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Anatomy & physiology of cornea

The cornea has five layers - epithelium, Bowman's layer, stroma, Descemet's membrane, and endothelium. The stroma makes up around 90% of the cornea thickness and contains collagen fibrils and keratocytes. The stroma's collagen is arranged in tightly packed, regularly spaced lamellae that run parallel to the corneal surface. Keratocytes maintain the stroma by synthesizing collagen and proteoglycans. The epithelium is stratified and squamous, consisting of basal, wing, and surface cells. It regenerates through cell migration from the limbus. Injury repair involves cell migration, proliferation, and remodeling.

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DEPT. OF OPTHALMOLOGY SHER-E-BANGLA MEDICAL COLLEGE HOSPITAL, BARISAL. DR. MD. NURUL ISLAM DO STUDENT SESSION – JULY, 2013 23-12-2014 sbj
The Cornea The cornea is a transparent avascular tissue with a smooth, convex outer surface and concave inner surface, which resembles a small watch-glass. To meet the diverse functional demands the cornea must be: - Transparent - Refract light - Contain the intraocular pressure - Provide a protective interface sbj
Dimensions Anterior surface: - Elliptical, about 11.7 mm horizontally and 10.6 mm virtically Posterior surface: - Circular, about 11.7 mm in diameter sbj
Contd… Thickness: - Centrally about 0.52 mm - Peripherally about 0.67 mm Surface area: - About 1.3 cm² (one-sixth of the globe) Optical zone: - Cornea is almost a sphere, the central 1/3rd is called optical zone about 5.4 mm sbj
Contd… Radius of curvature: - Anterior surface – about 7.8 mm - Post. Surface – about 6.5 mm Refractive power: +43.1 D (Air-tear = +43.6 D, Tear-cornea = +5.3 D, Cornea-aqueous = -5.8 D) Refractive index: 1.376 sbj
Contd… Topography: - Shape of cornea is important for fitting of contact lenses - Small spherical zone of ant. curvature (2-4 mm) is decentered up & outwards with visual axis, but correctly centered for pupillary aperture is termed as corneal apex or cap - Curvature varies from apex to limbus, greater flattening in nasally and superiorly - Cornea is flatter in men than in women - Cornea flattens slightly on convergence sbj
Composition of human cornea Water: 78 % Collagen: 15 % of which: Type-I : 50-55 % Type-III : 1 % Type-IV : 8-10 % Type-VI : 25-30 % Other protein: 5 % Keratan sulphate: 0.7 % Condroitin/dermatan sulphate: 0.3 % Hyaluronic acid: + Salts: 1 % sbj
Embryonic origin of Cornea: The formation of cornea is induced by the lens and the optic cup at the 7th weeks ofintrauterine life. • Corneal epithelium – Surface ectoderm • Bowman’s membrane – Mesenchyme • Stroma – Mesenchyme and Neural crest • Descemet’s membrane – Synthesized by endothelium • Endothelium – Neural crest sbj
Structure Behind the precorneal tear film there are five layers of cornea: 1. Epithelium 2. Bowman’s layer 3. Stroma 4. Descemet’s membrane 5. Endothelium sbj
Epithelium: The corneal epithelium is - - Stratified, squamous and non- keratinized - Continuous with conjuctival epithelium at limbus but having no goblet cells - 50-90 µm in thickness - Consists of 5 or 6 layers of nucleated cells resting on a basal lamina, namely- a. Basal cells b. Wing cells c. Surface cells sbj
Basal cells: - Deepest cell layer - Stand in a palisade manner on basal lamina - Germinative layer of the epithelium - Columner with rounded heads and flat bases - Nucleus is oval and oriented parallel to the cells long axis Contd… sbj
Wing or umbrella cells: - Second epithelial cell layer (1-2 layers of cells) - Polyhedral cells - Convex anteriorly forming cap over basal cells and send processes between them - Nucleus is oval and oriented parallel to corneal surface Contd… sbj
Surface cells: - Most superficial 2-3 layers of cells - Also polyhedral and become wider & flattened towards the surface - Flattened nuclei project backwards leaving the surface perfectly smooth Contd… - Most superficial cells are mostly hexagonal in shape and exhibit surface microvilli or microplicae sbj
