Orl 32 Tarns.docx

OPHTHA 32

PHYSIOLOGY AND BIOCHEMISTRY OF THE EYE

22 January 2019

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Dr. Rosemarie Garganta, DPBO, FPAO

Natakneng 2020

Mariano Marcos State University

Measurements of tear-film thickness have differed widely.  Original measurements of the precorneal tear film gave an average thickness of approx. 8-9 11m, with the aqueous layer constituting nearly all the thickness

Outline: PHYSIOLOGIC ACTIVITIES INVOLVED IN THE NORMAL FUNCTIONING OF THE EYES 1. Maintenance of clear ocular media A.Tear Film Three Layers Lipid or Oily Layer Aqueous Layer Mucin Layer Functions Tear Secretion Elimination of Tears Tear Dysfunction B. Physiology of the Cornea Layers Functions Transparency Source of Nutrients C. Physiology of the Crystalline Lens Functions Transparency Metabolism Source of Nutrient Pathway of Glucose Metabolism Accommodation D. Aqueous Humor and Maintenance of IOP Functions Composition Barriers Production, Mechanism Control of Production Drainage Clinical Implications of Breakdown of Barrier 2. Quiz

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PHYSIOLOGIC ACTIVITIES INVOLVED IN THE NORMAL FUNCTIONING OF THE EYES Maintenance of clear ocular media Maintenance of normal intraocular pressure The image forming mechanism Physiology of vision Physiology of normal binocular vision Physiology of pupil Physiology of ocular motility Three Layers: 1. Lipid or oily layer outer most layer; anterior layer of the tear film (approximately 100 molecules thick) contains polar and nonpolar lipids secreted by Meibomian glands and glands of Zeis and Moll

MAINTENANCE OF CLEAR OCULAR MEDIA Structures forming the refractive media of the eye A. Tear Film B. Cornea C. Aqueous Humor D. Crystalline Lens E. Vitreous Humor A. TEAR FILM The primary functions of the tear film are to:  provide a smooth optical surface at the air-cornea interface  serve as a medium for removal of debris  protect the ocular surface Human tears are distributed among the marginal tear strip (or tear meniscus), the preocular film covering the exposed bulbar conjunctiva and cornea (precorneal tear film) and the conjunctival sac (between the eyelids and bulbar conjunctiva). The precorneal tear film is a trilaminar structure consisting conceptually of an outer lipid layer, a middle aqueous layer, and an inner mucin layer.

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Meibomian glands  located in the tarsal plate of the upper and lower eyelids and are supplied by parasympathetic nerves that are cholinesterase-positive and contain vasoactive intestinal polypeptide (VIP)  Upper eyelid - approximately 30-40 Meibomian glands  Lower eyelid - 20-30 Meibomian glands  Each gland orifice opens onto the skin of the eyelid margin, between the tarsal gray line and the mucocutaneous junction The sebaceous glands of Zeis  located at the eyelid margin close to the eyelash roots  also secrete lipid, which is incorporated into the tear film 1 of 7

OPHTHA 32: PHYSIOLOGY AND BIOCHEM OF THE EYE

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. Because the polar lipids are charged compounds (phospholipids), they are located at the aqueous-lipid interface. The fatty acids of the phospholipids interact with the other hydrophobic lipids (cholesterol and wax esters, which make up the bulk of the lipid layer) through noncovalent, noncharged bonds. Tear lipids are not susceptible to lipid peroxidation because they contain extremely low levels of polyunsaturated fatty acids. Functions:  Prevents overflow of tears. (It maintains a hydrophobic barrier (lipid strip) that prevents tear overflow by increasing surface tension)  Retards the evaporation  Lubricates the eyelids thus preventing damage to the eye margin skin  Contribute to the optical properties of the tear film because of its position at the air-tear film interface 2. Aqueous layer intermediate; bulk of tear film secreted by main and accessory lacrimal glands Main Lacrimal Gland  divided into 2 anatomical parts, the orbital and the palpebral portions, by the levator aponeurosis  richly innervated by parasympathetic nerves containing the neurotransmitters acetylcholine and VIP The accessory lacrimal glands  The glands of Krause: constitute two-thirds of the accessory lacrimal glands; are located in the lateral part of the upper fornix; also present in the lower fornix  The glands of Wolfring are variably located along the proximal margin of each tarsus.  The accessory lacrimal glands are structurally like the main lacrimal gland; densely innervated, but the majority of nerves are unidentified. mostly water and small quantities of solids like sodium chloride, sugar, urea, protein, alkaline and salty in taste - Electrolytes and small molecules regulate the osmotic flow of fluids between the corneal epithelial cells and the tear film, buffer tear pH, and serve as enzyme cofactors in controlling membrane permeability.

