[H] Tear Film Lecture

What are the 4 main functions of the tear film?

1. Smooth optical refracting surface
2. Medium for debri removal
3. Protects ocular surface
4. Supplies oxygen, GFs, and other compounds to the cornea

[H] Tear Film Lecture

What are the 3 layers of the tear film in order from closest to the cornea to closest to the surface?

1. Mucin (posterior)
2. Aqueous (middle)
3. Lipid (anterior)

[H] Tear Film Lecture

Whay is the volume of the tear film higher in the unanesthetized vs. the anesthestized eye?

[H] Tear Film Lecture

What are the sources for the lipid layer?

• Meibomian glands
• Sebaceous glands of Zeis

[H] Tear Film Lecture

What are the 4 main properties of the lipid layer?

(hint: thickness? MP? polarity at each surface?)

• Very thin (0.1µm)
• A melting point of 35 degrees celcius
• A polar-aqueous interface
• nonpolar-air interface

[H] Tear Film Lecture

What are the four main functions of the lipid layer?

(hint: ROLP)

• Retard evaporation
• Optical properties: air-tear film interface
• Lipid strip prevents tear overflow
• Prevent damage to lid margin skin by tears

[H] Tear Film Lecture

What is important about the melting point of the lipid layer?

(hint: what is the temperature of the body?)

[H] Tear Film Lecture

What would happen if the lipid layer is not stable?

[H] Tear Film Lecture

What are the 5 nonpolar lipids found in the lipid layer in order of most to least abundant?

1. Wax esters (35%)
2. Cholesteryl esters (30%)
3. Triacylglycerols (4%)
4. Cholesterol (2%)
5. Fatty acids (2%)

[H] Tear Film Lecture

What are the two main types of polar lipids found in the lipid layer? How abundant are these lipids compared to other polar lipids? (hint: compare percentages)

• Polar lipids (16%) = phosphatidyl choline and phosphatidyl ethanolamine.
• Other (11%)

[H] Tear Film Lecture

Which layer of the tear film is the thickest?

Aqueous layer

[H] Tear Film Lecture

What are the 3 main sources of the aqueous layer?

• Lacrimal gland (orbital and palpebral portions, by levator aponeurosis)
• Glands of Krause (2/3 of accessory lacrimal glands)
• Glands of Wolfring

[H] Tear Film Lecture

What is the composition of the aqueous layer?

• Electrolytes
• Water
• Protein (soluble proteins and GFs)
• Other solutes

[H] Tear Film Lecture

What are the four main functions of the aqueous layer?

1. Supplying oxygen
2. Maintaining electrolytes
3. Antiinfective defense
4. Washing away debris

[H] Tear Film Lecture

Which Ig is found in body secretions? What type of tissue does it line, and is often secreted from in the eye?

• IgA
• Lines epithelium and can be secreted by epithelial cells

[H] Tear Film Lecture

Which Ig is found in blood vessels?

IgD

[H] Tear Film Lecture

Which Ig is involved in allergic responses as well as fighting off parasites? An increase in this Ig is associated with which eye condition?

• IgE
• Vernal Keratoconjunctivitis

[H] Tear Film Lecture

Which Ig crosses blood vessel walls, is the smallest Ig, and is found in the highest concentration in the body? Why does this Ig cross vessel walls?

• IgG
• To recruit other Igs to defend against infections

[H] Tear Film Lecture

Which Ig is involved in the first response to an infection/lesion, and is the largest Ig (pentamer)?

IgM

[H] Tear Film Lecture

Which tissues in the eye contain Igs?

Which Igs are involved in local host defense mechanisms of the eye, such as ocular inflammation?

• Plasma cells in the lacrimal glands and substantia propria
• Increased tear IgA and IgG

[H] Tear Film Lecture

How does IgA interact with other IgAs?

Through the J-chain. This forms IgA dimers throughout epithelium

[H] Tear Film Lecture

What are the 3 functions of electrolytes in the tear film?

