Anterior Segment of the Eye PDF

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Summary

Outer Coat – consists of: 1. Cornea – together with the sclera, forms the tough fibrous outer coat which encloses the ocular tissues and protects the eye from injury. Also provides the 2/3 of the refractive power of the eye - curved, transparent and the air-tear interface provides a refractive surfa...

Description

Outer Coat – consists of: 1. Cornea – together with the sclera, forms the tough fibrous outer coat which encloses the ocular tissues and protects the eye from injury. Also provides the 2/3 of the refractive power of the eye - curved, transparent and the air-tear interface provides a refractive surface for good optical quality. Gross Anatomy  Elliptical – when viewed from in front – long axis in the horizontal meridian – asymmetry produced by a greater degree of overlap of the peripheral cornea by opaque limbal tissue in the vertical meridian.  Due to difference in curvature between its posterior and anterior surfaces – cornea shows a regional variation in thickness – centrally the thickness is 0.52mm increasing towards the periphery to 0.67mm. Microscopic Anatomy  Epithelium – approx. 10% of the thickness of cornea (50µm). o Stratified squamous non-keratinised epithelium – consisting of 6 layers of cells: o Single row of basal cells – columnar cells – plasma membrane shows infolding and the cytoplasm contains prominent intermediate filaments. Cell junction: desmosomes (mediate adhesion between cells), hemi-desmosomes (involved in attachment of basal cells to the underlying stroma) and gap junctions (allow for inter-cellular metabolic coupling). o 2-3 rows of wing cells – polygonal – lateral extensions and a concave inferior surface to accommodate the apices of the basal cells. Prominent infolding which interdigitate with adjacent cells and numerous desmosomes – results in a strong intercellular adhesion. o 2-3 layers of superficial (squamous) cells – extensive finger-like and ridge-like projections (microvilli and microplicae) – show an extensive filamentous covering known as glycocalyx – important for spreading and attachment of the pre-corneal tear film. Complex network of tight junctions links the superficial cells.

o Non-epithelial cells – lymphocytes, macrophages and Langerhans cells. o Epithelium forms a permeability barrier to small molecules, water and ions and an effective barrier to entry of pathogens.  Bowman’s Membrane – anterior limiting membrane – 8-14µm. o Composed of randomly orientated array of fine collagen fibrils – merge with the fibrils of the anterior stroma.  Stroma - 500µm – accounts for 90% of thickness of cornea. o Composed of collagen fibrils embedded in a highly hydrated matrix of glycoproteins and proteoglycans. o Type I collagen – major stromal fibril-forming collagen. Lesser amounts of type III and V. o Collagen fibrils arranged in 200-250 layers (lamellae) running parallel to the surface – lamellae, approx. 2µm thick – extend from limbus to limbus. o Collagen fibrils of the central and mid-peripheral cornea have a preference for a superior-inferior and medial-lateral orientation, however at the limbus, fibrils form a well-defined annulus. o Narrow fibril diameter and constant separation – characteristic of corneal collagen, is a pre-requisite for transparency. o Proteoglycans – keratan sulphate and mixed dermatan/chondroitin sulphate – major corneal proteoglycans – play a role in corneal hydration. o Collagen and proteoglycans – maintained by keratocytes – cells occupy 3-5% of stromal volume and lie between collagen lamellae. Density decreases from superficial to deep stroma and increases from the centre to periphery.  Descemet’s Membrane – posterior limiting membrane – basement membrane of endothelium. o At birth, it’s 3-4µm – increases to a thickness of 10-12µm in adults. o Anterior 1/3rd represents that part produced in foetal life and is characterised by a periodic banded pattern. o Posterior 2/3rd is formed postnatally – has a more homogenous granular appearance.  Endothelium – monolayer of squamous cells that lines the posterior surface of the cornea. o Reduction in endothelial cells with age – due to limited capacity for mitosis to replace damaged cells.

o In tangential section, it appears as a mosaic of polygonal cells – in response to trauma, age, and prolonged contact lens wear – mosaic becomes less regular, shows greater variation in cell size and shape, as cells spread to fill gaps caused by cell loss. o Under electron microscope – lateral borders of the cells are markedly convoluted and adjacent cells are linked by tight junctions. Corneal innervation  Richly innervated surface tissue in the body – receives its predominantly sensory nerve supply from the nasociliary branch of the trigeminal nerve.  Nerves for cornea travel in the suprachoroidal space, before crossing the sclera to advance radially towards the cornea.  Unmyelinated nerve fibre bundles divide repeatedly and move anteriorly to form a rich plexiform network in the anterior third of the stroma.  Individual axons penetrate Bowman’s membrane and terminate in the epithelium. Corneal metabolism  Cornea requires a constant supply of oxygen and other essential metabolites (glucose, amino acids and vitamins) – its avascularity dictates that alternative routes exist for its metabolic needs: o From perilimbal vasculature – provision of oxygen and nutrients would appear limited – only significant for corneal periphery. o Tear film – open eye, oxygen is obtained from the atmosphere via diffusion across the pre-corneal tear film. Under steady state conditions – tears are saturated with oxygen – oxygen tension corresponding to the atmosphere (155mmHg at sea level). During eye closure – oxygen level in tears is in equilibrium with the palpebral vasculature. 5% increase in corneal thickness during sleep – returns to baseline levels within one hour f eye opening – related to tear film tonicity due to reduced tear evaporation rather than reduced oxygen availability. During the open eye, tears evaporate – creating a slight tear hypertonicity and subsequent corneal dehydration.

