Cornea and Sclera PDF

OBJECTIVES: 1. To gain knowledge on the parts, structures and functions of the eye; 2. Determine the functions of the parts of the eye 3. Analyze the result of coordinated functions of the eye parts which lead to the main function which is sight/clear vision.  GENERAL SHAPE AND DIMENS...

OBJECTIVES: 1. To gain knowledge on the parts, structures and functions of the eye; 2. Determine the functions of the parts of the eye 3. Analyze the result of coordinated functions of the eye parts which lead to the main function which is sight/clear vision.  GENERAL SHAPE AND DIMENSIONS OF EYEBALL  made up of the segments of 2 spheres of different size placed one in front of the other.  the shape is Oblate Spheroid ANTERIOR POLE OF THE EYE:  center of curvature of the transparent segment or cornea POSTERIOR POLE OF THE EYE: is the center of the posterior curvature of the eyeball, and it is located slightly temporal to the optic nerve. ANTERIOR POLE GEOMETRIC / OPTIC AXIS: A line that connects the two poles VISUAL AXIS: POSITION OF THE EYEBALL:  an imaginary line connecting  it is situated in the anterior part the fovea centralis of the retina of the orbital cavity, closer to the with the nodal point of the eye roof, and nearer the lateral than and continuing anteriorly through the medial wall. the cornea. DIMENSIONS volume of the eyeball 6-7 mL anterior to posterior pole 24 mm horizontal 23.5 mm Vertical 23 mm limbus to equator 16 mm limbus to posterior pole 32 mm circumference 69-81 mm LAYERS OF THE EYEBALL 1. FIBROUS LAYER (Coat/tunic)  Sclera  Cornea 2. MIDDLE VASCULAR/UVEAL LAYER  Choroid  Ciliary Body  Iris 3. INNER NERVOUS LAYER  Retina DISPLACEMENT OF THE EYEBALL  a space-occupying mass within the cone formed by the extraocular muscles quickly causes forward displacement of the eyeball. CORNEA  anterior , transparent one sixth of the globe  periphery merges with sclera to form corneoscleral junction (limbus corneae)  transparent tissue composed mainly of stroma (90%)  covered externally with regularly arranged stratified squamous epithelium  lined with single layer of endothelial cells  main refracting lens of the eye  thicker at its periphery CORNEA LAYERS OF THE CORNEA: A  Anterior Epithelium B  Bowman’s Membrane C  Corneal Stroma D  Descemet’s Membrane E  Endothelium A. ANTERIOR EPITHELIUM / CORNEAL EPITHELIUM (50 um)  consists of stratified squamous epithelium  5-6 cell layers thick  8-10 layers thick toward the periphery  capacity to regenerate  Major site of metabolic activity  Provides mechanical barrier to microorganisms and foreign matter  Desmosomes, which are abundant in the wing cell layers of normal corneal epithelium, adherens junctions throughout the different layers, and hemidesmosomes in the basal cell layers, provide structural integrity and anchoring function by linking the cytoskeletons of adjoining cells and maintain adhesion to the underlying substrate 3 LAYERS OF CORNEAL EPITHELIUM 1. SUPERFICIAL LAYER OF FLATTENED CELLS / SQUAMOUS CELLS  2 cell layers thick  Flat and have horizontal nuclei  Superficial surfaces have many microplicae & microvilli  adherence of the precorneal tear film  as surface cells age (disintegrate)  joined by tight junctions  exclude water and water soluble substances  2. WING CELLS (Polyhedral cells)  midzone layer formed by 2/3 layers of polyhedral cells  interlock with each other with large desmosomal attachments to adjacent cell.  consist of tenofilaments  polyhedral with a convex anterior surface  parallel with surface of cornea DESMOSOMES  cells are joined together from base to apex 3. BASAL CELLS / BASAL GERMINAL LAYER  deepest layer  one cell thick  compromise a single layer of columnar cells that rest on basement membrane  tall and columnar often show mitosis  move forward to be shed into the tear 7 days after HEMIDESMOSOMES  anchoring filaments B. BOWMAN’S LAYER (8 – 10 um)  interwoven collagen fibers  clear, acellular layer  modified portion of the stroma  resistant to deformation, trauma & passage of foreign bodies or microorganisms  no capacity to regenerate\when injured, forms scar tissue  not a true basement membrane C. CORNEAL STROMA / SUBSTANTIA PROPIA (200-250 Lamellae)  consists of 90% of the total cornea, made up of 200 collagenous lamellae  arranged nearly parallel to the corneal surface  constitute 90% of corneal thickness  equal in thickness (posteriorly)  enmeshed in glycosaminoglycans  transparent, fibrous and compact KERATOCYTES (Corneal Corpuscles)  wandering cells (lymphocytes)  fixed, long, flat cells  function as fibroblast D. DESCEMET’S MEMBRANE (10 – 12 um)  real basement membrane for the corneal endothelium  strong, homogenous membrane, highly elastic and very resistant  has capacity to regenerate  serves as the posterior limit of the stroma and is secreted by the endothelium  basement membrane of endothelial cells  stops abruptly at periphery and forms the anterior border line called Line of Schwalbe E. ENDOTHELIUM  innermost layer of single layer of mesothelial or hexagonal cells  consist of a single layer flattened cells that are polygonal in shape and whose plasma membranes interdigitate with one another * facilitates the pumping mechanism or active transport function thereby maintains corneal deturgescence or corneal state of partial dehydration. Nourishment of Cornea:  perilimbal vessels  direct oxygen from air  indirect oxygen from