Physiology

~50micrometers

Stratified squamous, non-keratinized epithelial tissue (5-7 cell layers thick)

• 1. barrier to pathogens
• 2. excess uptake of fluid from tears
• 3. UV filter
• 4. surface for tear film distribution
• 5. Transmission of O2, CO2 and metabolites**
• 6. optical refractive surface (with tear film)

70% (~47D)

• 1. Basal Layer
• 2. Wing Cells
• 3. Squamous cell layers

limbal stem cells

more columnar than cuboidal (shape depends on source of info)

• 1. highly mitotic (many mitochondria)
• 2. Produce basal lamina (40-60 nm thick)
• -thinner than bowman's

hemi-desmosomes

divide from stem cells (at limbus) and migrate inwardly

• type 7 and 4 collagen and cell adhesion molec (CAMs)
• involved in gross adhesion of basal cells to the basal lamina

• 1. laminin and heparin sulfate
• 2. integrins
• 3. fibronectin

Fuchs endothelial dystrophy (bw ages 40-50)

Meesman's K dystrophy

(non-progressive, fine dot-like glycogen containing opacities)

Epithelial basement membrane dystrophy (EBMD)

• 1. most common cause of recurrent K erosion
• 2. look for negative staining
• 3. fibrillin material accumulates bw basal lamina and bowman's causing and incr in thickness

1-3 layers; apoptosis (no mitosis happens here)

Squamous cell layers

cells are starting to degenerate, loosing their RNA and chromatin

Zonula occludens; makes layer a semi-permeable membrane

Light cells: are younger, smaller cells, that appear light due to their high level of microvilli

Cell-to-cell junctions

• Desmosomes (structural, cadherins)
• and
• gap junction (common in basal cell layer)

Zonula occludens

~7-10 day (continues cycle)

Vortex patten, curved (Y-axis of centripetal migration)

• Whorl keratopathy (medications: amiodarone or choloroquine)
• Fabry's dx (metabolic)

• mitotically, producing wing cells
• ( migrate superficially as more wing cells are produced, becoming squamous cells, X-axis of prolif)
• become less cuboidal and more squamous as move superficially

in wing cell layer, ending just before squamous cell desquamation into the tear filn (Z-axis cell loss)

• X=from deep to superficial
• Y=limbus to center
• Z= superficial to tear film

• May be mediated by the sympathetic nervous system, likely has a feedback control loop mechanis
• (intense pain signal may speed up the regeneration process)

• o The amount of drug instilled into
• the ocular surface
• o The active concentration of a drug
• is ½ that in the drop, as only about ½ gets to the site
• Higher concentration = more of the drug to get to its destination

• the epithelium more easily
• o Water soluble drugs perfuse through
• the stroma more easily

specific drugs that break down the epithelial junctions = increased permeability

• o Many drugs have a detergent or
• surfactant to increase permeability

• By mode of drug instillation (ung or
• gtts)
• celluvisc
• Closing eyes or punctal occlusion
• can increase duration of contact

• Stability of drug affected by pH,
• temperature, chemistry of drug

• Tight junctions provide a natural
• barrier, however ions may pass intercellulaly

• Lipophilic aka: hydrophobic, fat
• soluble molecules, non-polar
• o Nonionic molecules permeate
• epithelial cells
• o Ions have low diffusion permeability;
• polar

• to hydrophilic molecules because there is so much
• water in the stroma

• Since it is only 1 cell layer thick,
• this will not limit permeability
• o Determined by molecule size, as
• junctional gaps, and macula Occludens allow ions and non-ions to pass

• Detergents are surfactants that kill
• bacteria by breaking down lipid membranes and junctions

BAK, Chlorohexidene, EDTA

break down junctions and cause sloughing off of outer cells; inhibit wound healing, break down lipids and cell to cell junctions; cell membr

• - Causes an immediate increased
• permeability to NaFl by breaking down junctions
• § Inhibits healing, so prolonged
• contact is not indicated
• · Repeated use of 0.02% may cause
• irreversible corneal damage
• § Bacteriostatic: breaks down cell
• junctions and lipid bilayers
• § Commonly used in the best hand sanitizers

