Eye Cornea: Anatomy and Layers

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Questions and Answers

Which of the following is the primary function of the cornea within the eye's dioptric system?

  • Providing the majority of the eye's refractive power. (correct)
  • Protecting the inner structures of the eye from physical damage.
  • Controlling the amount of light entering the eye.
  • Facilitating color perception by filtering specific wavelengths of light.

A patient's corneal evaluation reveals an elliptical shape. Which of the following best describes the typical dimensions of the cornea?

  • Approximately 11mm vertically and 12mm horizontally. (correct)
  • Approximately 12mm in both vertical and horizontal dimensions.
  • Approximately 12mm vertically and 11mm horizontally.
  • Approximately 11mm in both vertical and horizontal dimensions.

Which statement accurately describes the variation in corneal thickness from the center to the periphery?

  • The cornea maintains a uniform thickness from the center to the periphery.
  • The cornea is thicker at the center (0.71mm) than at the periphery (0.53mm).
  • The cornea is thicker at the periphery (0.71mm) than at the center (0.53mm). (correct)
  • The cornea is thinner at the periphery than at the center due to the flattening effect towards the limbus.

What type of cells constitute the corneal epithelium?

<p>Non-keratinized, stratified squamous epithelium. (B)</p> Signup and view all the answers

The basal layer of the corneal epithelium is characterized by which of the following?

<p>It rests on a basement membrane and is a single layer of columnar cells. (D)</p> Signup and view all the answers

What is the primary function of hemidesmosomes and desmosomes in the corneal epithelium?

<p>To anchor the basal cells to the basement membrane and connect cells to each other. (C)</p> Signup and view all the answers

Which layer of the corneal epithelium is known as the germinal layer due to its mitotic activity?

<p>Basal layer (D)</p> Signup and view all the answers

In what way do surface cells contribute to the integrity of the tear film?

<p>By displaying microvilli and microplicae to stabilize the tear film. (B)</p> Signup and view all the answers

What is the approximate time frame for complete replacement of the corneal epithelium?

<p>7 days (C)</p> Signup and view all the answers

What results when Bowman's layer sustains damage?

<p>Formation of opaque scars because Bowman's layer cannot regenerate. (C)</p> Signup and view all the answers

What is the approximate thickness of Bowman's layer?

<p>8-14 μm (D)</p> Signup and view all the answers

Which statement accurately describes Bowman's layer?

<p>It's an acellular layer that resists injury but cannot regenerate. (B)</p> Signup and view all the answers

What percentage of the total corneal thickness does the stroma account for?

<p>90% (A)</p> Signup and view all the answers

Which of the following components are found within the corneal stroma?

<p>Collagen fibrils, keratocytes, and intercellular ground substance. (D)</p> Signup and view all the answers

Which of the following best describes the arrangement of collagen fibrils within the corneal stroma?

<p>Uniform diameter, uniformly spaced, and parallel to each other forming flat bundles. (A)</p> Signup and view all the answers

What is the role of keratocytes within the corneal stroma?

<p>Producing collagen and extracellular matrix components. (A)</p> Signup and view all the answers

Which of the following statements accurately describes glycosaminoglycans (GAGs) in the corneal stroma?

<p>They are hydrophilic molecules that help maintain corneal hydration and transparency. (A)</p> Signup and view all the answers

Descemet's membrane is best described as:

<p>The basement membrane of the corneal endothelium. (B)</p> Signup and view all the answers

How does Descemet's membrane change throughout a person's lifespan?

<p>It thickens, approximately doubling by age 40. (B)</p> Signup and view all the answers

Which of the following is true regarding Descemet's membrane?

<p>It will curl towards the anterior chamber if torn. (A)</p> Signup and view all the answers

What is the primary structural characteristic of the corneal endothelium?

<p>A single layer of flattened, epithelial-like cells (C)</p> Signup and view all the answers

What is a key distinguishing feature of the corneal endothelium cells?

<p>The cells are involved in active secretion and protein synthesis. (A)</p> Signup and view all the answers

How do adjacent cells fill gaps as endothelial density decreases with age or cell loss?

