Chapter 3 Physiology of the eye.docx

Anterior Eye Structures Eyelids All domestic animal species have a superior (upper) and inferior (lower) eyelid; most have a nictitating membrane (NM, third eyelid). The eyelids contain the meibomian glands; these are large sebaceous glands that secrete the outer, oily layer of the precorneal tear...

Anterior Eye Structures Eyelids All domestic animal species have a superior (upper) and inferior (lower) eyelid; most have a nictitating membrane (NM, third eyelid). The eyelids contain the meibomian glands; these are large sebaceous glands that secrete the outer, oily layer of the precorneal tear film (PTF). The conjunctiva which lines the inside of the eyelids and reflects onto the globe contains goblet cells that contribute the mucin to the PTF. accessory lacrimal glands are also present in some species. The normal blinking of the eyelids: maintains the physiologic thickness of the preocular tear film, aids movement of the tears both to and within the nasolacrimal system, and helps eliminate small particles the corneal and conjunctival surfaces. Reflex closure of the eyelids protects the anterior segment from external trauma. The eyelids determine the shape and width of the palpebral fissure, along with the associated medial and lateral canthal ligamentous and muscle attachments. a wide, round palpebral fissure is normal among brachycephalic breeds, a and a narrow, almond-shaped palpebral fissure is normal among dolichocephalic breeds. The shape of the palpebral fissure also depends on the relationship of the globe to the orbit. A small globe in a deep orbit allows a narrow palpebral fissure; the opposite occurs with a large globe in as hallow orbit. The NM aids in: protection of the conjunctiva and cornea by moving, either passively or actively, over the cornea when the globe is retracted. Contains one or more important accessory, tear-producing glands that contribute to the aqueous portion of the PTF. also helps to support the position of the lower eyelid through its mass in the ventromedial cul-de-sac, and it forms part of the lacrimallake in the medial canthus. In all animals, if the globe is retracted by the retractor bulbi muscles or decreases in size, the membrane passively begins to cover the eye. Eyelid closure is mediated by the efferent fibers of the facial nerve (CN VII) and their effects on the orbicularis oculi muscles. The oculomotor (CN III) innervates the levator palpebral superioris, which is responsible for opening the upper eyelid. Eyelid closure is the end result of three eyelid reflexes, the corneal, palpebral, and dazzle reflexes, and the menace response (Table 3., pg 125 NB). The corneal and palpebral reflexes are: primitive reflexes with a purely subcortical course. both are elicited by touch, with the afferent pathway being the ophthalmic branch of the trigeminal nerve (corneal) or the ophthalmic and maxillary branches of the trigeminal nerve (palpebral). the efferent pathway of these two reflexes as well as the menace response is the facial nerve stimulating the orbicularis oculi muscles, resulting in a blink. these reflexes are present immediately following birth or eyelid opening. By contrast, the menace response is cortically mediated and is initiated by a threatening gesture. The visual input results in a blink closure via the facial nerve or globe retraction via the abducens nerve. This response requires integration and interpretation and is a learned response Blinking and blink rates (Table 3.2, pg 126). Diurnal primates and birds have higher blink rates than nocturnal primates and birds A large comparative study showed that in general, larger mammals and primates blink more often than smaller mammals Eyelids in the Dog In dogs, the upper eyelid, which contains the cilia or eyelashes, is more mobile than the lower eyelid. Restrained dogs blink 10–20 times/min in comparison to unrestrained dogs (Table 3.2) Some 50% of dogs’ blinks are incomplete Puppies normally open their eyelids between 10 and 15 days of age Eyelids in the Cat Both eyelids lack cilia Kittens normally open their eyelids between 10 and 15 days of age; however, both eyes do not always open on the same day The NM of the feline species is large and active and may passively or actively cover part of the cornea or be drawn into the medial canthus. extends at least two-thirds of the way across the cornea and contains nine smooth muscles which lead to active retraction or protrusion The smooth muscle that draws the membrane into the medial canthus is innervated exclusively by postganglionic adrenergic sympathetic nerve fibers, with cell bodies located in the anterior cervical ganglion. Their axons follow the oculomotor nerve. NM shows no spontaneous activity, because the smooth muscle lacks tight junctions like that of the visceral smooth muscle. Each muscle cell is innervated by one or more axons, thus confirming that activation of the smooth muscle in the NM is neurogenic and that the myogenic conduction normally found in visceral smooth muscles does not occur (Marshall, 1974). Cats are the only common domestic animal in which sympathetic stimulation will cause the NM to move slightly. Horse Cilia are long and numerous on the upper eyelid, except near the medial canthus. blink at 19 blinks/minute, approximately 33% each of the blinks are minimal incomplete, moderate incomplete, or complete, with only 6% of the blinks having a complete squeeze. Lid closure is approximately twice as rapid as lid opening Eyelids are open at birth. Long tactile hairs, or vibrissae, are present on both the dorsal brow and lower eyelid. The vibrissae are long, stout, single shafts of hair that are usually thicker than adjacent skin hair; they may provide additional sensation for the eyelids. Cattle, Sheep, and Pigs Upper eyelid is the most mobile, and the majority of cilia are present on the upper eyelid. Pigs, rabbits, rodents, and some ruminants have a deeper structure, the Harder’s gland or harderian gland, in addition to the superficial gland of the NM. This gland secretes lipids, porphyrins, indoles, and growth factors and is thus also important for lubrication of the eye. In the pig, the meibomian glands are poorly developed, and the primary eyelid glands are sweat glands. Eyelids are open at birth. 4. Birds and Reptiles lower eyelid is larger and more mobile than the upper eyelid. There are no feathers corresponding to eyelashes on the lids. The superciliary line refers to feathers that correspond to the eyebrow and are often different colors than surrounding feathers. The superciliary or supraorbital ridge refers to the unfeathered bony protuberance just dorsal to the orbital rim that is seen in many raptors, such as eagles and hawks. This ridge is thought to provide shade to the eye. Birds blink with both eyelids or the NM alone. In contrast to mammals, the nearly transparent NM of birds is under direct skeletal muscular control. - NB Two muscles extraneous to the lid pull the NM over the entire cornea as many as 15–20 times per minute, even with the other eyelids closed Blinks in peacocks are strongly associated with gaze shifts The NM also contains a superficial tear gland, and some species have a deeper harderian gland. Chicks hatch with their eyes open. Tear Production and Drainage Both the optical and normal functions of the cornea depend on the integrity of the lacrimal system. Maintains: an optically uniform corneal surface by smoothing