tears  aqueous fluid-substance Central Cornea -- atmospheric oxygen for its aerobic metabolism Peripheral Cornea – nurtured by the superficial and deep corneal plexuses derived from the anterior ciliary arteries Corneal Endothelium – derives nourishment from aqueous THE CORNEA REQUIRES ENERGY:  To supply its metabolic needs  For continued mitotic activity of the epithelium  To regulate the flow of fluid and ions through the endothelium  MEASUREMENTS diameter of the cornea 11.7mm posteriorly (concave & circular) thickness of the cornea 0.5 – 0.6mm at the center thickness of the cornea at 0.7 the periphery radius of curvature of 7.7mm the anterior surface of the Cornea radius of curvature of the 6.9mm posterior surface of the cornea refractive index in air 1.00 refractive index in 1.33mm aqueous humor CORNEAL TRANSPARENCY IS ATTRIBUTED TO: 1. Regular anatomic structure 2. Tight junctions of the epithelial cells that are not permeable to aqueous solutions 3. Dynamic balance bet ions & water in the stroma that is maintained by an endothelial pump mechanism 4. Absence of blood vessels and pigments 5. Regular arrangement of epithelial and endothelial cells 6. Scarcity of cell nuclei in the stroma 7. Epithelial cells are not keratinized 8. Anterior surface of the tear film forms a regular refracting surface 9. Epithelial & Endothelial cells do not reflect light at their interfaces because of the same index of refraction 10. Collagen fibers are oriented in a two-dimensional lattice, with the distance bet each fiber approximately equal. 11. Each fibril has a diameter of less than one wavelength 12. Relative dehydration of the stroma is necessary for transparency BLOOD SUPPLY AND LYMPHATIC DRAINAGE OF THE CORNEA  The cornea is avascular and devoid of lymphatic drainage  The capillary blood vessels derived from the anterior arteries of the conjunctiva and sclera, ends at the circumference of the cornea NERVE SUPPLY OF THE CORNEA  these nerve fibers are derived from the ophthalmic division of the trigeminal nerve  mainly through the long ciliary nerves  the long ciliary nerves enter the sclera from the perichoroidal space at short distance posterior from the limbus  innervated by the nonmedullated sensory nerve fibers that mainly pass to the long and short ciliary nerves of the ophthalmic branch of the 5th CN FUNCTIONS OF THE CORNEA:  it is the most important refractive medium in the eye  this refractive power occurs on the anterior surface of the cornea, where the refractive index of the cornea is greatly different from that of the air CORNEOSCLERAL JUNCTION:  limbus corneae is the 1-2 mm wide transitional zone between cornea and sclera  contains trabecular meshwork  externally covered by conjunctiva TRABECULAR MESHWORK:  drainage system of anterior chamber  surrounds the circumference of anterior chamber DESCRIPTION:  white of the eye  Fibrous opaque coat  It occupies the posterior 5/6 0f the globe  Composed of dense white connective tissue consisting of bundles of collagenous fibers inbetween are flattened fibroblast and fine networks of elastic fibers.  It is covered by the fascia bulbi posteriorly and conjuctiva anteriorly. DIMENSIONS: 2 Large Opening 1. The Anterior Scleral Foramina  bridged by the transparent cornea  rich in elastic tissue 2. Posterior Scleral Foramina  bridged by sievelike structure, the lamina cribosa  MEASUREMENTS thickness at the 1mm posterior thickness behind 0.3-0.4mm the insertions of the aponeurotic tendons of the EOM at the equator 0.6mm at the corneoscleral 0.8mm junction radius of curvature 12mm LAYERS OF THE SCLERA 1. Episcleral Tissue  the outermost layer of loose fibro-elastic tissue continous externally with the dense connective tissue of the Tenon’s Capsule. 2.Scleral Stroma  consists of dense fibrous tissue intermingled with fine elastic fibers, colagen fibrils vary in diameter, run in whorls, loops, & arches, and many pass layer to layer at random, forming a feltlike matting of bundles. 3.Lamina Fusca  the innermost faintly brown color, due to the presence of melanocytes, has many grooves caused by the passage of ciliary vessels and nerves. BLOOD SUPPLY OF THE SCLERA  Anterior Ciliary Arteries NERVE SUPPLY OF THE SCLERA  Sclera receives an innervation from both the short and long ciliary nerves. FUNCTIONS OF THE SCLERA:  It allows the retina to present a smooth unwrinkled surface  Its tough, fibrous structure protects the intraocular contents from trauma & mechanical displacement.  It furnishes a firm surface for the attachment of the muscles  It aids in excluding extraneous light.  Its firmness & strength, w/IOP, preserves the shape of the eyeball and maintains the exact position of the optic system.  Its strength & firmness provide a rigid insertion for the EOM. ATTACHING STRUCTURES: A. Optic Nerve  perforates the sclera posteriorly about 3mm medial and 1mm above the posterior pole of the eyeball.  Site of this perforation is sometimes referred to as the posterior scleral foramina. B. Lamina Cribosa  part of the sclera pierced by the optic nerve fibers posteriorly.  weak, the rise of intra ocular pressure causes it to bulge. C. Sinus Venosus Sclerae/ Canal of Schlemm  circular canal situated posterior to the limbus lying within the sclera.  an oval channel  the outer wall of the canal is lined by the endothelium. D. Scleral Spur  a triangular ridge which attachments comes from the ciliary muscle. THAT’S ALL…