The Cytoskeleton

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The Cytoskeleton
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• The cytoskeleton is composed of three types of
  fibrous structures
• Microtubules
• Intermediate filaments
• Microfilaments

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• Structural support
• Internal framework maintaining position of the
  organelles
• Machinery required for movement of materials and
  organelles within cells
• Force generating elements responsible for
  movement of cells from one place to another

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• Microtubules
• Hollow
• Tubular structures 25nm in diameter
• Assembled from protein tubulin
• Form cytoskeleton, mitotic spindle, centrioles,
  core of cilia and flagella

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• The tubulin consists of alpha-beta tubulin
  heterodimers arranged in rows (protofilaments)
• Microtubules have a plus and minus ends.
• Typically the minus is for anchoring and the plus
  is for growing
• Microtubule Associated Proteins (MAPs) increase
  the stability and promote microtutule assembly

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• The motor proteins kinesins, dyneins move along
  microtubules and myosins move along
  microfilaments
• All require ATP for movement
• Typically kinesins and dyneins move along
  microtubules in opposite directions
• Kinesins towards plus end and dyneins towards
  minus end

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• Kinensin is thought to be involved in movement of
  ER derived vesicles, endosomes, lysosomes, and
  secretory granules and movement within axons.
• Nucleation of microtubules occurs at MTOC
  (microtubule organizing centers)
• For example the centrosome. A pair of centrioles
  are surrounded by electron dense pericentriolar
  material.

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• Centrioles contain nine evenly spaced fibrils,
  each containing three microtubules, A, B and C
  tubules.
• A tubule is connected to the center of the
  centriole by a radial spoke.
• Centrioles are in pairs and at right angles to
  each other.