Corneal Anatomy and Function Lecture 1 PDF

Corneal Anatomy and Functions Roy Joseph PhD MBA School of Optometry and Vision Sciences August 26th 2024 Schematic representation of the eye  Functions of Cornea Protective Transm...

Corneal Anatomy and Functions Roy Joseph PhD MBA School of Optometry and Vision Sciences August 26th 2024 Schematic representation of the eye  Functions of Cornea Protective Transmission outer coat of light Tensile (Tough Refraction of strength fibrous light (43-48D), tissue) dependents on Change in refractive Curvature index from cornea to of anterior aqueous corneal Curvature Corneal Change in surface thickness refractive of index from posterior air/tear film corneal to cornea surface  Corneal Anatomy 1/5 of the outer tunic of the eye Five layers (outside to inside) Diameter Epithelium 11.7 mm (vertical) Bowman’s layer 12.6 mm (horizontal) Stroma Thickness: Desmet’s membrane 0.52 mm Endothelium 0.71 mm at periphery (30% more thicker) Transparent zone joins the opaque scleral zone  Exqisitive Tensile strength Wound healing sensitivity capability Unique Features of Cornea Clarity Cornea is (transmits protected light between from 310-2500 nm) infection Aqueous by tear Avascular provides film and nutrition limbal Impermeable due to tight region junctions in superficial Derive oxygen epithelial cells from tear film  Five layers of cornea Corneal epithelium 50 m,10% of total Bowman’s layer, 8-4 m Stroma, 500 m, 90% of total Descemet’s membrane, 10 m Endothelium, 5 m  Schematic representation of corneal Epithelium Superficial cells Five-six- cell layers thick epithelium Nerves Basal epithelial cells Wing cells Bowman’s layer Stroma  Corneal Epithelium 10% of the total corneal thickness Three types of cell (50 M) Superficial 5-6 layers of squamous cells Wing cells Basal cells 1-2 layers, no cell cells 2-3 layers, no Human, rat and division no cell cell division Single division, turn over 7 rabbit: 5-7 layers days. Characteristic cell layer Oxen-15 layers wing processes Cell Most differentiated, division degenerative type. Gap junctions Cells have fewer and desmosomes Produced organelles No cell division No cell division by stem Secrets glycocalyx cells Tight junctions.  Corneal Epithelium Superficial cells Tight junctions Desmosomes Gap junctions (for cell (Impermeable) to cell Wing cells communication) Desmosomes Gap junctions (for cell to cell communication) Basal cells Hemidesmosomes Basement membrane Schematic representation of the Desmosomes and Hemidesmosomes Desmosomes: Bind adjacent cells Gap junctions: Basal cell communication junctions Hemodesmosomes: Bind basal cells to basement membrane Intercellulat space Occludin is a 65 kDa (522- amino acid polypeptide- human) integral plasma- membrane protein located at the tight junction. Together with the claudin group of proteins, it is the main component of the tight junctions. Claudins are small (20–27 kDa)) transmembrane proteins. Claudins are a family of proteins that are the most important components of the tight junction, where they establish the Paracellular barrier that controls the flow of molecules in the intercellular space between the cells.  Corneal Epithelial Stem Cell Niche Yazdanpanah, G.; Haq, Z.; Kang, K.; Jabbehdari, S.; Rosenblatt, M.L.; Djalilian, A.R. Strategies for reconstructing the limbal stem cell niche. Ocul. Surf. 2019, 17, 230–240. Corneal Epithelial Cell Regeneration Masood, F.; Chang, J.-H.; Akbar, A.; Song, A.; Hu, W.-Y.; Azar,D.T.; Rosenblatt, M.I. Therapeutic Strategies for Restoring Perturbed Corneal Epithelial Homeostasis in Limbal Stem Cell Deficiency: Current Trends and Future Directions. Cells2022, 11, 3247. Limbal region Stem cells Stem cells Basal Epithelial cells (derived from stem cells) Blood vessels Immunolocalization with antibody to alpha enolase (Binds only to limbal cells and identified as stem cells)  Stem Cells of Epithelial Basal Cells Stem cell reside in limbus and have three Characteristics Three characteristics of stem cells (1) Unlimited cell renewal (2) Asymmetrical division (One daughter cell remains as stem cell) (3) Irreversible process once committed to differentiation (daughter cells called transit cells) Limbal stem cells Corneal tumors Slow cell cycling, generally occurs Bind to-enolase antibody but not in limbal region retain 3H- to cornea-specific 64 kDa keratin thymidine (Uncertain about this now) Do not bind to antibodies to conjunctival and epithelial proteins Growing cells: Cell cycle Cell mass doubled Four successive phases M, G1, S, G2 phases. Interphase comprises DNA synthesis of G1, S and G2 phases in S phase. Other macromolecules Non-dividing are synthesized cells throughout (quiescent interphase. stage) Cells doubles DNA Thymidine its mass before synthesis M. uptake During G2 cell Cell Cycle: prepares for G1 mitosis (M). S Non-dividing cells enter G2 quiescent Go M state.  