Cornea and Sclera (1) PDF

Anatomy & Physiology of the Eye (7102203) Cornea and Sclera (1) Ms. Ghaidaa Shaqour, M.Sc. Lecturer, An-Najah National University Email: [email protected] Outlines Introduction Corneal Histologic Structure and Features Corneal Function Corneal Metabolism Corneal Innervation Corneal...

Anatomy & Physiology of the Eye (7102203) Cornea and Sclera (1) Ms. Ghaidaa Shaqour, M.Sc. Lecturer, An-Najah National University Email: [email protected] Outlines Introduction Corneal Histologic Structure and Features Corneal Function Corneal Metabolism Corneal Innervation Corneal Blood Supply Cornea and Sclera 2 Introduction The eye's outer connective tissue appears as two joined spheres. 3 Cornea and Sclera Introduction Watch this video! https://www.youtube.com/watch?v=4kpXKu5QKww 4 Cornea and Sclera Introduction The eye's outer connective tissue appears as two joined spheres. 1. Cornea Smaller Anterior Transparent sphere Radius: 8 mm 5 Cornea and Sclera Introduction The eye's outer connective tissue appears as two joined spheres. 2. Sclera Larger Posterior Opaque sphere Radius: 12 mm 6 Cornea and Sclera Introduction 7 Cornea and Sclera Introduction The eye's outer connective tissue appears as two joined spheres. The globe's dimensions: Anteroposterior: 24 mm Vertical: 23 mm Horizontal: 23.5 mm 8 Cornea and Sclera Corneal Histologic Features: Overview The cornea is the principal refracting component of the eye. Transparent and avascular. For optimal light transmittance. Anterior surface covered by the tear film Posterior surface borders the aqueous-filled anterior chamber. 9 Cornea and Sclera Corneal Histologic Features: Overview Continuous with the conjunctiva and sclera at its periphery. Composed of five layers: Epithelium, Bowman’s Layer, Stroma, Descemet’s Membrane, and Endothelium. 10 Cornea and Sclera Corneal Histologic Features: Overview 11 Cornea and Sclera Corneal Histologic Features: Overview Light micrograph of corneal layers 12 Cornea and Sclera Corneal Epithelium – Structure The epithelium is the outermost layer of the cornea. Stratified corneal epithelium is 5 to 7 cells thick (approx. 50 μm) Thickens at the periphery, continuous with the conjunctival epithelium. 13 Cornea and Sclera Corneal Epithelium – Surface Layer Two cells thick and display a very smooth anterior surface. It consists of nonkeratinized squamous cells This layer enhances tear film stability. Microvilli and microplicae (projections) increase surface area, improving tear film stability. 14 Cornea and Sclera Corneal Epithelium – Tight Junctions Tight junctions (zonula occludens) join the surface cells along their lateral walls Provide a barrier to intercellular movement of substances from the tear layer Prevent the uptake of excess fluid from the tear film. A highly effective, semipermeable membrane is produced Allowing passage of fluid and molecules through the cells but not between them. 15 Cornea and Sclera Corneal Epithelium – Middle Layer Made up of 2 to 3 layers of wing cells The diameter of a wing cell is approximately 20 μm These cells have winglike lateral processes Polyhedral Have convex anterior surfaces and concave posterior surfaces Fit over the basal cells 16 Cornea and Sclera Corneal Epithelium – Basal Layer The innermost layer of the corneal epithelium A single layer of columnar cells Cell diameters range from 8 to 10 μm These cells contain oval-shaped nuclei The rounded, apical surface of each cell lies adjacent to the wing cells, The basal surface attaches to the underlying basement membrane (basal lamina) 17 Cornea and Sclera Corneal Epithelium – Basal Layer The basal cells secrete this basement membrane Attaches the cells to the underlying tissue The basal layer is the germinal layer where mitosis occurs. 18 Cornea and Sclera Corneal Epithelium –Layers 19 Cornea and Sclera Corneal Epithelium – Epithelial Replacement Maintenance of the smooth corneal surface depends on replacement of the surface cells They are continually being shed into the tear film. Cell proliferation occurs in the basal layer. Basal cells move up to become wing cells, and wing cells move up to become surface cells. 20 Cornea and Sclera Corneal Epithelium – Epithelial Replacement Only the cells in contact with the basement membrane have the ability to divide The cells that are displaced into the wing cell layers lose this ability Stem cells located in a 0.5- to 1-mm-wide band around the corneal periphery are the source for renewal of the corneal basal cell layer. 21 Cornea and Sclera Corneal Epithelium – Epithelial Replacement A slow migration of basal cells occurs from the periphery toward the center of the cornea. Turnover time for the entire corneal epithelium is approximately 7 days More rapid than for other epithelial tissues Repair to corneal epithelial tissue proceeds quickly Minor abrasions heal within hours and larger ones often heal overnight 22 Cornea and Sclera Corneal Epithelium – Epithelial Replacement If the basement membrane is damaged: Complete healing with replacement basement membrane can take months. 23 Cornea and Sclera Bowman’s Layer The second layer of the cornea Approximately 8 to 14 μm thick A dense, fibrous sheet Made of interwoven collagen fibrils Randomly arranged in a mucoprotein ground substance. 