Ocular Anatomy Midterm 2

What percentage of the eye’s refractive power does the cornea provide?

75–80% of the eye’s refractive power.

Why is the cornea transparent and avascular?

To provide optimal light transmittance.

What role does the cornea play as a barrier?

Acts as a barrier between the eye and outside environment.

What are additional key features of the cornea’s structure and surfaces?

• Highly innervated

• Anterior surface covered by tear film

• Posterior surface borders anterior chamber

• Periphery continuous with conjunctiva and sclera

Is the cornea fully grown at birth?

Nearly at adult size at birth.

What are the anterior surface dimensions of the cornea?

• Horizontal diameter = 11.7 mm

• Vertical diameter = 10.6 mm

• Radius of curvature = 7.6 mm

• Wider than it is high (elliptical)

What are the posterior surface dimensions of the cornea?

• Horizontal and vertical diameter = 11.7 mm

• Radius of curvature = 6.5 mm

How much of the eye’s surface does the cornea comprise?

Approximately 1/6 of the eye’s entire surface.

Compare the anterior and posterior corneal surfaces.

The anterior surface is flatter than the posterior surface.

What is the corneal thickness in the periphery vs. the center?

• Periphery: 640–670 µm

• Central: 535–555 µm

What are the five layers of the cornea (anterior to posterior)?

• Epithelium (5–7 cells thick)

• Bowman’s layer (clear, acellular)

• Stroma (90% of thickness)

• Descemet’s membrane (basement membrane for endothelium)

• Endothelium (single cell layer)

How thick is the corneal epithelium and how many cell layers does it have?

5–7 cells thick (~50 µm), composed of 3 layers.

What are the three main layers of the corneal epithelium?

• Superficial/surface cell layer (2–3 cells thick)

• Wing cell layer (2–3 cells thick)

• Basal cell layer (single layer)

What is the turnover rate of the corneal epithelium?

7–10 days.

Where is the corneal epithelium continuous with the conjunctiva?

At the limbus.

What are the characteristics of surface squamous cells?

• Very smooth anterior surface

• Flattened nuclei, fewer organelles

• Secrete glycocalyx (adjoins mucin layer of tear film → tear stability)

• Apical microvilli increase surface area

What junctions join surface epithelial cells?

Tight junctions (zonula occludens) laterally at apical surface.

What is the function of tight junctions in corneal epithelium?

Prevent intercellular movement from tear film, prevent excess fluid uptake, act as semipermeable barrier (through but not between cells).

What adhesion exists between epithelial cells besides tight junctions?

Desmosomes.

What are the types of cell junctions?

Tight junctions, adhering junctions, desmosomes, hemidesmosomes, gap junctions.

What are tight junctions (zonula occludens)?

Seal adjacent cells like a ziplock, prevent leakage of solutes/water.

What are adhering junctions (zonula adherens)?

Mechanical strength, link cytoskeleton of adjacent cells, like velcro straps.

What are desmosomes (macula adherens)?

Strong adhesion via intermediate filaments, like rivets/spot welds, found in stressed tissues.

What are gap junctions?

Allow direct communication via ions/small molecules, like tunnels/bridges.

What are hemidesmosomes?

Anchor epithelial cells to basement membrane (not to other cells), like bolts to a foundation.

What are wing epithelial cells?

2–3 cells thick, transitional polyhedral structure, have wing-like processes, convex anterior, concave posterior, joined by desmosomes and gap junctions.

What are basal epithelial cells?

Single columnar layer, oval nuclei toward apex, apical surface adjacent to wing cells, interdigitations + desmosomes, joined via gap junctions.

What do basal epithelial cells secrete?

Their own basement membrane (different from Bowman’s).

How are basal epithelial cells anchored?

Hemidesmosomes → anchoring fibrils into Bowman’s (thru Bowman’s) and into the stroma.

How long does basement membrane healing take?

