Cornea and Conjunctiva

into what foramen does the cornea insert

anterior scleral foramen

average refractive power of cornea

43 D (2/3 of total power of eye)

function of cornea

transmit and refract light

barrier against pathogens and edema

main refractive element of the cornea

interface between air and tear film due to it having the largest difference in index of refraction between 2 layers

gives cornea 44 D of power

tear/cornea interface gives 5 D, and cornea/aqueous interface gives -6 D

where is cornea thickest

periphery

WTR vs. ATR

WTR: steepest meridian is vertical

ATR: steepest meridian is horizontal

what happens to astigmatism with age

shifts towards ATR

probably due to loss of lid tension (cornea is not being flattened vertically)

average anterior and posterior radius of curvature and diameter

anterior ROC: 7.8 mm

posterior ROC: 6.5 mm

diameter: 11.7 mm

average thickness of cornea and individual layers

cornea: 550 microns

epithelium: 52 microns

Bowman’s: 8-14 microns

stroma: 450 microns

Descemet’s: 5-15 microns

endothelium: 5 microns

(remember 55, 15, 450, 10, 5)

4 layers of corneal epithelium

surface layer, wing cells, basal layer, stem cells

histology of corneal epithelium

non-keratinized stratified squamous epithelium with 5-6 cell layers about 52 microns thick in total

what is the surface layer of the corneal epithelium composed of and how does it work

2 layers of non-keratinized squamous cells

how do the cells of the surface layer of the corneal epithelium function

contain plasma/cell membrane that secretes a glycocalyx

cells have microvilli and micro plicae that increase surface area and help stabilize tear film

what happens to the surface layer cells of the corneal epithelium as they age

slough off into the tear film

what do zonula occludens and desmosomes do

form tight barrier between cells to stop particles from moving between cells

what is the wing cell layer of the corneal epithelium composed of

2-3 cell layers with desmosomes that join cells to each other and to surrounding layers

what is the only mitotic layer in the corneal epithelium

basal layer

what is the basal layer of the corneal epithelium composed of

1 layer of columnar cells

what does the basal layer of the corneal epithelium do

secretes its own basement membrane (basal lamina)

what does the basal lamina attach to the basal layer of corneal epithelium with

hemidesmosomes

what does the basal lamina of the basal layer of the corneal epithelium attach to and how

attaches to basal layer via hemidesmosomes

attaches to Bowman’s via hemidesmosomes that penetrate Bowman’s and attach to ECM of stroma

what happens to the BM of the corneal epithelium with age

BM goes through reduplication and doubles in thickness by 60 years old

basal lamina vs reticular lamina

2 layers of a basement membrane

basal lamina: secreted by epithelial cells

reticular lamina: produced by underlying stroma cells in the corneawh

what 3 things increase the risk of recurrent corneal erosions (RCEs)

1. poor hemidesmosome attachments (BM)

2. EBMD

3. age-related BM thickening

what is the Palisades of Vogt

0.5-1.0 mm band around the limbus of the cornea at the same level as the corneal basal layer, where stem cells originate and migrate circumferentially to become basal cells

pathway of formation of the layers of the corneal epithelium

stem cells migrate circumferentially to become basal cells

basal cells produce BM and wing cells

wing cells migrate anteriorly to become epithelial surface layer

what conditions does limbal stem cell deficiency commonly affect and how

contributes to poor corneal epithelial maintenance

aniridia, Stevens-Johnson syndrome, alkali corneal burns

histology of Bowman’s layer

acellular layer made mostly of type 1 and type 5 collagen fibrils

how does Bowman’s layer respond to injury

Bowman’s is a tough layer that is resistant to injury

if injury does occur, Bowman’s cannot regenerate, resulting in a scar

Bowman’s layer (CAN/CANNOT) regenerate in response to injury

cannot

Bowman’s layer (IS/IS NOT) a basement membrane

is not

band keratopathy

calcium deposits in Bowman’s layer

how do pterygia relate to Bowman’s layer

they destroy Bowman’s layer as they progress onto the cornea

crocodile shagreen and what layers it involves

bilateral grayish white polygonal stromal opacities

involve stroma and possibly Bowman’s layer

Reis-Buckler’s dystrophy

rare corneal epithelial dystrophy that appears early in life and is secondary to damage in Bowman’s layer

where does initial damage occur in Keratoconus

Bowman’s

damage to what epithelial layer causes hydrops in Keratoconus

ruptures in Descemet’s

why does PRK cause post-op corneal haze

laser applied goes through Bowman’s layer

what layers does the flap created in LASIK surgery impact

surface epithelium and Bowman’s layer

what is the substantia propria

another name for corneal stroma

what is the make up of corneal stroma

dense regular connective tissue

composed of keratocytes (fibroblasts), collagen fibrils, ground substance, and water

75-80% of the stroma is made up of _________

water

what are keratocytes and what do they produce

fibroblasts (cells that create CT) of the cornea

produce collagen fibrils and ECM

collagen fibril organization in the corneal stroma

200-300 layers of uniformly spaced lamellae that run parallel to corneal surface

collagen is mainly type 1

differences in anterior vs. posterior stroma

anterior 1/3: more cross linking makes it more rigid and able to maintain the corneal curvature

posterior 2/3: more organized with larger lamellae that have less cross linking results in higher incidence of corneal edema

higher incidence of corneal edema in the (anterior/posterior) stroma

posterior (more organized lamellae with less cross linking)

ground substance function in corneal stroma

filler between keratocytes and collage fibrils

has GAGs that attract water and allow collagen to maintain even spacing and transparency

what is the function of GAGs in the corneal stroma

GAGs attract water that help the collagen lamellae maintain uniform spacing, allowing the cornea to be transparent