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Tear-film solutes include urea, glucose, lactate, citrate, ascorbate, and amino acid  enter the tear film via the systemic circulation, and their concentrations parallel those of serum levels. - Proteins in the tear film include immunoglobulin A (IgA) and secretory IgA (sigA). contain antibacterial substances like lysozyme, betalysin and lactoferrin Also present in tears is interferon, which inhibits viral replication and may be efficacious in limiting the severity of ulcerative herpetic keratitis. In addition, tears contain a wide array of cytokines and growth factors  play a role in the proliferation, migration, and differentiation of corneal and conjunctival epithelial cells; also regulate wound healing of the ocular surface. Functions:  supply oxygen to the avascular corneal epithelium  maintain a constant electrolyte composition over the ocular surface epithelium  provide an antibacterial and antiviral defense  smooth minute irregularities of the anterior corneal surface  wash away debris  modulate corneal and conjunctival epithelial cell function 3. Mucus Layer thinnest and innermost layer; mucin secreted by conjunctival goblet cells and the stratified squamous cells of the conjunctival and corneal epithelia and minimally by lacrimal glands of Henle and Manz - Goblet-cell mucin production is 2-3 f.lL/day, which contrasts with the 2-3 mL/day of aqueous tear production. - Both conjunctival and tear mucins are negatively charged, highmolecular-weight glycoproteins. coats the microplicae of the superficial corneal epithelial cells and forms a fine network over the conjunctival surface It contains mucins, proteins, electrolytes, and water. Functions:  converts the hydrophobic corneal surface into hydrophilic one, which is essential for the even and spontaneous distribution of the tear film  interact with the tear lipid layer to lower surface tension  stabilizing the tear film  trap exfoliated surface cells, foreign particles, and bacteria (by the loose mucin network covering the bulbar conjunctiva)  lubricate the eyelids as they pass over the globe Function of Tear Film 1. Keeps the cornea and conjunctiva moist 2. Provides oxygen to corneal epithelium 3. Washes away debris and noxious irritants. It carries tear constituents and debris to the puncta. 4. Prevents infection due to presence of antibacterial surfaces 5. Facilitates movement of the lids over the globe Tear Secretion tears are continuously secreted throughout the day Lacrimal secretory system: 2 components 1. Basic Secretion:  accessory lacrimal glands of Krause and Wolfing 2. Reflex Secretion:  main lacrimal gland; induced by:

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OPHTHA 32: PHYSIOLOGY AND BIOCHEM OF THE EYE

a) Physical irritation: superficial corneal and conjuctival sensory stimulation by mechanical, thermal or chemical means b) Psychogenic factor c) Bright light d) Induction with sensory nerve by a local neural reflex activates the parasympathetic and sympathetic nerves that innervate the tear glands and epithelium causing secretion Elimination of Tears tears flow downward and medially across the surface of the eyeball to reach the lower fornix then to the inner canthus drained to the nasal cavity via the lacrimal passages brought about by the active lacrimal pump mechanism contributed by the orbicularis muscle eyelid movement is important in tear-film renewal, distribution, turnover, and drainage when the eyelids close during blink, contraction of the muscles distends the fundus of the lacrimal sac, creating a negative pressure which siphons the tears through the punctum and canaliculi into the sac when the eyelids open, the muscle relaxes, lacrimal sac collapses and a positive pressure is created which forces the tear down the nasolacrimal duct into the nose Tear Dysfunction A qualitative or quantitative abnormality of the film may occur as a result of:  Change in the amount of tear film constituents  Change in the composition of the tear film - The amount or composition of the tear film can change because of aqueous deficiency, mucin deficiency or excess (with or without associated aqueous deficiency), lipid abnormality (Meibomian gland dysfunction), and/or ocular surface exposure. - For example, increases in tear-film osmolarity have been observed in patients with keratoconjunctivitis sicca (KCS, or dry eye syndrome) or blepharitis and in those who use contact lenses.  Uneven dispersion of the tear film because of corneal surface irregularities - The preocular tear film is dispersed unevenly with an irregular corneal or limbal surface (inflammation, scarring, dystrophic changes) or poor contact lens fit.  Ineffective distribution of the tear film caused by eyelid-globe incongruity - Eyelid-globe incongruity results from congenital, traumatic, or neurogenic eyelid dysfunction or absent or dysfunctional blink mechanism. Diagnostic test for tear dysfunction include:  Tear breakup time  Fluorescein staining  Lissamine green staining  Rose Bengal staining  Osmolarity test and  Schirmer test