out minor irregularities, removing foreign matter from the cornea and conjunctiva, lubricating the conjunctiva and cornea, providing nutrients to the avascular cornea, controlling the local bacterial flora. The PTF also undergoes constant evaporation and formation of transient “dry spots.” The rate of tear evaporation appears to be directly related to the rate of blinking, since the rate of blinking is faster than the development of these dry spots. Horse tear flow rate estimated to be 34 μL/min with a tear volume of 234 μL, which indicates a tear volume turnover rate of approximately 7 minutes Tear turnover and tear evaporation rates in humans are ~1 ± 0.4 μL/min and 0.14 ± 0.07 μL/min, respectively PTF divided into: Fig 3.1 bl 127 NB!! The outer layer (~0.1 μm) is a very thin, oily layer that forms a reversible, non-collapsible, multilayer film with the primary purpose of stabilizing the air–tear interface. The primary constituent of this lipid layer is the meibomian gland secretions (MGS), or meibum, a composite lipid-rich mixture. Up to 22 wt% comprises nonlipid components (proteins, salts, and polysaccharides). The main lipid classes found in canine MGS (cMGS): very long-chain cholesteryl esters, wax esters, (O-acyl)-omega-hydroxy fatty acids (OAHFA), cholesteryl esters of OAHFA Dogs have a relatively larger proportion of OAHFA than humans, which could be related to a higher tear film stability and lower blink rate in dogs versus humans The same types of molecules are found in the MGS of cattle, rodents, and marsupials This outer lipid layer – prevents evaporation of the underlying layers and overflow of tear film onto the eyelids, spreads over the aqueous subphase, imparts stability to the tear film, thickens the aqueous subphase, provides a smooth optical surface for the cornea, constitutes a barrier against foreign particles, provides some antimicrobial activity, seals the lid margins during prolonged closure, prevents maceration of the lid skin by the tears The lipid layer is produced by sebaceous glands (i.e., tarsal or meibomian glands) of the eyelids - undergo holocrine secretion (release of the entire cell and its contents). Androgens as well as neurotransmitters are likely involved in the regulation of MGS. Androgen receptor mRNA and protein have been isolated from rat, rabbit, and human meibomian acinar epithelial cells. Additionally, the neuropeptides, calcitonin gene-related peptide, substance P, neuropeptide Y, and vasoactive intestinal peptide have been identified in association with meibomian glands in humans and guinea pigs. A lipid layer may not be present in all species, as one was not detected in sea lions The middle aqueous layer (∼7 μm) is the thickest (>60% of the total tear film thickness) and performs the primary functions of the tear film. This layer is composed of ~98% water and ~2% solids, comprising predominantly proteins. The aqueous layer contains: inorganic salts, glucose, urea, proteins, glycoproteins, soluble mucins The lacrimal gland, gland of the NM, harderian gland, and accessory lacrimal glands in the conjunctiva all contribute to its formation. Destruction or excision of the lacrimal gland or NM gland results in a variable reduction in aqueous tear production Two-thirds of the aqueous tear production is produced by the lacrimal gland, approximately one-third by the gland of the third eyelid, and a very minor amount by the accessory lacrimal glands in the conjunctiva; however, there is variability between dogs. The aqueous portion is evaluated clinically primarily through use of the Schirmer tear test (STT), but the phenol red thread test can be used in very small animals. The deep, or mucin, layer (∼1 μm) is composed of tear mucins produced by the apocrine conjunctival goblet cells, as well as an underlying glycocalyx which is associated with the corneal and conjunctival microvilli. The fornix is rich in goblet cells in dogs, cats, and horses whereas the highest density in chinchillas and guinea pigs is in the palpebral conjunctiva All species have lower concentrations of goblet cells in the bulbar conjunctiva. In rats and mice, the goblet cells occur in clusters, while in rabbits, cats, dogs, and humans, they appear as single cells. Mucin is produced by goblet cells in response to mechanical, immune, histamine, antigenic, or (direct or indirect) neural stimulation. Two types of mucins: secretory and membrane bound The gel-forming mucin layer contains glycoproteins (20–40 million daltons and classified as MUC1-21), which are carbohydrate–protein complexes characterized by the presence of hexosamines, hexoses, and sialic acid. The glycocalyx comprises polysaccharides that are produced by the stratified squamous epithelial cells of the cornea and conjunctiva and project from the surface microvilli of those cells. They are considered membrane-spanning mucins In dogs, MUC16 is expressed at a higher level than MUC1 and MUC4, whereas rabbits have relatively equal expression of all three mucins. Additionally, the peripheral corneal epithelium has higher MUC1, MUC4, and MUC16 mRNA expression when compared with the central corneal epithelium. Membrane-associated mucins include promotion of water retention, provision of a dense barrier to pathogens and debris, participation in signal transduction, and direct interaction with the actin cytoskeleton. Mucins from the goblet cells and the corneal epithelial cells both play a critical role in lubricating the corneal surface, thus making its hydrophobic surface more hydrophilic (to permit spreading), and in stabilizing the PTF. The mucin layer as well as the integrity of the outermost layer of corneal epithelium are necessary for retention of the tear film on the cornea. Tears are a clear and slightly alkaline solution, with a mean pH of 8.3 (cattle), 8.1 (dog), and 7.8 (horse). In humans, horses, cattle, and rabbits, tear electrolyte concentration is similar to that of plasma, except for potassium, which is three to six times more abundant in tears, thus indicating an active transport mechanism Tear film osmolarity/osmolality is influenced by i) the rate of tear secretion, ii) evaporation, and iii) composition. It is similar in cats (329 mOsm/L), dogs (356 mOsmol/L), and rabbits (376 mmol/kg); humans (283 mmol/kg) and horses (284 mmol/kg) have a lower osmolarity. Even though the units are different, osmolarity and osmolality are interchangeable parameters in aqueous solutions such as the tear film. In human patients, increased osmolarity is correlated with a faster tear film break-up time and greater surface tension. Additionally, hyperosmolarity induces expression and production of inflammatory cytokines and activates several signaling pathways that activate inflammation cells The glucose concentration is lower in human tears than in plasma, but its concentration parallels that in plasma. Human patients with diabetes, the elevated glucose concentrations in tears appear to be related to the tissue fluids and are not from the lacrimal gland secretions. The PTF contains both nonspecific and specific antimicrobial substances. Nonspecific substances include: lysozyme, lactoferrin, α-lysine, complement. Specific antimicrobial substances: secretory immunoglobulins A, G, and M. Toll-like receptors that play a role in the defense against many types of microbial infections are expressed by the corneal and conjunctival epithelial cells in humans and horses Protein concentrations in canine tears