Low glucose concentration is low in tears – glycogen stores present in all corneal cells to provide glucose during conditions of metabolic stress. o Aqueous humour – oxygen tension here lies between 2080mmHg. Primary source for glucose and amino acids. Consumption rates of oxygen = 40:39:21 for epithelium, stroma and endothelium respectively. Reduced oxygen availability is associated with measurable and observable changes in corneal function – 10% of oxygen concentration is required to avoid oedema. Oxidative metabolism  Glucose is the main source of energy for the cornea – it’s a primary substrate for the generation of ATP – catabolised by: o Anaerobic Glycolysis – glucose oxidised to pyruvate and reduced to lactate (with a yield of 2 ATP molecules). Accumulation of lactate can result in stromal oedema via increased osmotic load. o Hexose monophosphate shunt – accounts for 35-65% of glucose utilisation. o Aerobic glycolysis (Krebs cycle) – results in 36 ATP – most active in the corneal endothelium – has the greatest energy requirement. Corneal Transparency  Cornea is highly transparent – transmitting more than 90% of incident light – it’s a typical CT consisting of a matric of collagen and proteoglycans – favours light scatter with consequent loss of transparency.  Collagen fibrils of the stroma disposed in a regular crystalline lattice – light scattered by fibrils is eliminated by destructive interference in all directions.  Disruption of short-range order between fibrils will lead to increases scatter and a loss of transparency.  Proteoglycans – keratan sulphate isoforms in the cornea – lumican (essential for transparency), keratocan and mimecan.

Hydration control

 Corneal hydration is an important determinant of corneal transparency – hydrophilic properties of the stroma are to a large part determined by proteoglycans that contribute to the fixed negative charge of the stroma.  Hydration is maintained at approx. 78% - if the cornea is allowed to swell  5% of this value, it begins to scatter significant quantities of light.  Maintenance of corneal hydration – dependent on the corneal endothelium – possesses both barrier property and a metabolically driven pump.  Barrier formed by focal tight junctions between adjacent cells – low electric resistance and allow passage of ions and small molecules.  Stromal swelling pressure is the driving force for water to “leak” across the epithelial and endothelial barrier layers – the leak is counterbalanced by pump mechanisms which reside in the epithelium and endothelium. The endothelium accounts for at least 90% of the pumping activity of the cornea.  A flux of bicarbonate ions is the predominant component of the endothelial ion transport system.  Hydration control – tight junctions between superficial epithelial cells form an effective permeability barrier to ions and polar solutes as well as active ion transport systems for Na+ and Cl-. Oedema  When corneas swell – light scattering increases with an ensued transparency loss due to the disruption of the regular collagen matrix.  Oedematous corneas show fibril aggregation with the result that large areas are devoid of collagen fibrils – occurs as a result of loss of GAGs.  Can result from also: o Retardation of carbon dioxide efflux. o Stromal lactate accumulation. o Breach of epithelial or endothelial barriers. o Inhibition of ion pumps. o Influx of water increases the separation between collagen fibrils causing increased light scatter. Corneal epithelial wound healing  Smooth and intact corneal epithelium is necessary – maintain clear vision.  Corneal epithelial repair involves, initial covering of the denuded area by cell migration, cell proliferation to replace lost cells and epithelial differentiation to reform the normal stratified epithelial architecture.

 Regeneration of the corneal epithelium is highly dependent on the integrity of the limbus.

2. Sclera  Forms the largest part of the fibrous coat of the globe.  Thickets posteriorly (1mm) and decreases gradually towards the equator to reach a minimum of 0.3mm under the tendons of the rectus muscles – from rectus insertions, it increases in thickness to 0.8mm before it blends with the cornea.  Contains a number of foramina for: o Optic nerve o Short posterior ciliary arteries and nerves o Long ciliary nerves o Vortex veins o Anterior ciliary arteries  Can be resolved into: o Episclera – loose vascular CT layer – lies beneath Tenon’s capsule (Fascia Bulbi – envelopes the globe from the limbus to the margins of the optic nerve) – to which is attached to fibrous bands. Connects sclera to the conjunctiva – highly vascular. o Scleral stroma – composed of densely packed collagen embedded in a ma5rix of proteoglycans – fibrils show a large variation in diameter and spacing and lamellar branch and interlace extensively = increased light scatter that is responsible for the opaque dull white appearance of the sclera. o Lamina fusca – pigmented layer located at the inner aspect of the sclera at the interface with the choroid.  The sclera is less hydrated than the cornea – shows smaller proportion of proteoglycans.  Largely avascular – nerve supply is rich....