Cornea and Sclera part 1 Flashcards
what are the horizontal and vertical dimensions of the cornea
horizontal is 12.6mm and vertical 11.7mm
what is the refractive component of the cornea
48 Diopters of plus power
what shape is the central part of the cornea
spherical or toroidal contour
what is the radius of curvature of the outer aspect of the cornea
7.8mm
what is the thickness of the central and peripheral portions of the cornea
the central is 0.52mm and peripheral is 0.65mm
99% of which part of the spectrum gets transmitted through the cornea
about 400nm
how many layers are there in the corneal epithelium
5-7 stratified, nonkeratinized, squamous epithelium
how many cells thick is the surface/apical layer
3-4
how many cells thick is the wing cell layer
1-3
how many cells thick is the basal cell layer
1 cell thick
which layer of the corneal epithelium has mitosis
the basal cell layer
which layer of the corneal epithelium is adhered to the basement membrane
the basal cell layer
what happens to the basal cells as cell division begins to occur
the daughter cells move towards the surface and begin to differentiate forming the wing layer
how long does it take for epithelial cell turnover
7 days
where do the basal cells originate from
stem cells in the limbal epithelium (palisades of vogt)
because basal cells have high metabolic activity, what structures are prominent here
mitochondria, endoplasmic reticulum, and golgi apparatus (high glycogen storage)
what is the mitotic rate of the epithelium per day
10-15%
does the cornea have a constant or intermittent cycle of shedding superficial cells and proliferating basal layer cells
constant cycle
which direction do the cells migrate in the corneal epithelium
centripetal migration
what are the smaller, light cells on the surface of the corneal epithelium
younger cells that have recently reached the cornea
what are the larger, darker cells on the surface of the cornea
mature cells that will be sloughed off
what are the holes that you see on the surface of the cornea
exfoliation holes or breaks in epithelium (cells in process of peeling of the surface)
when looking at the cornea with a scanning electron microscope, what does the cornea look like
an irregular array of polygonal cells, larger, smaller, and exfoliation holes
how are the basal cells adhered to the basement membrane and stroma
by hemidesmosomes
what are connected to hemidesmosomes in the basement membrane that pass through bowmans layer
anchoring fibrils
what type of collagen are anchoring fibrils
type 7 collagen
how deep do the anchoring fibrils penetrate into the strom
2 mirons deep
what is located at the ends of the anchoring fibrils
anchoring plaques (end plates)
what is destroyed in photorefractive keratectomy (PRK) and must be reassembled during the healing process
the connection between the anchoring fibrils and the hemidesmosomes
what is located in the superficial cells of the epithelium (apical cells) that completely encircle the cells to form a barrier
zonula occludens
where are gap junctions found
in all layers but more numerous in the basal layers than the superficial layers
what type of membrane do the zonula occludens form
highly effective semipermeable membrane
which ion is transported into the stroma and which is pumped out
Na+ is pumped from the tears into the stroma and Cl- is transported from the stroma into tears
what is EBMD
epithelial basement membrane dystrophy
what is a symptom of EBMD
painful recurrent epithelial erosions, susceptible to edema and infection
what causes recurrent erosions in EBMD
the basal cells have decreased hemidesmosomes and have an abnormal adhesion
what happens to the basement membrane in aging and diabetic patients
thickening of basement membrane
why do aging and diabetic patients have an increased risk of epithelial erosions
the anchoring fibrils cannot penetrate as deep through the thickened basement membrane into the stroma
why would the erosions in diabetic patients not heal as quickly as they should
if they have nerve damage the brain has a delay in receiving the information, the healing process is delayed
how thick is the basement membrane (basal lamina)
40-60 nm thick
what is the basement membrane made of
type 4 collagen, laminin, proteoglycan perlecan, fibronectin, and fibrin
does a corneal epithelium abrasion demand a slow or fast healing response
fast- must recover the exposed basement membrane with cells
what happens to mitosis after an abrasion occurs
mitosis stops and the attachment to the basement membrane is lost to conserve energy
after mitosis stops, what else happens to the cells
the cells enlarge and the epithelial sheet migrates by ameboid movement to cover the defect
what is the primary function of the corneal epithelium
form a barrier to invasion of the eye by pathogens and to uptake excess fluid by the stroma
what happens to the epithelium after the wound closes
mitosis resumes, protein synthesis by epithelial cells increases during cell migration and growth factors increase
how fast does a 6mm epithelial wound close
within 48 hours
what is the rate that an epithelial wound heals
60-80 microns per hour
is there an increase or decrease in glycogen levels in migrating cells
decrease in levels
is there an increase or decrease in glycolytic activity during cell mirgration
increase
are cells dependent on anaerobic or aerobic glycolysis
anaerobic- it is not efficient but fast
what has an increased expression to supply support for the healing process
glucose transporter GLUT1 mRNA and GLUT1 protein levels
what are 3 growth factors that increase during healing
EGF (epidermal growth factor) KGF and HGF (keratocyte and hepatocyte growth factor)
which growth factor is expressed by the corneal epithelium
the epidermal growth factor
which growth factor is expressed by stromal keratocytes
keratocyte and hepatocyte growth factors
which conditions have delayed healing or no normal epithelium adhesion is established
basement membrane dystrophy, diabetes, persistent or recurrent epithelial defects and severe injuries (alkali burns)
why is the peripheral epithelium faster to heal than the central epithelium
there is a greater blood supply in the limbus
if the basement membrane is NOT damaged, is there a delay in reestablishing formation of new hemisesmosomes
no it is formed quickly
if the basement membrane is damaged are the formation of new adhesion complexes formed quickly or slowly
slowly, there is a delay
how long is the new development of adhesion complexes delayed for