average 0.35 g/dL, with 93% globulin, 4% albumin, and 3% lysozyme, which is a ubiquitous antibacterial enzyme that hydrolyzes bacterial cell walls Lysozyme is produced by the conjunctival goblet cells and has antibacterial and antifungal properties; its concentration increases with conjunctivitis. Relative to humans and nonhuman primates, domestic animals have very low amounts of lysozyme (e.g., the horse has one-half to one fourth that of human tears) and the cat has none Lysozyme activity has not been detected in cattle, but it has been detected in sheep and goats ( Lactoferrin has been identified in the PTF of humans, dogs, cats, cattle, and other mammals, and reversibly binds the iron that would be available for bacterial metabolism an dgrowth. Immunoglobulin A (IgA) contributes to ocular defenses by coating bacterial and viral microorganisms leading to agglutination, neutralization, and lysis. IgA is present in greater concentrations in the PTF than immunoglobulins G and M. Cat tears have a 6.6 mg/mL total protein concentration with 9.7% IgA. The lacrimal nerve, a branch of the trigeminal nerve, is primarily sensory but also provides the lacrimal gland with its parasympathetic (release ACh and VIP neurotransmitters) and sympathetic (release norepinephrine and neuropeptide Y neurotransmitters) fibers. Both adrenergic and cholinergic distribution patterns around the acini and blood vessels of the canine lacrimal gland are similar; however, the cholinergic fibers appear to be greater in number than the adrenergic fibers. The acinar cells are primarily responsible for secretion of proteins in lacrimal gland fluid. These proteins are synthesized in the endoplasmic reticulum, modified in the Golgi apparatus, and stored in secretory granules. Stimulation of the cholinergic and adrenergic fibers in the lacrimal gland initiates the release of these proteins into the lacrimal fluid. This process requires a series of separate cellular pathways that use secondary messengers and is controlled by signal transduction. Lacrimation is stimulated by painful irritants, eye diseases, mechanical or olfactory stimuli of the nasal mucous membranes, and sinus diseases. Tear production as assessed with the external ocular surfaces anesthetized and the lower conjunctival fornix dried by Dacron swabs (STT II) measures ~50% of that measured without manipulation (STT I) in the cat and dog. Larger dogs also have greater wetting per minute than smaller dogs as measured with the STT I. Additionally, canine neonates have lower tear production than adults. In one litter of puppies, both the STT I and STT II increased significantly until 9 and 10 weeks of age, respectively. Clinical estimation of the rate of evaporation (and, indirectly, of the mucus component of the PTF) is performed through determining the time (in seconds) for the tear film to break Nasolacrimal drainage system eliminates used tear film and any excessive tears. PTF accumulates along the palpebral margin of each eyelid and is forced by blinking to move medially into the lacrimal puncta. When the tears are in the lacrimal pool and the facial muscles relax, the tears flow into the canaliculi by capillary action. Normal breathing movements also facilitate this flow into the canaliculi. Reflex blinking of the eyelids closes the lacrimal sac, which acts as a passive pump. Pseudoperistaltic motion of the nasolacrimal duct allows movement of the tears into the nasal cavity. In domestic animals, the lower canaliculus is considered to be the more important for tear drainage. Cornea Two critical optical properties: transparency and refractive power, Transparency The cornea serves as the most powerful refractive structure of the eye and thus needs to be transparent. Corneal clarity is a result of the lattice-like organization of the stromal collagen fibrils as well as the transparency of the cells within the cornea. The state of relative: i) dehydration, ii) hypocellularity, iii) unmyelinated nerve fibers, iv) a nonkeratinized epithelium, v) absence of blood vessels, and vi) pigment also contributes to corneal transparency. The corneal stroma comprises the bulk of the cornea and is responsible for 90% of its thickness. It is predominantly composed of water that is stabilized by an organized network of collagens, glycosaminoglycans (GAGs), and glycoproteins. Type 1 collagen is the most abundant form in the cornea; it aggregates into structural, each banded fibrils (fibre) with a uniform diameter of 25 nm in the central cornea that gradually increase to 50 nm at the periphery. Correspondingly, interfibrillar spacing is relatively constant in the central cornea at 20 nm and gradually increases in the paraxial cornea, before rapidly increasing at the limbus. The GAGs are important for maintaining this regular spacing between fibrils. The uniform thickness, small collagen fibrils arrange into parallel lamellae running at oblique angles to each other, and are separated by less than a wavelength of light (Fig. 3.2) In the normal cornea (A), a cross-section of the corneal fibrils demonstrates a nearly perfect lattice arrangement, with equidistant collagen fibrils permitting light transmission and concomitant transparency. By contrast, swelling of the cornea with edema (B) disrupts this highly ordered arrangement, resulting in light diffraction and an opaque, blue cornea. This formation results in a highly ordered lattice-like arrangement whereby short-range order results in corneal transparency via destructive interference. The parallel arrangement of the corneal collagen fibers extends from the center of the cornea to its periphery, where the fibrils develop a concentric configuration to form a “weave” at the limbus, which in turn provides strength and helps to maintain its curvature. Quiescent keratocytes lie between collagenous lamellae to form a closed, exquisitely structured syncytium. These three-dimensional, stellate-shaped cells comprise a cell body with multiple, extensive dendritic processes that interact with other keratocytes. Abundant corneal crystallins (~25–30% of the intracellular soluble protein), such as aldehyde dehydrogenase and transketolase, minimize refractive differences in the keratocyte cytoplasm, thus ensuring transparency of these cells. The thinness and even spacing of keratocytes in a clockwise circular arrangement throughout the stroma also minimize light scatter. Upon corneal wounding, transformation of keratocytes to activated fibroblasts and myofibroblasts results in a dramatic increase in cell volume and subsequent dilution of corneal crystallins with a concomitant increase in light scatter. Corneal scarring is now thought to be due to alterations in the light-scattering properties of keratocytes, in addition to changes to the extracellular matrix. The epithelium and endothelium are responsible for maintaining the cornea in a relatively dehydrated state. Loss of the corneal epithelium or endothelium results in a 200% or 500% increase in corneal thickness, respectively, due to stromal edema. Anatomic integrity of the epithelium and endothelium provides two-way, physical barriers against the influx of tears and aqueous humor (AH), respectively. However, the multiple-layered epithelium provides a relatively impermeable barrier versus the leaky, single-layered endothelium. The endothelium primarily maintains stromal deturgescence (state of dehydration) via active transport that is energetically maintained by