Eye Anatomy and Function: Sclera, Uvea, Cornea, and Lens

Lissamine Green Stain

  • Lissamine green stains the cornea and bulbar conjunctiva.
  • Cause: Dry eye.

Dry Eyes

  • Symptoms: Mild burning and grittiness due to corneal drying.
  • Signs:
    • Reduced tear meniscus on the lower eyelid.
    • Lackluster cornea.
    • Fluorescein and rose bengal staining of the inferior cornea and conjunctiva.
    • Reduced wetting on Schirmer’s test.
  • Causes: Abnormalities in any of the tear film layers.

Watery Eyes

  • Symptoms: Excess fluid production from the lacrimal gland.
  • Causes: Reflex watering due to inflammatory conditions such as foreign body, iritis, acute glaucoma, injury, etc.

Main Textbooks

  • Snell and Lemp: Clinical anatomy of the eye, Blackwell Scientific Publications.
  • Oyster: The Human Eye: structure and function, Sinauer Associates, Inc. Publishers.

Sclera and Uvea

  • Anatomically, the globe of the eye, stripped of its orbital attachments, has three distinguishable layers:
    1. Outer protective coat: Cornea and sclera.
    2. Middle vascular coat, pigmented to form a darker chamber: The uvea.
    3. Inner, light-sensitive coat: The retina.

Globe Layers Illustrated

  • Diagram showing the arrangement of:
    • Eyelid
    • Pupil, Iris, Sclera
    • Optic nerve
    • Medial, Inferior, and Superior rectus muscles
    • Macula, Retina, Lens, Cornea
    • Inferior oblique muscle
    • Conjunctiva

Uvea Layers Illustrated

  • Posterior Choroid
  • Choroid (uvea layer)
  • Sclera
  • Retina
  • Ciliary body (anterior uvea)
  • Iris

Sclera

  • The globe takes its shape from the outer coat (cornea and sclera).
  • 1/6 cornea and 5/6 sclera
  • Sclera (skleros = ‘hard’ in Greek): The "white of the eye" that envelops internal ocular structures and is a tough fibrous envelope.

Sclera Thickness

  • Adult sclera is approximately 1 mm thick posteriorly.
  • Equator: 0.5 – 0.6 mm.
  • Immediately posterior to the tendinous intersection of the recti: 0.3 mm.
  • Corneoscleral junction: 0.8 mm.

Sclera Functions

  • Protects intraocular contents from trauma and mechanical displacement.
  • Preserves the shape of the eyeball.
  • Provides a rigid insertion for extraocular muscles.

Sclera Whiteness

  • Not always maintained:
    1. Some races: pigment shows, especially towards the limbus region.
    2. Normal babies & malnourished children: bluish tinge due to thinness of the sclera (pigment cells in the choroid are visible).
    3. Older people: yellowish tinge appears due to accumulation of fatty degenerative products.

Anatomy of Outer Eye Layers

  • Diagram illustrating:
    • Conjunctiva and conjunctival plexus
    • Tenon's capsule
    • Superficial and deep episcleral plexus
    • Episclera
    • Sclera
    • Cornea
    • Fusion of Three layers at the limbus

Plexus Definition

  • Network of anastomosing or interlacing blood vessels or nerves.
  • An interwoven combination of parts or elements in a structure or system.

Layers of the Outer Eye

  • Most superficial layer: conjunctiva with the conjunctival vascular plexus, easily moved over underlying structures.
  • Underneath: Tenon’s capsule and the episclera, with an episcleral plexus of straight, radially arranged vessels.

Deepest Layer

  • The deepest layer is the scleral plexus, tightly adherent to the sclera.

Sclera Subdivisions

  • The sclera can be subdivided into three layers:
    1. Episclera: Outermost layer, loose connective tissue; a thin layer between the conjunctiva and sclera; connects to the Fascial Sheath of the eyeball (Tenon’s capsule) by fine strands of tissue; merges with the underlying scleral stroma. The episclera has a blood supply from the anterior ciliary arteries.
    2. Scleral Stroma: Dense fibrous tissue that intermingles with fine elastic fibers. Collagen fibres vary in diameter and orientations. The scleral stroma contains a few flat fibroblasts.

Episcleritis

  • Inflammation of the Episclera

Scleral Stroma Details

  • Photomicrograph showing the stroma and the lamina fusca.
  • Elongated fibroblasts between collagen bundles in the stroma.
  • Melanocytes in the lamina fusca.
  • Bundles of collagen fibers and nucleated fibroblasts.
  • Collagen bundles run parallel with the surface, but many are randomly arranged to form a feltlike structure.

Lamina Fusca

  • Innermost layer, faintly brown in color.
  • Contains melanocytes (pigment-producing cells in skin, hair, and eye) that determine color.

Scleral Vasculature

  • The sclera has few blood vessels, mostly towards the outer surface (episclera).
  • A number of vessels passing to and from the interior structures pass through it.

