Ocular Anatomy 2 Exam 1

Lens Anatomy and Physiology

• The lens is a transparent, avascular, and elliptic structure located in the posterior chamber of the eye.
• It comprises three parts: capsule, lens epithelium, and lens fibers.
• The lens is suspended from the ciliary body by zonular fibers and is malleable, allowing it to change shape during accommodation.

Lens Development

• Lens development begins on the 27th day of embryonic development, when the optic vesicles form and the adjacent surface ectoderm thickens to form the lens placode.
• The lens placode invaginates, forming the lens pit, which then becomes the lens vesicle as it separates from the surface ectoderm.
• The posterior lens epithelium differentiates into primary lens cells, which elongate anteriorly as fibers, filling the lumen and forming the embryonic nucleus.
• All lens growth after the development of the embryonic nucleus is attributed to secondary lens fibers.

Lens Composition and Structure

• The lens is composed of 1/3 protein and 2/3 water, with a pH of 6.9.
• The protein content varies among the lens, with a higher refractive index in the nucleus (1.50) and less in the outer cortical surface (1.37).
• The lens capsule is an elastic, acellular envelope that allows passage of small molecules and prevents large molecules from entering the lens.
• The lens epithelium is a single layer of cuboidal cells that secrete the anterior capsule throughout life and facilitate metabolic transport mechanisms.

Lens Fibers

• Lens fibers are formed from mitosis in the germinative zone and are initially nucleated and contain organelles.
• As they elongate, they lose their nuclei and organelles, and their cell membranes become compacted as insoluble protein in the nucleus.
• Fibers are hexagonal in shape and arranged in concentric rings, with a crescent-shaped cross-section (3 x 9 μm).
• Lens fiber cytoplasm contains a high concentration of crystallins (90%), which contribute to the gradient refractive index.

Divisions of the Lens

• The lens is divided into the embryonic nucleus, fetal nucleus, adult nucleus, and lens cortex.
• The embryonic nucleus is the center of the lens, formed by the elongating posterior epithelium of primary lens fibers.
• The fetal nucleus includes the embryonic nucleus and fibers surrounding it that are formed before birth.
• The adult nucleus includes the embryonic and fetal nuclei and fibers formed from birth to sexual maturation.
• The lens cortex contains fibers formed after sexual maturation, divided into superficial, internal, and deep zones.

Lens Sutures

• Lens sutures are formed by the lens fibers when they reach the poles and meet with other fibers in their layer.
• The anterior suture is formed by the joining of apical aspects of the fibers, forming an upright Y shape.
• The posterior suture is formed by the joining of the basal aspects of the fibers, forming an inverted Y shape.

Zonules of Zinn

• Zonules of Zinn are thread-like fibers that attach the lens to the ciliary body, also known as the suspensory ligament of the lens.
• They arise from the basement membrane of the nonpigmented ciliary epithelium in the pars plana and form two column-like structures on both sides of a ciliary process.
• Primary zonules attach to the lens, while secondary zonules attach to the primary zonules.

Accommodation

• Accommodation is the increase in refractive power of the eye when viewing a near object.
• It is achieved by the contraction of the ciliary muscle, which relaxes the zonules and increases the lens's curvature.
• The lens changes shape, increasing its refractive power, and the eye is able to focus on the near object.### Lens Physiology
• The primary function of the lens is refraction of light and transparency, which is achieved by the absence of blood vessels, few cellular organelles, and an orderly arrangement of fibers with short distances between components.
• The lens obtains nutrients from the surrounding aqueous humor and a small contribution from the vitreous.
• Metabolic activity in the lens is mostly anaerobic, with 70% of ATP production via anaerobic glycolysis, while aerobic glycolysis and the Krebs cycle are limited to the epithelium or superficial fibers with mitochondria.

Lens Metabolism

• The lens constantly pumps out water to maintain the correct constituents for optics, using an active Na/K pump that requires ATP.
• Both aerobic and anaerobic pathways require hexokinase to convert glucose to glucose-6-phosphate.
• If hexokinase is not present, glucose is converted into sorbitol (via aldose reductase), leading to an osmotic gradient that favors water movement into the lens, causing swelling, damage, and cataract formation.

Regulation of Lens Proteins

• Glutathione is the primary protector against oxidative damage and is transported into the lens by the aqueous humor or synthesized from lens epithelial cells.
• Ascorbic acid prevents oxidative damage (anti-cataract effect) and is present in the lens at a concentration of 3.5 mmol/liter or higher.

Oxidative Stress of the Lens

• Free radicals are a normal product of metabolic processes, and UV light absorption can also produce oxidative changes, causing the formation of free radicals.
• Glutathione is a reducing agent that detoxifies free radicals, preventing damage.
• Ascorbic acid also prevents oxidative damage.

Age Changes in the Lens

• There is a decrease in soluble lens proteins (alpha crystallins), ATP content, K ions, amino acids, and inositol with age.
• Glutathione activity decreases with age, leading to less detoxification of free radicals and cell damage.
• There is an increase in Ca, Na, and H2O with age, causing more permeability and disruption of ion balance.

Clinical Manifestations of Aging

• Presbyopia is a loss in accommodative ability, characterized by the inability to focus at near distances.
• Cataracts are any lens opacity, and the greatest cause of blindness, influenced by multiple factors affecting lens metabolism.