sodium potassium–activated adenosine triphosphatases Na+-K+-ATPases. Metabolism Steady-state hydration in the cornea occurs when the endothelial leak and pump rates are equivalent; this process is termed the “pump-leak” mechanism (Maurice, 1960). The leaky barrier function of the endothelium may at first seem counterintuitive, but most nutrients for the cornea, except oxygen, come from the AH. Thus, leakiness of the endothelium is essential to providing bulk fluid flow through a tissue that lacks blood and lymphatic vessels. Glucose transporters are found on both the apical and basolateral endothelial cell membranes that face the AH and stroma, respectively, to ensure transcellular glucose flux. The corneal epithelium converts glucose to glucose-6-phosphate, where it is subsequently metabolized to pyruvate via glycolysis. Most of this pyruvate is then metabolized into lactate, but some is diverted into the tricarboxylic acid cycle to produce ATP. Glucose is also stored in the epithelium as glycogen, which can be used for energy under stressful conditions such as corneal injury. The corneal epithelium and keratocytes in the anterior stroma obtain oxygen for aerobic glycolysis from the precorneal tear film, while the endothelium and keratocytes in the posterior stroma receive their oxygen from the AH. Upon eyelid closure, oxygenation of the anterior cornea is achieved by exposure to the palpebral conjunctiva and its vasculature. However, the palpebral conjunctiva has approximately one-third the atmospheric oxygen concentration, resulting in reduced corneal oxygenation. Consequently, the corneal epithelium will rely on anaerobic glycolysis for energetic needs in the absence of oxygen. If excessive lactate is produced by this process, corneal hydration occurs. Glucose is also metabolized by the corneal epithelium via the pentose phosphate shunt, which produces nicotinamideadenine dinucleotide phosphate (NADPH), an important free radical scavenger. Other metabolites from this pathway are ribose-5-phosphate (ribose-P) and reduced triphosphatepyridine nucleotide. Ribose-P is used in nucleic acid synthesis of DNA or RNA, whereas triphosphate-pyridine nucleotide is used by the corneal epithelium for lipid synthesis. Keratocytes primarily metabolize glucose via the pentose phosphate shunt as their metabolic needs are limited, and primarily relate to maintenance of the collagen fibrils and GAGs within the stroma. By contrast, the corneal endothelium has immense glucose needs (~5 times that of the epithelium) to sustain its pump mechanism. It uses similar glycolytic pathways as the epithelium: mainly anaerobic glycolysis, with the pentose phosphate and tricarboxylic acid pathways also making substantial contributions. Biomechanics The collagen lamellar architecture of the cornea varies dramatically between vertebrate species, with nonmammalian vertebrates exhibiting an orthogonal-rotational arrangement with a marked increase in lamellar branching in species such that birds >> reptiles > amphibians > fish; by contrast, the mammalian species exhibit a random pattern. In mammals without a Bowman’s membrane, the biomechanical behaviors of the cornea are primarily due to the collagen architecture in three composite-like regions: anterior stroma, posterior stroma, and Descemet’s membrane (Fig 3.3 pg 132) The variability in corneal collagen fiber organization and matrix properties observed between species likely contributes to their diverse mechanical properties. Sensitivity and Innervation Corneal sensitivity provides a critical protective function. Upon stimulation of the cornea, involuntary blinking occurs via intermediate relays from the ophthalmic branch of the trigeminal nerve to orbicularis oculi innervation from the facial nerve – a fundamental reaction termed the corneal or blink reflex. Concomitant with the blink reflex is reflex tearing from parasympathetic innervation to the lacrimal gland. Corneal sensitivity varies by: i) species, ii) region of the cornea, iii) dog and cat, skull conformation. For example, corneal sensitivity in dogs is highest, intermediate, and lowest in the dolichocephalic, mesatocephalic, and brachycephalic skull types, respectively. Similarly, the central cornea is less sensitive in brachycephalic cats than Domestic Shorthair (DSH) cats. Corneal sensitivity is greatest in the central cornea and lower in the peripheral cornea. Most corneal nerve fibers are sensory in origin and respond to mechanical, chemical, and thermal stimuli via the ophthalmic branch of the trigeminal nerve. However, a small proportion of nerves are sympathetic or parasympathetic in origin and derive from the superior cervical ganglion or ciliary ganglion, respectively. Mammalian corneas contain a dense limbal plexus, multiple radially directed stromal nerve bundles, a dense highly anastomotic subepithelial plexus, and a richly innervated epithelium (Fig. 3.4, pg 134) In the dog: corneal innervation arises from the corneal limbal plexus, which comprises a 0.8–1 mm wide, ring-like band, surrounding the peripheral cornea. plexus can be further subdivided into a predominantly perivascular, outer, periscleral zone and a denser and more highly branched inner, pericorneal zone. From the limbal plexus, nerve fibers enter into the stroma as 14–18 prominent, radially directed, superficial stromal bundles that are evenly distributed around the limbus The number of stromal bundles varies among mammals, with 6–8 and ~60 bundles in the rat and human, respectively Any myelinated axons included in stromal nerve bundle will lose their myelin as they traverse toward the central cornea. Each bundle in the canine cornea contains approximately 30 to 40 axons which undergo repetitive branching to form complex axonal trees that innervate the anterior cornea Immediately beneath the anterior basement membrane is a dense, anastomosing network of exceptionally thin, preterminal axons that comprise the subepithelial nerve plexus. The subbasal plexus arises from subepithelial nerve fibers entering the basal epithelium to form unique, preterminal arborizations termed epithelial leashes which exhibit a highly ordered distribution and give rise to a profusion of smaller, ascending branches The sub-basal nerve plexus of dogs, cats, and humans has a centripetal whorl-like pattern, whereas cattle and rabbits exhibit a horizontal pattern that is directed nasally The innervation of the epithelium is denser than any other surface epithelium such that injury to a single epithelial cell may be sufficient to stimulate nociception. While the anterior cornea is densely innervated, only sparse nerve fibers are present in the posterior cornea and are typically adjacent to the corneal endothelium. Thus, superficial corneal ulcers are typically more painful than deep stromal ulcers. The majority of sensory fibers are polymodal nociceptors. The remainder of the sensory fibers innervating the cornea comprise mechano-nocireceptors and cold thermal receptors, which are only activated in response to mechanical forces or changes in temperature, respectively Corneal nerves secrete a variety of neurotransmitters, including acetylcholine, vasoactive intestinal peptide (VIP), and neurotensin, as well as neuropeptides such as substance P and calcitonin gene-related protein (CGRP), which are critical to corneal epithelial proliferation and function. Impairment of corneal sensory innervation, termed neurotrophic keratitis, causes