B. PHYSIOLOGY OF CORNEA Cornea forms the main refractive medium of the eye Transparency of cornea Nutrients and metabolism of cornea Permeability of cornea

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Corneal wound healing CORNEA transparent, avascular a remarkable structure; has a high degree of transparency and excellent self-protective and reparative properties has a rich afferent innervations  long posterior ciliary nerves (branches of V1)  penetrate the cornea in 3 planes: o sclera o episcleral o conjunctival  peripherally, approximately 70-80 branches of the long posterior ciliary nerves enter the cornea and lose their myelin sheath 1-2 mm from the limbus Layers 1. Epithelium ~50 um thick and constitutes 5%- 10% of total corneal thickness composed of 4-6 layers  1-2 layers of superficial squamous cells  2-3 layers of broad wing cells  innermost layer of the columnar basal cells Surface projections (microvilli and microplicae) are present  coated with glycocalyx * Mucin glycoproteins - major constituents of glycocalyx; promote both stability of the tear film and wettability of the corneal surface Plasma membrane proteins and the lipids of corneal epithelial cell  heavily glycosylated  play an important role in cell-cell adhesion as well as in adhesion of the basal cells of the corneal epithelium to the underlying basement membrane  sugar residues play a role in wound-healing mechanisms  also have a role in pathogenesis of corneal infection by serving as attachment sites for microbes damage to this layer will cause transient, localized edema of the corneal stroma and will make the Bowman’s membrane susceptible to infections 2. Bowman’s membrane Immediately posterior to the epithelial basal lamina Acellular and it does not regenerate when damaged Removed in excimer laser surgery (photo refractive keratectomy or laser sub epithelial keratomileusis): development of corneal haze post op 3. Stroma Makes up 90% of the corneal thickness Keratocytes: stromal cells There is loss of kertocyte density with age; depending on age, keratocytes constitutes 10-40% of corneal volume The narrow and uniform diameter o collagen fibrils and their regular arrangement are characteristic of collagen of the corneal stroma and are necessary for the transparency of the cornea o When these interactions are disturbed, the ability of the cornea to remain transparent is profoundly affected

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Proteinase Inhibitors of cornea: play a key role in corneal protection by restricting damage during corneal inflammation, ulceration and wound healing 4. Descemet membrane Specialized basement membrane, 10-12 um thick, present between the endothelium and the posterior stroma  Secreted by endothelium and comprises an anterior bended portion and a posterior nonbended portion 5. Endothelium Single layer posterior to descemet membrane and is composed of polyglonal cells 20 um in diameter. In young adults, the normal endothelial cell count is approximately 3000/mm2 Number of endothelial cells decreases with age and there is a concomitant spreading and thinning of the remaining cells Functions as permeability membrane between the aqueous humor and the corneal stroma and as a pump to maintain the cornea in a dehydrated state by generating the negative hydrostatic pressure that also serves to hold free corneal flaps (eg. LASIK flaps) in place Derives sufficient oxygen from the aqueous humor to maintain normal pump function If endothelium is injured, healing occurs mainly via cell migration, reaarangement and enlargement ofthe residual cells Substantial cell loss or damage results in irreversible edema because human corneal endothelial cells have limited ability to divide after birth Infiltration of PMNs in response to severe corneals injury induces endothelial cells to become fubroblastic and to synthesize retrocorneal fibrous membrance (RCFM)  RCFM – forms between the descemet membrane and the corneal endothelium and causes a significant decrease in visual acuity 2 primary physiological functions 1. To act as a major refracting medium 2. To protect the intraocular contents  Corneal Removal – exposure of intraocular contents * It must maintain its transparency and replacement of tissues Cornea transparency is the result of 1. Peculiar arrangement of the corneal lamellae 2. Avascularity – no infection going on 3. Relative state of dehydration which is maintained by barrier effects of epithelium and endothelium and the active bicarbonate pump of the endothelium Source of Nutrients 1) Solutes: simple diffusion or active transport thru aqueous humor 2) Oxygen: derived from air thru the tear film; provided by the preocular tear film, eyelid vasculature, and aqueous humor Epithelium and endothelium are most actively metabolizing layers of the cornea Epithelium is 10x thicker than the endothelium – because it is more prone to trauma Epithelium can metabolize glucose aerobically and anaerobically into CO2 and H2O and lactic acid respectively