24 Cornea and Sclera Bowman’s Layer The fibrils have a diameter of 20 to 25 nm Run in various directions, And are not ordered into bundles Bowman’s layer sometimes is referred to as a “membrane,” But it is more correctly a transition layer to the stroma rather than a true membrane It differs from the stroma in that it is acellular and contains collagen fibrils of a smaller diameter 25 Cornea and Sclera Bowman’s Layer Bowman’s layer might provide biomechanical rigidity and shape to the cornea The pattern of the anterior surface is irregular and reflects the contour of the bases of the basal cells of the epithelium Posteriorly, the layer transition into stroma The fibrils gradually adopt a more orderly arrangement And begin to merge into bundles that intermingle with those of the stroma The posterior surface is not clearly defined. 26 Cornea and Sclera Bowman’s Layer Bowman’s layer is produced by the epithelium and is not believed to regenerate. Bowman’s layer is very resistant to damage E.g., by shearing, penetration, or infection No long-term effects have been documented in patients with Bowman’s layer removed by photorefractive keratoplasty 27 Cornea and Sclera Stroma The middle layer of the cornea Approximately 500 μm thick (about 90% of the total corneal thickness) 28 Cornea and Sclera Stroma- Collagen Fibrils The stroma (substantia propria) is composed of collagen fibrils, keratocytes, and extracellular ground substance The collagen fibrils have a uniform 25- to 35-nm diameter Run parallel to one another Forming flat bundles called lamellae 29 Cornea and Sclera Stroma- Collagen Fibrils The 200 to 300 lamellae are distributed throughout the stroma Lie parallel to the corneal surface. Each contains uniformly straight collagen fibrils arranged with regular spacing, sometimes described as a “latticework.” Adjacent lamellae lie at angles to one another, but all fibrils within a lamella run in the same direction Each lamella extends across the entire cornea And each fibril runs from limbus to limbus. Interweaving occurs between the lamellae. 30 Cornea and Sclera Stroma- Collagen Fibrils The arrangement of the lamellae varies slightly within the stroma. In the anterior one third of the stroma: The lamellae are thin (0.5 to 30 μm wide and 0.2 to 1.2 μm thick) They branch and interweave more than in the deeper layers In the posterior two-thirds of the stroma: The arrangement is more regular The lamellae become larger (100 to 200 μm wide and 1 to 2.5 μm thick) 31 Cornea and Sclera Stroma- Collagen Fibrils Anterior cornea has a higher incidence of crosslinking and is more rigid Helping to maintain corneal curvature In the innermost layer, adjacent to Descemet’s membrane, the fibrils interlace to form a thin collagenous sheet Contributes to the binding between stroma and Descemet’s membrane 32 Cornea and Sclera Stroma- Keratocytes Keratocytes or corneal fibroblasts Flattened cells that lie between and occasionally within the lamellae The cells are not distributed randomly A corkscrew pattern is recognizable from anterior to posterior The density higher in the anterior stroma 33 Cornea and Sclera Stroma- Keratocytes Keratocytes have extensive branching processes Joined by gap junctions along the lateral extensions, as well as the anteroposterior branches These are active cells Maintain the stroma by synthesizing collagen and extracellular matrix components Other cells may be found between lamellae Including white blood cells, lymphocytes, macrophages, and polymorphonuclear leukocytes 34 Cornea and Sclera Stroma 35 Cornea and Sclera Stroma- Ground substance Ground substance fills the areas between fibrils lamellae, and cells It contains proteoglycans (PG) Macromolecules consisting of a core protein with one or more attached glycosaminoglycan (GAG) side chains PG have a significant role in maintaining corneal tensile strength and the GAGs contribute to the relatively high stromal hydration 36 Cornea and Sclera Stroma- Ground substance GAGs are Hydrophilic Negatively charged carbohydrate molecules Located at specific sites around each collagen fibril. They attract and bind with water Maintaining the precise spatial relationship between individual fibrils 37 Cornea and Sclera Stroma- Transparency What contributes to stromal transparency? 1. The very regular arrangement of the stromal components 2. The small diameter of the fibrils The refractive index of the Fibrils is 1.411 The extracellular matrix is 1.365 38 Cornea and Sclera Stroma- Transparency Studies have shown that the distance between areas of different refractive indices can affect transparency. If the change in the index of refraction occurs across a distance that is less than one-half the wavelength of visible light (400 to 700 nm): destructive interference occurs and light scattering is reduced significantly 39 Cornea and Sclera Stroma- Transparency In the stroma the very specific spacing between the fibrils allows destructive interference of rays reflecting from adjacent fibrils. Although the components of the epithelium, Bowman’s layer, and Descemet’s membrane are arranged irregularly, the scattering particles are separated by such small distances that light scattering is minimal in these layers The cornea scatters less than 1% of the light that enters it 40 Cornea and Sclera