EYE REVIEW!

**Cornea and sclera are continuous, but have different morphology & function

Light enters eye through cornea, through anterior chamber, passes through pupil, lens, vitreous body, & image falls on the retina.

Between cornea & iris is the anterior chamber (filled with aqueous humor, continuously produced).

Vitreous body (NOT continuously produced) is a chamber between lens & retina.

Accommodation

• The lens changes shape to focus light on the back of the eye regardless of the distance of the object

• Cornea curvature is fixed, so focus comes from changes in the lens curvature through the ciliary muscles

• Lens with no tension: would be curved/round• normal state of lens: flattened by the tension of the

zonules/suspensatory ligaments. • To curve lens: ciliary muscles contract and ciliary body moves

closer to the lens. Zonules go slack. • To flatten the lens: ciliary muscles relax, ciliary body moves away

from the lens. Stretches the zonules.

• IMPORTANT POINTS: 1. Natural state of the lens is round. 2. Ciliary muscle is a sphincter

Lens

• 2-3000 lens fibers: extremely long cells with no nuclei, stretch anterior to posterior. Cytoplasm filled with crystallin, arranged in a regular lattice

• Anterior surface: layer of cuboidal cells with nuclei, merges with nucleated cells at the lens margin (proliferate throughout life to become lens fibers)

• Lens capsule: thick basement membrane surrounding lens. Attachment site for the zonules.

• Posterior chamber: small space between zonules and lens, filled with aqueous humor.

• Transparent because of anucleate nature and fibers containing crystalline proteins.

Cornea• Transparent part of the outer/scleral coat of the eye.• Highly innervated (protects from harm), avascular, gets O2 & nutrients from aqueous humor and

outside surface. Kept moist by tears.• 5 layers:

– Corneal epithelium: 3-7 or 4-6 cells thick, non-keratinized stratified squamous– Bowman’s Membrane: thick, retractile CT layer that separates stroma&epithelium– Corneal stroma: fibroblasts & ~200 thin, precisely org. lamellae of type I collagen– Descemet’s membrane: thick elastic basement membrane– Corneal endothelium: highly metabolically active (mitochondria & pumps for fluid-h20+proteoglycans in

stroma would destroy org.), no stem cell population

• Limbus: transition between cornea & sclera. Stem cells for proliferation of corneal epithelium. • Conjunctiva: epithelium at limbus. stratified columnar epithelium with goblet cells. Extends from

limbus to cover interior eyelids. • Sclera: Deeper layers of the cornea become opaque sclera. Covers eye with a tough coat, pierced

at lamina cribrosa at posterior pole for optic nerve exit • Transparency from: regular and highly organized ECM, tight junctions in corneal epithelium, and

fluid pumping by corneal endothelium (removes h20)

Cornea’s optical properties

• Cornea transparent surface that bends/refracts the light and focuses it on the back of the eye

• Responsible for most of the eye’s ability to refract and focus light

• Refraction of the cornea is fixed.

Iris• Colored structure surrounding the pupil.• Controls amount of light entering the eye• Controls the size of the pupil through the sphincter pupillae

(shrinks pupil) and a diffuse dilator pupillae (enlarges pupil)• Spongy stroma with melanocytes: faces anterior chamber

• Pigmented epithelium faces posterior chamber. This

epithelium becomes the ciliary body laterally.

Ciliary Body• 2 roles:

1. Smooth muscle allows changing in lens shape2. Epithelium produces aqueous humor

• Aqueous humor: low-protein plasma-like substance made continuously by the epithelium of ciliary body. Nourishes cornea, lens, iris, corneal endothelium, & stroma. Secreted into posterior chamber, flows around iris through pupil to “the angle”

• “the angle”: angle between cornea and iris.drains through trabeculae in edge of cornea and through CT until it enters Canal of Schlemm. Then to episcleral venous system.

• Posteriorly, ciliary body is in continuity with the choroid. • Choroid: vascular pigmented tissue supplying nutrients to the outer retina. (inner retina

nourished by central retinal artery).• Ciliary body joins modified retina at the ora serrata. • Bruch’s membrane: a thick common basement membrane between choroid & RPE.

photoreceptors & RPE receive O2 and nutrients by diffusion from choroidal circulation through this membrane.