Choroid Anatomy

• The choroid is composed of the lamina fuscha (suprachoroidal lamina), choroidal stroma, choriocapillaris, and Bruch's membrane.
• The choroidal stroma contains melanocytes, fibroblasts, macrophages, lymphocytes, and mast cells, with collagen fibrils arranged circularly around the vessels.

Choroid Function

• The choroid is responsible for providing nutrients to the outer retinal layers.
• The choroidal vessels are innervated by the autonomic nervous system, with sympathetic stimulation causing vasoconstriction and decreased choroidal blood flow, while parasympathetic stimulation causes vasodilation and increased choroidal blood flow.

Ciliary Body Anatomy

• The ciliary body is composed of the supraciliaris (supraciliary lamina), ciliary muscle, ciliary stroma, and ciliary epithelium.
• The ciliary epithelium consists of two layers: the pigmented ciliary epithelium and the non-pigmented ciliary epithelium.

Ciliary Body Function

• The ciliary body is responsible for producing and secreting aqueous humor, with the non-pigmented ciliary epithelium being the site of active secretion.
• The ciliary muscle is responsible for accommodation, with contraction of the longitudinal fibers causing the lens to become more spherical and increasing its power.

Iris Anatomy

• The iris is composed of four layers: the anterior border layer, stroma and sphincter muscle, anterior epithelium and dilator muscle, and posterior epithelium.

Iris Function

• The iris acts as a diaphragm to regulate the amount of light entering the eye.

• The iris muscles are innervated separately, with the sphincter muscle being parasympathetically innervated and the dilator muscle being sympathetically innervated.### Functions of the Ciliary Body

• Aqueous humor maintains the pressure and shape of the eye

• Provides a transparent and colorless refractive index

• Provides nutrition for the avascular cornea, lens, anterior vitreous, and trabecular meshwork (TM)

• Serves as a collection bin for metabolic waste products of surrounding tissues and clears out inflammatory products and blood from the globe

Aqueous Humor Composition

• Aqueous humor has 20 times greater ascorbate (Vitamin C) concentrations than plasma
• Ascorbate acts as an antioxidant to protect the cornea and lens against oxidative damage
• Aqueous humor has 200 times less protein than plasma
• Consequence of the tight junctional barrier
• Low concentration of proteins causes minimal light scatter and thus maximum light transmission

Waste Products in Aqueous Humor

• Waste products from the cornea and lens, including Cl-, amino acids, and lactate
• High concentration of lactate, a metabolic waste product of the anaerobic glycolysis of the lens and cornea

Aqueous Humor Production

• 2.5 μl of aqueous humor is produced per minute
• Aqueous humor production follows the circadian rhythm, with a higher rate during the day and a 50% decrease at night
• The osmolarity of aqueous humor is slightly hyperosmotic to plasma
• Aqueous humor viscosity is relative to water (1.025-1.040)

Blood-Aqueous Barrier

• Controls the secreted aqueous humor
• Fenestrated capillaries permit large molecules to exit the blood
• Tight zonular junctions of the non-pigmented epithelium (NPE) prevent molecules from passing between the cells
• Forcing molecules to pass through the cell to enter the posterior chamber
• Protein is well-controlled, with a small content in the aqueous humor

Iris

• Iris is freely permeated by the aqueous humor
• Iris capillaries are not fenestrated, preventing large molecules from leaking out of the iris blood vessels
• The zonula occludens junction in the endothelial cell maintains the barrier function

Choroid

• Provides nutrients to the outer retina
• Is an egress for catabolites from the retina, passing through Bruch's membrane into the choriocapillaris
• Dark pigmented to absorb excess light (as in the RPE layer)
• Suprachoroidal space provides a pathway for the posterior vessels and nerves that supply the anterior segment

Blood Supply to the Uvea

• Short posterior ciliary arteries enter around the optic nerve and form the choroidal vessels
• Long posterior ciliary arteries and anterior ciliary arteries join to form the major circle of the iris, supplying the iris and ciliary body
• Venous return for most of the uvea is through the vortex veins

Uveal Innervation

• Sensory innervation is provided by the nasociliary branch of the trigeminal nerve
• Sympathetic fibers from the superior cervical ganglion via the ophthalmic and short ciliary nerves innervate the choroidal blood vessels (vasoconstriction), iris dilator (contraction), and ciliary muscle (relaxation)
• Parasympathetic fibers from the ciliary ganglion innervate the ciliary muscle (contraction), iris sphincter muscle (contraction), and choroidal vessels (vasodilatation)

Aging Changes

• Iris: loss of pigment from the epithelium, pigment deposition, and dilator muscle atrophy
• Ciliary body: connective tissue increase within the layers of the ciliary muscle, ciliary muscle contraction diminution with age, and aqueous humor formation decrease
• Choroid: drusen, decreased choroidal blood flow, and decreased choroidal thickness

Clinical Applications

• Iridodialysis: blunt trauma to the eye or head, resulting in a thin iris root tear away from the ciliary body
• Iris synechiae: abnormal attachment between the iris surface and another structure
• Pigmentary dispersion syndrome: pigment granules are shed from the posterior iris surface, dispersed into the anterior chamber, and deposited on the iris, lens, corneal endothelium, and TM
• Presbyopia: loss of the ability to accommodate due to normal age-related change
• Thyndall phenomenon: usually, the beam of light in the anterior chamber is dark, but with the presence of particles, it will be visible due to uveal inflammation, flare, and cells