• Regulate osmotic flow
• Buffer tear pH
• Serve as enzyme cofactors (Fe2+, Cu2+, Mg2+, Ca2+)

[H] Tear Film Lecture

Compared to blood serum, how much Na+ and K+ are found in the tear film?

Which compound is responsible for regulating pH?

Which solutes found in tears are found in the same concentrations in serum?

• Na+ found in tears is about the same concentration as in serum
• K+ is about 5-7x the amount in serum
• Bicarbonate regulates pH
• Urea, glucose, lactate, citrate, ascorbate, and amino acids

[H] Tear Film Lecture

What are the 3 main factors that trigger reflex tearing?

1. Physical irritation
2. psychogenic factors (emotions)
3. Bright light (Optic nerve involvement)

[H] Tear Film Lecture

How is tear secretion controlled through the autonomic nervous system?

Parasympathetic and sympathetic innervation of tear glands and epithelia

[H] Tear Film Lecture

Which hormones are found in tear secretions?

• Peptide hormones (cAMP produced through Gs and Gi systems)
• alpha-melanocyte-stimulating hormone
• VIP (Vasointestinal peptide)
• Steroid hormone: Androgen (intracellular receptor)

[H] Tear Film Lecture

What are the 3 functions of the mucin layer?

1. Coats the microplicae
2. Forms a fine network over the conjunctival surface
3. Contains mucins, proteins, electrolytes, and water

[H] Tear Film Lecture

Which part of the mucus layer is responsible for imparting viscosity to the tear film?

What are the 3 main types of proteins found in the mucin layer?

1. Epithelial glycocalyx
2. Proteins, glycoproteins and lipoproteins

[H] Tear Film Lecture

The mucin layer is formed of both hydrophhobic and hydrophillic portions. Which part is hydrophillic? Hydrophobic? Which do each represent in the "bottle-brush" model?

Oligosaccharides (sugars) = hydrophilic (hairs)

Proteins = hydrophobic (handle)

[H] Tear Film Lecture

What are the 3 functions of mucins?

1. Convert corneal epithelium to a hydrophillic layer (spread)
2. Stabilize the tear film
3. Trap exfoliated surface cells, foreign particles, and bacteria

[H] Tear Film Lecture

What are the sources of mucins? How much mucin does each source produce?

• Conjunctival goblet cells (2-3μl/day)
• Stratifies squamous cells of conj and K epithelia (aqueous tear production of 2-3ml/day)
• Minimally by lacrimal glands of Henle and Manz

[H] Tear Film Lecture

Which type of mucin has been found to be reduced in some cases of dry eye?

Mucin 16 reduced in the mucin membrane

[H] Tear Film Lecture

What is the mucin deficiency hypothesis of dry eye?

That microplica produce mucins, and that mucins are supposed to interact with microplica normally, but when mucin does not interact with microplica there is an uncovered portion of the tear film and dry eye results

[H] Tear Film Lecture

What are the main causes of mucin dysfunction involving both deficiencies and excess of mucins?

What happens in both deficiency and excess of mucins in tear film?

• Deficiency = Avitaminous A and conjunctiva destruction
• Excess = Hyperthyroidism, allergies, vernal, and GPC
• In deficiencies the cornea will dry and melt leading to blindness. In excess, the tear film becomes sticky.

[H] Tear Film Lecture

Which 9 proteins are involved in antiinfection?

• Lysozyme
• Lactoferrin
• Phospholipase A2
• Lipocalins
• defensins
• Interferons
• Interleukins
• Glycoproteins
• Albumin

[H] Tear Film Lecture

How do lysozymes work to decrease incidence of infections in the eye?

By destroying g+ bacteria by hydrolyzing peptidoglycan of the outer cell wall

[H] Tear Film Lecture

How do lipocalins work as antiinfectives in the tear film?

They remove harmful lipophilic molecules, and are antifungal

[H] Tear Film Lecture

How does lactoferrin work as an antiinfective in the tear film? (3 factors)

• Synergizes with lysozyme
• Binds to lipoteichoic acid on the bacterial surface
• Discourages growth of iron-dependent bacteria by binding iron.

[H] Tear Film Lecture

What are the 6 main GFs found in the tear film?