Up to 6 weeks.

How thick is Bowman’s layer?

8–19 µm.

What is Bowman’s layer composed of?

Dense interwoven collagen fibrils in mucoprotein ground substance, random, not bundled.

Is Bowman’s layer a membrane?

Sometimes called one, but really a transition to stroma.

How does Bowman’s layer differ from stroma?

Acellular, smaller fibril diameter, irregular anterior surface.

How does Bowman’s merge with stroma?

Posteriorly fibrils gradually order into bundles merging with stroma.

Does Bowman’s regenerate?

No, injured → replaced with epithelial cells or scar tissue.

How resistant is Bowman’s layer?

Very resistant to shearing, penetration, infection.

What happens to nerves in Bowman’s?

Corneal nerves pass through and lose Schwann covering, leaving “naked nerves.”

How thick is corneal stroma?

450–500 µm (~90% of corneal thickness).

What is corneal stroma composed of?

Collagen fibrils, keratocytes, extracellular ground substance (proteoglycans, GAGs).

Describe stromal collagen fibrils.

25–35 nm diameter, uniform, parallel, form lamellae (200–300).

How do stromal lamellae run?

Parallel to corneal surface, fibrils run limbus to limbus, adjacent lamellae angled with interweaving.

How do lamellae differ in stroma?

Anterior 1/3 = thin, more branching/interweaving. Posterior 2/3 = more regular, thicker lamellae.

Where is cross-linking greater in stroma?

Anterior cornea → more rigid, maintains curvature.

How is posterior stroma arranged?

Compact fibrils, more regularly arranged, interlace with Descemet’s.

What are keratocytes?

Corneal fibroblasts, flattened, lie within/between lamellae, more in anterior stroma, connected by gap junctions, become active after injury.

What is the role of keratocytes outside injury?

Maintain stroma by making collagen, GAGs, matrix metalloproteinases.

What other cells can be found in stroma?

WBCs, lymphocytes, macrophages, PMNs (↑ in pathology).

What is stromal ground substance?

Space filler between fibrils, lamellae, and cells; contains GAGs.

What do stromal GAGs do?

Attract water (hydrophilic, negative charge), maintain hexagonal lattice spacing, regulate collagen arrangement.

What are proteoglycans?

Core protein + attached GAGs; regulate collagen spacing, tensile strength, hydration.

What are the 4 proteoglycans of cornea?

Decorin (chondroitin/dermatin sulfate, anterior stroma), Lumican, Keratocan, Mimican (keratan sulfate, posterior stroma).

What role does Lumican play?

Regulates collagen fibril diameter.

How much light entering cornea is scattered?

<1%.

What contributes to corneal transparency?

Regular fibril spacing and diameter, controlled distance between fibrils.

What decreases stromal transparency?

Scarring, high IOP, neovascularization, edema, keratocyte activation.

What is Descemet’s membrane?

Basement membrane of endothelium, produced throughout life, thickens (doubles by age 40).

What are the 2 layers of Descemet’s?

Anterior (3 µm, embryonic, banded lattice) and posterior (non-banded, secreted through life).

What are properties of Descemet’s?

Elastic fibril arrangement, curls if torn, resistant to trauma/enzymes, regenerates if damaged.

What marks Descemet’s termination?

Schwalbe’s line.

What is corneal endothelium?

Innermost layer, single flattened cell layer (5 µm thick), basal surface on Descemet’s, apical microvilli extend into anterior chamber.

What is the shape of endothelial cells?

Mostly hexagonal, forming an endothelial mosaic.

What cell junctions are in endothelium?

Interdigitations, gap junctions, tight junctions (macula occludens).

How leaky is endothelium?

Slightly leaky; allows glucose, amino acids in, but pumps water out to maintain hydration.

What maintains corneal hydration in endothelium?

Ionic pumps (esp. Na⁺/K⁺ ATPase).

What organelles are abundant in endothelial cells?