what is the main GAG in the corneal stroma

keratin sulfate

what is descemet’s membrane

basement membrane produced by the corneal endothelium that increases in thickness with age

what is descemet’s membrane composed of

type 4 collagen

descemet’s membrane (IS/IS NOT) a basement membrane

is

what is the difference between bowman’s layer and descemet’s membrane in their trauma response

both layers are very resistant to trauma

while bowman’s definitely does not regenerate in response to trauma, descemet’s may regenerate in response to trauma

where does descemet’s membrane terminate and what does it become

terminates at the limbus

becomes Schwalbe’s line

Haab’s striae

folds in descemet’s membrane in congenital glaucoma

Hassall-Henle bodies

small areas of thickened descemet’s membrane in the peripheral cornea

extend into anterior chamber

called guttata when they appear centrally

where is Dua’s layer located

between posterior stroma and descemet’s membrane

histology of corneal endothelial layer

1 layer of squamous cells about 5 microns thick

what pumps do endothelial cells have and what is their function

Na+/K+ ATP pumps

maintain corneal hydration and transparency by regulating water and ion flow between the aqueous humor and stroma

endothelial cells are rich in __________

organelles and mitochondria

endothelial cells are linked at their (apical/basal) borders by ____________ junctions

apical (face anterior chamber)

maculae occludens (also a few zonula occludens)

why do endothelial cells have maculae occludens junctions

they are weaker junctions that create a weak barrier, allowing for amino acids, glucose, and nutrients from the aqueous humor to enter the cornea

endothelial cells (DO/DO NOT) replicate

do not

what happens to endothelial cells with age

they decrease in number

change shape and change size to compensate for loss of density

change in shape of endothelial cells with age

pleomorphism

change in size of endothelial cells with age

polymegathism

why does stromal edema occur with age

loss of endothelial cells causes loss of Na+/K+ pumps

Na+/K+ regulate water and ion flow

what is the vasculature of the cornea

cornea is avascular

3 sources cornea gets its nutrients from

1. diffusion from aqueous humor

2. limbal conj and episcleral capillary networks

3. palpebral conj capillary networks

main source of oxygen for the cornea when the eye is open

tear film

has oxygen that has diffused from the atmosphere

main source of oxygen for the cornea when the eye is closed

palpebral conjunctival blood networks

what happens to the cornea when it is deprived from oxygen

corneal neovascularization

what factors encourage corneal neovascularization to occur

increase in cytokines and growth factors, including VEGF

where do new vessels stem from in corneal neovascularization

endothelial cells of limbal capillary network

what is corneal innervation responsible for in the cornea

proper wound healing

pain sensation

what cranial nerve is responsible for innervating the cornea

CN V1 (trigeminal)

what nerves provide corneal innervation

CN V1 > nasociliary nerve > long posterior ciliary nerves and short posterior ciliary nerves

LPCNs and SPCNs innervate the cornea

LPCN formation vs. SPCN formation

LPCNs branch right off of the nasociliary nerve

After the nasociliary nerve travels through the ciliary ganglion, SPCNs form

SPCNs form after the __________ nerve travels through _____________

nasociliary

ciliary ganglion

path for LPCNs and SPCNs to penetrate the cornea

SPCNs and LPCNs form a myelinated network of 60-80 nerves and enter the mid-stroma

travel 2-4 mm inside the stroma anteriorly

lose myelin sheath and penetrate Bowman’s to enter the epithelium

when do SPCNs and LPCNs lose their myelin sheath

as they penetrate Bowman’s layer to enter the corneal epithelium

where are nocireceptors found and what is their role

receptors found in corneal nerves when they penetrate corneal epithelium

mediate pain

why is the cornea so sensitive

corneal nerves lose their myelin sheath as they penetrate the corneal epithelium, leaving the nerves naked and highly sensitive

neurotrophic keratitis

damage of CN V1 causes poor corneal innervation

characterized by poor corneal wound healing and lack of corneal sensitivity (role of corneal nerves)

ulcer with no pain

what diseases can cause neurotrophic keratitis

any diseases that damage CN V1

commonly herpes simplex, herpes zoster, CVA, diabetes

3 locations of corneal nerve networks

1. epithelium

2. anterior stroma/Bowman’s

3. mid stroma

nerve network in corneal epithelium

intraepithelial plexus

nerve network in anterior stroma/bowman’s

subepithelial plexus

nerve network in mid stroma

stromal plexus

where in the cornea are there no corneal nerves and why

posterior stroma, descemet’s, or endothelium

corneal nerves enter at the level of the mid stroma and travel anteriorly

where do corneal nerves enter the cornea

mid-stroma

4 main functions of the conjunctiva

1. protects the tissues of the lids and orbit

2. allows for lots of eye movement without damaging the soft tissues

3. antimicrobial

4. produces the mucin layer of the tears

what layer of the tear film does the conjunctiva produce

mucin layer

2 layers of the conj

stratified non-keratinized epithelial layer

submucosa

how does the histology of the conj epithelium change between the palpebral and bulbar conj

epithelial cells are cuboidal/columnar in the palpebral conj, and become squamous in the bulbar conj

conjunctival epithelial cells (increase/decrease) as you get closer to the limbus

decrease

what does does the stratified non-keratinized epithelial layer of the conj consist of

superficial cells containing melanin, microvilli, and goblet cells

what is the conjunctival submucosa

deeper layer of the conj made of loose CT and separated into 2 layers

2 layers of conjunctival submucosa

outer lymphoid layer

deep fibrous layer

contents and role of outer lymphoid layer of conjunctival submucosa

IgA, macrophages, mast cells, lymphocytes, PMN leukocytes, eosinophils, Langerhans cells

defends the eye against pathogens