• Tumor growth factor β
• Epidermal growth factor
• β fibroblast growth factor
• Interleukin-1α
• Interleukin-1β
• Tumor necrosis factor α

[H] Tear Film Lecture

What are the main functions of GFs in the tear film?

1. Proliferation, migration, and differentiation of K and conj epithelial cells
2. Wound healing of the ocular surface

[H] Tear Film Lecture

What are the 4 qualitative/quantitative causes of tear dysfunction?

• Change in the amount of tear film constitutions
• Change in the composition of tear film
• Uneven dispersion of the tear film because of corneal-surface irregularities
• Ineffective distribution of the tear film caused by eyelid-globe incongruity

[H] Tear Film Lecture

What are the 3 main deficiencies causes abnormalities of the tear film?

• Lipid deficiency
• Aqueous deficiency
• Mucin deficiency

[H] Tear Film Lecture

What are the 9 multifactorial causes of tear film dysfunction?

• Age
• Hormonal status
• Genetics
• Sex
• Immune status
• Innervation status (Bell's palsy)
• Nutrition
• Pathogens (systemic meds can decrease tear production, e.g. antihistamines and PGs)
• Environmental stress

[H] Tear Film Lecture

What percentage of patients seeking treatment at an ophthalmologist's office have symptoms consistent with dry eye disease (DED)?

30%

[H] Tear Film Lecture

How many men and women in America have dry eye? At what age range was this estimate taken for?

• 3.23 million women and 1.68 million men (so a ratio of 3:1 women to men)
• Total of 4.91 million Americans
• 50 years and older

[H] Tear Film Lecture

What percentage of American dry eye subjects reported dry eye symptoms some of the time or in response to certian environmental, workplace, or recreational activities?

As high as 20%

[H] Tear Film Lecture

What percentage of LASIK patients reported DED symptoms after surgery?

Whay is dy eye associated more with LASIK than PRK?

• In more than 50%
• Because nerves take longer to reconnect rather than regrow in the case of PRK

[H] Tear Film Lecture

What are the 5 main types of diagnostic tests for dry eye?

• TBUT (fluorescein often used)
• lissamine green staining (assess conj. and K health)
• rose bengal staining (assess conj. and K health)
• osmolarity tests (↑osmo = ↑proteins, ↓osmo = unusual aqueous layer)
• Schirmer tests (tests tear production. Exhibits wide intrasubject, day-to-day, and visit-to-visit variation)

[H] Tear Film Lecture

How do artificial tears compare to real tears?

[H] Tear Film Lecture

What are the options for therapy of mild dry eye?

Two or more of the following options used concurrently:

• Artificial tears
• punctal plugs
• Omega-3 fatty acid supplementation

[H] Tear Film Lecture

What are the options for therapy of moderate dry eye?

Two or three of the following may be used to acheive control:

• mild dry eye options
• gel formulation at bedtime
• Restasis trial for 3-6 months

[H] Tear Film Lecture

What are the options for therapy of severe dry eye?

Since punctal plugs have the potential to concentrate inflammatory cytokines, try first to get ocular surface tissues at least partially rejuvenated prior to plugs. Options include:

• More viscous and preservative-free artifical tears used often
• Lotemax qid for a week, then bid for a month (to observe for significant inflammatory component)
• Long-term Restasis if Lotemax brought relief
• Oral doxycycline 100mg/day for two weeks, then 50mg/day for 6 months. 
• Omega-3 fatty acid supplementation for 3-6 months
• Punctal plugs
• Moisture sheilds or moisture goggles

[H] Tear Film Lecture

What is the active component in Restasis?

Cyclosporin

[H] Tear Film Lecture

What are 4 the functions of blinking?

• Tear renewal
• Tear distribution
• Tear turnover
• Tear drainage

[H] Tear Film Lecture

How do the upper eyelids function?

How do the lower eyelids function?

1. Upper eyelids clear anterior surface of debris and any insoluble mucin and expresses secretions from MGs.
2. Lower eyelids move horizontally in a nasal direction and pushes tear fluid and debris toward superior and inferior puncta.