<p>By the remaining cells spreading and enlarging, leading to polymegathism. (B)</p> Signup and view all the answers

Below what endothelial cell density does edema and swelling of the stroma occur?

<p>800 cells/mm² (D)</p> Signup and view all the answers

What is unique about the cornea in regards to blood supply?

<p>It is avascular, relying on other sources for nutrients and oxygen. (C)</p> Signup and view all the answers

How does oxygen reach the corneal tissue?

<p>By diffusion from the anterior surface of the air-cornea interface and from the aqueous humor. (D)</p> Signup and view all the answers

Which division of the trigeminal nerve innervates the cornea?

<p>Ophthalmic division (D)</p> Signup and view all the answers

Which statement accurately describes corneal nerves?

<p>All levels, except the Descemet's Membrane (DM) are richly supplied. (B)</p> Signup and view all the answers

What percentage of wet weight does water represent in the corneal epithelium?

<p>70% (A)</p> Signup and view all the answers

Which of the following best describes the primary source of glucose for the cornea?

<p>Aqueous humor. (A)</p> Signup and view all the answers

Which metabolic process does the corneal epithelium primarily rely on for energy production?

<p>Aerobic glycolysis (A)</p> Signup and view all the answers

How does the cornea obtain oxygen when the eyelids are closed?

<p>From the palpebral conjunctiva (B)</p> Signup and view all the answers

Which of the following is a key metabolic adaptation observed in the cornea?

<p>The storage of glucose as glycogen in the epithelium. (D)</p> Signup and view all the answers

Which statement correctly describes the importance of corneal hydration for transparency?

<p>Corneal transparency is closely related to corneal water content. (D)</p> Signup and view all the answers

What is the function of corneal epithelium cells to transparency?

<p>To be closely packed (A)</p> Signup and view all the answers

What primarily maintains the relative state of dehydration necessary for corneal transparency?

<p>The barrier function of the endothelium and the endothelial pump. (A)</p> Signup and view all the answers

How does the endothelial pump function to maintain corneal transparency?

<p>By actively transporting water from the stroma back into the aqueous humor. (D)</p> Signup and view all the answers

What is Sattler's veil?

<p>A visual disturbance caused by epithelial loss due to increased IOP (B)</p> Signup and view all the answers

Which term describes a superficial opacity involving Bowman's membrane and superficial corneal stroma?

<p>Nebula (B)</p> Signup and view all the answers

Which term describes an opacity involving less than one third of the stroma?

<p>Macula (C)</p> Signup and view all the answers

How the corneal epithelium response when there are wounds?

<p>It responds immediately to reestablish barrier action and protect underlying tissues (A)</p> Signup and view all the answers

Flashcards

Cornea Definition

The projecting transparent anterior surface of the outer protective coat of the globe.

Is the cornea vascular or avascular?

Lacks blood vessels; gets nutrients from tears and aqueous humor.

Cornea Function

It provides the bulk of the refractive power of the eye's dioptric system.

Cornea coverage area

Approximately 1/13 of the surface of the eyeball.

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Corneal Curvature

The radius of curvature is about 8mm.

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Corneal Shape

Approximately 11mm vertically and 12mm horizontally.

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Cornea Layers

Epithelium, Bowman's layer, Stroma, Descemet's membrane, Endothelium

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Corneal epithelium type

Non-keratinised, stratified squamous epithelium.

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Basal Cell Layer

The innermost part of the epithelium; a single layer of tall columnar cells resting on a basement membrane.

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Basal Cell anchoring

Anchored by hemidesmosomes and desmosomes to the basement membrane.

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Mitosis location

The germinal layer where Mitosis occurs

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Wing Cell Layer

4-5 cell layers, polyhedral, and forms the bulk of the corneal epithelium.

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Surface Cell Layer

The final 2-3 layers which are eventually lost to the tears but replaced by cells moving up from beneath.

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Surface Cell Features

Display microvilli & microplicae projecting into tear film, maintaining stability. Share a continuous junctional complex with tears - zonula occludens.

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Epithelium Functions

Maintenance of transparency and regeneration after abrasion.

