Anatomy of the Eye: Tarsal Plates, Conjunctiva, Iris

Tarsal Plates

• Composed of dense connective tissue
• Collagen fibrils of the tissue are uniform in size and run vertically and horizontally
• Contain Meibomian glands for tear production
• Support eyelid shape, eye protection and facilitate eye movement

Conjunctiva

• Composed of two layers: stratified epithelium and connective tissue stroma
• Epithelium continuous with skin epithelium at the mucocutaneous junction of the lid margin
• Epithelium is 2-3 cells thick in the upper eyelid and 3-4 cells thick in the lower eyelid
• Epithelium changes to stratified squamous near the limbus
• Connective tissue layer provides support

Iris

• Iris arteries originate from the major circle of the iris located in the ciliary body near the iris root
• Iris vessels follow a radial course from the iris root to the pupil margin
• Iris capillaries are not fenestrated and form part of the blood-aqueous barrier
• Iris stroma is continuous with the stroma of the ciliary body
• Sphincter muscle is a circular muscle located in the pupillary zone of the stroma
• Sphincter muscle contraction constricts the pupil (miosis) and is innervated by the parasympathetic system

Anterior Epithelium and Dilator Muscle

• Anterior iris epithelium is composed of myoepithelial cells
• Apical portion is pigmented cuboidal epithelium
• Basal portion contains elongated, contractile, smooth muscle processes
• Dilator muscle extends from the iris root to below the midpoint of the sphincter
• Dilator muscle fibers are arranged radially and contraction enlarges the pupil (mydriasis)
• Dilator muscle is sympathetically innervated

Posterior Epithelium

• Posterior iris epithelium is a single layer of heavily pigmented, columnar cells
• Posterior iris epithelium loses pigment as it continues into the ciliary body as the nonpigmented epithelium
• Posterior iris epithelium is continuous with the pigmented epithelium of the ciliary body at the pupillary margin, forming the pigmented pupillary ruff

Lens

• Lens capsule is a thin, acellular membrane surrounding the lens
• Lens contains crystallins which contribute to the refractive index gradient
• Crystallin concentration is higher in the nucleus than in the cortex
• Anterior lens epithelium is involved in capsule secretion and metabolic transport
• Germinal zone near the equator is where cell division occurs and new lens fibers are created
• Lens fibers elongate and differentiate, forming concentric layers of the lens

Ultraviolet Radiation

• The cornea absorbs wavelengths below 300 nm
• The lens absorbs wavelengths between 300 and 400 nm
• The retina receives wavelengths greater than 400 nm
• Lens absorbs almost all UV light and unstable free radicals cause molecular changes
• Lens epithelium is susceptible to damage from free radicals

Ciliary Body

• Ciliary body is a ring-shaped structure
• Non-pigmented ciliary epithelium is continuous with the posterior iris epithelium
• Lateral walls of non-pigmented epithelial cells are joined by desmosomes, gap junctions and zonula occludens: these form part of the blood-aqueous barrier
• Non-pigmented epithelial cells are involved in secretion of aqueous humor and act as a diffusion barrier between blood and aqueous humor

Retina

• Retinal pigment epithelium (RPE) is the outermost retinal layer and is a single cell layer
• RPE cells contain melanin granules that extend from the apical to the middle portion of the cell
• RPE cells become larger and more cuboidal as the layer nears the ora serrata
• RPE cells are involved in light absorption and the visual cycle

Explore the intricate structures of the eye, including the tarsal plates, conjunctiva, and iris. This quiz covers the composition, functions, and significance of each component in eye anatomy. Test your knowledge and understanding of these essential features.