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Glucose  primary metabolic substrate for the epithelial cells, stromal keratocytes, and endothelium  metabolized in the cornea by all 3 metabolic pathways:  tricarboxylic acid (TCA) cycle  more active in the endothelium than in the epithelium  anaerobic glycolysis  hexose monophosphate (HMP) shunt  In the epithelium and endothelium, it breaks down 35%- 65% of the glucose  Keratocytes of the stroma metabolize very little glucose; lacks 6-phosphogluconate dehydrogenase, an important enzyme in the HMP pathway  Stroma  receives glucose primarily from the aqueous humor by carrier-mediated transport through the endothelium  Epithelium  receives glucose by passive diffusion through the stroma  Preocular tear film and limbal vessels  supply approximately 10% of the glucose used by the cornea Pyruvic Acid  the end product of glycolysis, is converted either to carbon dioxide and water (via TCA cycle under aerobic conditions) or to lactic acid (under anaerobic conditions)  Production of lactic acid increases in conditions of oxygen deprivation, as in the case of tight-fitting contact lenses of low oxygen permeability  Accumulation of lactic acid in cornea has detrimental consequences to vision, such as edema (due to an increase in an osmotic solute load) or stromal acidosis, which can change endothelial morphology and function Human corneas possess a remarkably high level of aldehyde dehydrogenase and transketolase  Contribute to the optical properties of the cornea  Protect corneal cells against free radicals and oxidative damage by absorbing UVB irradiation C. PHYSIOLOGY OF CRYSTALLINE LENS LENS Transparent structure playing main role in the focusing mechanism of vision focuses incident light onto the sensory elements of the retina  To do so, the lens must be transparent and must have an index of refraction higher than that of the surrounding fluids Maintenance of transparency  depends on the precise organization of the cellular structure of the lens  must be maintained while the lens changes shape during accommodation High refractive index – due to the presence of a high concentration of proteins especially of the soluble proteins called crystallins Considering the lens's mode of growth and the stresses to which the lens is chronically exposed, it is remarkable that in most people, lenses retain good transparency  sixth or seventh decade of life – humans typically do develop visually significant opacities

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Lens Transparency Factors that play significant role in maintaining outstanding clarity and transparency of lens a) Avascularity b) Tightly packed nature of lens cells c) The arrangement of lens proteins d) Semipermeable character of lens capsule e) Pump mechanism of lens fiber membrane that regulate the electrolyte and water balance in the lens, maintaining relative dehydration f) Auto-oxidation and high concentration of reduced glutathione in the lens maintains the lens proteins in a reduced state and ensures the integrity of the cell membrane pump  Cataract – lens not transparent – Senile etiology: still unknown – DM etiology: Pump Mechanism In early stages of embryonic development, the lens is opaque, but as the vascular suppyly is lost, the lens becomes transparent Absence of the chromophores that absorb visible light Presence of highly organized structure that gives minimal light scatter Coupling of the epithelial layer, cortex and nucleus allows the control of ion levels, water content, and pH needed to maintain lens transparency Basic Mechanism of Transparency Loss  Loss of transparency of previously clear fibers  Formation of opaque fibers  Fibrous metaplasia  Epithelial opacification  Accumulation of pigments  Formation of deposits of extracellular materials  Loss of lens transparency results in blurred vision (without pain) for both near and distance vision Lens Metabolism Lens requires continuous supply of energy (ATP) for  active transport of ions and amino acids  maintenance of lens dehydration  for continuous protein synthesis * Most of the energy produced is utilized in the epithelium of lens which is major site of all active transport process * 10-20% of the ATP generated is used in protein synthesis Energy Production primarily through anaerobic glycolysis in metabolically active cells in the anterior lens  this process is necessitated by the fact that the oxygen tension in the lens is much lower than that in other tissues, given that oxygen reaches the avascular lens only via diffusion from the aqueous humor most of the glucose entering the lens is phosphorylated to glucose-6-phosphate by hexokinase, the rate-limiting enzyme of the glycolytic pathway most glucose-6-phosphate passes through glycolysis  2 molecules of ATP are formed per original molecule of glucose small proportion of glucose-6-phosphate  metabolized through the pentose phosphate pathway (HMP)  This pathway is activated under conditions of oxidative stress because it is responsible for replenishing the supply of NADPH that becomes oxidized through the increased activity of glutathione reductase under such conditions

