Lecture 10: Lens Structure and Function PDF
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This lecture covers the structure, functions, and composition of lens tissues, cells, and associated proteins. The lecture also details the function of proteins in the lens and maintenance mechanisms within the lens cells.
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Learning Objectives Identify the lens capsule, epithelium and fibers on a diagram of the lens L ist the composition and major functions of the lens capsule Composition: THICK BM ○ Slow turnover rate, deposition of matrix material Interactin...
Learning Objectives Identify the lens capsule, epithelium and fibers on a diagram of the lens L ist the composition and major functions of the lens capsule Composition: THICK BM ○ Slow turnover rate, deposition of matrix material Interacting networks of fibrous proteins ○ Laminin (heterotrimeric protein) ○ Type IV collagen (proteins linked head-to-head) ○ Nidogen (glycoprotein) ○ Perlecan (proteoglycan) Functions: Passive exchange of metabolic substrates in and wastes out Selectively filters various intermediate-sized molecules based on size and charge Maintains transparency Focuses light on retina Structure and protection utline the structure and organization of the lens epithelium O Structure: Simple cuboidal epithelium Organization: Covers anterior part of lens Connected by desmosomes and tight junctions List the functions of the lens epithelium Regulates homeostatic functions of lens Metabolic engine, sustains physiological health of lens Progenitors for new lens fibers escribe the structure, formation and organization of lens fiber cells D Structure: Long, thin, transparent cells Formation: Equatorial epithelial cells differentiate into fiber cells Form bulk of lens Organization: Connected by gap junctions Tightly packed → maintains transparency List the requirements for maintaining intact, transparent lenses Specific arrangement of cells Lack of organelles Densely packed lens crystallin proteins Proteins and metabolic activities organized to protect crystallins from damage Cytoskeleton → specific shape to cells Prevention and repair of photooxidative damage Energy metabolism = glycolysis and PPP Membrane protein channels maintain osmotic and ionic balance escribe the shape and organization of alpha crystallin proteins in the lens D Shape: Quaternary structure Doughnut-shaped Organization: Intertwining polypeptides ⇒ Spherical particles chain-like structure escribe the mechanism by which alpha crystallin proteins protect the lens from denatured protein D aggregation Molecular chaperones ○ Bind to denatured proteins in lens, prevents precipitation ○ Present denatured proteins to larger chaperones for refolding Describe the structure of beta/gamma crystallin proteins Compact, globular proteins → form dimers and multimers Tertiary structure Tightly packed escribe the role played by beta/gamma crystallin proteins in maintaining appropriate refractive D indexes across the lens Tightly packed → forms colloid (biological glass) Very stable, resists denaturation (at high T = 70-80ºC) Half-life of denaturing step = 19 years From center to equator ○ Decreasing: [macromolecule], refractive index, [gamma crystallins] escribe the composition, arrangement, location and function of the following cytoskeletal elements D in lens cells: Actin microfilaments ○ Composition: Proteins → actin, spectrin, tropomodulin, adducin ○ Arrangement: Rearrange as fiber cells elongate Maintain hexagonal shape of differentiating lens fiber cells ○ Location: Cytoplasm ○ Function: Forms terminal web to stabilize ends of lens fibers at sutures Microtubules ○ Composition: Tubulin proteins ○ Arrangement: Greater concentration in outer portion of lens Lower concentration in inner portion (center) of lens ○ Location: Accumulated on outer portion of lens ○ Function: Establish and maintain elongated shape of fiber cells Migration of chromosomes in dividing lens epithelial cells Required for transport of organelles and vesicles Intermediate filaments ○ Vimentin Composition: Arrangement: tetramer of vimentin polypeptides forms IF Location: anterior epithelial cells Function: Movement of cells into wound area Repair damage of lens epithelium ○ Filensin & Phakinin Composition: Beaded filaments (replace vimentin-containing IF) → composed of Filensin proteins and Phakinin proteins Arrangement: Location: Function: Maintains size, shape, stiffness of lens Vital for: formation of correct beaded filament structure and maintaining formed filaments in soluble state escribe the structure and function of aquaporin-0 molecules in the membranes of lens cells D Structure: Tetramer ○ F orms “thin junctions” with adjacent cells → thin junctions similar to tight junctions but ONLY for water Functions: Water channels Adhesion protein Allow circulation of nutrients, ions, waste products and water throughout avascular lens escribe the structure and function of connexin proteins in the membranes of lens cells D Structure: Hexamers ○ Forms connexon channels/gap junctions with adjacent cells Function: Allow passage of: ○ Ions, nucleotides, glucose, small peptides, aa, secondary messengers (cAMP, cGMP) Allow circulation of nutrients, ions, waste products and water throughout avascular lens utline the metabolic pathways found in lens cells, including the location of specific pathways in O specific cell types Glycolysis, PPP, TCA cycle → anterior epithelial cells Glycolysis, fermentation → cortex and nucleus lens fiber cells ○ Produce a lot of lactate due to insufficient oxygen in lens utline the challenges faced by lens cells with respect to nutrient supply, waste management, and O maintenance of cell volume Bulk of lens relies on anaerobic glycolysis & fermentation to satisfy energy requirements Cannot rely on passive diffusion to deliver nutrients or transport wastes Lack Na+/K+-ATPase and K+ channels → cannot use transport system to maintain slight negative membrane potential and control steady-state volume Describe the process of microcirculation in the lens, including The roles of active and passive transport All components/molecules involved in the process
