Lecture 8: Lens Anatomy & Physiology PDF
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This document provides learning objectives and detailed descriptions of the major functions, structure, and dimensions of the human lens. The document also covers accommodative abilities and gradient power index relating to the lens and other relevant layers. It is likely part of a larger educational resource.
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Learning Objectives L ens Be able to describe the major functions of the lens Transmit visible light Refract and focus light onto retina Malleable with variable refractive power ○ Contributes to ~18-20D of total power (⅓) → total = 60D (...
Learning Objectives L ens Be able to describe the major functions of the lens Transmit visible light Refract and focus light onto retina Malleable with variable refractive power ○ Contributes to ~18-20D of total power (⅓) → total = 60D (cornea + lens) Main UV radiation filter (absorbs short wavelengths of light) ○ 50% transmission at 390nm ○ 0% transmission at 360nm e able to describe and identify the normal dimensions of the lens (shape, anterior and posterior B radius of curvature, poles, lens thickness, lens diameter, equator) Shape: biconvex Anterior radius of curvature: 8-14nm Posterior radius of curvature: 5-8 nm (steeper/more convex) Poles: 3.5-5mm thick (center of anterior and posterior surfaces) Lens thickness: increases with age; 0.023 nm/year Lens diameter: birth = 6.5mm; adult = 10mm (nasal to temporal distance; grows throughout life) Equator: largest lens circumference (outermost edges) Be able to describe the accommodative abilities of the eye and its gradient power index Gradient power index ○ n increases from anterior surface to center ○ Index of fraction decreases from center of lens towards posterior surface ○ Higher crystallin concentration = higher n Accommodation ○ Unaccommodated = P ~20D ○ Increase in refractive power → steepening of posterior and anterior surfaces during accommodation ○ Amplitude decreased with age (8-12yo: ~14D; 50yo: ~0D) Be able to identify and describe the layers of the lens: Lens Capsule ○ Outermost layer ○ Transparent and acellular ○ Modified BM, covers entire lens secreted by lens epithelium cell Provides tructure, zonules attach to lens ○ Collagen type 4 ○ Thicker where zonules attach; thinnest at posterior surface Forms protective barrier to bacteria and inflammatory cells ○ ○ Anterior produced by anterior lens epithelium Lens Epithelium ○ Only in anterior surface ○ Metabolically active ○ Single layer of simple cuboidal epithelium over anterior surface ○ Apical: inner, near lens fibers ○ Basal: outer, towards lens capsule ○ Posterior = present in embryos; form primary lens fibers ○ Columnar towards equator; covert to lens fibers ○ Secrete anterior lens capsule (BM) throughout life ○ Transport of nutrients and waste (Na+/K+ pumps) ○ Zones: Central: Flattened and hexagonal; anterior surface Germinal/proliferative: Columnar and smaller in area, higher density New cells generated here → migrate towards equator to become new lens fibers ⇒ ⇒ Main area of cell division cell mitosis (create new fibers secondary lens fibers) Transition zone and equator Elongate and rotate, parallel to cortical surface ○ Lens bow = nucleus move anteriorly and form this Lens Fibers ○ Major content of the lens ○ Innermost layer ○ Cortex (outermost layer of fibers), nucleus (innermost layer of fibers) ○ Single fiber → pole to pole on each equatorial side ○ Where fibers meet = sutures formed ⇒ ○ Junctions: (attach fibers laterally) allows for smooth movement Ball-and-socket Tongue-in-groove ○ Crystallins = ~40% of fiber’s weight Be able to describe the relationship between the lens with the zonules of Zinn Attachment side = lens capsule Helps mold shape of lens when accommodating ○ No zonules = capsule would return to spherical shape Zonules = suspension system Zonular lamellae = where zonules insert and merge into capsule, near equator Be able to identify and describe division of the lens: Embryonic nucleus ○ Primary lens fibers (from posterior lens epithelium) ○ First 2 weeks - 2 months of embryonic development ○ Highest index of refraction Fetal Nucleus ○ Secondary lens fibers; grow around embryonic nucleus Meet in 3 branches → Y sutures Anterior: upright Posterior: inverted Lens sutures = where lens fibers meet at poles Asymmetric with age (grows more branches) ○ 2 months of pregnancy - birth Adult Nucleus ○ From secondary lens fibers ○ Birth - sexual maturation ○ Stellate sutures Lens Cortex ○ From secondary lens fibers ○ Sexual maturation - death ○ Stellate sutures ○ Continues to form → epithelial cell mitosis in germinal region Be able to describe how the ciliary body and lens interact during accommodation Unaccommodated ○ Viewing distant object ○ CM relaxed ○ Diameter of ciliary ring = large ○ Lens stretches/flattens (anterior lens zonules → tension on lens) Accommodated ○ Stimulation of cones due to retinal blue ○ Increased refractive power to focus image on retina ○ CM contracts (towards lens) ○ Diameter of ciliary ring decreases ○ Tension of zonules on lens decreases ○ Lens returns to spherical shape → increase in dioptric power Be able to describe the clinical implications of the lens changes due to age Presbyopia ○ Decreased ability of lens to focus at near with age Theories: Lens capsule = thicker and less elastic with age L ens nucleus = more rigid Anterior shift of lens = more tension on zonules = difficult to accommodate Cataracts: Nuclear Sclerosis, Cortical, Posterior Subcapsular ○ Opacification of the lens ○ Nuclear: Forms in the nucleus (center of lens) Natural aging changes ○ Cortical Spoke-like pattern, in cortex Spokes extend outer to inner (center of lens) ○ Posterior subcapsular Beneath posterior capsule Significant vision decrease Blocks line of sight ⇒ Systemic condition diabetes or steroid-use itreous V Be able to describe the major functions of the vitreous Mechanical support for surrounding tissues Shock absorber due to viscoelastic properties ○ Protects retina in rapid eye movement and physical activity Transmits and refracts lights Storage area for metabolites for retina and lens ○ Avenue for movement of substances in the eye Be able to list the major composition of the vitreous Water (98-99%) Collagen (Type 2) Hyaluronic acid (GAG) Other: salts, soluble proteins, glucose, lactic acid, ions Be able to describe the vitreous zones: Vitreous Cortex (Outer Zone) ○ Outermost zone/hyaloid surface ○ Anterior cortex Anterior to vitreous base Surrounded by CB, posterior chamber, lens ○ Posterior cortex Posterior to vitreous base Attaches to perpapillary hole (ONH), premacular “hole”, prevascular fissures Intermediate Zone Surrounds Cloquet’s Canal ○ ○ Begins at anterior cortex ○ Inserts into posterior cortex ○ Less collagen compared to outer cortex Cloquet Canal ○ Part of the retolental tract ○ Center of vitreous ○ Remnant of hyaloid artery and primary vitreous ○ Termination = Area of Martegiani Be able to describe the locations and function of the vitreal attachment sights Vitreous base → STRONGEST ○ Attached to CB (pars plana) and peripheral retinal (ILM) ○ Dense collagen network; at transitional zone between ora serrata and pars plana Posterior lens (retrolental ligament of Wieger) ○ Between posterior lens and anterior vitreous ○ Strength decreases with age Optic disc ○ Peripapillary circular adhesion ○ Strength decreases with age Macula ○ Annular area of adhesion (ring attachment) ○ Vitreomacular adhesion or traction Retinal vessels → WEAKEST ○ Vitreous strands extend and attach to BV Be able to identify the strongest and weakest attachment sights ○ STRONGEST: VITREOUS BASE ○ WEAKEST: RETINAL BV Be able to describe the clinical implications of the vitreous changes due to age Vitreous Syneresis ○ Liquification ○ Rearrangement of collagen fibers ○ Vitreous shrinks ⇒ ○ Increased risk of retinal detachment pulls away from attachment sites Posterior Vitreous Detachment ○ Separation of posterior vitreal cortex from ILM ○ Common in elderly population, increases with age ○ 50% symptomatic by 50 ○ Flashes/floaters ○ Retinal detachment