Scleral Apertures

  • There are three main sets of apertures in addition to those presented by the cornea and optic nerve:
    1. Posteriorly: The optic nerve is surrounded by a ring of ‘ciliary’ vessels and nerves passing through the sclera.
    2. Medially: About 4 mm behind the equator, four large ‘vorticose’ (vortex) veins leave, one in each quadrant of the globe.
    3. Anteriorly: Some smaller ciliary vessels and nerves which accompany the recti pass through the sclera just behind the limbus.

Posterior Sclera Vessels

  • Diagram showing:
    • Venae vorticosae (V.V.)
    • Long posterior ciliary artery and nerve (L.P.C.)
    • Short posterior ciliary vessels (S.P.C.)
    • Central retinal vessels in optic nerve (C.R.V.)

Scleral Vascular Ring

  • Some posterior vessels (arteries) anastomose (reconnection of two streams) within the sclera to form a continuous vascular ring around the optic nerve (circle of Zinn).

Optic Nerve and Sclera

  • In the posterior half of the globe, where the optic nerve leaves the eye, the outer scleral collagen fibres turn outward to form the outer (dural) sheath of the nerve.
  • Some collagen fibres cross the optic nerve to form part of a supporting network, the lamina cribrosa.
  • The lamina cribrosa is NOT part of the sclera.

Uvea

  • Within the sclera is the dark veined coat, the uvea.
  • Anterior part: forms the mobile, coloured structure visible through the cornea, the iris; central aperture is the pupil through which light enters the eye.
  • Beyond the iris and concealed behind the anterior sclera, the uvea thickens to form the ciliary body.
  • Thins again towards the equator to become the choroid.

Choroid

  • The choroid is a thin soft brown coat lining the inner surface of the sclera.
  • Extremely vascular.
  • Extends from the optic nerve, posteriorly, to the ciliary body anteriorly.
  • About 0.22 mm thick at the posterior pole and gradually thins anteriorly to 0.1 mm.

Choroid Layers

  • The choroid may be divided into three layers:
    1. Vessel layer: Loose connective tissue containing melanocytes and numerous large and medium sized blood vessels. The arteries are branches of the short posterior ciliary arteries and the veins are much larger and converge to the vorticose veins.
    2. Capillary Layer: Consists of a network of capillaries. They are fed by arteries in the vessel layer and drained by the veins in the vessel layer. The capillaries are supported by connective tissue containing melanocytes.
    3. Bruch’s Membrane: 2 to 4 \,\mu m thick, consists of 5 different components:
      1. The basement membrane of the endothelium of the capillaries of the capillary layer
      2. An outer layer of collagen fibres
      3. A meshwork of elastic fibres
      4. An inner layer of collagen fibres
      5. Basal membrane of the pigment epithelium of the retina

Choroid Function

  • Main function is to nourish the outer layers of the retina with its blood vessels.
  • Also serves to conduct many blood vessels forward to the anterior regions of the eye.
  • Large number of pigment cells in the choroid absorb excess light entering the retina and prevents reflection.

Cornea Introduction

Cornea Functions

  • Lens: curved
  • Window: transparent
  • Protection: strong

Cornea Properties

  • Isolated from blood supply (avascular).
  • Temperature: 38\degree C \pm 1 \degree C.

Cornea Anatomy

Cornea Layers

  • Five layers:
    • Epithelium
    • Bowman's layer
    • Stroma
    • Descemet's membrane
    • Endothelium

Cornea Transverse Section

  • Illustration of the transverse section of the human cornea showing all five layers.

Cornea: Epithelium

  • Most anterior layer.
  • 5-6 cells thick (50 \,\mu m).
  • Most regular squamous stratified epithelium in body.
  • With tear film provides optically smooth front surface to cornea.
  • Squamous: stratified epithelium that consists at least in its outer layers of small scale like cells.

Epithelium Cell Types

  • Three types:
    • Squamous
    • Wing
    • Basal

Epithelium Cell Details

  • Diagram showing:
    • Squamous cells
    • Wing cells
    • Basal cells
    • Bowman's layer
    • Nerve fiber
    • Stroma

Epithelium Cell Cycle

  • Undergo 10-day cycle:
    1. Basal
    2. Wing
    3. Squamous
  • Programmed cell death – apoptosis. Different to cell death from infection or injury
  • Cells bound together by desmosomes.
  • Epithelium is a passive barrier, stopping water from entering the stroma below.
  • Basal cells have gap junctions.

Epithelium Repair

  • Continual loss of cells from surface.
  • Only basal cells capable of division.
  • Repair by means of basal cells and limbal stem cells.

Bowman's Layer

  • Modified portion of stroma.
  • No cells; Narrow collagen fibrils.
  • 12\,\mu m thick.
  • Some species – no Bowman's layer!