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Bowman's membrane merge

Basal membrane of the epithelial cells merges with the anterior limiting membrane.

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Bowman's Layer Damage

It shows a good deal of resistance to injury or infection. If damaged, opaque scars are formed and it cannot be regenerated.

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Stroma composition

Collagen fibrils, Keratocytes and Intercellular ground substance.

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Keratocytes Functions

Have the ability to produce collagen and extracellular matrix components and have phagocytic abilities.

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Ground substance

Glycosaminoglycan (GAG).

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Descemet's Membrane

Basement membrane of the endothelium that is strong and homogenous. Has very resistance membrane of ≈ 10µm thick.

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DM Production

It produces the basal membrane material & can reform it if the membrane is damaged in minor trauma and DM is produced constantly.

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DM Component

Composed of very regular strata of fine collagen fibrils.

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DM Elasticity

Does not consist of true elastic tissue but shows elastic tendencies due to arrangement of the collagen fibrils.

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Endothelium

Most posterior layer of the cornea which consists of a single layer of flattened epithelial-like cells of 5µm thickness.

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Corneal Endothelium Cells

Polyhedral sided cells but mostly hexagonal (70-80%) which form endothelial mosaic.

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Endothelial pump

Actively transports material from the anterior chamber of the corneal stroma vice-versa in the maintenance of corneal hydration.

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Endothelial cells division

Endothelial cells do not divide and replicate which leads to cell loss.

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Corneal Blood Supply

The cornea is avascular with blood supply by conjunctival and episcleral capillary networks located in the limbus. Absence of blood vessels is a factor in corneal transparency.

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Corneal Oxygen

Oxygen diffuses in from the anterior surface of the air-cornea interface and the aqueous humor diffusion.

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Nerve supply to cornea

Superficial cells, Wing cell, Basal cells

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Innervation Nerve

The ophthalmic division of the trigeminal nerve by way of long ciliary nerves.

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Epithelium Composition

Corneal epithelium represents 10% of the total wet weight of the cornea and water is 70% of the wet weight.

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Stromal Composition

Includes collagen, protien, glycosaminoglycans

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Corneal Metabolites

Cornea is avascular. It receives its metabolites from the vessels of the limbus, via the tears & aqueous.

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Glucose pathways

  • Embden-Meyerhof pathway (Glycolysis) - into lactate, -tricarboxylic acid cycle (Krebs cycle) - into CO₂ and H₂O
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Corneal Transparency factors

Uniform and regular arrangement of corneal epithelium, regulars and arrangement of collagen fibrils and avascularity of cornea

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Transparency

Stroma transmits >90% of the incident light and regular arrangement and lattice structure of the fibrils is responsible for its transparency

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GAG roles?

corneal hydration thru' interactions with the electrolytes & water.

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Endothelium pump

active sodium-potassium adenosine triphosphatase (Na-K ATPase) pumps.

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Study Notes

Cornea

  • The cornea is the transparent, projecting anterior surface of the eye.
  • It is avascular.
  • Provides the bulk of the refractive power of the eye's dioptric system, approximately 45D out of 60D.
  • It covers about 1/13 of the eyeball's surface area, and circumferentially 1/6 of the globe.

Dimensions

  • The radius of curvature is about 8mm.
  • The refractive index is 1.376.
  • The cornea is elliptical: approximately 11mm vertically and 12mm horizontally.
  • The anterior surface's central cornea radius of curvature is 7.8mm, while the posterior surface is 6.5mm.
  • It flattens towards the limbus and is thicker at the periphery (0.71mm) than at the center (0.53mm).

Layers

  • The cornea has five layers: epithelium, Bowman's layer, stroma, Descemet's membrane, and endothelium.

Corneal Epithelium

  • It is non-keratinized stratified squamous epithelium.
  • It is a continuation of the conjunctival epithelium.
  • It has a uniform thickness of ≈50μm.
  • It consists of 5-6 layers of cells and it is subdivided into basal layer, wing cell layer and surface cell layer.

