Eye Histology

Describe the three layers of the eye

3 concentric layers

• outer supporting layer, the corneoscleral coat (colorless and blue) (aka fibrous layer)

• middle vascular coat or uvea (pink)

• inner photosensitive layer, the retina (yellow)

Describe the two parts of the fibrous layer

• Location?

• Function? (1);

• Describe properties

• What’s the limbus

Parts:

• Sclera

  • posterior 5/6 of the eyeball

  • maintain eye shape

  • Dense, opaque fibrous coat

• Cornea:

  • Ant. 1/6 of eye

  • Transparent part of fibrous coat

• Limbus:

  • Corneoscleral Junction

  • Contains Scleral Venous Sinus (canal of Schlemm)

    • on internal surface of the sclera

Describe the Sclera

• Episclera:

• Stroma

• Lamina Fusca

• Lamina cribrosa

• Collagen Arrangement:

Sclera Histological Features:

• Episclera:

  • Loose connective tissue (vascular)

• Stroma

  • irregularly arranged, thick collagen fiber bundles and many fibroblasts

• Lamina Fusca

  • suprachoroid lamina w/ melanocytes.

• Lamina cribrosa

  • specialization where optic nerve fascicles and vessels penetrates sclera

• Collagen Arrangement:

  • circles or figure eight patterns spiraling around openings for central retinal artery and vein

  • Perforated disc appearance.

Describe the Limbus

• CT present here

• Describe blood vessels found here

• Describe Contents of Internal Scleral Sulcus

• What is the Scleral spur

Limbus:

• CT:

  • Loose conjunctival subepithelial connective tissue

• Blood Vessels:

  • conjunctival capillaries terminate here.

• Contents of Internal Scleral Sulcus:

  • houses trabecular meshwork (with spaces of Fontana) and canal of Schlemm

    • Endothelium-lined channel

    • Pathway of aqueous humor outflow

      • thus, control of intraocular pressure

• Scleral spur:

  • origin of corneoscleral trabecullae

Describe the Cornea

• Properties

• Function

• list the 5 layers

Cornea:

• Properties:

  • Transparent

  • regular epithelium

  • avascular

  • regular arrangement of stromal components

• Function:

  • Major refractive element of eye

    • Curve irregularities = astigmatism

• 5 layers:

  • Epithelium

  • Bowman's membrane

  • Stroma (substantia propria)

  • Descemet's membrane

  • Endothelium

Describe the five layers of the Cornea

• Epithelium: (5-6 layers)

  • Histology:

  • External surface contains/function:

  • Cont. w/:

  • Regenerative capacity:

  • Innervation:

• Bowman’s Membrane:

  • Histology:

  • Function:

  • Location:

  • Regenerative Capacity:

• Stroma (substantia propria):

  • Histology:

  • Transparency influenced by:

  • Cont. W/:

• Descemet’s membrane

  • basement membrane for:

  • Extends into:

Corneal Endo:

• Histology:

• Function:

• Damage →

Epithelium: (5-6 layers)

• Histology:

  • Stratified squamous nonkeratinized epithelium

• External surface contains/function:

  • microvilli

  • microplicae

  • help stabilize precorneal tear film

• Cont. w/:

  • conjunctival epithelium

• Regenerative capacity:

  • new cells generated in limbal region → migrate centrally

  • minor wounds heal rapidly

• Innervation:

  • Numerous free nerve endings: touch, pain

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Bowman’s Membrane:

• Histology:

  • Acellular region

  • has Fine, randomly arranged collagen fibrils

• Function:

  • helps anchor corneal epithelium

    • alongside basal lamina on external surface

  • forms barrier for infection

• Location:

  • Ends @ limbus

• Regenerative Capacity:

  • Does not regenerate well → corneal scarring

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Stroma (substantia propria):

• Histology:

  • collagen fibers and fibroblasts

  • flattened, collagenous lamellae

    • parallel to corneal surface

    • oriented at right angles

  • Flattened modified fibroblasts (keratocytes) between layers

• Transparency influenced by:

