Anatomy of the Cornea (Lecture 2)

What are the 5 main functions of the cornea?

1. Transparency

2. Major refractive surface

3. Mechanical strength- collagen

4. Barrier- Ocular biodefense system

5. Highly innervated- Protection

How does the cornea get its transparency function?

- Avascular

- Structural arrangement

- Hydration

How does the cornea get its mechanical strength function from collagen?

Maintains ocular contour

What does the arrangement of collagen do for the cornea?

Maintains transparency

How does the cornea act as a barrier?

Participating in the ocular biodefense system

Who does the cornea participate in the ocular biodefense system with?

Conjunctiva & tears

How does the cornea act as protection?

Highly innervated

What happens when the cornea is touched?

Causes reflex tearing & reflex blinking (sensory protective mechanism)

What are the adjacent structures that interact with the cornea?

1. Tear film

2. Aqueous humor

3. Limbus

What structure interacts with the front of the surface of the cornea?

Tear film

What structure interacts with the back of the surface of the cornea?

Aqueous humor

What structures interact with the cornea to create the junction with the conjunctiva & sclera?

Limbus

What describes the anterior surface of the cornea?

- Convex

- Aspheric

- Prolate

Where is the steepest part of the cornea?

In center where it is nearly spherical

What do we see with the thickness of the cornea peripherally?

Flattens out

What is the structure of the cornea shaped most like?

An egg than a basketball

How does the posterior surface compare to the anterior surface?

Steeper than the anterior

What does the posterior surface make the cornea into?

A minus lens

What describes the thickness of the anterior surface?

Thinner in the center & thicker in the periphery

What is the vertical diameter of the anterior surface?

10.6 mm

What is the horizontal diameter of the anterior surface?

11.7 mm

What shape is the anterior surface?

Oval

Why is the horizontal diameter longer than the vertical diameter?

Often steeper vertically than horizontally (WTR)

What shape is the posterior surface?

Round

What is the diameter of the posterior surface?

11.7 mm

What is the average thickness of the cornea in the center? (pachymetry)

544 +- 34 micrometers

Where are thicker areas of the cornea?

Periphery is thicker than the center

How does increased hydration impact the thickness of the cornea?

Thickens the cornea

How does the thickness of the cornea flucuate?

On diurnal basis

What time is the cornea thicker?

A.M.- During the night when sleeping, the eyelids are closed, tears become isotonic with the cornea

Why is the cornea thicker at night (when we are sleeping)?

No fluid being pulled out of the cornea from tears

When is the cornea thinner?

During the day

What causes the cornea to be thinner during the day?

- Open eyelid causes hypertonic tears (tears evaporate)

- Cornea thins

What is the temperature of the cornea?

Cooler external surface

What kind of arrangement is seen with the cell of the cornea?

Uniform

What type of cells make up the structure of the epithelium of the cornea?

Non-keratinized stratified squamous (5-7 layers)

What do we see with the superficial cells of the corneal epithelium?

Superficial cells exfoliate (slough off- need to be constantly replaced by the deeper cells in the epithelium)

What describes the vascularity of the corneal epithelium?

Avascular

What type of function does the corneal epithelium serve as?

Anterior Barrier function

What is the corneal epithelium continuous at?

The limbus with conjunctiva epithelium

What do the surface of the squamous epithelial cells contain?

1. Glycocalyx

2. Microplicae & microvilli

3. Tight junctions

What is found within the glycocalyx covering of the squamous layer?

- Glycoproteins

- Glycolipids

- Mucin

What does the glycocalyx covering interact with?

Mucin layers of tears

What describes the surface of the glycocalyx covering?

Smooth surface

What does the glycocalyx covering maintain for the epithelium?

Hydrophilic properties

What does the glycocalyx covering enhance?

Tear film stability

What is the function of the microplica & microvili of the squamous layer?

- Increase surface area

- Anchor tears to cornea

Where are tight junctions found within the squamous layer?

In between squamous epithelium cells

What is the function of the tight junctions within the epithelial cells?

Forms anterior corneal barrier- preventing unwanted things to get into cornea

Where are the wing cells of the epithelium located?

Beneath the single layer of squamous cells

What describes wing cells?

2-3 layers of cells with wing-like lateral extensions Interdigitations

What are key functions of the wing cells?

Do not divide (differentiate from a wing cell to a squamous cell as the cells move toward the surface of the epithelial layer)

What describes the innervation of wing cells?

Highly innervated

What do wing cells differentiate into?

Squamous cells

Where are basal cells found in the epithelium?

Found at the bottom or base of the epithelium as a single layer

Why are the basal cells very mitotically active?