Basal Cell Layer

  • The innermost part of the epithelium
  • It is a single layer of tall columnar cells with oval-shaped nuclei.
  • Cells are approximately 20 µm high and 10 µm wide, resting on a basement membrane.
  • The cells are anchored to their basement membrane via hemidesmosomes and to each other by desmosomes.
  • Spaces between the cells form a lymph space.
  • It is the germinal layer, where mitosis occurs.
  • These cells are continuous with the same layer in the conjunctiva.

Wing Cell Layer

  • Mitosis in the basal layer produces new cells that move into the wing cell layers.
  • These are also called umbrella cells.
  • They have a polyhedral shape with rounded heads and concave bases and also have winglike lateral processes.
  • Consists of 4-5 cell layers at the periphery, centrally only 2-3 layers exist.
  • This layer forms the bulk of the corneal epithelium

Surface Cell Layer

  • It forms the final 2-3 layers and produces a smooth anterior surface.
  • From the basal layer to the surface, the nucleus flattens, becoming an ovoid.
  • These cells are ultimately lost to the tears, replaced by cells moving up from beneath.
  • It displays microvilli and microplicae (folds) that project into the tear film, stabilizing it.
  • The layer has a continuous junctional complex with tears called zonula occludens.
  • It's rich in glycogen, enzymes, and acetylcholine.

Function of the Corneal Epithelium

  • The main function is maintenance of transparency.
  • Despite its avascularity, the corneal epithelium has a very active power of regeneration after abrasion.
  • It takes 7 days for replacement of the entire corneal epithelium.
  • If the basement membrane is damaged, it will take months for the basement membrane to regenerate.

Bowman's Membrane

  • Basal membrane of the epithelial cells merges with the anterior limiting membrane (BM).
  • The anterior limiting membrane (BM) is an acellular tough membrane situated between the epithelium and stroma.
  • It is 8–14 µm thick.
  • Composed of randomly arranged collagen fibrils ≈ 25nm diameter, lying in a mucopolysaccharide matrix.
  • Separated from the epithelium by a sharply defined border, but the demarcation from the stroma is ill-defined.
  • Not a true membrane and not truly elastic (does not consist of elastic tissue) and does not regenerate.
  • It shows a good deal of resistance to injury or infection.
  • If damaged (either due to infection or injury), opaque scars form and cannot be regenerated.

Stroma (Substantia Propria)

  • It forms 90% of the total corneal thickness.
  • ≈0.5mm/500 microns thick.
  • It is composed of a modified connective tissue of collagen fibrils, keratocytes and intercellular ground substance
  • Collagen fibrils have a 25 to 35 nm diameter
  • Collagen fibrils are uniform and run parallel to each other to form flat bundles called lamellae
  • forms 200-300 lamellae in the corneal stroma
  • Lamellae consist of evenly spaced collagen fibrils oriented parallel to the corneal surface and continuous with the sclera at the limbus.
  • Transparency depends on the regular spacing of the collagen fibrils (diameter & distance of the corneal fibrils)
  • Cells present between lamellae are -Keratocytes, Wandering macrophages -Lymphocytes and Polymorphonuclear leucocytes
  • Keratocytes are corneal fibroblasts that have a flattened cell shape and multiple cytoplasmic processes
  • They lie between lamellae and sometimes within lamellae
  • They produce collagen and extracellular matrix components and have phagocytic abilities
  • The ground substance/extracellular matrix fills the areas between fibrils, lamellae and cells
  • It contains proteoglycans, a macro molecule with a carbohydrate glycosaminoglycan (GAG)
  • GAGs are hydrophilic and negatively charged molecules
  • GAGs help provide high stromal hydration
  • GAGs help maintain corneal transparency

