Learning Objectives•Understand basic form and function of the cytoskeleton•Distinguish microtubules, intermediate filaments, microfilaments•Understand cilia and flagella•Know the importance of the cilium in photoreceptor developmentTwo ‘Categories’ of Organelle1. Membranouse.g.Plasma membraneNucleusMitochondria2. Non-Membranouse.g.CytoskeletonRibosomesThe cytoskeleton consists of three kinds of protein filaments:•Microfilaments (also called actin filaments) •Intermediate filaments •MicrotubulesCytoskeleton: an elaborate arrangement of protein fibers that serve such functions as: •definition and maintenance of cell shape •provides mechanical strength •locomotion •chromosome separation in mitosis and meiosis•intracellular transport of organellesCytoskeletonA. Microfilaments -strands of protein(actin)1. Connect organelles to membranes2. Influence cell motility and shapeB. Intermediate filaments - egkeratins1. Structural stabilityC. Microtubules - strands of tubulin1. Influence cell structure and shape2. Motility - organelle movement - cilia/flagel•Made of the protein actin•Actin monomers polymerize to form long, thin fibers.•~ 6-8 nm diameterActin filaments (microfilaments)•form a band just beneath the plasma membrane •provide mechanical strength•link membrane proteins (e.g., cell surface receptors) to cytoplasmic proteins •anchor the centrosomes at opposite poles of the cell during mitosis •pinches dividing animal cells apart during cytokinesis•Organization and function of microvilli•generate locomotion in cells such as white blood cells and the amoeba •As “thin filaments” actin interacts with myosin ("thick") filaments to generate force in skeletal muscleCopyright ©2009 American Physiological SocietyMcDonough, A. A. Motoring down the microvilli. Am J Physiol Cell Physiol 297: C1331-C1332 2009;Apical region of a renal (kidney) proximal tubule cell illustrating dense apical microvilli10 nm in diameterProvide a supporting framework within the cell.Intermediate filamentsSeveral types .•keratins are found in epithelial cells •nuclear lamins form a meshwork that stabilizes the inner membrane of the nuclear envelope•neurofilaments strengthen the long axons of neurons •vimentins provide mechanical strength to muscle (and other) cells.~ 25 nm hollow cylinders made by a ring of 13 "protofilaments" Built from dimers of alpha tubulin and beta tubulinA. Centrioles/CentrosomesCell motility/Cell divisionCentriole:Barrel-shaped microtubule structuremade from 9 microtubule tripletsCentrosome:Orthogonal arrangement of two centriolesMicrotubules are dynamicThey grow by the polymerization of tubulin dimers (powered by the hydrolysis of GTP)They shrink with the release of tubulin dimers (depolymerization).The growing end is termed the “plus” end; the shrinking end is termed “minus”.Microtubules participate in a number of cell activities that involve motion. Cells have protein "motors" that move along the microtubule using from ATP. There are two major groups of microtubule motors:kinesins (most of these move toward the plus end of the microtubules) dyneins (which move toward the minus end).Cilia and FlagellaCilia and flagella are built from arrays of microtubulesCilia and FlagellaCilia and flagella are built from arrays of microtubulesFlagella are whip-like structures that undulate to move cells. (e.g. sperm motility)Cilia are hair-like structures that can beat in synchrony. Cilia sweep fluids across cells in the trachea and Fallopian tubes.Flagella and cilia have a 9 + 2 or sometimes 9 + 4, or 9 + 0 arrangement of microtubules. Cross-sections of sperm tail (flagella). (The Biology Project, U of Arizona 1977)•9 fused pairs of microtubules on the outside of a cylinder•2 unfused microtubules in the center.Annual ReviewsThe primary cilium enables the cell to respond to mechanical stimuliRetinascienceblogs.comThe photoreceptor outer segment is a specialized ciliumInsinna and Besharse Dev Dyn.2008 Aug;237(8):1982-92.Insinna and Besharse Dev Dyn. 2008 Aug;237(8):1982-92.Cilium in a developing rod photoreceptorSteinberg et al 1980 J. Comp. Neurol. 190:501-18Diagram of photoreceptor disc surface formation by outgrowth of plasma membrane from the ciliumConnecting cilium between rod inner and outer segment (9 + 0)Insinna and Besharse Dev Dyn.2008 Aug;237(8):1982-92.Insinna and Besharse Dev Dyn.2008 Aug;237(8):1982-92.Similarities between cilia in c. elegans and rodsInsinna and Besharse Dev Dyn.2008 Aug;237(8):1982-92.Failure of OS formation in zebrafish embryos depleted of Kinesinprotein Kif17. a. Normal. b. Mutant1999. Mutations in a gene called RP-1 are associated with 3% of retinitis pigmentosa cases. The gene is retina-specific. Its function is unknown.Pierce et al. (1999) Nature Genetics 22:2482004. RP-1 is discovered to be a photoreceptor cilium-associated protein.Liu, Zou and Pierce (2004) J Neurosci