decreased corneal healing, increased epithelial permeability, and recurrent corneal ulcers The neuropeptides CGRP and substance P also act as neurogenic mediators of inflammatory responses by inducing vasodilatation, plasma extravasation, and cytokine release following their release from depolarized nociceptor endings. This neurogenic inflammation impacts both the injured cornea and the non-injured conjunctiva, iris, and ciliary body, as nerve impulses from stimulated nociceptors travel not only centripetally through the axon to the central nervous system, but also to non-stimulated peripheral axon branches of the trigeminal nerve. This reflex is likely responsible for the clinical signs of conjunctival hyperemia and anterior uveitis, including miosis, ocular hypotension, and aqueous flare associated with an isolated corneal lesion. Iris and Pupil Pupillary functions include: regulating light entering the posterior segment of the eye, increasing the depth of focus for near vision, and minimizing optical aberrations by the lens. The iris muscles consist of both a constrictor (sphincter) that encircles the pupil and radial dilator muscle. The sphincter muscle is an annular band of smooth muscles near the pupillary margin of the iris and is derived from neural ectoderm. The dilator muscle, also derived from neural ectoderm, consists of a series of myoepithelial cells that extend from near the pupillary margin to the base of the iris and are contiguous posteriorly with the pigmented epithelium of the ciliary body. Constrictor muscle, which is the stronger of the two, is innervated by the oculomotor nerve (CN III) which provides primarily parasympathetic control = miosis Dilator muscle is innervated primarily by sympathetic nerves. The constrictor muscle causes = mydriasis The sympathetic activity in the iridal dilator muscle and ciliary body musculature is mediated by a combination of β-receptors (β1 and β2) and α-receptors (α1 and α2; Components of the pupillary light reflex (PLR) are listed in Table 3.4, pg 135. Species differences of the α- and β-receptors have been demonstrated among humans, rabbits, nonhuman primates, cats, and dogs, and they are summarized in Table 3.5, pg 135. These receptors alter the effects of drugs on the eye. feline pupils constrict with the use of timolol, a nonselective β-adrenergic antagonist, because the feline iris sphincter muscle has primarily β-adrenergic nerve fibers Because β-adrenergic nerve fibers are inhibitory to the sphincter muscle, the miosis in response to topically applied timolol is suspected to be the result of its antagonism of inhibitory input to the sphincter muscle. Most synapses in the ciliary ganglion are involved in relaying impulses that result in accommodation; the remainder are concerned with constriction of the pupil. Pure mu opioid agonists such as morphine act on subcortical cells (i.e., oculomotor nuclear complex) to cause constriction of the canine pupil and dilation of the feline pupil because of the release of catecholamines from the adrenal glands. Endogenous prostaglandin F2α appears to be involved in maintaining muscle tone in the sphincter muscle of the iris. Prostaglandins most likely act directly on these muscles, and they appear to act to a lesser extent on the dilator muscles of the canine iris. Exogenous prostaglandin analogues cause miosis in cats, dogs, and horses, and the receptors have been detected in the iris and ciliary body of several mammals Neuropeptide Y has a modulatory role on the iris dilator muscles that enhances adrenergic-induced contractions. By itself, however, neuropeptide Y does not have any potent contractile qualities. Iris color, or the amount of melanin, influences the effects of many drugs, as melanin can bind drugs, increasing their time to onset and duration Pupil shape also varies among species. Vertical pupils: terrestrial mammals and reptiles that are ambush predators and are diurnal. Horizontally elongated pupils prey species. On dilation, the vertical sides of the domestic feline pupil expand to produce a circular pupil. The constrictor muscle fibers are vertically oriented, and therefore contraction leads to a vertically oriented slit pupil. In prey with horizontal pupils, changing head pitch causes torsional movement of the eye, such that the pupil’s long axis maintains rough alignment with the horizon. In young horses, the pupil is more circular than in adults. Under illumination, the ends of the oval pupil of mature horses do not constrict, but the dorsal and ventral borders do. In bright daylight, the superior granula iridica occludes the central papillary opening, resulting in two apertures and assisting with focusing through the creation of Scheiner’s disc phenomenon. With very low illumination or administration of a mydriatic, the dorsal and ventral borders of the pupil dilate, thereby forming a circular pupil. The equine pupil responds relatively slowly to a light stimulus in comparison to that of cats and dogs. The avian pupil is circular and highly motile. The consensual pupillary reflex is usually absent (because of total decussation of nerve fibers at the optic chiasm), but occasionally a strong beam of light may traverse the posterior ocular layers and the thin medial orbital bones to stimulate the opposite retina. As the constrictor and dilator muscles are mainly striated with varying amounts of nonstriated fibers, the pupil is not affected by traditional mydriatic agents, but it can be dilated by various neuromuscular-blocking drugs. Dogs demonstrate intrinsically photosensitive retinal ganglion cells (ipRGC) that contribute to the PLR. Melanopsin, the ipRGC photopigment, has peak sensitivity at 480 nm. The ipRGCs are slow to activate and have a high threshold. Below the ipRGC threshold, PLRs to blue and red stimuli are similar. However, when the ipRGCs are strongly activated with a blue stimulus, pupil constriction persists for at least 60 seconds after the stimulus is removed. Additionally in dogs, the stimulus intensity required to elicit a threshold PLR is approximately 10-fold lower than that required to elicit a scotopic threshold ERG response in the dark-adapted state Nutrition of Intraocular Tissues Nutrients are provided and waste is removed by two systems of blood vessels (i.e., retinal vessels, uveal vessels), the formation and egress of AH, and the vitreous body. Intraocular tissues lack a typical lymphatic system, and the uveal tract (i.e., iris, ciliary body, and choroid) provides this function. Ocular Circulation The choroid, ciliary body, and iris are supplied by the uveal vessels. The outer retina in some animals (e.g., dogs, cats, ruminants, and pigs) and almost all or the entire retina in others (e.g., horses, guinea pigs) is nourished by diffusion from the uveal vessels in the choroid. The inner retina is supplied by retinal vessels in many species. Blood vessels supplying the cornea and lens in the embryo regress before birth or shortly thereafter, leaving the AH as the primary source of nutrients for the cornea and lens. Birds have a unique structure, the pecten oculi, which is a heavily pigmented, highly vascularized, and usually fanlike structure projecting from the surface of the optic nerve into the vitreous. A similar structure occurs in reptiles, termed the conus papillaris. The avian pecten likely functions as an important source of nutrients for the inner retina. The pecten may also control the intraocular pH, which