Descemet's Membrane

  • It is the basement membrane of the endothelium
  • It is a strong, homogenous, and very resistance membrane of ≈ 10µm thickness.
  • It is sharply defined from the cornea stroma.
  • The endothelium produces the basal membrane material and can reform it if the membrane is damaged in minor trauma.
  • DM is produced constantly and it thickens throughout life.
  • It doubles by age 40 (5µm at young and will increase to 15μm over a lifetime)
  • Consists of 2 laminae:
    1. Anterior lamina - banded appearance, a latticework of collagen fibrils, 3µm thick secreted during embryonic development
    1. Posterior lamina - Nonbanded, Homogenous, Secreted by the endothelium throughout life
  • It does not consist of true elastic tissue but shows elastic tendencies due to the arrangement of the collagen fibrils
  • It curls towards the anterior chamber if torn.
  • It can be regenerated if damaged
  • It represents a barrier to perforation in deep corneal ulcers

Endothelium

  • It terminates near limbus as Schwalbe's line.
  • It is the most posterior layer of the cornea.
  • It consists of a single layer of flattened epithelial-like cells.
  • Approximately 5µm in thickness.
  • Cells are polyhedral: 5 & 7 sided cells but mostly hexagonal (70-80%) which form endothelial mosaic.
  • No basement membrane.
  • Cell membranes are interconnected with each other with various junctional complexes.
  • These complexes are zonular occludens, macula occludens and macula adherens
  • Barriers formed by these junctional complexes are slightly leaky which allows the entrance of nutrients from the aqueous humor.
  • Endothelial cells actively transport material from the anterior chamber of the corneal stroma and back in the maintenance of corneal hydration
  • Rich in mitochondria than in any other cells of the eye except the retinal photoreceptor cells.
  • It is involved in active secretion and protein synthesis.
  • Endothelial cells do not divide and replicate.
  • Adjacent cells spread to fill the gaps caused by cell loss.
  • The regular hexagonal arrangement is replaced by fewer cells (cell density decreases) & increase in size (polymegathism) & shape (pleomorphism) with age
  • Damage to corneal endothelial cells & cell density less than 800 cells/mm² leads to edema & swelling of the stroma with resultant loss of transparency.

Dua's Layer

  • According to a 2013 paper, this layer is located in the cornea, but had not been detected previously.
  • It lies between the corneal stroma and descemet's membrane, making it the fourth caudal layer
  • Approximately 15 micrometers thicks
  • It is thin, very strong and impervious to air.

Blood Supply

  • The cornea is avascular.
  • Blood supply is conjunctival (anterior ciliary artery) and episcleral capillary networks located in the limbus.
  • Absence of blood vessels is a factor in corneal transparency.
  • Oxygen diffuses in from the anterior surface of the air-cornea interface and the aqueous humor diffusion.
  • Can become vascular in pathological condition, i.e. during infection, blood vessels grow into the cornea from the limbus & conjunctiva (neovascularization).

Nerve Supply

  • The cornea is one of the most sensitive tissues of the body.
  • Densely innervated with sensory nerve fibres via the ophthalmic division of the trigeminal nerve by way of 70-80 long ciliary nerves that pierce sclera posterior to limbus.
  • These nerves form the annular plexus (pericorneal plexus).
  • Branches from the annular plexus travel radially to enter the corneal stroma, losing their myelin sheaths.
  • All levels, except the DM, are richly supplied.
  • No nerve endings are located in the Descemet's membrane or the endothelium.
  • Cornea nerves do not display any specialized endings
  • Only sensory neurons have been identified in the human cornea
  • end with the terminal button & are free nerve endings (sensitive to pain & temperature).

Biochemical Composition

  • The number of cells and morphology varies among the different corneal layers and this is related in chemical dissimilarities
  • The cornea is a heterogeneous tissue
  • The epithelium represents 10% of the total wet weight of the cornea filled with 70% water and 30% solids
  • The 30% solid consists of nuclei acids (DNA & RNA), lipids in the cell membrane, & proteins
  • Epithelium demonstrates high activities of enzymes of glycolysis, Kreb's cycle, ATP, Electrocytes (Na+, K+, Cl-), High concentration of acetylcholine and cholinesterase
  • The stroma has few keratocytes per unit volume and low enzymatic activities with 75% water on wet wt basis.
  • Approximately 25% solids, collagen, other proteins, and glycosaminoglycans (GAG) or mucopolysaccharides.
  • Cornea collagen has a high nitrogen, glycine, proline & hydroxyproline content
  • GAG approximately constitutes 4% of the dry weight of the cornea
  • GAGS are localized in the interfibrillar space and play a role in `corneal hydration through interactions with the electrolytes & water
  • GAG acts as anions & binds cations and water
  • Transparency of the cornea is dependent on the regular spacing of the collagen fibers, which in turn is regulated by GAG & proteoglycans maintaining precise spatial relationship between the collagen fibrils