Ultrastructural features: - Epithelial cells shows usual organelles like other actively metabolizing cells - Moderately abundant mitochondria in wings & middle layer cells but small and scarce in basal cells - Wing & superficial cells have high glycogen content Contd… sbj
- Tonofibrils: cells contain a cytoplasmic meshwork of electrondense intermidiate filaments composed of cytokeratins - The plasma membrane of contiguous cells interdigitative to each other - Adhesion is achieved by – • Tight junctions & desmosomes – surface cells • Desmosomes – wings & superficial cells • Desmosomes & Hemidesmosomes – in basal cells Contd… sbj
Contd… sbj
- In the basal cells there are anchoring filaments which pass through the hemidesmosomal structure to be inserted into basal lamina - Langerhans cells (cells of immune recognition system) present near periphery. They are almost absent at central cornea but aggregate in response to infection Contd… sbj
Basal lamina: - The basal lamina is secreted by the basal cells - 0.5 - 1 µm wide - Ultrastructurally it is distinguished in to two patrs – i. Lamina lucida (superficial) ii. Lamina densa (deep osmiophilc) Contd… sbj
- The lamina consist of collagen and glycoprotein constituents which integrated with Bowman’s layer by array of short anchoring filaments - Lipid solvent, stromal oedema and inflamation may loosened the cohesion between Bowman’s zone and lamina - With old age, in diabetes and in some corneal disorders it becomes thickened and multilamellar Contd… sbj
Epithelial Turnover: - Early studies suggested that the epithelium replaced approximately weekly by division of basal cells and the oldest shed from the surface - It is now recognized that the germinative region lies at the limbus, the stem cells, and cells migrate at a very slower rate (123 µm/week) to the center of the cornea which may be as long as a year Contd… sbj
- The XYZ hypothesis: 1. Thoft R. and Friend J. (1983) proposed on the basis of experimental evidence that both limbal basal and corneal basal cells are the source for corneal epithelial cells, and there is a balance among division, migration & shedding. Contd… sbj
The corneal epithelium is maintained by a balance among sloughing (Z) of cells from the corneal surface, cell division (X) in the basal layer and renewal of basal cells by centripetal migration (Y) of new basal cells originating from the limbal stem cells. Contd… sbj
Epithelial Repair: - Repair of corneal epithelial injury like abration follows a distinctive sequence of events - Injury (abration) Cells at wound edge retract, thicken and lose attachment Travel in an amoeboid movement to cover the defect Cells at wound edge ruffle and send out filopodia and lamellipodia towards the center of wound Contd… sbj
Migration process is halted by contact inhibition They then anchor and Mitosis resumes to re-establish epithelial thickness Surface tight junctions re-establised Adhesion with Bowman’s layer within 7 days (if basal lamina intact) • The healing process occurs rapidly, rate of cell migration is 60 – 80 µm/hr Contd… sbj
- In case of total epithelial loss including total limbus, cornea is covered with vascularized conjunctival type of epithelium by adjacent conjunctiva - If a small part of limbus with stem cell is retained then conjunctival type of epithelium is gradually disappear and metabolic behavior of corneal epithelium re-established very slowly Contd… sbj
Bowman’s layer: (Ant. Limiting lamina) - Modified region of anterior stroma - Acellular homogeneous zone - 8 – 14 µm thick - Ant. surface is smooth & parallel with corneal surface - It delineates the anterior junction between cornea and limbus sbj
Ultrastructural features: - Ultrastructurally it is a felted meshwork of fine collagen fibrils of uniform size in a ground substance - Posteriorly it becomes blended & interweaving with fibrils of ant. stroma - Compact arrangement of collagen gives it great strength and relatively resistant to trauma both mechanical and infective Contd… sbj
