CYTOSKELETON

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CYTOSKELETON
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Actin in terminal web and microvilli cores

• Kidney and the small intestine
• Web of actin and associated proteins help to
  shorten and then lengthen the microvilli
• keeps the surface from becoming clogged

Freeze fracture/deep etch SEM
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Cytoskeleton

• Gives the cell mechanical strength, controls its
  shape, drives and guides its movement
• Four classes
• Muscle thick filament
• Microfilaments
• Intermediate filaments
• Microtubules

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Muscle thick filament

• Composed of bundles of the contractile protein
  myosin
• Large amounts in muscle cells, lesser amounts in
  other cells

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Comparison of cytoskeletal components
Rope-like bars Long, hollow cylinders
Helical polymers of actin about 10 nm made
from tubulin flexible, concentrated in
about 25nm the cortex of the
cell about 7nm
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Appearance of cytoskeletal components in cultured
fibroblast cell
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Ultrastructure of cytoskeletal components
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Microfilaments

• Are the thinnest at 7nm in diameter
• Also called actin
• Ubiquitous, multifunctional
• Responsible for cell crawling
• Needed to produce contractile forces
• Interacts with myosin for muscle contraction

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Placement of actin in cells
Contractile ring In dividing cells
Microvilli cores And terminal web
Leading edge of Motile cell
Stress fibers And cell cortex
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Actin bundles

• Actin filaments are thin and flexible
• Usually found in bundles rather than individually
• Usually associated with the plasma membrane

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Core of microvillus and terminal web
TW terminal web of actin
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Arrangement of actin in moving cell

• Cell pushes out in front
• Protrusions adhere to surface
• Rest of cell drags itself forward
• Can see sheet-like lamellipodia, and
• Thin, stiff filipodia

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Intermediate Filaments

• Strengthen cell against mechanical stress
• Keratin is found in epithelial cells
• Vimentin is found in connective tissue, muscle
  and support cells of the nervous system
• Neurofilaments are found in nerve cells

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Keratinocyte of epidermis

• Presence of desmosomes and tonofilaments help
  keep the skin strong in the presence of
  mechanical stress

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Epidermolytic syndromes

• Genetic diseases from mutations in the keratin
  genes.
• Deformed keratin leaves skin highly vulnerable to
  mechanical injury and even gentle pressure can
  rupture the cells, causing skin to blister.

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Microtubules

• Most rigid of cytoskeletal elements
• May occur singly or in groups
• Responsible for movement of materials within cell
• Gives cells shape
• Form cilia and flagella
• Helps cells divide

Negative stain
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Placement of tubulin in non-metaphase and
metaphase cell

• Microtubules help with intracellular traffic in
  non-dividing cell
• Aid in splitting apart sister chromatids during
  cell division

Fluorescently labeled antibody against tubulin
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Microtubules, mitotic spindle and centrioles

• Centrioles organize the microtubules for cell
  division

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Treating cancer by halting cell division

• Colchicine, colcemid, vincristine and vinblastine
• Bind to tubulin and inhibit mt polymerization
• This blocks mitosis
• Taxol
• Stabilizes microtubules, preventing disassembly
• Also blocks cell division

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Cilia and Flagellum

• Microtubules grouped together for movement
• Microtubules occur as doublets and will have a
  central doublet
• Associated dynein
• This arrangement is called a 9x2 arrangement

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Cilia

• Motile structures which project in parallel rows
  from certain epithelial structures
• 7-10 mm in length (may be up to half cell length)
• 300 cilia per cell
• Requires ATP from mitochondria for movement

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Cilia

• 9X2 doublet arrangement and other movement
  machinery
• Beating movement of cilia is usually for
  cleansing and movement purposes along the surface
  of epithelial cells

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Cilia in cross and longitudinal sections

• Cilia and flagella must attach in the cell via
  the basal body

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Cilia of rod or cone cell in eye

• Outer segment considered a highly modified cilium
  due to presence of basal body and cilia like
  arrangement of microtubules
• Function support (?)

Outer segment
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Microtubules help shape cells

• Platelets
• Neuronal axons and dendrites
• Sperm head

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Kartageners syndrome

• Autosomal recessive disorder in males
• Defective or absent dynein in cilia flagella
• Bronchiectasis persistent respiratory
  infections
• Permanent dilation of bronchi or bronchioles
• Sterility
• Inversion of vicera
• Heart on right side, liver on left
• Ciliary movement may be required during
  development for organ placement