Sclera and Cornea Anatomy & Physiology

What makes up a major component of connective tissue in the eye?

collagen

What is the structure of collagen?

3 alpha chains that spiral around one another to form a triple helix

What type of collagen is the most common type found in the cornea and sclera?

type 1

What kind of matrix are collagen fibrils embedded in?

proteoglycan/water matrix

What GAGs are predominant in the cornea?

keratan sulfate and dermatan sulfate

What is the role of fibroblasts?

- produce collagen and proteoglycans

- repair collagen fibers

What are the fibroblasts in the cornea called?

keratocytes

What are the fibroblasts in cartilage called?

chondroblasts

What are the fibroblasts in bone called?

osteoblasts

Under an electron microscope, what kind of pattern do collagen fibrils form? What are the dark and light bands seen?

- banding pattern

- dark bands = gap between individual collagen molecules

- light bands = overlapping of collagen molecules

What is the diameter of collagen fibrils in the scleral stroma?

25-230 nm

What is the diameter of collagen fibrils in the corneal stroma?

25-35 nm

What are proteoglycans made up of?

core protein + glycosaminoglycans (GAGs)

What kind of charge do GAGs have? Therefore, what kind of molecules do they attract?

GAGs have a very negative charge, therefore attract positively charged molecules, like Na+

Why will tissues with large amounts of GAGs take up a considerable amount of water?

since GAGs are negatively charged, they attract Na+ which water follows

What creates the gel surrounding collagen fibrils?

water and GAG/proteins

What are the three layers of the sclera?

1) episclera

2) scleral stroma

3) lamina fusca

What is the episclera? Which structure is it physically linked to?

outermost layer of the sclera; linked to Tenon's capsule by connective tissue strands

As you move towards the back of the eye, does the sclera thin or thicken?

thins

What is the scleral stroma?

thick, dense connective tissue layer that is continuous with the corneal stroma at the limbus

What is the lamina fusca?

thin, pigmented layer on the inner side of the sclera. this connective tissue connects the sclera to the choroid

What is the diameter of scleral collagen fibrils?

30-150 nm (larger and more variable than corneal fibrils)

Compared to the cornea, what is the organization of the scleral collagen fibrils? What does this contribute to?

scleral collagen fibrils are not as organized as corneal -> contributes to lack of transparency

What happens to light transmission when atoms are not regularly spaced (e.g. sclera)?

destructive interference of in-phase light wave fronts -> less transmission, less transparency (scattering)

What is the weakest point of the sclera?

lamina cribosa, where the optic nerve exits

How much of the eye does the sclera cover?

~85%

Where are the thickest and thinnest points of the cornea at?

thickest at the limbus and thinnest at the apex

What is the radius of curvature of the cornea?

~8 mm

How thick is the cornea at the center and at the periphery?

center: 0.52-.0.53 mm thick

periphery: 0.71 mm thick

Which side of the cornea has a smaller radius of curvature?

posterior side

Does the cornea have vasculature?

no

What is the shape of the cornea's curvature?

- not spherical and not constant

- flattens out as you move from center toward periphery

- radii of curvature increases from center to periphery

Which part of the cornea has a constant curvature?

optical zone (center of cornea), ~4mm

Since the radius of curvature increases from center to periphery, what happens to the power?

power decreases towards the periphery

What is the size of the eye and the cornea in an adult?

24 mm eye, 12 mm cornea

What is the size of the eye and the cornea at birth?

16 mm eye, 10 mm cornea

How long does it take the cornea and eye to grow to adult size?

cornea: 3 years

eye: 15 years

What is the total refractive power of the cornea?

+43.1 D

What is the refractive power of the air/tear interface?

+43.6 D

What is the refractive power of the tear/cornea?

+5.3 D

What is the refractive power of the cornea/aqueous?

-5.8 D

From anterior to posterior, what are the layers of the cornea?

- epithelium (50 microm)

- bowman's layer (8-4 microm)

- stroma (500 microm)

- descemet's membrane (10 microm)

- endothelium (5 microm)

How many cell layers is the corneal epithelial cell layer composed of?

6-8 cell layers

How thick is the corneal epithelial layer?

~50 microm thick (~10% of total cornea thickness)

What kind of cells is the corneal epithelium composed of?

- 5 layers of flattened squamous cells

- 2-3 layers of cuboidal wings cells

- a single layer of columnar basal cells

What is the basal cell layer connected to?

the underlying basement membrane anterior to the Bowman's layer

Is the corneal epithelium constantly renewing?

yes

What does the migration of epithelial cells look like in renewal?

columnar cells (coming from limbus) --> cuboidal wing cells --> flattened squamous

Which corneal epithelial cells do desmosomes bind together? Which specialized proteins do they use?

bind adjacent squamous cells together as well as underlying wind cells through cadherins

What is the purpose of desmosomes in the cornea?

serve as a barrier to prevent aqueous content of tears from passing

What are tight junctions? Which corneal epithelial cells are they present in?

tight junctions serve a similar role to desmosomes; present in squamous cells and occasionally wings cells

What are basal cells bound together by?

bound together by desmosomes and gap junctions

What are basal cells bound to the basement membrane by?

hemidesmosomes (via integrins)

What are cellular rivets?

cell-cell anchoring junctions (desmosomes and hemidesmosomes)

The corneal epithelium contains a dense array of free terminals of nerve fibers from which nerves?

long ciliary nerves

The nerve endings from long ciliary nerves in the corneal epithelium have cell bodies located in _____________________.

trigeminal ganglion

What is the Bowman's layer?

anterior-most part of the cornea stroma

What is the Bowman's layer produced by?

stromal keratocytes and epithelium

What collagen types make up the Bowman's layer?

type I and VII

What is the organization of Type VII collagen fibrils in the Bowman's layer?

