Cytoskeleton Tutorial
Microtubules, microfilaments & intermediate
filaments
The Cytoskeleton
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The cytoskeleton is unique to eukaryotic cells. It is a
dynamic three-dimensional structure that fills the cytoplasm. This structure
acts as both muscle and skeleton, for movement and stability. The long fibers
of the cytoskeleton are polymers of subunits. The primary types of fibers
comprising the cytoskeleton are microfilaments, microtubules, and
intermediate filaments.
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Microfilaments
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Microfilaments are fine, thread-like protein fibers, 3-6
nm in diameter. They are composed predominantly of a contractile protein
called actin, which is the most abundant cellular protein. Microfilaments'
association with the protein myosin is responsible for muscle contraction.
Microfilaments can also carry out cellular movements including gliding,
contraction, and cytokinesis.
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Microtubules
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Microtubules are cylindrical tubes, 20-25 nm in diameter.
They are composed of subunits of the protein tubulin--these subunits are
termed alpha and beta. Microtubules act as a scaffold to determine cell
shape, and provide a set of "tracks" for cell organelles and
vesicles to move on. Microtubules also form the spindle fibers for separating
chromosomes during mitosis. When arranged in geometric patterns inside
flagella and cilia, they are used for locomotion.
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Intermediate Filaments
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Intermediate filaments are about 10 nm diameter and
provide tensile strength for the cell.
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Examples of the cytoskeleton
in epithelial cells
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In the epithelial (skin) cells of the intestine, all three
types of fibers are present. Microfilaments project into the villi, giving
shape to the cell surface. Microtubules grow out of the centrosome to the
cell periphery. Intermediate filaments connect adjacent cells through
desmosomes.
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External
cell movement
Cellular movement
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Cellular movement is accomplished by cilia and flagella.
Cilia are hair-like structures that can beat in synchrony causing the
movement of unicellular paramecium. Cilia are also found in specialize
linings in eukaryotes. For example, cilia sweep fluids past stationary cells
in the lining of trachea and tubes of female oviduct. Flagella are whip-like appendages that undulate to move cells. They are longer than cilia, but have similar internal structures made of microtubules. Prokaryotic and eukaryotic flagella differ greatly.
Both flagella and cilia have a 9 + 2 arrangement of
microtubules. This arrangement refers to the 9 fused pairs of microtubules on
the outside of a cylinder, and the 2 unfused microtubules in the center.
Dynein "arms" attached to the microtubules serve as the molecular
motors. Defective dynein arms cause male infertility and also lead to
respiratory tract and sinus problems. Below are two cross-sections of sperm
tails (flagella).
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