[H] Tear Film Lecture

What tissues are part of the lacrimal functional unit (LFU)?

What are the protective cells of the LFU?

What is the main challenge of the LFU?

• All tissues of the ocular surface, secretory glands, eyellids and outflow channels of the nasolacrimal pathway are linked via a neural network called the LFU.
• Mast cells, neutrophils, macrophages, dendritic cells, basophils, and eosinophils.
• Lack of concordance

[R] Cornea, Sclera, and Limbus

What are the two main functions of the cornea?

1. Transmission of light. Must be transparent
2. Refraction of light. Must bend light to help focus on retina.
   

[R] Cornea, Sclera, and Limbus

What are the diameter demensions of the cornea?

What is the radius of curvature of the cornea?

What is the thickness of the cornea (central and peripheral? (fig. 2-1)

What borders the cornea?

1. 11mm vertically, and 12mm horizontally
2. r = 8mm, but anterior surface has a greater r than back surface. (anterior = 7.8mm, posterior = 6.5mm)
   
3. Central is thinner than peripheral: central = 0.53mm, peripheral = 0.71mm.
4. The limbus
   

[R] Cornea, Sclera, and Limbus

What is the relationship between radius of curvature and surface steepness?

Inverse relationship: Short radius of curvature gives a steep surface.

[R] Cornea, Sclera, and Limbus

What is regular corneal astigmatism?

What is WTR astigmatism?

What is ATR astigmatism?

What is oblique astigmatism?

1. Principle meridians are 90° apart.
2. WTR = vertical is steepest meridian (r = shortest at 90°)
3. ATR = Horizontal is steepest meridian (r = shortest at 180°)
4. Oblique = Steepest meridian is at 45° or 135°

[R] Cornea, Sclera, and Limbus

What is irregular corneal astigmatism? How is it corrected?

• Principle meridians are not 90° apart
• Must be corrected with CLs
  

[R] Cornea, Sclera, and Limbus

For describing epithelial histology, how are the top, sides, and cell surfaces adjacent to the basement membrane decribed?

• Apical = top
• Lateral = side
• Basal = Usually resting on a basement membrane. Connects epithelial cells to CT.
  

[R] Cornea, Sclera, and Limbus

Concerning the basement membrane of epithelial tissue, what secretes the basal lamina?

What secretes the reticular lamina?

• Basal lamina is secreted by epithelium
• Reticular lamina is secreted by CT
  

[R] Cornea, Sclera, and Limbus

What is the name for the belt-shaped intercellular junctions?

What is the name for the "spot connection" shaped intercellular junctions?

What is the function of intercellular junctions?

• Zonular
• Macular
• Joining cells to eachother
  

[R] Cornea, Sclera, and Limbus

How would you describe adhering intercellular junctions? (hint: size of intercellular space, and type of molecule)

• Narrow intercellular space
• Transmembrane adhesion molecules

[R] Cornea, Sclera, and Limbus

What are the 3 main functions of occluding intercellular junctions?

• Focal fusion of outer leaflet of plasma membranes
• Forms a semipermeable membrane when found in an epithelial layer
• Seals off intercellur space
  

[R] Cornea, Sclera, and Limbus

What is a terminal bar?

Adjacent zonular occluden (ZO) and zonular adheren (ZA)

[R] Cornea, Sclera, and Limbus

1. Describe the characteristics of a desmosome(hint: shape of attachement, type of attachment, extent of attachment).
   
2. How is it different from a hemidesmosome?
3. What movement of materials do desmosomes permit?
   

1. Strong, circular attachment plaque with filaments extenting into the cellular cytoplasm
2. A hemidesmosome is an attachment of the basal surface underlying the CT, and do not connect adjacent cells
3. Only movement of materials around these structures, not through them.
   

