Lecture 6: Corneal Anatomy PDF

‭Learning Objectives‬ ‭Normal dimensions of the cornea (diameter, radii of curvature, pachymetry or corneal thickness)‬ ‭‬ ‭Diameter‬ ‭○‬ ‭Anterior:‬ ‭‬ ‭Horizontal: 12mm; vertical: 11mm‬ ‭○‬ ‭Posterior:‬...

‭Learning Objectives‬ ‭Normal dimensions of the cornea (diameter, radii of curvature, pachymetry or corneal thickness)‬ ‭‬ ‭Diameter‬ ‭○‬ ‭Anterior:‬ ‭‬ ‭Horizontal: 12mm; vertical: 11mm‬ ‭○‬ ‭Posterior:‬ ‭‬ ‭Circular: 11.7mm‬ ‭○‬ ‭Corneal pachymetry (thickness):‬ ‭‬ ‭Central: 540-570µm (555µm expected)‬ ‭‬ ‭Peripheral: 710 µm‬ ‭○‬ ‭Radius of curvature:‬ ‭‬ ‭Anterior: 7.8mm; posterior: 6.5mm *smaller number = steeper curvature‬ → ‭ posterior = STEEPER‬ ‭a.‬ ‭Define with-the-rule vs against-the-rule, and oblique astigmatism‬ ⇒ ‭i.‬ ‭With-the-rule and against-the-rule meridians are 90º apart‬ ‭1.‬ ‭With: steepest meridian = VERTICAL (x180)‬ ‭2.‬ ‭Against: steepest meridian = HORIZONTAL (x090)‬ ⇒ ‭ii.‬ ‭Oblique astigmatism steepest meridian 45º or 135º‬ ‭*Rx = opposite axis‬ ‭b.‬ ‭What is regular vs irregular astigmatism?‬ ‭i.‬ ‭Regular astigmatism = meridians are 90º apart‬ ‭ii.‬ ‭Irregular astigmatism = meridians are NOT 90º apart‬ ‭Identify the layers of the cornea and the purpose of each layer‬ ‭‬ ‭Epithelium‬ ‭○‬ ‭Stratified, non-keratinized cells‬ ‭○‬ ‭Surface cells, wing cells, basal cells, BM, regeneration‬ ‭‬ ‭Bowman layer‬ ‭○‬ ‭Thin layer, BELOW epithelium‬ ‭○‬ ‭Dense, irregular CT‬ ‭○‬ ‭Acellular‬ ‭○‬ ‭Transition layer‬ ‭○‬ ‭Produced before birth by epithelium & CANNOT regenerated‬ ‭○‬ ‭Nerves lose their Schwann cell sheath and pass naked into the epithelium‬ ‭‬ ‭Stroma‬ ‭○‬ ‭THICKEST corneal layer‬ ‭○‬ ‭~500µm thick‬ ‭○‬ ‭Regular, dense CT‬ ‭○‬ ‭Composition: collagen fibrils, cells (keratocytes), ground substance‬ ‭‬ ‭Keratocytes: fibroblasts (immune response)‬  ‭ ‬ ‭Synthesize collagen and ECM‬ ‭‬ ‭Connected bia gap junctions‬ ‭‬ ‭Ground substance‬ ‭‬ ‭Maintains spacing and stromal hydration‬ ‭‬ ‭Proteoglycans: increase strength and volume‬ ‭○‬ ‭Glycosaminoglycans (GAGs): negatively charged side chains of‬ ‭proteoglycans (attracts water)‬ ‭‬ ‭Hydrophilic: spacing between fibrils within lamellae‬ ‭‬ ‭Too much water: corneal edema‬ ‭○‬ ‭Organization important for light transmission and corneal transparency‬ ‭*Dua’s layer‬ ‭‬ ‭Descemet’s membrane‬ ‭○‬ ‭BM of endothelium → attached by hemidesmosomes‬ ‭○‬ ‭Regenerates and thickens throughout life → thickness doubles by 40 yo‬ ‭○‬ ‭Dense CT‬ ‭○‬ ‭Elastic properties (no elastin)‬ ‭○‬ ‭Resistant to proteolytic enzymes and trauma‬ ‭○‬ ‭Schwalbe’s Line‬ ‭‬ ‭Termination line of membrane, near limbus (before TM begins)‬ ‭○‬ ‭Intercellular junction:‬ ‭‬ ‭Hemidesmosomes‬ ‭ ‬ ‭Endothelium‬ ‭○‬ ‭Intercellular junctions:‬ ‭‬ ‭Lateral interdigitations (finger-like projections; maintain structure)‬ ‭‬ ‭Gap junctions‬ ⇒ ‭‬ ‭Macula occludens “slightly leaky” barrier‬ ‭a.