  • regular diameter and spacing of collagen fibers

    • (maintained by glycosaminoglycans)

• Cont. W/:

  • sclera at limbus

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Descemet’s membrane

• basement membrane for corneal endothelium

• Extends into trabecular meshwork

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Corneal Endo:

• Histology: Simple squamous epithelium

• Function:

  • Allows passage of nutrients from aqueous humor

    • critical for corneal hydration (stroma)

• Damage →

  • swelling of stroma with loss of transparency

What are the 3 parts of the vascular layer

3 parts: choroid, ciliary body, iris

Describe the Choroid

• Properties:

• Location:

• Consists of:

• Function:

• Attachment:

• Bruch's membrane (lamina vitrea)

  • Function:

  • Clinical Significance:

• Suprachoroid

  • Location:

  • Consisting of:

  • potential space?

Choroid:

• Properties:

  • dark brown

  • highly vascularized

• Location:

  • between the sclera and retina

  • (posterior 5/6 of eyeball)

• Consists of:

  • choriocapillaris: net-like capillary beds

    • Fenestrated

    • Greatest density near macula

  • Bruch's membrane

    • a thin, amorphous hyaline membrane

• Function:

  • choriocapillaris → supplies outer layers of retina

• Attachment:

  • Firmly to pigmented layer of retina

  • Loosely to Sclera

• Bruch's membrane (lamina vitrea)

  • acellular membrane

  • Function:

    • Acts as Basal Lamina for:

      • retinal pigment epithelium, collagenous and elastic layers,

    • Acts as basement membrane for:

      • endothelium of choriocapillaris

  • Clinical Significance:

    • Drusen: Areas of diffuse or discrete thickening

• Suprachoroid

  • Location:

    • transition layer between choroid and lamina fusion of sclera

  • Consisting of:

    • interconnected layers

      • melanocytes,

      • fibroblasts,

      • connective tissue fibers

  • has surrounding potential (suprachoroidal) space

1. Describe the Ciliary Body

   • Structure:

   • Cont. W/:

   • Extends from? 2 Parts?

   • Function:

   • Annular smooth muscle fibers

   • 3 parts:

2. Describe Ciliary Epithelium

   • Histology:

   • 2 Layers? 1: Cont. of? 2. Function?:

3. Describe the Stroma

   • What is it?

   • Contains:

   • Cont. of:

4. Describe the Ciliary muscle

   • Innervation:

   • Mech (via radial/circular muscle fibers):

   • 3 Groups:

     • Attached to?

     • function?

5. What is the function of ciliary processes?

Ciliary Body

• Structure:

  • Ring Shaped; surrounds Lens

• Cont. W/:

  • choroid behind and the iris in front

• Extends from

  • scleral spur: ora serrata (irregular anterior border of retina)

    • Ant. 1/3 = (pars plicata)

      • contains: 70 radially arranged folds- ciliary processes

    • Post. 2/3: (pars Plana)

• Function:

  • suspend lens → control thickness

    • via zonular fibers of suspensory ligament

• Annular smooth muscle fibers

  • found w/in external part of ciliary body

• 3 parts:

  • ciliary epithelium

  • ciliary body stroma

  • ciliary muscle

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Ciliary Epithelium

• Histology:

  • Cuboidal, columnar epithelium

• 2 Layers:

  • Outer pigmented layer

    • Continuation of retinal pigment epithelium

  • Inner NON-pigmented layer:

    • Function

      • Secrete aqueous humor (ciliary process)

      • Produce vitreal collagen and hyaluronin

        • pars plana

      • Produces and anchors zonular fibers

        • pars plana

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Stroma

• What is it?