is affected by acidic retinal waste products Several marine mammals, the bottlenose dolphin (Tursiops truncatus), spotted seal (Phoca largha), and California sea lion (Zalophus californianus), have an ophthalmic rete from which the retinal and choroidal arteries are derived. A higher degree of thermal emission in this area than adjacent areas of the skin suggests that the rete might conserve ocular heat so that photoreceptor and ocular muscle function can be maintained in a cold ambient temperature. Additionally, the rete may have a flow-damping effect by maintaining resistance to blood flow in the orbit Ocular Blood Flow Blood flow through all tissues is dependent on: The vascular pressure promoting flow, the resistance of blood vessels, and the viscosity of the blood Perfusion pressure is the difference in pressure between the arteries and the veins. IOP approximates = venous pressure, so perfusion pressure is the difference in pressure between the small arteries entering the eye and the IOP. Of clinical importance is that the perfusion pressure to the eye is reduced by lowering the blood pressure or raising the IOP, as occurs in glaucoma. Hemodynamics in the rabbit ophthalmic artery demonstrates that autoregulation maintains normal blood velocity and resistance when the IOP is below 40 mmHg. However, at higher pressures the autoregulatory capacity is limited Anterior Uveal Blood Flow In most species: the major arterial circle of the iris is formed by the nasal and temporal long posterior ciliary arteries. The iris and ciliary body receive approximately 1% and 10%, respectively, of the ocular blood flow In humans and rabbits, additional iridal blood flow occurs from the anterior ciliary arteries via the extraocular muscles (EOM). Blood flow to the ciliary body in most species: The iridal major arterial circle, branches of the anterior ciliary arteries, and branches of the long posterior ciliary arteries. The cat and monkey iris and ciliary body have autoregulation of their blood flow. Carbon dioxide dilates the anterior uveal vessels, and sympathetic α-adrenergic receptors cause vasoconstriction in the anterior uvea. Parasympathetic muscarinic receptors and prostaglandins, however, cause vasodilation. Prostaglandins E1 and F2α appear to cause a two- to threefold increase in blood flow to the anterior uvea when applied topically. Choroidal Blood Flow Blood supply to the choroid is supplied by the short posterior ciliary arteries, but some of the peripheral choroid receives blood from the major arterial circle of the iris. The choroidal capillaries are fenestrated and large (diameter 15–50 μm). These vessels are highly permeable and permit glucose, proteins, and other substances in the blood to enter the choroid. Within the choroid, these proteins create a high osmotic pressure gradient that assists in the removal of fluids from the retina. The short posterior ciliary arteries appear to supply well-defined territories within the choroid. As a result, these “watershed zones” can develop with marked elevations of IOP, and appear in the dog as pyramidal-shaped areas of choroidal and retinal degeneration extending from the optic nerve head. The rate of uveal blood flow is rapid (1.2 mL/min in the cat), with a mean combined retinal and choroidal circulation time of 3–4 seconds. With this high rate of blood flow, oxygen extraction from each millimeter of blood is low (∼5%–10%). The oxygen content of choroidal venous blood is 95% of that in arterial blood. Reduced flow rates result in higher oxygen extraction, so that total extraction is reached. This protects the oxygen supply to the retina, and it also protects the eye from light-generated thermal damage. Choroidal vessels have little to no autoregulatory mechanism, but carbon dioxide is a potent vasodilator of choroidal vessels. Choroidal vessels are under the strong influence of sympathetic stimulation, which can result in a 60% reduction of choroidal blood flow. The α-adrenergic drugs cause vasoconstriction of choroidal vessels, but β-adrenergic drugs have no effect. In rabbits, α- and β-adrenergic blockade causes choroidal vasodilation and vasoconstriction, respectively. Retinal Blood Flow The retina receives 4% of the ocular blood flow in the monkey In cats, 20% of the oxygen consumed by the retina is delivered through the retinal circulation and the remaining 80% is via the choroidal circulation Blood flow in the innermost retina is practically unaffected by moderate changes in perfusion pressure. Autoregulation of retinal blood flow is extensive in the cat, monkey, and pig, and protects the retinal circulation from large variations in perfusion pressure (Alm & Bill, 1972; Both metabolic and myogenous (muscle) autoregulation are present in the eye. Metabolic control of retinal blood flow is similar to that of blood flow to the brain. In the brain, increased PO2 and decreased PCO2 cause vasoconstriction, and decreased PO2 and increased PCO2 cause vasodilation. In the cat, maximum retinal vasodilation occurs with an increased PCO2 of 75–80 mmHg, so as to increase flow from 15 to 50 mL/min. Retinopathy of prematurity occurs when immature eyes with developing blood vessels are exposed to higher oxygen concentrations than the normal physiologic in utero hypoxia. The increased oxygen causes vasoconstriction and inhibition of vascular development, with obliteration of vessels. With the hypoxic injury is a concomitant, rebound release of angiogenic factors that lead to pathologic angiogenesis. Signs include Vaso proliferation, retinal edema, hemorrhages, and possible retinal detachment. Retinal vessels have α-adrenergic-binding sites that, when stimulated, cause vasoconstriction, thus increasing retinal vascular resistance. Retinal arteries most likely autoregulate through a myogenic mechanism, which is activated based on stretch. During sympathetic stimulation, myogenic autoregulatory responses appear to increase. Opening and closing of capillary beds in many tissues occur with varying metabolic needs. Choroidal blood flow is constant through all capillary beds; however, regional blood flow in the retina decreases from the optic disc to the periphery. The vascular endothelial cells may produce nitric oxide, endothelins, prostaglandins, and renin-angiotensin products in response to chemical stimuli (e.g., acetylcholine, brachykinin), changes in blood pressure and blood vessel wall stress, changes in local oxygen levels, and other stimuli Blood Flow of the Optic Nerve Head Provided primarily by branches from the short posterior ciliary arteries. In humans, cats, and rabbits, optic nerve head blood flow possesses autoregulation over a wide range of IOPs (∼30– 75 mmHg), but in humans, this autoregulation is most efficient when IOP is 6–30 mmHg Ocular perfusion pressure is the relationship between systemic blood pressure and IOP, determines blood flow in the optic nerve head. The autoregulatory capacity of optic nerve head blood flow is more susceptible to an ocular perfusion pressure decrease induced by lowering the blood pressure, compared with that induced by increasing the intraocular pressure. Axons from the retinal ganglion cells exit the eye under the effect of IOP, passing through the lamina scleral cribrosa at progressively decreasing tissue pressures to become the optic nerve. The retrolaminar pressure for these axons in the dog is approximately 7 mmHg and relates directly to the cerebrospinal pressure. Both