- Convex ridges may generate over surface if its tension is relaxed during indentation, hypotony or manipulation causes ant.corneal mosaic, polygonal or chicken-wire pattern over surface - No regeneration and replaced by coarse scar tissue - It is perforated many places by nerve to epithelium Contd… Fig.: Ant.corneal mosaic sbj
Stroma: (Substantia propria) - About 500 µm thick (about 90% of corneal thickness) - Consists of regularly arranged lamellae of collagen bundles, lie in proteoglycan ground substance with – - 200 – 300 bundles – centrally - 500 bundles – peripherally - Width about 9 – 260 µm - Height about 1.15 – 2 µm - Small population of cells – keratocytes present sbj
- Arrangement of lamellae – - Lamellae are arranged in layers, parallel with each other & with corneal surface - In deeper stroma the lamellae form strap-like ribbons which run approximately at right angles to those in consecutive layers - At the periphery this right-angular arrangement is slightly changed where it gets scleral fibres - At the limbus the bundles appeared to take a circular course Contd… sbj
Contd… Fig.: Arrangement of stromal lamellae sbj
Ultrastructural features: - Each lamellae comprises of a band of collgen fibrils arranged in parallel with each other - Fibrils show typical 64 nm periodicity of connective tissue collgens with a microperiod of 6 nm - There is a unique uniformity of fibril diameter, it is 22 (±1) nm from ant. to post. - There is remarkable regularity of seperation both within and between lamellae Contd… sbj
- The keratocytes occupy 2.5 – 5 % of total stromal volume and is responsible for synthesis and maintaining of collagen & proteoglycan substance of stoma - Keratocytes: - Long, thin, flattened cells (maximally 2µm thick) running parallel to corneal surface - Having long flattened nuclei, sparse cytoplasm but contains full component of organells - Position – between the lamellae Contd… sbj
- There stellate processes extened for great distance and frequent contacts are made with other keratocytes in same horizontal plane forming gap junctions - Lymphocytes, macrophages and polymorphonuclear leucocytes (very rarely) also found in stroma ocationally Contd… sbj
Stromal repair: - Repair of stroma after small injuries involves: - Keratocytes activation Migration & transformation into fibroblasts Production of scar tissue Initial fibrils are large & irregular Contd… sbj
Remodelling of scar tissue occurs, it ensues – 1. Thinning of fibrils 2. Reformation of lamellae over months 3. Increase in tranperency - Larger wounds provoke rapid vascular response and leaving vascularised scar along with lymphatic channels Contd… sbj
Descemet’s membrane: (Post. Limiting layer) - It is the basal lamina of corneal endothelium - First appears at 2nd month of gestation and synthesis continue throughout adult life Thickness – at birth :- 3 – 4 µm at childhood :- about 5 µm at adult :- 10 – 12 µm - There is a distinct structural difference between fetal & postnatal components sbj
- It is a strong resistant sheet - It thickens with age and in some corneal degenerative conditions - Major protein of DM is Type IV collagen Contd… sbj
Ultrastructural features: - In adult human, the anterior 1/3rd of DM corresponds to fetal part, which is like a laminated structure and shows an irregular banded pattern in cross section - In tangential section it appears to consist of superimposed plates forming a lamellar pattern Contd… sbj
- Posterior 2/3rd formed after birth and consist of a homogeneous fibrogranular material - The zone adjoining the endothelium is the most recently formed - Modified hemidesmosomes attachment present in between DM & endothelium Contd… Descemet’s membrane sbj
Hassal-Henle warts: - It is the peripheral excrescence produced by focal overproduction of basal lamina like material in aging cornea - No clinical abnormality in corneal function Descemet’s warts of central cornea is called Cornea Guttata, it is associated with increased permiability of endothelium Contd… sbj
