Ocular: Cornea 1 And 2 Flashcards
Major concepts of the cornea:
-part of fibrous tunic
-transparent
-tough
-highly innervated
-avascular
-major refractive component of the eye
Cornea is the anterior ____th of the eye
1/6th
Area of cornea:
1.3 cm2
Is the cornea or the sclera steeper?
Cornea
Radius of curvature of posterior cornea:
6.5 mm
Radius of curvature of anterior cornea:
7.8mm
Range of horizontal corneal dimensions anteriorly:
11.7-12.6mm
**appears elliptical
Range of vertical corneal dimensions anteriorly:
10.6-11.7mm
Vertical and horizontal dimensions of cornea posteriorly:
11.7 horizontally and vertically
*cornea appears circular
Overall dioptric power ranges of the cornea:
+43.1D to +48.8D
-3/4th refractive power of eye
Index of refraction of the cornea
1.376
Astigmatism
Unequal curvature of the cornea that prevents light from being focused to a single point on the retina
Regular astigmatism:
Flat and steep meridians are 90 degrees apart
Types of regular astigmatism:
-with the rule
-against the rule
-oblique
With the rule astigmatism
Steeper in vertical meridian
Against the rule astigmatism
Steeper curvature in horizontal meridian
Oblique astigmatism
Steeper curvature in oblique axis
Irregular astigmatism
Steepest and flattest meridians are not 90 degrees apart
Lenticular astigmatism
Unequal curvature of the lens
Average central corneal thickness:
O.52mm
Average peripheral corneal thickness:
0.70mm
The thicker a persons cornea the _______ their IOP pressure measurement will be
Higher
The thinner a persons cornea, the _________ their IOP measurement will be
Lower
True IOP
the actual fluid pressure inside the eye measured by a tool that accounts for the biomechanical properties of the cornea
Pachymeter
Measures central corneal thickness using ultrasound
When might you want to use a Pachymeter on a patient ?
If the IOP measurement seems off, its used to measure true IOP esp if cornea is too thick or too thin
Bulbar limbus:
Fascia bulbi (tenon’s capsule) strongly adheres to episclera and conjunctiva
Corneoscleral limbus:
-Juncture of cornea and sclera
-1-2mm wide
Conjunctival limbus:
Conjunctiva extends 1mm beyond corneoscleral limbus
Optical zone of cornea is the central ___mm
4mm
5 layers of the cornea:
-anterior epithelium
-bowman’s layer
-stroma
-descemets membrane
-endothelium
The tear film sits ________ to the cornea and consists of ___ layers
Anterior, 3 layers
Three layers of the tear film:
-lipid/oily layer
-aqueous layer
-mucous layer
Where does lipid/oil layer of tear film come from?
Meibomian glands in the palpebrae and caruncle
Where does aqueous layer in the tear film come from?
Lacrimal gland
Where does mucous layer in the tear film come from?
Goblet cells in the conjunctiva
Why is the oily layer important?
Prevents the aqueous layer from evaporating
Why is the mucus layer important?
Decreases the surface tension and provides a smooth, wearable surface for the aqueous layer
Rate of tear production:
1.2 microliters/minute
Volume of tears:
6.2 +/- 2 microliters
Corneal epithelium is made up of what type of tissue? And how thick is it?
Stratified squamous epithelium, 50-60 microns thick
Corneal epithelium is continuous with the…..
Bulbar conjunctival epithelium
Corneal epithelium consists of ____________ and ______________cells and rests on a ______________membrane
Melanocytes and langerhan’s cells, basement membrane
Epithelium is composed of ___ - ___ layers and what type of tissue makes it up?
5-7 layers, non-keratinized, non-secretary stratified squamous epithelium
Apical (superficial) layer of epithelium is 1-2 layers of _________, ___________ cells near the surface
Flattened, squamous cells
2-3 layers of _____ cells attach to each other by desmosomes and gap junctions
“Wing” cells
The basal layer of cells in the epithelium are the only layer of cells in the corneal epithelium that undergo ________ __________ and are __________shaped
Mitotic division, columnar
Why are microvilli and microplicae important?
They pick up O2 from the air that is dissolved in the tears and assist in maintained the mucous lateral for adherence of aqueous layer
Loss of microvilli and microplicae can occur following…
Use of contact lenses
Loss of surface microvilli can lead to …
Transient dry eye symptoms until new cells reach the surface and develop microvilli
Where are tight junctions found on the apical surface of epithelium?
Between the lateral surfaces of apical cells!!
How to wing cells attach to adjacent cells?
They attach through desmosomes
How do wing cells communicate with adjacent cells?