[R] Cornea, Sclera, and Limbus

1. Describe the structure of a gap junction (hint: what is it composed of?)
   
2. What is the function of a gap junction?
   

1. A transmembrane channel/pore called a connexon, which is made up of 6 protein connexins
2. Permits passage of small molecules, nutrients, and ions
   

[R] Cornea, Sclera, and Limbus

1. What are the 5 layers of the cornea?
2. which layers are connective tissue?
   

1. Epithelium, Bowman's layer, stroma, Descemet's membrane, and endothelium
2. The three middle layers (Bowman's, Stroma, and Descmet's)
   

[R] Cornea, Sclera, and Limbus

1. What type of epithelium is the cornea composed of?
2. Describe the surface cells; shape, # of layers.
3. Describe the middle cells; shape, # of layers
4. Describe the most posterior basal cells; shape, # of layers.
   

1. Stratified squamous
2. Flat cells, 2 layers/2 cell thickness
3. Wing-shaped cells close to cuboidal shape, 2-3 cell thickness/layers
4. Single layer, columnar in shape
   

[R] Cornea, Sclera, and Limbus

1. Is the surface layer of the cornea keratinized?
2. What type of projections does the suface layer contain?
3. What does the surface layer secrete?
   

1. No, it is nonkeratinized
2. Microvilli (finger-like shape) and microplicae (ridge-shaped)
3. Secrete glycocalyx of mucin tear layer
   

[R] Cornea, Sclera, and Limbus

1. What type of intercellular junctions are found in the corneal epithelium?
2. What is their function?

1. Zonular occludens (ZO), and desmosomes
   
2. ZO: Semi-permeable membrane
   

[R] Cornea, Sclera, and Limbus

1. How is the wing cell layer of the corneal epithelium often described?
2. What types of intercellular junctions are found in this layer?
   

1. Transition layer
2. Desmosomes and gap junctions
   

[R] Cornea, Sclera, and Limbus

1. What is the basal layer of the corneal epithelium often called? Why is it called this?
   
2. What does this layer secrete?
3. What is the turnover rate of the corneal epithelium?
4. What types of intercellular junctions does this layer have?
   

1. Germinative layer - where mitosis occurs.
2. Secretes basal lamina portion of basement membrane (BM)
3. 7 days
4. Desmosomes, gap junctions, and hemidesmosomes (fibrils attach to plaque in stroma)
   

[R] Cornea, Sclera, and Limbus

1. What are the 2 main causes of recurrent corneal erosions?
2. What are the 2 main signs/symptoms of recurrent corneal errosions?
3. How are recurrent corneal erosions treated?
   

1. Malformed hemidesmosomes, and epithelial BM dystrophy (EBMD)
2. Areas of sloughed epithelium especially when waking up in the morning (lid movement can cause epi to be pulled off), and pain that can be severe
3. bandage CL, or even a corneal puncture that stimulates BM adhesion
   

[R] Cornea, Sclera, and Limbus

1. How often does the corneal epi undergo cell division?
2. Cell division takes place in which layer?
3. How are these cells replenished?
4. How is the barrier function maintained in the epi?
   

1. Constantly
2. Basal layer
3. Basal cells replenished from stem cells in the limbus
4. maintained because the basement layer has ZO
   

[R] Cornea, Sclera, and Limbus

1. What can cause the loss of Bowman's layer?
2. What is the composition of Bowman's layer? (hint: type of tissue, cell type)
3. How does this layer compare to the stroma?
4. How is this layer produced and regenerated?
5. What other theoretical role does this layer play?
   

1. PRK surgery, but not LASIK. Also traumatic eye injury
2. Dense irregular CT, acellular
3. smaller and less regular fibers than stroma
4. Produced prenatally and cannot be regenerated
5. May play a role in protection against UV light
   

[R] Cornea, Sclera, and Limbus

What is a pterygium and how does it affect Bowman's layer?

Conj tissue that invades the corneal tissue. Bowman's layer is lysed.

[R] Cornea, Sclera, and Limbus

1. What is the composition of the substantia propria of the stroma? (hint: type of tissue, type of fibrils, orientation, and extent)
2. What are the cells in the stroma called?
3. What material fills between fibrils, lamellae, and other cells?
4. What are GAGs? (charge, and function)
   

1. Regular dense CT that forms collagen fibrils of Lamellae (bundles) oriented parallel to the surface and extend from limbus to limbus. Interweaving occurs between lamellae.
   