short- and long-term elevations in IOP also produce tissue changes within the lamina cribrosa that influence blood flow and axon vitality. Ocular Barriers Schematic demonstration of the anatomy and the biological membranes and barriers of the eye. Panels A, B, C and D represent the corneal epithelial barrier (CEB), the blood aqueous barrier (BAB), the biostructures of retina, and the blood-retinal barriers (BRB) both inner endothelial and outer pigmented epithelial barriers. The hemostasis of eye is performed by several static and dynamic barriers. Tear film is the first physiologic impediment against installed topical pharmaceuticals (0). The cornea forms an excellent obstacle preventing topical drugs to reach the anterior chamber of the eye (1). The conjunctival/scleral route is the most permeable path to the hydrophilic drugs and macromolecules (2). The systemically administered small compounds are able to penetrate from the iris blood vessels into the anterior chamber (3). The administered drugs reached to anterior chamber are subjected to aqueous humor outflow (4). These drugs can be carried away from anterior chamber by venous blood flow (BAB function) after diffusing across the iris surface (5). The systemically administered drugs must cross the BRBs. These drugs must cross the outer retinal barrier, " retinal pigment epithelia (RPE) " and the inner retinal barrier, " retinal capillary endothelia (RCE) " (6). For intravitreal delivery, drugs can directly be injected into the vitreous (7). Drugs can be removed from the vitreous away by the retinal blood vessels (8). Drugs within the vitreous can be diffused into the anterior chamber (9).   The blood–ocular barriers contain endothelial and epithelial tight junctions with varying degrees of “leakiness.” These barriers prevent nearly all protein movement and are effective against low-molecular-weight solutes such as fluorescein and sucrose. The two primary barriers within the eye are the blood–aqueous barrier (BAB) and the blood–retinal barrier (BRB). Other lesser barriers of the eye exist as well: The zonula occludens of the corneal epithelium: prevents the movement of ions and therefore fluid from the tears into the stroma, prevents some evaporation, and protects the cornea from pathogens. The macula occludens of the corneal endothelial cells: prevents bulk flow of AH into the corneal stroma but allows moderate diffusion of small nutrients and water. Blood–Aqueous Barrier The blood-aqueous barrier is formed by an epithelial barrier located in the nonpigmented layer of the ciliary epithelium and in the posterior iridial epithelium, and by the endothelium of the iridial vessels. Both these layers have tight junctions of the "leaky" type. Depends primarily on the: tight junctions in the non-pigmented ciliary body epithelium, the nonfenestrated iris capillaries, and the posterior iris epithelium. The anterior BAB in the iris allows transcellular transport by means of vesicles. Paracellular transport is controlled by tight junction extensions. The anterior surface of the iris does not serve as a barrier as it does not have a continuous cellular layer. The epithelial portion of the BAB is the inner, nonpigmented ciliary epithelium, and it controls the flow of fluid into the posterior chamber. The BAB is less effective than the retinal epithelial barrier, because protein can pass into the AH through leakage in other parts of the anterior uvea. Both the ciliary body and choroidal blood vessels are highly fenestrated and thus leak most of their plasma components, including protein, into the stroma. No barrier is present between the AH and the vitreous humor, which allows the diffusion of solutes from the posterior aqueous into the vitreous humor, or between the anterior uvea and the sclera. Breakdown of the BAB is seen clinically as an aqueous flare in anterior uveitis or secondary to loss of AH, as in anterior chamber paracentesis. Blood–Retinal Barrier Schematic illustration of the blood-retinal barrier (BRB) structure. (A) The outer BRB insulates the outer retina from the bloodstream deriving from the choriocapillaris (CC), whose endothelium shows multiple fenestrations (a). The molecules reach the neuroretina through the Bruch’s Membrane (BM), composed of five layers, which forms a size-selective barrier (a’), and through the retinal pigment epithelium (RPE), characterized by tight junctions (a’’). (B) The inner BRB insulates the inner retina from the retinal vasculature, composed by a deep, intermediate, and superficial plexus; the endothelial cells of the retinal vessels form an effective barrier because of the presence of tight junctions among them (b). The tight junctions between the RPE comprise the epithelial portion of the BRB. The non-fenestrated retinal capillary endothelium with tight junctions between the cells comprises the endothelial portion of the BRB. The most permeable point of the BRB is the optic nerve head, at which substances from the choroid can pass into the nerve. The choroidal capillaries are highly permeable to permit passage of all low-molecular weight compounds and proteins. Thus, nutrients from the choroidal blood supply pass readily into the retinal pigment epithelium, where numerous transport systems account for the selectivity of the barrier and elaborate transcellular pathways exist to pass them on into the retina. This high protein permeability of the choroidal vessels also elevates osmotic pressure, which helps fluid to pass out of the retina. The transport of water from the retina to the choroid is driven by the active transport of chloride to prevent water accumulation in the subretinal space. No significant barrier exists between the vitreous body and the retina. Fig. 1. The blood-retinal barrier (A) The retina is a multi-layered tissue in the posterior 3 segment of the eye, and is shown by the H&E stained micrograph on the left. The cell types 4 comprising the inner BRB (endothelial cells, pericytes, astrocytes, and Müller cells) and 5 outer BRB (retinal pigment epithelial cells) are overlaid on the retinal micrograph, and are 6 magnified on the right. Tight junctions between retinal capillary endothelial cells and retinal 7 pigment epithelial cells form the basis of the inner and outer BRB, respectively. The 8 endothelial cells of inner retinal capillaries are not fenestrated, whereas those of the 9 choroidal capillaries are (depicted below the retinal micrograph). (B) Protein components of 10 the intercellular junctions. Aqueous Humor and Intraocular Pressure In the normal eye, IOP is generated by flow of AH against resistance, and is necessary to maintain the appropriate shape and optical properties of the globe. AH is a clear, colorless liquid that fills the anterior and posterior chambers as well as the pupil. It has a refractive index of 1.335, which is slightly denser than water, and is a critical constituent of the eye’s optical system. As AH is formed by the ciliary body processes, it enters the posterior chamber and flows through the pupil into the anterior chamber, where it leaves the eye through the corneoscleral trabecular and uveoscleral outflow pathways. The rate of AH formation equals the outflow, so the IOP is maintained relatively constant, and the refractive surfaces of the eye are kept in a normal position. This continuous flow of AH supplies the avascular cornea and lens with nutrients and also removes their waste products. A convection current exists within