Scientific Basis of Vision Iris, Ciliary Body and Lens Shiva Swamynathan

1. Scientific Basis of Vision Iris, Ciliary Body and Lens Shiva Swamynathan Department of Ophthalmology University of Pittsburgh School of Medicine

2. Uveal Tract – Vascular middle layer of the eye Iris (Anterior), Ciliary Body (Middle), Choroid (Posterior) Iris: Pigmented diaphragm located between anterior and posterior chambers. Regulates the amount of light reaching the retina. Ciliary Body: Regulates the composition and production of aqueous humor, affecting the ionic environment and metabolism of the lens, iris, TM and cornea. Choroid: Vascular layer of the eye located between the retina and the sclera. Provides nourishment and oxygen to outer layers of the retina.

3. Where is the uveal tract attached to the sclera? The scleral spur The exit points of vortex veins The optic nerve.

4. Anatomy of the Iris Sphincter Pigmented epithelium Anterior Border Stroma Dilator Anterior Border- An incomplete layer of endothelial (mesothelial) cells. The Irideal Stroma accounts for most of the mass of the iris. Composed of connective tissue, blood vessels and nerve fibers. Pigmented cells containing melanin are scattered within the stroma. The posterior boundary of the stroma, peripheral to the sphincter muscle, is demarcated by a sheet of smooth muscle, the dilator muscle. Thepigmented epithelium is composed of two layers of densely pigmented cells.

5. Blood Supply to the Iris Source: The major arterial circle in the ciliary body. The blood vessels of the iris run in a radial direction. The anterior border layer contains very few vessels.

6. Innervation of the Iris The iris is supplied by the ciliary nerves, which pierce the sclera around the optic nerve and run forward in the perichoroidal space. After reaching the iris the ciliary nerves form a plexus around its attached margin, from which the nerve fibers ending in the anterior surface of the iris, Sphincter and Dilator are derived.

7. Iridial Muscles- Sphincter and Dilator (derived from neural ectoderm) Sphincter muscle. Encircles the pupil, and constricts the pupil in bright light or during accommodation. Controlled by parasympathetic nerve fibers. Dilator muscle. Arranged radially, these fibers extend from the base of the iris to the sphincter muscle. Controlled by sympathetic nerve fibers, although some parasympathetic fibers are associated with it. Main functions of iris and ciliary body smooth muscle Contraction/relaxation Receptor characterization Second messenger formation and regulation Arachidonic acid release and eicosanoid biosynthesis

8. Ciliary Body • Produces aqueous fluid that fills the anterior chamber, maintaining IOP • The ciliary muscle facilitates lens accommodation Pigmented epithelium is leaky while non-pigmented epithelium facing the aqueous is responsible for the blood-aqueous barrier What is the rate of aqueous humor production in humans? About 2-3 ml/min.

9. How does aqueous humor enter the posterior chamber? Actively or Passively? Actively- Energy dependent secretion carbonic anhydrase-II activity dependent. Passively- Diffusion and Ultrafiltration. Active secretion accounts for the bulk of aqueous humor What is the effect of Carbonic Anhydrase-II inhibitors on aqueous flow? CA-II inhibitors reduce the rate of entry of sodium and bicarbonate into the aqueous, thus reducing the aqueous humor formation.

10. Aqueous fluid outflow The aqueous fluid produced by the ciliary body flows between the iris and lens, through the pupil to the drainage angle at the junction of the iris and the cornea, and exits the eye through the trabecular meshwork and Schlemm’s canal, interscleral channels and episcleral vein. True/False: Aqueous humor is rich in macromolecules False. Aqueous humor is practically free of macromolecules, allowing for optical clarity. For example, it contains about 1/500 of the albumin present in the plasma.

11. Eicosanoids Signaling molecules with hormonal activity, made by oxygenation of 20-carbon essential fatty acids. They control inflammation and serve as messengers in the CNS. They include compounds such as prostaglandins, prostacyclins, thromboxanes, and leukotrienes. Eicosanoids are derived from either omega-3 (ω-3) or ω-6 EFAs. Unlike the ω-3's, ω-6 eicosanoids are generally pro-inflammatory. Anti-inflammatory drugs such as aspirin and other NSAIDs act by downregulating eicosanoid synthesis. They play critical roles in regulating aqueous production and outflow.

12. Prostaglandin Synthesis • In response to cytokines, neurotransmitters or pharmacologic treatments, phospholipase-A2 is activated, releasing arachidonic acid from membrane phospholipids • Free arachidonic acid is converted to prostaglandin H2 endoperoxide intermediates by cyclooxygenase-I (Cox1; ubiquitously expressed) or Cox2 (expressed in pro-inflammatory conditions) • Free arachidonic acid can be metabolized through 5’-lipoxygenases and cytochrome P-450 pathways to generate leukotrienes and epoxides, respectively. • Phospholipase A2 can be inhibited by corticosteroids; • Cox1 by non-steroidal anti-inflammatory drugs (NSAIDS); and the 5-lipoxygenase pathway by nordihydroguaiaretic acid (NDGA).