more than 12 months
how thick is bowman’s layer
12 microns thick
what type of collagen is in Bowman’s layer
type 1 collagen
is the collagen in Bowman’s layer randomly or regularly arranged
randomly arranged
True or False: bowman’s layer acellular
True
what does the stroma consist of
an extracellular matrix comprised of lamellar arrangement of collagen fibrils
are the collagen in the stroma parallel or perpendicular to the corneal surface
parallel
what separates the collagen fibrils in the stroma
a matrix of proteoglycans
what 2 things produce and maintain the extracellular matrix
keratocytes and fibroblasts
how many lamellas of collagen fibers are in the stroma
200-250
which direction do the collagen fibers lie in the anterior stroma
obliquely
which direction do the collagen fibers lie in the posterior stroma
orthogonally
at the limbus, the collagen fibers run circumferentially around the cornea, how wide is the annulus formed
1.5-2.0 mm
what types of collagen are in the stroma
mostly type 1, also type 5 and 6 collagen
what is the refractive index for the collagen fibers in the stroma
1.411
what is the refractive index for the extrafibrillar matrix
1.365
how much light is scattered because of the highly uniform size and spacing of the collagen fibers
only 10%
what is Maurice’s proposal
the corneal transparency is a consequence of a crystalline lattice arrangement and the light scattered by individual fibrils os uniform diameter is canceled by destructive interference with scattered light from adjacent fibers
do the requirements for Maurice’s proposal apply when talking about the cornea
no
do the sizes of the collagen vary in size in the stroma
yes but within a small range
why is there weak scatter of light in the stroma
the collagen have a diameter of a small fraction of the wavelength of visible light
how far apart must the collagen fibers be to remain transparent
less than 1/2 the wavelength of visible light
if the cornea swells from damage to epithelial or endothelial barriers, what happens to the transparency
it is lost
if there is a “lake” of water in the stroma where there aren’t collagen fibers, what 3 things can it cause
- increase divergence of refractive index
- increase in distance between collagen fibrils
- all resulting in a loss of light transmittance
is the collagen fibril diameter larger in the anterior or posterior cornea
the anterior
is the density of the fibrils larger in the anterior or posterior cornea
the posterior
why is there a twofold increase in light scatter by the anterior cornea compared to the posterior cornea
because the fibril diameter is larger and the density is smaller
what is the rate of evaporation on the corneal surface
2.5 microliters per cm^2 per hour
what percentage of the cornea thins during the day compared to at night
5%
what disorder causes a compromised endothelial pump function and worse edema in the morning
Fuch’s dystrophy
what is dellen
localized area of corneal drying and evaporation
what may persistant dellen reflect
a decrease in stromal fluid flow when stroma hydration is abnormal or minimal flow of water in the cornea
what 3 conditions may disrupt tears to spread evenly
coloboma, pterygium, or a pingeula
if IOP is high but remains under 50mm Hg, will the cornea remain a constant thickness
yes because the stromal swelling pressure is in a similar range
if IOP is higher than 50mm Hg or there is abnormal endothelial function, will the cornea remain a constant thickness
no there is epithelial edema and increased stromal thickness
what is the relationship between IOP and stromal swelling pressure
IP (stromal imbibitions pressure) = IOP - SP
as there is an increase in corneal thickness, what happens to stromal swelling pressure
it decreases
how would you restore tensile strength after a stromal wound
re-synthesis and cross linking of collagen, alterations in proteoglycan synthesis, and gradual wound remodeling
what cells show up within hours around areas of cellular necrosis in a penetrating corneal wound
polymorphonuclear cells - granulocytes and then monocytes (white blood cells)
how many years can tensile strength gradually increase up to
the 4th year post-op
why do incisions in avascular cornea far from the limbus heal more slowly
they are farther from the blood supply than a peripheral incision
why would you want to prescribe a steroid for corneal wound healing
to control the speed of recovery or control the amount of scaring
what is a disadvantage of prescribing steroids after a corneal wound or surgery
the wound heals slower
how thick is the corneals endothelium, Descemet’s membrane
10-15 microns thick
what happens to Descemet’s membrane as we age
it thickens
what fibers is descemet’s membrane composed of
type 4 collagen, laminin, and fibronectin
what secretes Descemet’s membrane
the endothelial cells
what is a descementocele
herniation of descemet’s membrane (the membrane stays intact after severe corneal ulcerations and it pushes decemet’s membrane up after destruction of the epithelium and stroma)
what is Descemet’s membrane highly resistant to
proteolytic enzymes
what happens when Descemet’s membrane is lost
because it doesn’t regenerate, the cells enlarge to compensate for loss, they change shape and spread over the missing areas
what is Fuch’s dystrophy
a disease of the endothelial cells and an abnormal Descemet’s membrane is secreted
what happens to the endothelial cells as Fuch’s Dystrophy progresses
the endothelial cell function decreases and cells are lost
what is Guttata
collagenous material formed on the posterior surface of Descemet’s membrane
what happens to the endothelial cells in Guttata
thinning and enlargement of endothelial cells
what happens to endothelial cells as we age
the cell density decreases
what is the newborn endothelial cell density
greater than 5500 cells/mm^2
what is the adult endothelial cell density
2500-3000 cells/mm^2
what is the minimum endothelial cell density
400-700 cells/mm^2
what do endothelial cells contain
large nucleus, mitochondria, endoplasmic reticulum , and golgi apparatus
endothelial cells are metabolically active in what 3 things
transport, synthesis, and secretory
what junctions are in the endothelial cells layer
tight junctions- macula occludens and gap junctions
what is the purpose of the macula occludens
they do not completely circle the cells and provide a leaky barrier between aqueous and stroma
where are the gap junctions in the endothelial cells
between lateral membranes for intercellular communication
