

26. ENDOTHELIUM Single layer of hexagonal, non replicating cells Cell density at birth:- 6000/mm³ Falls by 26% in 1st year Further 26% lost in next 11 year (about 2400-3000 cells/mm³ in young adults) Rate slows and stabilizes in mid-age Defect left by dying cells is filled by enlargement of remaining cells – Polymegathism

27. There is considerable functional reserve for endothelium (corneal decompensation occurs only after >75% of adult age cells are lost i.e cell count <500cells/mm³ ) At 500 cells/mm³ corneal edema develops and transparency impaired Cells of endothelium bound together by tight interdigitating junctional complexes Attached to DM by hemidesmosomes Endothelial cells maintain corneal transparency throughout life by pumping excess fluid out of stroma (Na+/K+ ATPase pump system) Abundant mitochondria and other organelles

28. BLOOD SUPPLY OF THE CORNEA Avascular structure Anterior ciliary artery (from ophthalmic artery) forms vascular arcade in limbal region that anastomoses with vessels from facial branch of external carotid artery These loops are in the subconjunctival tissue which overlap the cornea and do not invade them

29. NERVE SUPPLY OF CORNEA

30. Subepithelial plexus Intraepithelial plexus forms 3 nerve plexus Nerve fibres pass anteriorly Penetrate the pores in bowman’s membrane, lose their schwann’s sheath, divides into filaments under basal layer of epithelium which extends between the cells of all the layers of epithelium and forms ENDS IN MID STROMA Stromal plexus

31. FUNCTIONS OF CORNEA powerful refracting surface that transmits light in an orderly manner for proper image formation Protect intraocular contents and maintain structural integrity of globe Absorption of topically applied drugs Wound repair after anterior segment surgery or trauma

32. This Photo by Unknown Author is licensed under CC BY