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Carbohydrate Cataracts associated with diabetes mellitus and galactosemia True diabetic cataract  rapidly developing bilateral "snowflake" cataract that appears in the lens cortex of patients with poorly controlled type 1 DM  people with type 2 DM do not typically develop this type of cataract but have a higher prevalence of age-related cataract with a slightly earlier onset Classic galactosemia  caused by a deficiency of galactose-1-phosphate  infants develop bilateral cataracts within a few weeks of birth unless milk (lactose) is removed from the diet Cataracts are also associated with a deficiency of galactokinase. Under certain conditions in which sugar levels are elevated significantly. Sources of Nutrient Supply Avascular, it is dependent for its metabolism on chemical exchanges with the aqueous humor All nutrients must be obtained from the surrounding fluids Pathway of Glucose Metabolism Glucose is very essential for lens metabolism Metabolism activity of the lens is largely limited to epithelium and cortex 80% of glucose is metabolized anaerobically by the glycolytic pathway, 15% by Pentose Hexose Monophosphate and a small property via oxidative Kreb’s Citric Acid Cycle Sorbitol Pathway: important in the production of cataract in DM and galactosemic patients Accommodation  In an emmetropic eye, parallel rays of light away from infinity are brought to focus on the retina, with accommodation being at best o Emmetropic – normal vision; good distance on near and far  Eyes have been provided with a mechanism by which we can even focus the diverging res coming from a near object on the retina in a bid to see clearly: accommodation  When the eye is at rest (unaccommodated), the ciliary ring is large and keeps the zonules tense, and the lens is flat o Lens is suspended 360o by zonules  Contraction of the ciliary muscle causes the ciliary ring to shorten and releases zonular tension of the lens capsule. The lens then alters its shape to become more convex or conoidal due to the configuration of the anterior lens capsule which is thinner at the center and thicker at the periphery. o Ciliary muscle relaxed, zonules are tensed  unaccommodation o Ciliary muscle contracted, zonules are relaxed  accommodation D. AQUEOUS HUMOR AND MAINTENANCE OF INTRAOCULAR PRESSURE (IOP) Aqueous humor is a clear, watery fluid filling the anterior chamber (0.25 ml) and posterior chamber (0.06 ml) It is secreted by the nonpigmented ciliary epithelium (NPE) from a substrate of blood plasma Role on IOP maintenance  IOP – pressure of the eyeball; measured by Goldmann Tonometry

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OPHTHA 32: PHYSIOLOGY AND BIOCHEM OF THE EYE

Important metabolic role by providing substrates and removing metabolites from the avascular cornea and transparent lens Because the aqueous humor is devoid of blood cells and of more than 99% of the plasma proteins, it provides an optically clear medium for the transmission of light along the visual path. Physiology Process  Production  Drainage  Maintenance of IOP Functions: 1. It maintains a proper IOP 2. It plays important metabolic role by providing substrates(eg, glucose, amino acids) and by removing metabolites (eg, lactic acid, pyruvic acid) from the avascular cornea and lens 3. It maintains optical clarity – aqueous humor is essentially protein-free 4. It takes the place of lymph that is absent in the eyeball Composition:  Water: 99.9%  Solids: 0.1%  Proteins  Amino acids  Non-colloidal contents: glucose, urea, lactic acid, ascorbate, etc.  *oxygen