Mitochondria (high energy demand, second only to photoreceptors).

Do endothelial cells divide?

No; when damaged, cells spread/migrate → thinner.

How does endothelial density change with age?

Decreases with age. Children: 3000–4000 cells/mm². Age 80: 1000–2000. Minimum for function = 400–500.

What is pleomorphism?

Variability in endothelial cell shape.

What is polymegathism?

Variability in endothelial cell size.

How does cell loss affect endothelial function?

Disrupts pumps, lets excess aqueous into stroma.

How is cornea innervated?

70–80 branches from long/short ciliary nerves (CN V1 nasociliary).

Where do corneal nerves enter?

Peripheral mid-stroma.

What happens to corneal nerves as they enter cornea?

Lose myelin but retain Schwann cells until Bowman’s, then lose Schwann covering. Terminate in epithelium unmyelinated.

What are the 3 corneal nerve plexuses?

Mid-stromal, sub-epithelial, sub-basal.

How dense is corneal innervation compared to skin?

400x more dense → explains painful abrasions.

What are the corneal nerves sensitive to?

Touch, temperature, pain.

What effect does long-term contact lens wear have?

Decreases corneal sensitivity.

Does the cornea have blood supply?

No, it is avascular.

How is the cornea nourished?

Diffusion of glucose + O2 from the aqueous (posterior), conjunctival and episcleral capillaries at limbus (anterior).

Why is cornea immune privileged?

No blood vessels → antigen processing absent except in inflammation/mechanical irritation.

What are the 2 primary functions of cornea?

Refract light and transmit light.

What factors affect corneal refraction?

Anterior curvature, index change air→cornea, corneal thickness, posterior curvature, index change cornea→aqueous.

What minimizes corneal light scatter?

Smooth surface (epithelium + tear film), absence of blood vessels, stromal spacing, optimal hydration (78%).

What percentage of visible light does cornea scatter?

<1%, mostly at epithelium/endothelium.

How is corneal hydration maintained?

78% water, controlled by epi/endo barrier, stromal anionic molecules (GAGs), and ion/water transport in the endothelium.

Where does corneal oxygen come from?

Open-eye: atmosphere via tear film. Closed-eye: 2/3 from limbal capillaries, remainder from the aqueous.

How do nutrients (glucose) enter cornea?

From the aqueous humor through leaky endothelium.

Why does endothelium need high energy?

To power ~1.5 million pumps per cell.

How often is epithelium replaced?

Every 7–10 days.

What is the process of epithelial renewal?

Basal cell mitosis → wing cells → surface cells → apoptosis/shedding into tears.

New cells are in the basal layer → move more and more anterior.

What cells provide renewal at limbus?

Limbal stem cells

• Palisades of Vogt involved

How is UV radiation absorbed by cornea?

<300 nm absorbed by epithelium/Bowman’s, 300–320 nm by stroma.

This helps protect the eye from UV damage.

What protects cornea from UV damage?

Vitamin C and glutathione scavenging free radicals.

What are age-related corneal changes?

• Overall thickness constant

• Bowman’s thins (30% lost by 80)

• Transparency decreases → opacities

• Descemet’s thickens

• Endothelial density decreases

• Increased polymegathism/pleomorphism

• Anterior cornea shifts to against-the-rule astigmatism (from with-the-rule astigmatism)

• Corneal nerves are lost → ↓ sensitivity.

What are the structural characteristics of the lens?

The lens is avascular, transparent, and not innervated.

Where is the lens located within the eye?

It is in the posterior chamber, anterior to the vitreous, and posterior to the iris and anterior chamber.

What supports and connects the lens to surrounding structures?

It is suspended by zonular fibers from the ciliary body, sits in the patellar fossa, and is attached to the vitreous via Weigert’s ligament.

How does the lens contribute to vision?

It helps focus light rays on the retina and adjusts shape for accommodation.