Gap junctions
basal cells, like wing cells, also are attached to adjacent basal cells via _____________, and communicate via _____________
Desmosomes, gap junctions
How do basal cells attach to basement membrane?
Hemidesmosomes break, leaving anchoring fibrils form basal cells holds attachment
Why are anchoring fibrils important for basal cells?
They form the complex responsible for the firm attachment of the epithelium to underlying stroma
Corneal abrasion:
-scratch on the eye’s cornea
-usually painful but not severe
-commonly caused in sports by a scratch from fingernail
What are palisades of Vogt? Why are they important?
-Series of ridges in the corneoscleral limbus
-they house stem cells for corneal epithelium
Bowman’s layer is believed to provide ___________ _________ and is made up of ________ _________ ___________ ___________
Structural support, randomly arranged collagen fibrils
Is bowman’s layer acellular?
YES
How thick is bowman’s layer?
8-12 micrometer thick
Bowman’s layer contains ________ for passage of corneal nerve fibers
Channels (or pores)
Can the bowman’s layer be striped away from the stroma? Why or why not?
Cannot be because it is a continuous sheet
Can bowman’s layer regenerate?
NO
When is bowman’s layer formed?
Prenatally
If bowman’s layer is damaged, what replaces it?
Thickened epithelium (aka epithelial facet)
Does bowman’s layer contribute to bulbar conj?
NO
Why is bowman’s LAYER no longer called bowman’s MEMBRANE?
Because bowman’s “membrane” was found to be PAS negative, which is NOT a true basement membrane
What percent of corneal thickness is made up by the stroma?
90%
How thick is the stroma?
450-500 micrometers
What is the stroma made up of?
Collagen, keratocytes, and ground substance; where the collagen is organized in lamellae
In the stroma, what pulls aqueous humor into the cornea?
Glycosaminoglycans (GAGS)
In the stroma, what is interspaced between lamella and why?
Keratocytes, to maintain ground substance and collagen
How do keratocytes communicate with each other?
Gap junctions
Descemet’s membrane is the _____________ ____________ of endothelium
Basement membrane
Descemet’s membrane thickness throughout life from __micrometers to ___ micrometers
3 to 15 micrometers
Does descemets membrane regenerate if damaged?
YES
In Descemet’s membrane what is laid down before birth?
The anterior banded zone
In the Descemet’s membrane, what is laid down after birth?
Posterior unbanded zone
Where does Descemet’s membrane terminate?
At the schwalbe’s line ABRUBTLY
Hassall-Henle warts
transparent growths on the peripheral cornea and is normal with age
Corneal guttata
Transparent growths in the central cornea that indicate abnormal function of endothelium and can indicate endothelial dystrophy
Corneal endothelium is __-___micrometers thick
4-6
Corneal endothelium is a monolayer composed of about_________to _______cells
400,000 to 500,000 cells
Corneal endothelium cells appear ____ and ______ in shape and their apex faces the ________ chamber
Flat, hexagonal, anterior chamber
Corneal endothelium is linked together by ____________ and form ____________ to communicate with adjacent cells
Macula occludens, gap junctions
How does corneal endothelium maintain hydration?
By pumping aqueous humor out of the cornea
Loss of cells in the corneal endothelium can cause..
Change sin size and shape
If the pump action in corneal endothelium is not occurring what results?
Stroma edema
As corneal endothelial cells become stressed they __________
Dropout
When cells dropout from corneal endotheialium, the remaining cells compensate by…
Expanding in size and shape
When cells in corneal endothelium dropout, there is…
Less pump available to counterbalance leak of aqueous humor into corneal stroma
Is cornea vascular or avascular?
AVASCULAR
Where are nutrients derived from?
Tear film and aqueous humor, but also rely on nutrients from vasculature at peripheral cornea
Vasculature at peripheral cornea where we can get nutrients:
-superficial pericorneal plexus
-deep pericorneal plexus
Is cornea innervated?
YES! One of the most highly innervated structures in the body
Corneal sensitivity is ________ times greater than that of integument
300-600 times
Sensory innervation from the cornea is conveyed via:
Long posterior ciliary nerves
Where do the LPCN course and form?
In the suprachoroidal space and form an annular plexus of smaller branches within the corneoscleral limbus
Small radial branches extend into the middle and anterior stroma and form a _________________ below the bowman’s layer
Subepithelial plexus
If nerve fibers in the subepithelial plexus lose their myelin sheath they become naked and pass through the bowman’s layer to enter corneal epithelium where they form an ______________ ______________
Intraepithelial plexus
Where are sensory fibers not present within the cornea?
Posterior stroma, Descemet’s membrane, and corneal endothelium