‬ ‭Histology of each layer‬ ‭i.‬ ‭Layers of the corneal epithelium and intercellular junctions‬ ‭1.‬ ‭Surface cells (flat)‬ ‭a.‬ ‭Flattened, nucleated squamous cells‬ ‭b.‬ ‭Microvilli and microplicae‬ ‭c.‬ ‭Secrete glycocalyx that adheres to mucin layer‬ ‭d.‬ ‭Intercellular junctions: desmosomes and zonula occludens‬ ‭2.‬ ‭Wing cells (transition layer)‬ ‭a.‬ ‭Middle/transition layer‬ ‭b.‬ ‭Intercellular junctions: desmosomes and gap junctions‬ ‭3.‬ ‭Basal cells (columnar cells)‬ ‭a.‬ ‭Germinative layer → undergo mitosis‬ ‭b.‬ ‭Single layer of columnar cells‬ ‭c.‬ ‭Secretes basal lamina portion of basement membrane‬  ‭d.‬ I‭ntercellular junctions: desmosomes, gap junctions, hemidesmosomes‬ ⇒ ‭(anchor corneal epithelium to rest of surface fibrils attach to plaques‬ ‭in the corneal stroma)‬ ‭e.‬ ‭Replenished from stem cells in the limbus‬ ‭ii.‬ ‭Zonula vs macula occludens‬ ‭1.‬ ‭Zonula occludens: intercellular junctions‬ ‭a.‬ ‭Functions as a barrier to protect the cornea from foreign invaders‬ ‭2.‬ ‭Desmosomes → specialized macula adherens‬ ‭a.‬ ‭Circular attachment plaque‬ ‭b.‬ ‭Filaments extend into cell cytoplasm‬ ‭c.‬ ‭Adhesive mucoprotein fills the intercellular space‬ ‭d.‬ ‭Resists shearing of tissues, strongly connects cells‬ ‭iii.‬ ‭Acellular vs cellular layers‬ ‭1.‬ ‭Acellular‬ ‭a.‬ ‭Bowman layer‬ ‭b.‬ ‭Descemet membrane‬ ‭2.‬ ‭Cellular‬ ‭a.‬ ‭Epithelium‬ ‭b.‬ ‭Stroma‬ ‭iv.‬ ‭Stromal fibril organization‬ ‭1.‬ ‭Collagen stroma → collagen fibrils‬ ‭a.‬ ‭Extend from limbus to limbus‬ ‭b.‬ ‭Fibrils organized into flat bundles = lamellae‬ ‭i.‬ ‭Individual fibril runs parallel‬ ‭ii.‬ ‭Lamellae = parallel to ocular surface; stacked on top of each‬ ‭other‬ ‭c.‬ ‭Anterior: thinner, more interweaving, cross-linking (more rigidity →‬ ‭maintain corneal curvature)‬ ‭d.‬ ‭Posterior: thicker, regular arrangement‬ ‭v.‬ ‭Endothelial structure and function‬ ‭1.‬ ‭Single, thin layer of cells‬ ‭2.‬ ‭Basal → adjacent to Descemet membrane‬ ‭3.‬ ‭Apical → adjacent to anterior chamber‬ ‭4.‬ ‭Metabolically active *makes energy to perform active transport (water OUT to‬ ‭maintain equil.)‬ ‭5.‬ ‭Regular, mosaic pattern‬ ‭6.‬ ‭Does NOT regenerate‬ ‭7.‬ ‭Maintain corneal transparency‬  ‭b.‬ ‭Define a basement membrane and review complications with changes in structure‬ ‭i.‬ ‭Separates the epithelium from the CT‬ ‭ii.‬ ‭Provides an anchor for the epithelial cells‬ ‭iii.‬ ‭Two layers: basal lamina (secreted by epithelium) and reticular lamina (secreted by CT)‬ ‭iv.‬ ‭Thickness doubles by age 60‬ ‭1.‬ ‭Thickens = exceed length of anchoring fibrils → weakening attachment to the‬ ‭rest cornea‬ ‭v.‬ ‭Descemet membrane = BM of endothelium‬ ‭1.‬ ‭Hassall-Henle bodies: where dropout normally occurs‬ ‭a.