Contd… Fig.: Cornea Guttata in Fuchs’ dystrophy sbj
Posterior embryotoxon: • Schwalbe’s line may hypertrophied in congenital anomalies and appears as visible shelf on gonioscopy, is called posterior embryotoxon Contd… Schwalbe’s line: • The peripheral rim of DM is the internal landmark of corneal limbus and also it is the anterior limit of drainage angle, is called Schwalbe’ line sbj
Repair of Descemet’s layer: After traumatic interuption of DM (Path./Mech.) Endothelium spread its cells to resurface the defect Synthesis of fresh basal lamina which is structurally identical to normal descemet’s layer Contd… sbj
Endothelium: - It is a single layer of hexagonal, cuboidal cells attached posterior aspect of DM - It is nuroectodermal in origin - Corneal endothelial cells production is relatively fixed, it is about 500000 sbj
- Endothelial cells density – - about 6000 cells/mm² at birth - 26% lost in 1st year - Further 26% lost over next 11 years - Rate of cell loss slows and stabilizes around middle age and then it is about 2500 cells/mm² - If cells density falls upto 500 cells/mm² corneal oedema devlops and transparency reduced Contd… Endothelium of a healthy cornea Endothelium of a rigid contact lens wearersbj
- At birth cells are 10 µm in height, with age it becomes flattened to 3- 5 µm and 18-20 µm width - Single oval nucleus located centrally - Cells shape is hexagonal in youth with age it become polymorphic Contd… sbj
Ultrastructural features: - The anterior cell membrane (Basal) is attached with DM by modified hemidesmosomes - The posterior cell membrane (Apical) facing Anterior chamber shows 20-30 microvilli - Lateral borders produce a complex interdigitation with neighboring cells Contd… Fig.: 3-D view of deep cornea showing part of endothelium, DM, Stroma sbj
- Cell junctions with surrounding cells at lateral surfaces – - Ant. 2/3rd – maculae adharentes - Post. 1/3rd & apicolateral edges – macculae occludentes - Endothelium is rich in subcellular organeles – large number of mitochondria, both rough and smooth endoplasmic reticulum, free ribozomes, these reflects that endothelium is extremely active metabolically Contd… sbj
Nutrition to endothelium: - Endothelium gets its nutrition & O₂ from aqueous - Essential nutrients (such as glucose & amino acids) pass across its surface to supply the cellular needs of all the corneal layers Contd… sbj
Fluid regulation: - The state of relative deturgescence of stroma is maintained by this delicate monolayer of cells by two ways – - Providing a barrier function to the ingress of salt and metabolites to the stroma - Actively reducing the osmotic pressure of stroma by metabolically pumping the bicarbonate ions out of the stroma to aqueous Contd… sbj
Endothelial Repair: - Physical & chemical damage to endothelium results in loss of cells - Neighboring cells move over to fill the gap by sliding process and enlargement of cells occur - Thus, after injury, the endothelial cell density falls, the cell area increases and the cell height decreases Contd… sbj
Blood supply of Cornea: - The cornea is an avascular structure - Small loops derived from the anterior ciliary vessels invade its periphery for about 1 mm. - Actually, these loops are not in cornea but in the subconjunctival tissue which overlaps the cornea. sbj
Nerve supply of Cornea: - Cornea is rich in sensory nerve supply derived from ophthalmic division of trigeminal nerve via anterior ciliary nerves and nerves to the surrounding conjunctiva – Ant.ciliary nerve enter the pericoroidal space a short distance behind the limbus Connect with each other & conjunctival nerve and form pericorneal plexus 60-80 myelinated branches pass into cornea sbj
After 1-2 mm lose myelin sheaths and divide into anterior and posterior groups Anterior nerves (40-50) pass through stroma and form plexus subjacent to Bowman’s layer Nerve fibres then penetrate Bowman’s layer and form subepithelial plexus Fibres then divide dichotomously to form a parallel network which run for upto 2 mm Contd… sbj