13. True or False? • Prostaglandin analogs enhance outflow rather than blocking the formation of aqueous humor. • CA-inhibitors, b-blockers and a2 agonists decrease the formation of aqueous humor. • NSAIDs bind irreversibly to cyclooxygenases blocking biosynthesis of prostaglandins from arachidonic acid. • COX1 is widely and constitutively expressed. • COX2 is expressed at low levels in normal physiologic conditions and upregulated in response to pro-inflammatory signals. • COX2 inhibitors (Vioxx, Celebrex, Bextra, etc) increase the risks of cardiovascular toxicity and complications. • Prostaglandin receptors are G-protein coupled 7-transmembrane domain containing membrane proteins

14. Neurotransmitters and Receptors in the Iris and Ciliary Body Muscle Receptor Agonists Sphincter Cholinergic Ach, Muscarine (&Ciliary) Nicotine Dilator Adrenergic Norepinephrine, Phenylephrine… • Sphincter and ciliary muscles contain cholinergic muscarinic type receptors and are innervated by the parasympathetic third cranial oculomotor nerve. • Dilator muscles contain a-adrenergic type receptors and are innervated by sympathetic nerves from the superior cervical ganglion. Adrenergic impulses are transmitted to the muscles by norepinephrine • Irideal muscles also contain sensory nerves. Sensory neurotransmitters substance P and calcitonin gene-related peptide (cgrp) may regulate inflammatory reactions and irideal muscle tone.

15. Effect of miotic and mydriatic agents on iris and ciliary muscles Muscle Receptor Agonists Sphincter Cholinergic Ach, Muscarine (&Ciliary) Nicotine Dilator Adrenergic Norepinephrine, Phenylephrine… Miotic agents Cholinergic agonists stimulate the sphincter (Constrict the pupil) and result in increased accommodation. Adrenergic blockers block the dilator. Mydriatic agents Adrenergic agonists stimulate the dilator (Dilate the pupil) Cholinergic blockers block the sphincter and act as cycloplegics by blocking ciliary muscles.

16. Mode of action of anti-glaucoma agents

17. The choroid • Posterior segment of the uveal tract, located between retina and sclera • Provides nutrients and oxygen to the outer layers of the retina • Blood from the choroidal vessels drains via the four vortex veins • Choroid is bounded by Bruch's membrane and the sclera Retinal Pigment Epithelium Bruch’s Membrane Choriocapillaris Medium Choroidal Vessels Large Choroidal Vessels Suprachoroidal space Sclera

18. Lens • Lens consists of an external capsule, anterior single cell layered epithelium, equatorial differentiating zone, and the fiber cells in the rest of the tissue. • Lens fiber cells are protein-filled sacks lacking nucleus and organelles. • Lens nucleus consists of the central fiber cells produced from embryonic through adolescence stage (among the oldest cells in the body). • Lens cortex consists of the outer fibers laid down after adolescence. New fiber cells are added to the lens margin throughout life.

19. Lens Fiber Cell Membranes • Tightly packed with fairly low fluidity • High amount of saturated fatty acids • High cholesterol:phospholipid ratio, and a • High concentration of sphingomyelin • Lipids contribute about 1% of total lens mass Name the major lens-specific integral membrane protein Aquaporin-0 or Major Intrinsic Protein (Aqp0 or MIP).

20. Ionic Balance in the Lens • A sodium-potassium ATPase pump, an intrinsic membrane protein hydrolyzes ATP, to transport Na+ out and K+ in to the lens. • Na+-K+-pumps are found primarily in the anterior surface of the lens, in the epithelium and outer, immature fiber cells. • Intercellular Communication in the Lens • Through gap junctions consisting of connexin 43 in the epithelial cells and Cx-46 and -50 in the fiber cells. • MIP (Aqp0) also helps in intercellular communication. • Primary Source of Energy in the Lens • Anaerobic glycolysis is the primary source of energy in the lens. • Pentose phosphate pathway is used in oxidative stress conditions, to replenish NADPH.

21. Crystallins Abundant, stable and water-soluble proteins responsible for the transparent and refractive properties of the lens • Two critical properties of lens crystallins • Should remain water-soluble at high concentrations • Should be stable and stay active for a long time • Two classes of crystallins • Ubiquitous (Present in all vertebrates) • e.g.,a-Crystallinandb/g-crystallins • Taxon-specific (Different in different species) • e.g., e-crystallin • What is the native molecular mass ofa-crystallin? • Larger than 600kDa; can be as high as 1500-2000kDa. • aA- andaB-crystallinsare both around 20kDa monomers. • a-crystallin is a super-aggregate of these monomers.

22. How is spherical aberration avoided by the lens? Glass bead in water Fish lens in water Decreasing gradient of refractive index from the centre to the periphery of the lens resulting from continual lens development minimizes the spherical aberration

23. True/False • b- and g-crystallins are structurally related. • True • aB-crystallin is a widely and constitutively expressed member of the small heat shock proteins family and is inducible by heat and other forms of stress. • True • g-crystallin tends to be concentrated in the nuclear region of the lens, as it is abundantly expressed early in development. • True • Most taxon-specific crystallins are oxidoreductases which bind pyridine nucleotides. Reduced nucleotides absorb UV light, protecting the retina from oxidative damage. • True • In the normal lens, concentrations of sodium are low (~10mMol/L) and potassium, high (~120mMol/L) relative to aqueous humor, which contains high sodium (~150mMol/L) and low potassium (~5mMol/L). • True • The lens displays a smooth gradient of refractive index, which is lowest in the oldest cells in the center and the highest in the newest cells at the periphery. • False. Refractive index is the highest at the center and the lowest at the periphery • aA-crystallin has chaperone-like activity, which is absent in the aB-crystallin. • False. aB-crystallin also has chaperone like activity.

24. True/False 1. The lens is avascular and nourished by diffusion from the aqueous and vitreous. True 2. The lens capsule is thicker at the anterior, compared to the posterior of the lens. True 3. Lens epithelial cells are metabolically active and regulate the water and ion balance of the entire lens. True 4. Elimination of cellular organelles is necessary to reduce light scatter. True 5. Adult lens is surrounded by a single celled epithelial layer. False 6. Mature lens fibers support active transcription. False

25. Questions/Comments? Room 1025, EEI Phone: 412-802-6437 Swamynathansk@upmc.edu