- thin, multi-branching sheets connected by anchoring plaques

- continuous with the anchoring fibrils from hemidesmosomes that join that columnar basal cells of the epithelium to the basement membrane

What is the organization of Type I collagen fibrils in the Bowman's layer?

large type I fibrils interweave in an irregular pattern

How much of the corneal thickness does the stroma compose?

90%

What is the stroma comprised of?

collagen fibrils and fibroblasts

What is the role of the specialized fibroblasts in the corneal stroma?

produce collagen fibrils during development and maintain the connective tissue in the mature eye

What is the diameter of the collagen fibrils in the stroma and what are they typically grouped in?

- 25-35 nm

- grouped into flat bundles called lamellae

How many lamellae are distributed throughout the corneal stroma? In which direction do they run compared to the corneal surface?

- 200-300

- all lamellae run parallel to the corneal surface

How is transparency achieved in the cornea?

- regular arrangement of collagen fibrils (constructive interference of light waves)

- maintenance of a constant hydration/water content by the endothelium and to a lesser extent, the epithelium

What kind of arrangement are the collagen bundles of the stroma organized in?

"crystal lattice" - regular spacing and uniform diameter contribute to optical clarity

What is the Descemet's membrane?

structure between endothelium and stroma, considered a basement membrane of endothelium

How many types of collagen does the Descemet's membrane have? Which is the most dominant?

5 types of collagen with type VIII most dominant

What is located in between the collagen fibril lattice that connects that Descemet's membrane to the stroma and endothelium?

Are there any anchoring junctions between Descemet's membrane and endothelium?

no

What is a guttata?

thickening of Descemet's membrane in the central cornea

What are Hassall Henle bodies?

thickening near periphery of cornea (common in aging)

What is a Descemetocele?

herniation of/in descemet's membrane that causes erosion of the corneal epithelium

How many cells thick is the corneal endothelium?

1 cell thick (5 microm) - made up of a single layer of metabolically active hexagonal cells

What does corneal epithelium contain to pump water out of the corneal stroma and into the aqueous humor? What does this help maintain?

- lots of mitochondria (ATPases)

- helps maintain hydration of stroma and with transparency

What happens to endothelial cells with age?

cells are not replaced, but shape and size become irregular to fill in lost cells

What is pleomorphism?

variation in shape of cells

What is polymegathism?

change in cell size

Compared to normal endothelium, how do contact lenses affect the endothelium?

disrupts the endothelium, changing size and shape of cells, which can affect corneal transparency

In development, which layers of the cornea are present first?

epithelium and endothelium (6th week)

What developmental layer is corneal epithelium derived from?

surface ectoderm (as well as conjunctiva)

What developmental layer is endothelium derived from?

neural crest cells separating from the edges of the optic cup

What developmental layer are stromal cells derived from?

neural crest cells (7-7.5 weeks)

By three months, which layers of the cornea are formed?

epithelium and endothelium, but are not final adult structures until 7 months

How are corneal lamellae of the stroma organized anterior to posterior?

anterior 1/3: thinner

posterior 2/3: more packed

Will corneal epithelial or endothelial damage cause greater optical aberration and image disruption?

endothelial (reduces ATPase action)

What is keratoconus?

cone shaped cornea

What are the two types of keratoplasty?

1. penetrating keratoplasty

2. lamellar keratoplasty

What is a penetrating keratoplasty?

replacing the entire cornea

What is a lamellar keratoplasty?

replacing a portion of the cornea

What is radial keratotomy?

- a technique to modify the power of the cornea by putting slits in the periphery of the cornea

- flattens the optical zone

- some correction for astigmatism is possible but severe damage and slow healing can lead to scarring (risky!)

Deep wounds to the cornea are first filled with which cells? Is repair fast or slow

epithelial; slow

Which arteries give rise to the anterior ciliary arteries?

arteries to the four rectus muscles

- SR, IR, MR give rise to 2

- LR gives rise to 1

How many anterior ciliary arteries (episcleral) typically are there?

7

What structures do the anterior ciliary arteries supply?

- conjunctiva

- cornea

- limbus

- ciliary body

- iris

Where do the anterior ciliary arteries bifurcate?

after passing over the rectus muscle tendons

What are the superficial branches of the anterior ciliary arteries?

episcleral arteries: corneal arcades, conjunctiva arteries

Where are the superficial marginal plexus and corneal arcades located?

within episcleral arterial circle