And give rise to fine free nerve terminals to superficial epithelial layers The posterior groups of nerves (40-50) pass posteriorly to innervate the posterior stroma excluding Descemet’s membrane Contd… sbj
Sentinel of the eye: • The adrenergic fibres from cervical sympathetic, supply the limbus also supply almost whole of the eye and its appendages, giving warning of injury for instance by a foreign body, its called ‘sentinel of eye’ Contd… sbj
Corneal Nutrition & Metabolism - Cornea requires energy for normal metabolic activities as well as for maintaining transparency and dehydration - Energy is generated by the breakdown of glucose in the form of ATP - Most actively metabolizing layers are epithelium & endothelium sbj
Sources of Nutrients: - Oxygen – mainly from atmosphere through tear film, with minor amounts supplied by the aqueous and limbal vasculature - Normal Po₂ in tears is 155 mm Hg - In aqueous is about 40 mm Hg - Minimum 25 mm Hg Po₂ is needed for maintaining deturgescent state and transparency Contd… sbj
- Glucose, amino acid, vitamins, and other nutrients supplied to cornea by aqueous humor, a lesser amounts from tears or limbal vessels - Glucose also derived from glycogen stores in corneal epithelium - Epithelium consumes O₂ 10 times faster than stroma Contd… sbj
Metabolic pathways: - Three processes or pathways – 1. Penntose shunt (Hexose monophosphate shunt) – occurs both In hypoxic and normoxic condition - Forms NADPH and Pentose (Ribose 5-P) from gulcose which are used in nucleic acid synthesis 2. Glycolysis (Embden meyerhof pathway) – anaerobic process, glucose/glycogen converted to pyruvate yelding 2 ATPs 3. TCA or Krebs or citric acid cycle – in aerobic conditions pyruvate is oxidized to yield 36 ATP, water, CO₂ Contd… sbj
Contd… Fig.: Metabolism of Glucose in cornea sbj
Corneal Transparency The cornea transmits nearly 100% of the light that enters it. Transparency achieved by – 1. Arrangement of stromal lamellae Two theories – i) Maurice (1957): The transparency of the stroma is due to the lattice arrangement of collagen fibrils. He explained, because of their small diameter and regularity of separation, back scattered light would be almost completely suppressed by destructive interference sbj
ii) Goldman et al. (1968): He suggest, a perfect crystalline lattice periodicity is not always necessary for sufficient destructive interference. He explained, if fibril separation and diameter is less than a third of the wavelength of incident light, then almost perfect transparency will ensue. This is the situation which obtains in normal cornea. Contd… A B sbj
Other factors of corneal transparency - 2. Corneal epithelium & tear film • Epithelial non-keratinization • Regular & uniform arrangement of corneal epithelium • Junctions between cells & its compactness and also tear film maintain a homogenicity of its refractive index 3. Relative deturgescence state of normal cornea 4. Corneal avascularity 5. Non myelenated nerve fibres Contd… sbj
Factors affecting corneal Hydration: i. Stromal swelling pressure exerted by GAGs ii. Barrier function of epithelium and endothelium iii. Hydration controled by active pump mechanisms of the corneal endothelium • The enzyme pump systems are – • Na⁺/K⁺ ATPase pump system • Bicarbnate dependent ATPase • Carbonic anhydrase enzyme • Na⁺/H⁺ pump iv. Evaporation of water from corneal surface v. Intraocular pressure sbj
Drug permeability across the Cornea: Factors affecting drug penetration through the cornea are – 1. Lipid and water solubility of the drug 2. Molecular size, weight and concentration of drug 3. Ionic form of the drug 4. pH of the solution 5. Tonicity of the solution 6. Surface active agents 7. Pro-drug form sbj
Corneal wound healing: • Discussed earlier of this presentation with every layers.. sbj
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