• § Bactericidal; static: disrupts
• membranes

• § Also breaks down cell-to-cell
• junctions but does this by chelating metal ions
• § Chelates calcium which makes
• bacteria unable to survive
• · The calcium is also required for
• maintenance of tight junctions
• · Chelation is where a molecule binds
• a metal ion so that it becomes unusable to other functions; sequestering

lipid solubility

stability

• - Tear pH of 7.3-7.7 is within the
• comfort zone
• § Tear pH of <6.6 and >7.8 will
• cause discomfort
• § Tear pH of <6.5 and >8.5
• indicates that endothelial damage is possible

• 1. Biphasic (stimuli sensitive)--see notes
• 2 . Small molecules
• 3. Increased contact time with cornea
• 4. Vehicle with surfactant / detergent
• to break up epithelial integrity

• · The cul-de-sac holds 20-30uL
• · The volume of tear file is 7-9uL
• · Excess drops will run over (50uL)
• · Quick dilution with reflex tearing
• · Inflamed / infected eye – increased
• protein in tears will bind to drug and decrease bioavailability of the drug

long term EW can thin epith by up to 5.6%

corneal stroma

90%, ~500um

78%

• 68% is
• collagen, 9% is proteoglycans, 10% is keratocyts, and the remainder is
• random ions, salts, and glycoproteins

• organized in lamellae
• (perpendicular, stacked, oblique), as regular dense connective tissue

• modified fibroblasts
• Functions:
• · Production and organization of collagen and proteoglycans
• · Turnover of the extracellular matrix, which takes 12 months or more
• o Matrix metalloproteinase, MMP, are
• enzymes produced by keratocytes to degrade the extracellular material so that
• it can be renewed
• · Communication throughout the cornea
• via keratocyte-to-keratocyte gap junctions
• · Provision of energy
• · Tethering of neighboring lamellae
• · Stromal wound healing

bw the collagenous lamellae of the stroma

• When developing, they are very active and have many organelles, but once the stroma is mature, they have
• lost most of their organelles and are fairly transparent and less active.
• -There are also enzymes (ALDH1 and TKT) and crystalline that helps keep them transparent. When there is a wound, keratocytes mobilize to heal it and stop production
• of these enzymes and are therefore not transparent during healing.
• They are also extremely flat and a very small percentage of stroma.

• Keratocytes
• They can extend up to 100um and don’t have many organelles in their processes; organelles in the processes are within nodules

Collagen

• 1. structural support
• 2. an anchor for overlying epithelial tissues, & basal lamina
• 3. bed for wound repair
• 4. In the cornea, its arrangement aids
• corneal transparency

• Since collagen is hydrophobic, this
• prevents most interactions between molecules, with crosslinking being the
• exception
• · The collagen is staggered
• which gives a banded appearance due to the gaps
• o This occurs every 64um

mainly 1 , some types 5, 6, 7

1.411

• -organizes into lamellae, of which
• there are 200 to 250 arranged parallel to one another in the stroma
• -highly organized
• -Each lamella is 2um thick, and runs from limbus to limbus

-At the limbus, the lamellae turn toform a 1.5-2.0mm wide annulus running circularly around the limbus; this functions to maintain tension on the central cornea, controlling corneal curvature

• - Lamellar structure of the stroma makes
• a partial corneal transplants possible

Keratoconus

• -Size: Stroma – 30nm diameter fibrils
• (22-32nm); Sclera - ~120nm diameter fibrils (variable)

-Arrangement: Stroma – form into parallel lamellae (sheets), very regular; Sclera – interwoven connections of various sized fibrils, makes sclera stronger than stroma

• -Spacing: Stroma – 42-44nm between
• fibrils; Sclera – variable spacing, larger than cornea

• Proteoglycans
• water-soluble glycoproteins that are composed of a single
• protein core or chain, with glycosaminoglycan (GAG) side chains

sulfated; negatively charges the proteoglycan

• causing attraction of water and repulsion of nearby
• collagen fibrils, accounting for the interfibrillary distance in the stroma

KS proteoglycans

• 1. Lumican, the most common, which regulates collagen fibril diameter and spacing; it also regulates production of collagen by inhibiting fibrillogenesis
• · Essential for maintaining corneal
• transparency
• 2. Keratocan is found almost exclusively in the cornea
• 3.Mimecan

• CD and DS proteoglycans
• -They tend to be more negativelycharged than KS proteoglycans, so the interfibrillary distance is larger in the
• periphery