2. cells = fibroblasts = keratocytes (specific name of fibroblast in the cornea)
3. Ground substance
4. Glycosaminoglycans (GAGs) = proteoglycans. Negatively charged (hydrophilic), and function to attract and bind with water, maintaining the precise spatial relationship between individual fibrils, and are one reason for relatively high stromal hydration.
   

[R] Cornea, Sclera, and Limbus

1. What is the Theory of Transparency?
2. How is transparency maintained in the cornea?
3. How is transparency lost in the cornea?
   

1. That light should pass straight through the cornea, and scattered light causes glare
2. Fibers are arranged in a regular lattice pattern that causes destructive interference, which reduces light scatter as long as the distance between different refractive indices is less than 1/2 the wavelength of light.
3. If spacing of fibers is disrupted
   

[R] Cornea, Sclera, and Limbus

1. What is the composition of Descemet's Membrane? (type of tissue and cell type)
2. What are the properties of Descemet's?
3. What is its relationiship with the corneal endothelium?
4. How does it change with age?
   

1. Dense CT, acellular
2. Elastic properties (but no true elastic fibers), highly resistant to proteolytic enzymes.
3. BM of endothelium
4. Thickens with age
   

[R] Cornea, Sclera, and Limbus

1. What is the line called where Descemet's membrane ends?
2. What shape is it?
3. It is the termination of the membrane at what portion of the cornea?
   

1. Schwalbe's line
2. Circular line
3. Corneal periphery
   

[R] Cornea, Sclera, and Limbus

1. What is the arrangement of the corneal endothelium?
2. 70-80% of the cells are what shape in the endothelium?
3. How does the endothelium change with age? (hint: does it replicate?)
   

1. Regular endothelial mosaic
2. Hexagonal
3. Does not replicate, cells just spread out
   

[R] Cornea, Sclera, and Limbus

1. What are the 3 intercellular junctions that the corneal endothelium contains?
2. Which type of intercellular junction does it for sure not contain?
   

1. Lateral interdigitations (increase strength of cornea and increase surface area, and contain 1.5 milllion Na+/K+ pumps), gap junctions (provide intercellular communication), occluding junctions (different from tight epithelial junctions, slightly leaky barrier described as a series of small gaps to allow passage of water)
2. No hemidesmosomes (now calling them this, but a special type)
   

[R] Cornea, Sclera, and Limbus

1. How does Descemet's membrane normally change with age?
2. What dysfunctional changes can occur in Descemet's membrane with age?
   

1. Hassell-Henle bodies form that are bumps/lumps of the endo associated with aging
2. Guttata is abnormally increased space between membrane of central cornea seen when focused on basal layer of cornea on slit-lamp. Indicates dysfunction.
   

[R] Cornea, Sclera, and Limbus

1. What is keratoconus,a nd what are the 2 general causes?
2. What affect does this condition eventually have on the stroma?
3. What other condition does keratoconus lead to, and how can it be corrected?
4. What is Munson's sign?
   

1. Corneal dystrophy that initially disrupts the BM and Bowman's layer. Cause is either metabollic or nutritional.
2. Eventually causes significant stromal thinning
3. Produces irregular astigmatism that cannot be corrected with spectacles, but RGP CLs may slow progression.
4. V-shaped indentation in the lower eyelid when a keratoconus patient's gaze is directed downward.
   

[R] Cornea, Sclera, and Limbus

1. What is thought to prevent blood vessels from growing in the cornea?
2. How much is the cornea innervated by nerves?
3. What is lost from the nerves within 1-2mm of entering the cornea?
4. What is the majority of receptors found in the cornea?
   

1. There may be an antiangiogenic agent present in the cornea. 
2. Dense sensory innervation (70-80 bundles of axons enter corneal periphery)
3. Loss of myelin sheath
4. Nociceptors (pain receptors) for corneal reflex
   

[R] Cornea, Sclera, and Limbus

1. What part of the cornea contains 900-1200 axons?
2. Which plexus is located underneath the epi of the cornea?
3. In which plexus is the Schwann cell covering lost as the fibers pass through Bowman's?
   