They are dividing to produce wing cells which differentiate into squamous cells

What do the basal cells act as?

Limbal stem cells

What are the basal cells only in contact with?

Basement membrane

Where are hemidesmosomes found?

Along the basal surface of epithelial cells to the basal lamina portion of the basement membrane

What do basal cells secrete?

Basal lamina

What is the role of plectin in the basement membrane?

Attaches keratin intermediate filaments to the inner plaque of the hemidesmosomes

What is attached to the plectin?

Transmembrane proteins: integrin & type XVII collagen

What do Integrin & XVII collagen do?

Bind to the glycoprotein laminin in lamina lucida of the basal lamina

What is the function of integrin & XVII collagen?

Serve as anchoring filaments between the hemidesmosomes & laminin

What is laminin's role in basement membrane structure?

Laminin (anchoring filaments) attach to type IV & type VII collagen meshwork in the lamina dense of the basal lamina

What molecules does laminin anchor?

Anchors the integrin molecules & type XVII collagen to the type IV & type VII to the lamina densa

What is the role of type VII collagen in the basement membrane structure?

Serves as an anchoring fibril with type IV anchoring plaques located within Bowman's layer & the most superficial stroma

What are the steps of the basal epithelial cells get attached to Bowman's Layer and the Superficial Stroma?

1. Attaching cytoskeletal system of basal epithelium to its hemidesmosomes

2. Attachment of transmembrane proteins to the extracellular matrix

3. Attachments of laminin in the extracellular matrix

4. Attachment of lamina densa to Bowman's layer & the corneal stroma

What is the function of the Dendritic Langerhans cells in the epithelium?

Antigen-presenting cells (APCs)

Where are dendritic langerhans cells found in the epithelium?

Numerous in periphery of cornea (usually absent in central cornea)

What role do dendritic langerhans cells have as antigen-presenting cells?

- Can receive information that comes into the epithelium

- Can transfer that antigen to a T lymphocyte

- Helps the corneal epithelium participate in the bio-defense mechanism of the eye

What is involved within the process of epithelial regeneration?

1. Limbal stem cells

2. Transient amplify cells

3. Basal epithelial

4. Wing cells differentiate

5. Squamous cells

Where are limbal stem cells found?

On edge of cornea

What do the limbal stem cells divide into?

One new stem cell & one new transient amplifying cell

Where do the transient amplifying cells that are formed migrate to?

Toward the center of the epithelium of the cornea producing new basal epithelial cells

What is the function of the newly formed basal epithelial cells?

Undergo mitosis forming new wing cells

What happens to the squamous cells within epithelial regeneration?

Shed/exfoliated & so new cells need to be produced

What is the turnover rate of the epithelial layer?

7-14 days

Layers of the Cornea Diagram

What is the composition of Bowman's Layer?

Randomly arranged, small diameter collagen (22 nm)

What describes the cellularity of Bowman's layer?

Acellular- modified anterior stroma

What is Bowman's layer strongly attached to?

Stroma

What is the function of Bowman's Layer?

Provides shear strength (resists sliding of stromal sublayers)

What function as a barrier does Bowman's layer serve as?

Prevents larger substances from getting into the deeper layers of the cornea

What is the composition of the stroma?

- Collagenous protein

- Cells- keratocytes

- Ground substance- non-collagenous protein

How does the size of collagen compare to Bowman's layer?

Smaller in diameter but larger than Bomman's layer

What is the structure of Type I Collagen?

- Relatively uniform in diameter (~30 nm)

- Laid down in sheets (lamellae)

Why is the distance between fibers consistent centrally?

Very uniform because surrounding each fibril is the ground substance (matrix) which contains proteoglycans & glucosaminoglycans

What is the role of the proteoglycans within Type I collagen?

Help to keep a precise arrangement in the space between the collagen fibrils

Where do we see the fibers of the type I collagen widen?

Peripherally

How is lamellae arranged?

Unique arrangement of collagen fibrils in adjacent lamellae helps to maintain corneal transparency

Where do we see more disorganization of the lamellae?

Anteriorly & peripherally

Where do the lamellae extend from?

Limbus-to-limbus

What are the functions of the lamellae?

- Tensile strength

- Shape stability

- Transparency of stroma

What are circumcorneal annulus?

Collagen fibrils within a lamellae turn & run circumferentially around the cornea at the limbus

What are the circumcorneal annulus believed to help?

Maintain curvature of the cornea

What are the keratocytes?

Corneal fibroblasts (fibrocytes)

What do keratocytes form?

A communicating network

What type of junctions do keratocytes form?

- Tight junctions

- Gap junctions

What do keratocytes allow for?

Functional syncytium so the cells can communicate with one another