Metabolism

  • The cornea is avascular and it receives its metabolites from the vessels of the limbus, via the tears & aqueous.
  • Tears is an important source for amino acids & O2.
  • Most of the O₂ in the cornea is consumed by epithelium and endothelium.
  • Epithelium gets much of its O2 from limbal capillaries or precorneal tear film.
  • Endothelium gets most of its O2 from aqueous humor
  • The cornea is mainly aerobic
  • When the eyelids are closed, O₂ is obtained by corneal epithelium from the palpebral conjunctiva.
  • Glucose can be stored in the epithelium as glycogen broken down in emergency situations (surgical wound).
  • Energy is needed for maintence of transparency and dehydration
  • Energy in the form of adenosine triphosphate (ATP) is generated by the breakdown of glucose and utilization of oxygen
  • Glucose is broken down in 2 different pathways: -Embden-Meyerhof pathway (Glycolysis) - into lactate -tricarboxylic acid cycle (Krebs cycle) - into CO₂ and H₂O
  • Glycolysis is the conversion of glucose to pyruvate in the absence of oxygen; ATP then is generated
  • Under anaerobic conditions, the corneal epithelium produces lactic acid; Glucose + 2P₁ + 2ATP =2 lactate + 2ATP + 2H2O
  • In aerobic conditions, pyruvate enters Kreb's cycle with the production of 36 mol ATP /mol glucose

Transparency

  • Normal cornea is a transparent structure
  • Corneal transparency occurs due to both anatomical annd physiological factors

Anatomical Factors that Affect Corneal Transparency

-*Uniform and regular arrangement of corneal epithelium -corneal epithelial are closely packed and uniformly arranged with homogeneity in refractive index Tight -intercellular junction -tear film keeps epithelial surface smooth -*Regular arrangement of collagen fibrils (corneal lamellae) in stroma -stroma transmits more than 90% of the incident light -regular arrangement and lattice structure of the fibrils is responsible for its transparency Maurice's theory -collagen fibrils of uniform diameter are packed regularly and each fibrils is separated from the others -form lattice structure; scatter from individual fibrils gets canceled by destructive interference -in a perfect lattice, fibrils are 150nm apart -corneal clouding may be due to the occurrence of spacing of fibrils greater than 200nm

  • *Avascularity of cornea -corneal avascularity factors are unknown -when vessels are present due to pathology and neovascularization: loss of transparency

Physiological Factors that Affect Corneal Transparency

  • Relative state of dehydration
  • corneal transparency closely related to corneal water content -cornea is maintained in a state of deturgescence or partial hydration at 78% water -*disturbance to this leads to opacity

Two Mechanisms Contribute to Maintenance of Stromal Deturgescence

  • Barrier function of the endothelium -*passive barrier but less efficient than epithelium barrier function
  • Endothelial pump -*pump function consisting of active sodium-potassium adenosine triphosphatase (Na-K ATPase) pumps -*actively remove fluid from the stroma, pumping it back into the aqueous

Response to Corneal Injury

  • Epithelium responds immediately to reestablish barrier action & protect underlying tissues
  • Bowman's membrane has no mechanism for repair
  • Stroma has Keratocytes that are special fibroblast: produce new reparative collagen & proteoglycan, but cornea loses its transparency
  • Descemet's Membrane is rescreted by endothelium
  • Endothelium has no mechanism for repair

Conditions due to Injury

  • Epithelium = no opacity
  • Bowman's Membrane & Superficial stroma = Nebula
  • ⅓ stroma = Macula
  • More than ½ stroma = leucoma
  • In cases of Increased IOP & occasionally during CL wear: corneal epithelium loses its clarity & causes the visual disturbance known as sattler's veil

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