  • Vascular connective tissue layer extending into ciliary processes

• Contains:

  • melanocytes and fibroblasts

• Cont. of:

  • Choroid

---

Ciliary muscle

• Innervation:

  • postganglionic parasympathetic from ciliary ganglion

• Mech (via radial/circular muscle fibers):

  • Contraction (displaced towards optic axis) → suspensory ligament relaxed → lens thickens → near vision

  • Vice Versa for Far Vision

• 3 Groups:

  • Outer longitudinal muscle fibers

    • Attached to scleral spur

    • Function:

      • distends spaces of trabecular meshwork → aqueous outflow

  • Middle radial muscle fibers

    • Attached to circular fibers and scleral spur

  • Inner circular muscle fibers

    • Anteriorly located, sphincter arrangement

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Ciliary processes

• Function:

  • produce Aqueous Humor;

    • nourishes avascular lens and cornea

Describe the Iris

• Location

• Function

• Base of Iris?

  • cont. w/?

  • Connected to?

• Describe the substances that gives the iris pigments (2)

• Describe the muscles of Iris:

  • Orientation?

  • Function?

  • Innervation?

• Shape?

• Describe the anterior zone: What separate what here?

• List the four layers:

Iris:

• Location:

  • Contractile diaphragm located in front of the lens

  • Surrounds pupil

• Function:

  • Controls amount light entering eye

• Base of Iris:

  • cont. w/ ciliary body

  • connected to inner border of cornea by trabecular meshwork.

• Pigments:

  • Melanocytes and blood vessels in stroma

  • very dark posterior pigment epithelium

• Muscles of Iris:

  • Sphincter pupillae:

    • annulus of smooth muscle;

    • contraction decreases pupil size (miosis);

    • supplied by parasympathetic fibers

  • Dilator pupillae:

    • radial smooth muscle;

    • increases pupil size (mydriasis);

    • supplied by sympathetic fibers

• Shape:

  • Cone shaped

  • margin anterior to root Margin and root are thinnest

• Anterior Surface:

  • ciliary zone and pupil zone separated by collarette

    • Radially-oriented collagenous trabeculae (elevations) in ciliary zone

    • some surround ovoid crypts

• Layers:

  • Anterior border layer,

  • Stroma,

  • Dilator pupillae muscle,

  • Posterior pigment epithelium

Describe the four layers of the Iris

1. Anterior border layer

   • Histology:

2. Stroma:

   • Histology:

     • Communicates w/

   • Contains:

3. Dilator pupillae muscle (anterior epithelial layer)

   • Histology:

   • Orientation:

   • Basal vs apical portion:

4. Posterior pigment epithelium

   • Histology:

   • Extends:

   • Layers forms:

Anterior border layer

• Histology:

  • No epithelial cells

  • Uneven discontinuous layer consisting of:

    • dense collection of fibroblasts + melanocytes + (few) collagen fibers

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Stroma:

• Histology:

  • Loose CT containing:

    • fibroblasts + melanocyte + collagen fibers,+ network of blood vessels.

  • Communicates w/

    • aqueous humor of anterior chamber

      • via openings in anterior border layer

• Contains: sphincter pupillae

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Dilator pupillae muscle (anterior epithelial layer)

• Histology:

  • Layer of myoepithelial cells

    • derived from ant. layer of pig. epithelium

• Orientation:

  • Radially Oriented

• Basal vs apical portion:

  • Basal: Muscular

  • Apical: Epithelial

    • lightly pigmented

    • apposed to posterior pigment epithelial cell

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Posterior pigment epithelium

• Histology:

  • Large cuboidal epithelial cells that are densely pigmented.

• Extends:

  • onto anterior surface at pupillary margin. (short distance)

• Layers forms:

  • radial folds most apparent near pupil margin

Describe Aqueous Humor

• Flow Pathway

• Describe Glaucoma:

  • Pathophysiology

  • Types

Flow:

• Post. → Ant. chamber (through zonule and pupil)

• iridocorneal angle → trabecular meshwork

• Drains into scleral venous sinus (canal of Schlemm)

Glaucoma:

• Pathophysiology:

  • Increased resistance to aqueous humor outflow → increased intraocular pressure

• Types:

  • Open angle

    • Obstruction of trabecular meshwork

  • Close Angle:

    • Iris physically blocks inner surface of anterior chamber angle

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Note: Both forms may be primary or secondary.