• -Decorin is a
• CD/DS proteoglycan found in the cornea that is also involved in inhibition of fibrillogenesis

CD and DS proteoglycans

• -Increase of CD in stroma
• -Increase of DS in stroma

-Decrease of KS (or none) in stroma

stroma

stroma

• 1. lattice
• 2. granular
• 3. macular

Bowman's layer (not a membrane)

Comes from modified fibroblasts that are part ofthe first mesenchymal wave

◦ types 3, 5, 6, and 7 are more common in Bowman’s◦ -It does not have as much Type 1 collagen as therest of the stroma

Reis-Buckler's dystrophy

• ◦ It is composed of interweaving(non-parallel)collagen fibrils that are attached to fibrils in the anterior◦ stroma◦
• o The interweaving is irregular andthe fibrils haverandom termination◦
• o This organization gives the layer toughness(tougher than the rest of stroma)

NO; acellular

Bowman's

• Altered structure
• Altered refractive index
• Energy transfer
• Produces heat
• Produces altered frequency
• Produces broken bonds (photopigment)
• Destructive interference

Corneal endothelium

endothelium

4-6um thick from base to apex

• polygonal; 70-75% are hexagonal which is preferential as it is thermodynamically very stable and
• strong (think benzene rings!)

~20um

500,000, decrease 0.6% per year

>5500cells/mm2

2500 to 3500cells/mm2

400-700 cells/mm2

• Variation of cell size and # of hexagonal cells
• -Specular microscopy (looks at the reflected light and allows measurement of cell size and number)

1. CV of normal endothelium is approximately 0.25 and any increase in this value indicates a more variable cell area, acondition called polymegathism

2. In a healthy young cornea, 70-80% ofendothelial cells are hexagonal at their apices and a reduction in thispercentage indicates that endothelial cells are losing their shape, a conditioncalled pleomorphism

• have large nucleus, many mitochondrion (very active), many ER and Golgi (produces a
• lot of protein)

Amitotic: cell not replaced, but cells spread out and cover that area

• § Less energy is required (can use for running
• pumps)
• § Less chance of mutations
• § Would slough off endothelial cells like the
• epithelial cells do and where would they go?

• 1. Age
• 2. Surgery (see details in notes)
• 3. Keratoconus
• 4. DM
• 5. Ocular HTN (causes a decr in cell density)
• 6. CL use

interdigitations,Macula Occludens, and gap junctions

3 Na+ for every K+

• Hydrogen, bicarbonate, and water are also
• transported
• o Water follows along with the sodium, helping the corneal stroma to retain its dehydrated state

Descemet's membrane

3-4 um

• at birth, This
• portion of Descemet’s is composed of collagen types 4 and 8, and remains distinct throughout life as the anterior
• banded layer

posterior un-banded layer

10-15 um

• anteriorly to the stroma via collagen fibrils that run between thelayers
• § Posteriorly, it connects to the endothelial
• cells via Fibronectin adhesion, NOT
• hemi-desmosomes

• Hassal-Henle bodies
• § Small white hyaline outgrowths on Descemet’s membrane, on the inside surface of the
• cornea in its periphery; normal aging change
• § CL patients may present with them sooner than
• normal

• 1. Guttata
• 2. Fuch's dystrophy
• 3. Posterior polymorphous dystrophy
• 4. Descemetocele
• 5. Descemet's rupture

• interfibrillary distance increases, thickness
• increases, and light scatter increases (the only factor that affects thickness is hydration)
• stromal hydration increases linearly with stromal thickness

70%

• · Tendency of stroma to imbibe water (take inwater) and swell
• Structural capacity for swelling, based uponits extracellular natureo Sincethere are lots of fibers etc. (not completely cellular); there is space for itto swell·
• IOP forces water from aqueous into the stroma (bulk flow)·
• Homeostatic characteristic of stromao Waterheld into place due to the negative charge of GAG’s

• 1. Structural limits on swelling·
• 2. Action of pumps·
• 3. Tear film osmolarity has a slight effecto As thetear film evaporates, water is evaporating, and solutes are left behind à tear film is hypertonic and fluid leavesstroma to make the tear film isotonic again·
• 4. Cell-to-cell barriers and junctions

the force necessary to prevent swelling at a given level of hydration

55-60 mmHg

• decreases
• (inverse relationship)

Imbibation pressure