Vitreal Cavity

• Filled with vitreous humor– Hyaluronic acid– Proteoglycans– Type II collagen– Water

• Produces pressure that gives eye its form

Retinal Pigment Epithelium

• Separated from choroid by Bruch’s membrane• Simple cuboidal epithelium resting on thick, Bruch’s

membrane.• Pigment in cells absorb light• Sockets in which ends of photoreceptors nestle• Transport nutrients to photoreceptors and transport waste from

photoreceptors. • 3 roles:

– Isolates retinal neurons from blood– Phagocytoses membrane that is shed by photoreceptors– Reduces light scatter

Retina

• Photoreceptors: contain photopigment in discs located within outermost segment. When light interacts with the photopigment, conformational change and neural signal

• Blood supply: central retinal artery enters through optic disk and ramifies inner surface of retina

• CRA sends branches to inner 2/3 of the retina, but not the photoreceptors

• Capillary network of the choroid, the choroicapillaris supplies photoreceptors through Bruch’s membrane and the RPE

• 2 types of photoreceptors: – Rods: sensitive in dim light, not wavelength sensitive– Cones: sensitive in bright light, differential sensitivity to

wavelengths… color!

Retinal cells• Inside out structure: light must

pass all cell layers to reach the photoreceptors

• Simplest circuit: photoreceptor --> bipolar cell --> retinal ganglion cell

• Lateral connections: horizontal cells (between photoreceptors and bipoolar cells in the outer plexiform layer) & amacrine cells (between bipolar cells and ganglion cells in the inner plexiform layer)

• Muller cells: neuroglia. have somata in the inner nuclear layer and cytoplasm through the whole retinal thickness.

ONL

INL

GCL

OPL

IPL

Photoreceptor

Ganglion Cell

Bipolar Cell

Amacrine Cells

Horizontal Cells

Fovea

• Specialized for the highest acuity, high resolution vision

• Small area, temporal to the optic disk. Most cells other than cones are displaced laterally, allow tight packing of photoreceptors sensitive to wavelength in daylight. Blood vessels excluded.

• Center of gaze

Optic Disk

• 1) RGC axons exit the eye• 2) retinal blood vessels enter the eye• RGCs travel across the retinal surface to become the optic

nerve head/optic papilla/optic disk, where they become myelinated.

• Only axons.. No photoreceptors--blindspot in your visual field.

• Axons make a right angle, penetrate sclera through perforations called the lamina cribrosa. Central retinal artery travels to retina through the center of the nerve and the central retinal vein is alongside the artery.

Eye Development

Moore & Persaud, The Developing Human, 5th Ed

• The optic stalk grows out of the brain (diencephalon) and forms the optic cup

• Optic cup: a primordial retina (an undifferentiated neuroepithelium) separated from primordial RPE by a fluid-filled space that is in continuity with the 3rd ventricle

• When optic cup comes into contact with ectoderm, it induces formation of the lens vesicle, which then invaginates and pinches off.

• Retina and RPE are derived from brain, optic nerve is part of the central nervous system.

• Retinal neurons & neuroglia are generated from multipotent stem cells in the neuroepithelium along inner/outer walls of optic cup. They migrate through neuroepithelium to the vitreal margin. First cells generated are RGCs, last are photoreceptors. Photoreceptors are still generated in early postnatal life. Retina isn’t in its final form until well after birth.

What is glaucoma?

• Glaucoma: elevated intraocular pressure from overproduction of aqueous humor or blockage in drainage. High pressure in the anterior chamber transduced through vitreous body, pressure on retina. Can damage neural retina by impeding blood flow in reitinal arterioles, pressure on RGC axons. Cupping of lamina cribrosa and optic disc.

Clinical Correlations• Glaucoma: elevated intraocular pressure from overproduction of aqueous humor or blockage in

drainage. High pressure in the anterior chamber transduced through vitreous body, pressure on retina. Can damage neural retina by impeding blood flow in reitinal arterioles, pressure on RGC axons. Cupping of lamina cribrosa and optic disc.– Open angle glaucoma: increased production by ciliary body epithelium– Closed angle glaucoma: iris closes the angle, blocking drainage

• Cataract: opacity of the lens, comes from UV light changing the conformation of crystallin in the cytoplasm of lens fibers.

• Presbyopia: lose ability to accommodate b/c of lens hardening, contractility of ciliary muscle can’t change shape of lens. Increases with age.

• Retinal detachment: separation of retina from pigment epithelium

• Papilledema: swelling of optic nerve head from increased intracranial pressure (from tumor or hemorrhage)