Describe the Retina

• Two Layers

  • contents of each

• Two Divisions:

  • Location?

  • Extends?

  • Contains?

Two Layers:

• Inner neural layer

  • Sensory and neural cells

    • Photoreceptors (rods/cones);

    • bipolar cells,

    • ganglion cells;

    • interneurons (horizontal and amacrine cells);

    • supporting cells (Müller cells)

  • Optic Disc (blind spot)

  • Macula Lutea — yellow elliptical area (1.5mm diameter) toward center of retina;

    • contains: Fovea Centralis (foveola)

      • central depression

      • area of most acute vision;

      • contains only cones

• Outer pigmented layer

  • Retinal pigment epithelium

  • Fused with choroid

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Divisions:

• Optic part of retina (pars optica)

  • light sensitive;

  • extends from optic disc → Ora Serrata

    • irregular border slightly posterior to ciliary body.

• Non-neural part of retina (pars caeca)

  • anterior to ora serrata

  • Extends:

    • forward over ciliary processes and posterior iris

  • Contains:

    • pigmented and columnar epithelium

Describe the two layers of the Retina

• Outer Pig. Layer:

  • cont. w/

  • Function?

  • Hisgo. features:

    • Epithelium made up of?

    • Adherent to?

    • Function of Apical Microvillia?

• Neurosensory Layer:

  • Overview

• What is the first Layer?

• What is the Second Layer?

  • Differentiate between Rods and cones

Outer Pigmented Layer:

• Cont. w/:

  • pigment epithelium of pars plana

• Function:

  • Absorb excess visible light and UV

  • Forms: blood-retinal barrier layer

    • via tight junctions

  • Adhesion to neurosensory retina

  • Phagocytosis

    • of shed photoreceptor membrane disks

• Histological Features:

  • epithelium

    • Cuboidal, columnar epithelium

      • More columnar in central

      • flattened in periphery

  • Strongly adherent to Bruch's membrane

  • Apical microvillia

    • embrace photoreceptor cells;

    • increase SA for metabolic exchange,

    • aid in phagocytosis

    • maintain adhesion to neural retina

NOTE: Site of retinal detachment — a potential space, subretinal space, found here.

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Neurosensory Layer:

• Overview:

  • 3 layers of cell bodies, 2 layers of synapses

  • ten layers in all

Layers:

1. Retinal pigment epithelium

2. Photoreceptor Layer

   • Rods:

     • 120 million per eye

     • Function:

       • Highly Sensitive

       • Dim light Vision

       • no color differentiation

     • Histo Features:

       • Long slender cells

       • Membrane bound lamellae (disks) in outer segment containing rhodopsin

   • Cones:

     • 7 million/eye

     • Function:

       • Less sensitive to light

       • sensitive to' colors

     • Highest Density: Fovea

     • Histo Features:

       • Conical shaped outer segments

       • Shorter than rods

       • Disks in outer segment containing iodopsin

       • free communication with interphotoreceptor space

Describe layers 3, 4, 5,6 of Retinal Layer

• 3:

  • Function

  • Formed by?

  • Describe Muller Cells?

• 4:

  • Contains?

• 5:

  • Layers of?

  • Composed of?

• 6:

  • Contains?

• 3: External limiting membrane

  • Function:

    • mechanical support

  • Formed by

    • adherens junctions between apical ends of Müller cells and photoreceptor inner segments

  • Müller cells

    • Principal supporting glial cells of retina

    • Radial orientation

    • Function:

      • Envelop blood vessels, neuronal cell bodies and processes

• 4: Outer nuclear layer

  • Contains nuclei of rods and cones

• 5: Outer plexiform layer

  • Layer of synapses

  • Composed of process of photoreceptors, bipolar cells, and horizontal cells.

• 6: Inner nuclear layer

  • Contains nuclei of bipolar, horizontal, amacrine and Müller cells.