‬ ‭Normal, physiological aging change; peripheral cornea‬ ‭2.‬ ‭Guttata: abnormal dysfunction of endothelium; central cornea‬ ‭c.‬ ‭Identify the cell regeneration of the layers and timing‬ ‭i.‬ ‭Movement of epithelial cells (inner to outer): basal cells → wing cells → surface cells →‬ ‭shed into tear fil‬ ‭ii.‬ ‭Barrier functions maintained → ZO not complete until surface layer‬ ‭iii.‬ ‭Full epithelial turnover: 7-10 days‬ ‭iv.‬ ‭Minor superficial corneal abrasions: ~24-48 hrs (BM not affected)‬ ‭v.‬ ‭BM regeneration: months‬ ‭vi.‬ ‭Descemet membrane: does NOT regenerate‬ ‭d.‬ ‭Maintaining corneal clarity‬ ‭i.‬ ‭Constructive vs destructive interference‬ ‭1.‬ ‭Constructive:‬ ‭a.‬ ‭Additive; increases light scattering → less transparency‬ ‭2.‬ ‭Destructive:‬ ‭a.‬ ‭Decreases light scattering → contributes to transparency, why we don’t‬ ‭get glare‬ ‭ii.‬ ‭What happens to the corneal structure with loss of transparency? (to the extent of‬ ‭causes described in lecture)‬ ‭1.‬ ‭Loss of transparency = disorganized stromal organization‬ ‭a.‬ ‭Corneal edema = corneal lamellae NOT evenly spaced, too much water‬ ‭b.‬ ‭Scattered light = glare‬ ‭Describe how the cornea prevents infection and limits inflammation‬ ‭‬ ‭Due to anti-angiogenic properties‬ ‭‬ ‭No blood vessels limits local immune and inflammatory responses‬ ‭Describe the innervation of the cornea‬ ‭a.‬ ‭Where are the nerves enter and where are the nerve endings located?‬ ‭i.‬ ‭(70-80 bundles of axons) enter peripheral cornea in mid-stroma‬  ‭.‬ ‭Nerves thicker in corneal periphery‬ 1 ‭ii.‬ ‭Stromal plexus: 900-1200 axons‬ ‭iii.‬ ‭Subepithelial plexus‬ ‭iv.‬ ‭Intraepithelial plexus (Myelin sheath lost → Bowman layer)‬ ‭b.‬ ‭Which cranial nerve does innervation come from? Specifically, what branch?‬ ‭i.‬ ‭CN5 → ophtahlmic branch (V1)‬ ‭Describe the corneal blood supply and where the cornea receives oxygen‬ ‭‬ ‭Corneal blood vessels‬ ‭○‬ ‭NO BV → antiangiogenic property (no BV growth)‬ ‭‬ ‭Corneal blood supply‬ ‭○‬ ‭Bulbar conj and episceral vessels at limbus & palpebral conj networks‬ ‭a.‬ ‭Where is the majority of the oxygen supply from?‬ ⇒ ‭i.‬ ‭Cornea nourished diffusion from the tear film and aqueous humor‬ ‭ii.‬ ‭Open eye: tear film‬ ‭iii.‬ ‭Closed eye: palpebral vasculature‬ ‭iv.‬ ‭Desirable hydration = 78% hydrated‬ ‭1.‬ ‭Dehydration = spacing between corneal stroma‬ ‭b.‬ ‭Define immune-privileged‬ ‭i.‬ ‭Eye limits local immune and inflammatory responses to preserve vision‬ ‭ii.‬ ‭Transplants do not require tissue matching/immunosuppressive medications‬ ‭iii.‬ ‭Cornea and aqueous = anti-angiogenic properties‬ ‭Be able to identify layers of the cornea and conj/sclera, and structures as inner or outer‬ ‭‬ ‭Sclera is OUTER to choroid‬ ‭Identify the function of the limbus‬ ‭‬ ‭Limbus = transition zone between the cornea, conjunctiva, and sclera‬ ‭a.