Bowman’s Layer 8-14 m thick, very resistant to Contains interwoven Secreted by shearing, collagen fibrils epithelial cells penetration or (randomly arranged, Secreted during infection diameter 20-25 m) embryogenesis, and mucoproteins and does not Transitional layer and ground regenerate of stroma rather substance than a membrane If injured, Collagen(Type replaced by Differ from VII) epithelial cells of stroma (acellular and smaller Laminin stromal scar diameter collagen tissue Heparin sulfate fibrils) Fibronectin Only found in primates Fibrin Light micrograph of Bowman’s Layer Epithelium Bowman’s layer Stroma 1.Layer in between the epithelium and the stroma 2. Superficial stromal lamellae curve forward to merge with the Bowman’s layer.  Corneal Stroma 500 m thick (constitutes 90% of Each collagen fibril corneal thickness) (diameter: 30 nm) is Contains separated from each Collagen fibrils other by an equal Keratocytes (flattened fibroblasts) distance (64 nm) Intercellular ground substance Scattered light (Proteoglycan+GAG- interact and cause Glycoproteins) destructive Nerve axons interference. Contains GAGs 200-300 superimposed layers of lamella that runs parallel (glycoaminoglycans (made of collagen fibrils that run ) as ground full length of cornea). substance.  Stroma Keratocyte cells Ground substance Flattened cells, found Collagen Fills area between fibrils between and within lamellae and cells. lamellae Proteoglycans Synthesize collagen, glycosaminoglycan and Glycosaminoglycans proteoglycans (GAG’s; Negativelycharged) Other cells (increase GAG’s attach to water during pathogenesis) and help in maintaining Blood cells, precise location in lymphocytes, between fibrils. macrophages and In central cornea: polymorphonuclear Keratin sulfate (major) lymphocytes Chondroitin sulfate (minor)  Cut obliquely Cut in cross-section Corneal stroma collagen Cut in cross-section Longitudinal  Collagen Fibrils of stroma Present in lamellae Narrow uniform diameter (30 nm) 1/3 of stroma (anterior):Thin and Regularly spaced interwoven. (55-60 nm), Lattice arrangement Posterior: 2/3 more regular arrangement Run parallel to each other, limbus Interlamellar adhesive to limbus strength at 50% stromal depth: 14.2 g wt/mm of Produced by tissue(centrally) and 2X keratocyte peripherally. This reflects force required to break proteoglycan bonds between collagen lamellae.  Schematic representation of the Keratocytes Keratocytes Six fibroblasts lying between the stromal lamellae Cell to cell Contact Half life of collagen 100 days.  Collagen fibrils arranged in a lattice Wavelength of light 2000 A0 = 200 nm Collagen fibrils (30 nm Spacing 64 m diameter) Destructive interference  Comparison of the average number of collagen fibers Keratin sulfate falls in concentration Collagen fibers number Fiber diameter Midcornea Increasing diameters of collagen from cornea to sclera  Transparency of Cornea Stromal Components Light scatter components of epithelium, Bowman’s Light scatter Scatter 10% light layer and occurs when Non uniform refractive Desmet’s cornea indices; Dry collagen = membrane contains 1.55 and Ground Components fluctuations in substance = 1.354 arranged refractive index that are Lattice arrangement of irregularly distributed fibrils (equidistance and Scatter particles over distance uniform diameter Light separated by such larger than ½ scattered by individual a small distance of wavelength fibrils is cancelled by that light of light (2000 destructive interference scattering is A°) with scattered light from minimal Electromagnetic spectrum VIBGYOR 2500 nm 400-700 nm Spatial 280-315 nm: 315- 499 fluctuation in Absorbed by nm: cytoplasm cornea Absorbed