the anterior chamber whereby AH circulates downward adjacent to the air-cooled cornea and upward near the lens where the temperature is warmer. This thermal circulation is responsible for the deposition of cellular material – termed keratic precipitates – on the inferior aspect of the corneal endothelium. Aqueous Humor Formation The ciliary body has several critical functions: production of AH by active secretion, ultrafiltration, and diffusion; generation of IOP through the aqueous dynamic process; influencing through its musculature the conventional (i.e., corneoscleral trabecular meshwork or pressure-sensitive) AH outflow; provision of blood and nerve supplies for the anterior segment; control of accommodation via its musculature; formation of the BAB; and provision of the entry for nonconventional (i.e., uveoscleral or pressure-insensitive) AH outflow. rich in antioxidant systems, with significant concentrations of catalase, superoxide dismutase, and glutathione peroxidase types I and II. Major drug detoxification center in the eye, with its microsomes containing the cytochrome P450 proteins, which catalyze many drugs. In avian species, the ciliary body musculature is composed of distinct anterior and posterior components that alter the corneal curvature for corneal accommodation and move the ciliary body anteriorly for lenticular accommodation The bilayered ciliary epithelium, consisting of the outer pigmented epithelium (PE) and inner non-pigmented epithelium (NPE), is the site for AH secretion. At their apical borders, the PE and NPE connect via gap junctions to form an intricate network (Fig. 3.5, pg 140). Pathways for unidirectional secretion ( A ) and possible reabsorption ( B ) across the ciliary epithelium. PE, pigmented ciliary epithelial cells; NPE, non-pigmented ciliary epithelial cells. ((A) From McLaughlin CW et al: Am J Physiol Cell Physiol 293:C1455, 2007. Used with permission.) Adjacent NPE cells are joined by tight junctions to form a barrier that inhibits paracellular diffusion. AH is formed by three basic mechanisms: diffusion, ultrafiltration, and active secretion by the NPE. The processes of diffusion and ultrafiltration form the “reservoir” of the plasma ultrafiltrate in the stroma of the ciliary body, from which the AH is derived via active secretion by the ciliary epithelium. Diffusion of solutes, such as carbohydrates, occurs from a region of higher concentration to that of lower concentration. By contrast, ultrafiltration occurs when movement of a compound across a cell membrane is increased by a hydrostatic force; in this case, from differences between ciliary body capillary pressure and IOP. Energy-dependent active transport is required to secrete solutes against a concentration gradient across the basolateral membrane of the NPE; it is the most important factor in AH Formation. Two enzymes critical in this process, Na+-K+-ATPase and carbonic anhydrase (CA), are abundantly present in the NPE. The Na+-K+-ATPase is membrane-bound and is found in the highest concentrations along the basolateral interdigitation of these cells. Inhibition of the Na+-K+-ATPase with cardiac glycosides (e.g., oubain) or vanadate causes a marked decrease in aqueous formation as differences in osmolarity between plasma and AH are small, thereby making the rate of aqueous production dependent on the rate of solute transfer. Due to the primary active secretion of sodium, other molecules and ions cross over the epithelium by secondary active transport. As a consequence, increased concentrations of ascorbate, amino acids, and chloride are observed in AH relative to plasma in most mammalian species. Electroneutrality is maintained by anions accompanying the actively transported sodium; channels allow passage of chloride on the basolateral NPE membrane and a passive transporter exchanges bicarbonate for chloride. As a result, high osmolarity is produced on the basolateral aspect of the NPE, thus initiating diffusion of water out of the cells by aquaporins. To continue the process, sodium and chloride constantly enter the PE via Na+/H+ and Cl-/HCO3- antiports and a Na-K-2Cl cotransporter. Chloride is just as critical as sodium in driving AH formation, such that A3 adenosine receptor agonists, which enhance chloride release, increase IOP in mice. CA is abundant in the cytoplasm and on the basal and lateral membranes of the NPE and PE and catalyzes the following reaction: CO2 +H2O HCO3+ H . Formation of bicarbonate by CA is essential for secretion of AH, such that inhibition of CA results in decreased active transport of sodium by the NPE; it is unclear how this process occurs, although several hypotheses exist. Topical and systemic CA inhibitors substantially decrease AH production, therefore reducing IOP, and are thus useful in the management of glaucoma Aqueous Humor Composition As an ultrafiltrate of plasma, the compositions of AH and plasma are similar, with a few notable exceptions: a low protein concentration, high ascorbate and lactate concentrations, and reduced amounts of urea, glucose, and nonprotein nitrogen occur within AH versus plasma (Fig. 3.5) Thus, breakdown of the BAB modifies the composition of the AH, primarily by the addition of proteins and prostaglandins, and increases light scattering. The resultant Tyndall effects makes a slit-lamp beam evident within the anterior chamber, an observation clinically known as aqueous flare. With the addition of proteins, the aqueous composition closely approximates that of plasma and is termed plasmoid aqueous. Plasmoid aqueous in domestic animals forms fibrin clots easily due to high concentrations of the glycoprotein fibrinogen. Unless treated pharmacologically, these fibrin clots can cause numerous complications, including anterior and/or posterior synechiae or adhesions between the iris and the cornea and/or lens. Ascorbate concentration in the AH exceeds that in plasma due to an active transport mechanism. This high concentration of ascorbate might assist in protecting the anterior segment structures from oxidative damage induced by ultraviolet radiation. Furthermore, ascorbate is a cofactor in electron transfer reactions, is a reducing agent in hydroxylation reactions, helps regulate production of GAGs in the trabecular meshwork, and might play a role in the storage of iridal catecholamines In most mammalian species, the concentration of amino acids in the AH is higher than that in the plasma, suggesting active transport of amino acids is occurring across the ciliary epithelium. In the dog, however, amino acid concentrations are less in AH than in plasma Carbohydrate concentrations in the AH are ~80% of plasma since they enter by diffusion and are subsequently metabolized by the lens and cornea. Thus, the concentration of diffusible substances in blood can impact its amount in the AH. For example, the concentration of glucose in AH is markedly increased in diabetic patients due to elevated plasma concentrations. Urea concentration in the AH is also ~80% that of plasma. Results of previous studies in the dog have indicated that urea penetrates the BAB very slowly; therefore, a steady concentration as compared to that in plasma is never reached, thus resulting in a lower amount of urea in the AH. By contrast, immunoglobulins, enzymes, and lipids are present in much lower concentrations in AH versus plasma due to the BAB. The major cations in the AH are sodium, potassium, calcium, and magnesium, with sod