The aqueous humor composition is in dynamic equilibrium, determined both by its rate of production and outflow and by continuous exchanges with the tissues of the anterior segment The aqueous contains the following 1. Inorganic ions and organic anions: Inorganic ions - sodium, potassium, magnesium Organic anions – lactate and ascorbic acid 2. Carbohydrates: glucoe and inositol People with diabetes mellitus have increased glucose levels in aqueous  higher concentrations in the lens and short-term refractive and longerterm cataract implications 3. Glutathione and urea 4. Proteins: albumin and transferrin 5. Growth-modulatory factors: play substantial role in modulating the proliferation, differentiation, functional viability and wound healing of ocular tissues 6. Oxygen and carbon dioxide: the corneal endothelium depends critically on the aqueous oxygen supply for the active-fluid transport mechanism that maintains corneal transparency Barriers Blood-aqueous or blood-retina (depending on their location in the eye)  the composition and amounts of all materials entering and leaving the eye, except for materials

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that exit through the Schlemm canal, can be carefully controlled pertuberations of these barriers  mixing of blood and ocular fluids  plasmoid aqueous, retinal exudates, retinal edema

Production: Ciliary body The formation of aqueous is largely a product of active secretion by the inner NPE and involves membraneassociated Na+,K+ -ATPase.  CA (carbonic anhydrase) II is present in both pigmented epithelium (PE) and NPE. Its inhibitors reduce the rate of entry of sodium and bicarbonate into the aqueous  reduction in aqueous flow.  Carbonic anhydrase inhibitors and beta-blockers are used systemically and topically in the treatment of glaucoma to reduce the rate of aqueous humor formation The aqueous humor is secreted by the ciliary epithelium at a flow rate of 2-3 f.LL/min. The ciliary epithelium is a bilayer of polarized epithelial cells lining the surface of the ciliary body  NPE (Non-pigmented epithelium)  faces the aqueous humor through the cells' basal plasma membrane  establish the blood-aqueous barrier by the presence of tight junctions proximal to the apical plasma membrane  preventing the free passage of plasma proteins and other macromolecules from the stroma into the posterior chamber.  PE (Pigmented Epithelium)  faces the stroma, also through the cells' basal plasma membrane.  Considered a leaky epithelium  it allows solutes to move through the intercellular space between the PE cells. Mechanism of Production 1. Ultrafiltration: plasma substances pass out from the capillary wall – passive mechanism 2. Secretion: active transport 3. Diffusion: passive mechanism  involves the movement of ions such as sodium across a membrane toward the side with the most negative potential Control of Aqueous Humor Production  The diurnal variation in IOP certainly indicates that same endogenous factors influence the aqueous humor formation  Ultrafiltration and diffusion are dependent on the level of blood pressure in the ciliary capillaries, the plasma osmotic pressure and the level of IOP Drainage:  Aqueous humor flows from the posterior chamber into the anterior chamber through the pupil (pupillary aperture)  From anterior chamber, it is drained out by 2 routes:

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Trabecular (conventional) outflow: 90%  Free flow occurs from trabecular meshwork to schelmm’s canal Uveoscleral (unconventional) outflow: 10%  Passes across the ciliary body into the subarachnoidal space and drained by the venous circulation in the ciliary body, choroid and sclera.

Clinical Implications of Breakdown of the Blood-Aqueous Barrier With compromise of the blood-aqueous barrier in conditions such as ocular insult (trauma or intraocular surgery), as well as uveitis and other inflammatory disorders, the protein content of aqueous humor may increase 10-100 times, especially in the highmolecular- weight polypeptides. The levels of inflammatory mediators, immunoglobulins, fibrin, and proteases rise, and the balance among the various growth factors is disrupted  The clinical sequelae include: fibrinous exudate and clot (with or without a macrophage reaction and formation of cyclitic membranes) synechiae formation (peripheral and posterior), as well as an abnormal neovascular response, which further exacerbates breakdown of the barrier. Chronic disruption of the blood-aqueous barrier is implicated in the abnormal hyperplastic response of the lens epithelium, corneal endothelium, trabecular meshwork, and iris, and in the formation of complicated cataracts. Degenerative and proliferative changes may occur in various ocular structures as well. The use of anti-inflammatory steroidal and nonsteroidal drugs, cycloplegics, protease activators or inhibitors, growth factor and anti-growth factor agents, and even surgical intervention may be necessary to combat these events.

Test Your Memory . . . 1. Layers of tear film and the glands that produced (6 pts) 2. Corneal transparency is the result of (2 pts) 3. Functions of Aqueous humor (2 pts)

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