‬ ‭Define the palisades of Vogt‬ ‭i.‬ ‭Thickened epithelium with radial projections around corneal periphery‬ ‭ii.‬ ‭Concentrated in superior and inferior limbus‬ ‭iii.‬ ‭Location of stem cells‬ ‭b.‬ ‭Be able to draw the histological borders of the limbus‬ ‭i.‬ ‭Anterior border:‬ ‭1.‬ ‭External scleral sulcus: located at edge of cornea‬ ‭2.‬ ‭Internal scleral sulcus: located at edge of TM and Schlemm’s canal‬ ‭3.‬ ‭Termination of Bowman layer to Descemet’s membrane‬ ‭ii.‬ ‭Posterior border: posterior scleral spur to conj surface‬ ‭Describe the differences between the structure of the cornea and the sclera‬ ‭a.‬ ‭Color, thickness, composition‬  ‭i.‬ ‭Sclera:‬ ‭1.‬ ‭Color:‬ ‭a.‬ ‭White (healthy)‬ ‭b.‬ ‭Blue (thing or transparent)‬ ‭c.‬ ‭Yellow (liver damaged or with fat deposition → jaundice or alcoholism)‬ ‭2.‬ ‭Thickness:‬ ‭3.‬ ‭Composition: Type I collagen, irregular pattern of lamellae (fewer GAGs)‬ ‭ii.‬ ‭Cornea:‬ ‭1.‬ ‭Color: Transparent‬ ‭2.‬ ‭Thickness: 540-570µm; 550µm expected‬ ‭3.‬ ‭Composition: epithelium, Bowman layer, stroma, Descemet’s membrane,‬ ‭endothelium‬ ‭a.‬ ‭Acellular and cellular aspects‬ ‭b.‬ ‭Permeability‬ ‭i.‬ ‭Sclera is MORE PERMEABLE to drug delivery compared to cornea → organization and‬ ‭lack of tight junctions‬ ‭c.‬ ‭Scleral apertures/foramen‬ ‭i.‬ ‭Scleral foramina (openings in scleral tissue)‬ ‭1.‬ ‭Anterior scleral foramen: cornea‬ ‭2.‬ ‭Posterior scleral foramen: ON, lamina cribrosa, meshwork of scleral fibers‬ ‭ii.‬ ‭Scleral apertures (where nerves, arteries, veins pass through)‬ ‭1.‬ ‭Anterior apertures: near lumbus; anterior ciliary arteries‬ ‭2.‬ ‭Middle apertures: near equator; vortex veins‬ ‭3.‬ ‭Posterior apertures: around ON; short and long posterior ciliary arteries and‬ ‭nerves‬ ‭d.‬ ‭Describe the lamina cribrosa‬ ‭i.‬ ‭Weakest area of sclera‬ ‭1.‬ ‭High IOP = damage = loss of NFL = glaucoma‬ ‭ii.‬ ‭Meshwork of scleral fibers‬ ‭ e able to identify which layers continue past the corneal boundaries and which layers they transition‬ B ‭to, and which new layers begin past the limbus‬ ‭‬ ‭Sclera‬ ‭○‬ ‭Continuous with corneal stroma at limbus‬ ‭‬ ‭Episclera‬ ‭○‬ ‭Outer to sclera‬ ‭○‬ ‭Inner to conj and Tenon’s capsule‬ ‭‬ ‭Tenon’s capsule‬ ‭○‬ ‭Outer to episcleral tissue‬  ‭ ‬ ‭Between the inner conjunctiva and outer episcleral‬ ○ ‭‬ ‭Bulbar conj‬ ‭○‬ ‭Continuous at fornix with palpebral conj‬ ‭ ‬ ‭Limbus‬ ‭○‬ ‭Cornea epithelium → bulbar conjunctival epithelium‬ ‭○‬ ‭Cornea stroma → scleral stroma‬ ‭○‬ ‭Cornea endothelium → TM epithelium‬ ‭○‬ ‭Terminates: Bowman’s, Descemet’s‬ ‭○‬ ‭Begins: conj stroma, episclera, Tenon’s capsule‬ ‭a.‬ ‭What attaches to the scleral spur posteriorly?‬ ‭i.‬ ‭Attaches to the TM anteriorly‬ ‭ii.‬ ‭Attaches to the CM posteriorly‬