Corneal Stroma

  • Provides:
    • Strength
    • Elasticity
    • Form of cornea

Collagen Structure

  • Collagen triple helix.
  • 1.5 nm
  • Triple alpha Helix.
  • Alpha helical coiled coil
  • Intermolecular cross-link

Collagen Fibril Structure

Stroma Components

  • Collagen fibrils withstand radial tension.
  • Proteoglycans withstand compression along AB.

Corneal Lamellae

  • Contain parallel collagen fibrils.
  • Run at various angles from limbus to limbus.
  • Typically 1.5-2.5 \,\mu m thick.
  • Human stroma contains about 200 lamellae in its thickness.
  • Posterior lamellae don’t interweave.
  • Anterior lamellae do interweave.

Keratocytes

  • Mostly between lamellae.
  • Flattened cells with long pseudopodia in contact with each other.
  • Occupy about 10% of stromal volume.
  • Responsible for slow turnover.

Keratoblasts Details

  • Produce collagen fibres and help maintain fibril order
  • Important in wound healing

Stroma Composition

  • Composition of the bovine corneal stroma:
    • Water: 78%
    • Collagen: 15%
    • Other proteins: 5%
    • Proteoglycans: 1%
    • Salts: 1%

Proteoglycans

  • Long repeating sulphated disaccharides.
  • Highly negatively charged – attract water.
  • Attached to protein core.
  • Act as collagen spacers.

Descemet's Membrane

  • 6-12 \,\mu m thick.
  • Structureless band – gives mechanical strength and a base for endothelium.
  • 6 \,\mu m thick

Limbus

Limbus Details

  • Diagram illustrating:
    • Limboscleral junction
    • Corneolimbal junction
    • Pathologist's limbus
    • Histologist's limbus

Limbus Functions

  • Nourishment of peripheral cornea.
  • Corneal wound healing.
  • Immunosurveillance.
  • Contains pathways of aqueous humour outflow.
  • Contains circumferential or tangential collagen fibrils.
  • Site of surgical incisions.

Swelling Tendency

  • STROMA
  • Contains GAGs which create a stromal swelling pressure of about 50mmHg in normal human

Corneal Swelling Effect

  • Fixed-ve charge
  • Donnan effect
    • Creates a swelling pressure

Corneal Transparency

  • Short range: the first- or second-nearest neighbours of an atom or molecule.
  • Long range order of atoms or molecules - crystalline lattice.
  • Short range order.

Transparency Theories

  • Several theories:
    • Uniform Refractive Index – cornea is transparent because the refractive index (RI) of the collagen fibrils matches the refractive index of the matrix between them.
    • Not true – RI of human corneal collagen = 1.411 and RI of interfibrillar matrix = 1.365

Diffraction Grating Theory

  • Collagen fibrils are arranged in a crystalline lattice with perfectly regular spacing.
  • As fibrils all have the same diameter they act like a three- dimensional diffraction grating whose spacing (interfibrillar centre- to-centre distance) is much less than wavelength of light.
  • Not true. X-ray diffraction (Sayers, Elliott, Meek) showed no long-range order

Short Range Order Theory

  • Transparency would result from destructive interference of all the scattered light as long as there is sufficient short-range order in the positioning of the uniform diameter collagen fibrils.
  • Only the straight through light would interfere constructively and thus be transmitted by the cornea

Light Interference Diagram

  • Destructive interference
  • Constructive interference
  • Destructive interference

Animations

  • KEY TO ANIMATIONS
    • Plane wave
      • black line = peak
      • grey line = trough
    • Scattered wave – red = peak, grey = trough
    • Calculated Intensities = square of the amplitude

The Lens

Accommodation

  • Human accommodation – deformation of lens
  • Some species – Axial movement of lens or retina
  • Deformation itself is not accommodation

Lens Structure

  • The lens: transparent, biconvex structure situated between iris & vitreous body.

Lens Measurements

  • Anterior radius of curvature 8-14mm
  • Posterior radius of curvature 4.5-7.5mm
  • Axis length 4mm (unaccommodated) – grows at 0.023mm/year
  • Equatorial diameter (adult) 10mm

Lens Position

  • Equator: encircled by ciliary processes of ciliary body and lies 0.5 mm from them.

Lens Support

  • The lens is kept in position by the suspensory ligaments (Zonules of Zinn).

Zonules of Zinn Details

  • Diameter 1-2 \,\mu m
  • Comprised of fibrillin (connective tissue with elastic properties)

Accommodation Process

  • Accommodation for close objects: ciliary muscles contract.
  • Relieves the tension of the radiating fibres of the zonule; the lens assumes more globular shape.
  • At the same time: sphincter pupillae muscle contracts, the pupil becomes smaller; light through the thickest, central part of the lens reaches the retina.

Ciliary Muscle Action

  • Relaxed vs Contracted