1. Stromal plexus
2. Subepithelial plexus
3. Intraepithelial plexus (called naked nerve endings)
   

[R] Cornea, Sclera, and Limbus

1. Where is the sclera located?
2. What is the composition of the sclera? (type of tissue, vascularity)
   
3. What is the sclera continuous with?
4. Describe the 3 different colors the sclera can be depending on the physical state of the patient
   

1. Possterior 5/6 of globe (light scatter not reduced here)
   
2. Dense irregular CT, avascular
3. Corneal stroma (more fibroblasts in sclera)
4. White = healthy, Blue = thin or transparent (can occur with osteogenesis), Yellow = with fat deposit or liver dysfunction (jaudice)
   

[R] Cornea, Sclera, and Limbus

1. What are the 2 foramina of the sclera and where are they located?
2. What are the 3 main apertures of the sclera, where are they located, and what do they contain?
   

1. Anterior scleral foramen is located in the corneal area. Posterior scleral foramen is located where the optic nerve passes through, and is bridged by the lamina cribrosa
   
2. Anterior apertures are located near the limbus and are the passages for the anterior ciliary arteries. Middle apertures are located near the equator and are passages for vortex veins. Posterior apertures are located around the optic nerve and are the passages for the long and short posterior ciliary arteries and nerves.
   

[R] Cornea, Sclera, and Limbus

1. What is the location and composition of the episclera?
2. What is the location and composition of Tenon's capsule?
3. What is the location of the bulbar conj, and what is it continuous with?

1. Located outer to sclera and composed of loose vascular CT
2. Located outer to episclera and composed of a dense CT capsule
3. Located outer to Tenon's capsule and is continuous with palpebral conj at fornices
   

[R] Cornea, Sclera, and Limbus

1. What is the composition of the bulbar conj?
2. What specialized cells does it contain and what do these cells secrete?
3. What is another name for the stroma of the bulbar conj and why is it known to be immunologically active?
   

1. Stratified non-keratinized columnar epi
2. Contains goblet cells that produce mucin tear layer
3. Also called submucosa, blood vessels and lymphatics found here
   

[R] Cornea, Sclera, and Limbus

1. What is the location of the limbus?
2. What is the anterior border of the limbus?
3. What is the posterior border of the the limbus?
4. What 3 landmarks are associated with the limbus?
5. The limus is the transition site of which 3 layers?
   

1. Annular region surrounding corneal periphery
2. Line drawn from termination of Bowman's to terminationi of Descemet's (where BM ends)
3. Line from posterior scleral spur to conj surface and perpendicular to it
4. External scleral sulcus, internal scleral sulcus, and Palisades of Vogt
   
5. Cornea, conj, and sclera
   

[R] Cornea, Sclera, and Limbus

1. The trabecular meshwork and Schlemm's canal is located in which region of the limbus? Where do these systems eventually drain into?
2. The grey "pegs" that extend into the corneal limbus (extend down within deeper corneal tissue), prominantly posteriorly and superiorly, and contains conj stem cells is known as what?
   

1. internal scleral sulcus, drains into venous system
2. Palisades of Vogt
   

[R] Corneal Physiology

1. What is the physiological function of the cornea?
2. What light is the cornea most sensitive to? What does exposure to this light cause?
3. What are the main causes of exposure to this light?
   

1. Transmission of light (from UV 310nm to IR 2500nm), absorption of short wavelength UV rays, and refraction of light.
2. 270nm light, which causes photokeratitis (inflammation from UV exposure).
3. Welder's are, tanning beds, and snowblindness (Ouch!)
   

[R] Corneal Physiology

1. What is the refractive power of the cornea?
2. Refraction of light is determined by what 3 factors?
3. Refraction and transmission of light require minimal ______ and ______, determined by no ______ _______ in the cornea and proper _________.
   