  • Capillaries (outer plexus) from retinal arteries end in this layer

Describe Bipolar Cells

• Function

• Connectivity

Describe Horizontal Cells

• Cell Process?

• Synapses on?

• Function?

Describe Amacrine Cells:

• Dendritic Processes?

• Function?

Bipolar Cells:

• function:

  • Transmit signals from photoreceptor to ganglion cells (single axon passes inward to synapse)

  • Contributes to sensitivity of rod system

• Connectivity:

  • Fovea: One Cone to One BP

  • Periphery: 50-100 rods to One BP

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Horizontal cells

• Cell processes project horizontally

• Synapse on multiple photoreceptor pedicles

• Function: Integrative

  • release inhibitory neurotransmitter (GABA)

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Amacrine:

• Many dendritic processes

  • Terminates on synaptic complex between BP and gang, cells

Function: Modulatory

• (GABA, 5-HT, Ach, DOMV peptide) often inhibitory

Describe layer 7-10

• 7: Contains?

• 8: Function?

• 9: contains? Myelination?

• 10: Composed of? Separates?

• 7: Inner plexiform layer

  • Contains processes of bipolar, ganglion, and amacrine cells.

• 8: Ganglion cell layer

  • Output cells of neural retina

    • Axons coalesce → optic nerve

      • up to 7 layers of cells combined

• 9: Optic nerve fiber layer

  • Contains axons of ganglion cells coursing toward optic disc;

  • unmyelinated until enter optic nerve

• 10: Internal limiting membrane

  • Composed of basal lamina of Müller cells

  • Separates vitreous and retina

Describe the Chambers of the Eye:

• Ocular:

  • Location?

  • Divides into

• Vitreous:

  • Location

  • Contains

  • Composed of?

  • Function

Ocular chamber

• Location:

  • anterior to the lens and suspensory ligament

• Divided into: anterior and posterior chamber by iris

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Vitreous Chamber

• Location:

  • posterior 4/5 of eye between lens and retina

• Contains:

  • transparent, avascular, gelatinous substance

    • vitreous body

• Composed of:

  • water (-99%), collagen fibers, and hyaluronan

• Function:

  • Holds retina in place,

  • supports lens,

  • transmits light

Describe the Lens

• Prop.

• relationship w/ ciliary

• what happens w/ age

• 3 parts?

Lens:

• Properties:

  • Avascular, biconvex ellipsoid structure

  • Normally Transparent

• Ciliary Muscles Relationship:

  • alters shape of lens → change refractive power

• W/ Age:

  • Loses elasticity

  • may lose transparency

    • Cataracts

• 3 parts:

  • Capsule

  • Anterior epithelium

  • Lens fibers

Describe the 3 parts of the lens

• Capsule

  • what is it?

  • produced by

  • location?

  • Thickest @

  • Describe Peri-equatorial region

    • Function?

    • Histology?

    • Produced/maintained by?

• Epithelium

  • Histo

  • Location

  • Transformation

• fibers

  • Migration?

  • Deeper older cells?

  • Contains?

Lens Capsule

• What is it?

  • Thickened, smooth basement membrane

• Produced by

  • lens epithelium and lens fibers

• Location:

  • Completely envelops lens

• Thickest:

  • pre and post equatorially (17-28 prn)

• Peri-equatorial region

  • Where zonules attach

  • Histo

    • Dense, glassy bundles

      • each contains series of fine fibers composed of microfibrils

  • Produced/maintained by:

    • pigmented epithelium of pars plana

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Lens Epithelium

• Histo:

  • Simple cuboidal epithelium

• Location:

  • only on anterior surface of lens

• Transformation:

  • Equatorial lens epithelial cells → elongate → transform into lens fibers throughout life

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Lens fibers

• Migration:

  • apical part of lens fiber → anteriorly and basal part posteriorly

• Deeper (Older) cells

  • lose their nuclei

• Contain: crystallins

  • specialized proteins