1. 43D-48D
2. Curvature of various structure (ant. and post. corneal curvatures), interfaces between different indices of light, and length of pathway through structures.
3. scattering, distortion, blood vessels, hydration
   

[R] Corneal Physiology

1. Cell proliferation requires _______ including ______ and _____ ____. The _____ layer provides corneal epi replacement. Some glucose goes through the _____ __________ ____ providing building blocks for _____ ___ synthesis.
2. Where does the majority of the corneal glucose supply come from? How does it pass through the endothelium?
   

1. nutrients, glucose, amino acids, Basal, hexose monophosphat shunt, nucleic acid
2. Aqueous. Passes through the leaky endothelium easily.
   

[R] Corneal Physiology

Appropriate hydration of which layer of the cornea is necessary to preserve transparency? Why?

The stromal layer because 90% of the cornea is made up of the stroma.

[R] Corneal Physiology

1. Corneal ATP is generated by metabolism of what nutrient?
2. How does glucose primarily enter the cornea?
3. How does oxygen primarily enter the cornea?
4. What type of metabolism produces 2ATP/glucose molecule?
5. What type of metabolism produces 36ATP/glucose molecule?
   

1. Glucose
2. From tear film
3. From tear film and subconj vessels, and from perilimbal vessels when eyes are closed.
   
4. Anaerobic
5. Aerobic
   

[R] Corneal Physiology

1. Where do the sensory corneal nerves terminate?
2. What happens to the nerve endings as epi cells turnover?
3. Which type of innervation is 3-4 times higher in the cornea than in any other epi tissue?
   

1. At free nerve endings tightly surrounded by epithelial cells
2. Nerve ending pattern changes as they retract, reinsert or shift position
3. Sensory
   

[R] Corneal Physiology

Why is it important that sympathetic nerve fibers have recently been identified in the cornea?

[R] Corneal Physiology

What are the 5 theorized functions of acetylcholine and acetylcholinesterase in the cornea?

• They might:
• Be transmitters for pain
• Contribute to tissue reaction occuring in response to pain
• Mediate inflammatory response
• Play a role in transport processes and affect cellular permeability
• Regulate cell mitosis (ACh can stimulate cGMP production → cGMP stimulates mitosis)
  

[R] Corneal Physiology

1. What is the neurotrophic effect of sensory nerves in the cornea?
2. Animal denervation studies  of the ophthalmic division of the trigeminal nerve shows that it leads to what 5 conditions?
   

1. It means that sensory nerves may affect cell function, such as corneal metabolism.
2. increased epi layer permeability (breakdown in ZO), decreased cellular adhesion (breakdown in adhesens), impaired wound healing (epi damage), reduced mitosis, and reduced cell migration.
   

[R] Corneal Physiology

1. What is the primary sensation of touch on the cornea?
2. How sensitive is the cornea?
3. What is the effect of CLs on the cornea?
4. How is cornea sensitivity measured?
5. How does the cornea interpret temperture changes?
   

1. Primary sensation is pain
2. High sensitivity
3. Contact lenses decrease corneal sensitivity
4. Clinically “measured” by blink response: Aestheiometer
5. Temperature sensitivity: Registers changes but does not accurately recognize hot/cold

[R] Corneal Physiology

1. How long does it take for central corneal nerve damage to heal?
2. How long does it take for peripheral corneal nerve damage to heal?
   

1. Normal nerve pattern is present by week 4
2. Reinnervation takes longer than 60 days and density of pattern is lower than normal

[R] Corneal Physiology

1. What prevents an intracellular water influx from tears?
2. How do ions move through these structures?
3. Which 2 ions flow through channels, and in what direction?
4. What mechanism moves Na, K, and Cl ions through these structures?
5. What type of ATPase pumps are located in this structure, and at which portion?
   
6. What are aquaporins and where are they located in the cornea?
   

1. ZO barrier
   
2. Ions move across the apical surface
3. Channels allow Na+ to pass into epi from tears and Cl- to pass into tears
4. Co-transporter mechanism moves Na, K, and Cl ions
5. Na+/K+ ATPase pumps located in basolateral membrane actively move ions
6. Aquaporins, water transport channels, are present in epithelial cell membranes