Cytoskeleton
Dr. Gauri Haval
Assistant Professor
Department of Zoology
Abasaheb Garware College,
Pune 411004.

• All cells have to be able to rearrange their
internal components as they grow, divide, and
adapt to changing circumstances. These
spatial and mechanical functions depend on a
remarkable system of filaments called the
cytoskeleton.
• The cytoskeleton’s varied functions depend on
the behavior of three families of protein
filaments—actin filaments, microtubules, and
intermediate filaments

Microfilaments
Microfilaments are made up of
actin filaments
• Actin microfilaments are thin
threads that function in cell
division and cell motility.
• Actin filaments determine the
shape of the cell’s surface and
are necessary for whole-cell
locomotion; they also drive the
pinching of one cell into two.
• The surface area intestinal cell
is increased by its many
microvilli, cellular extensions
reinforced by bundles of
microfilaments.
• These actin filaments are
anchored to a network of
intermediate filaments

• Microfilaments are helical polymers of
protein actin. They are flexible with a
diameter of 8 nm.
• They are organized to form bundles
forming two dimensional networks and
three dimensional gels.
• They are dispersed throughout the cell
but concentrated in cortex.
• In muscle cells, actin filaments (orange)
lie parallel to thick myosin filaments
(purple).
• Myosin acts as a motor molecule.
• The teamwork of many such sliding
filaments enables the entire muscle cell
to shorten.
• Myofibrils are highly specialized and
efficient motility machines built from
• actin and myosin filaments,
Single actin filament
Stress fibres (green) terminating in
adhesion
Striated muscle

• (b) In a crawling cell
(ameboid movement), actin
is organized into a network
in the gel-like cortex (outer
layer).
• This contraction forces the
interior fluid into the
pseudopod, where the actin
network has been
weakened.
• The pseudopod extends
until the actin reassembles
into a network.
Microfilaments and Motility

Microtubules: Largest cytoskeletal filament in the cell
• Microtubules are
structurally more complex
than microfilaments.
• Microtubules are polymers
of the protein tubulin.
• Tubulin subunit is a
heterodimer formed of two
globular proteins α tubulin
and β tubulin.
• A microtubule is a hollow
cylindrical structure built
from 13 parallel
protofilaments, each
composed of αβ-tubulin
heterodimers stacked head
to tail and then folded into
a tube.
The persistence length of a microtubule is several
millimeters, making microtubules the stiffest and
straightest structural elements found in most
animal cells. the regular, parallel orientation of
their subunits gives microtubules structural and
dynamic polarity with plus ends growing and
shrinking more rapidly.

• Many animal cells have a single,
well-defined MTOC called the
centrosome, which is located near
the nucleus and from which
microtubules are nucleated at their
minus ends, so the plus ends point
outward and continuously grow and
shrink, probing the entire three-
dimensional volume of the cell.
• Embedded in the centrosome are
the centrioles, a pair of cylindrical
structures arranged at right angles to
each other in an L-shape
configuration

• Cilia and flagella are highly specialized and efficient motility
structures built from microtubules and dynein. Both cilia and
flagella are hair like cell appendages that have a bundle of
microtubules at their core
• By their undulating motion, they enable the cells to which
they are attached to swim through liquid media
Flagella on sperm Cilia on Protozoan

The 9+2 arrangement of microtubules in a flagellum or cilium.

• The basal body anchoring the cilium or flagellum to the cell
has a ring of nine microtubule triplets.
• The nine doublets of the cilium extend into the basal body,
where each doublet joins another microtubule to form the
ring of nine triplets.
• The two central microtubules of the cilium terminate above
the basal body
• The dynein arms of one microtubule doublet grip the
adjacent doublet, pull, release, and then grip again.
• The action of the dynein arms causes the doublets to bend.
• Dynein use the energy of ATP hydrolysis to move
unidirectionally along a microtubule. The motor dynein
moves toward the minus end of microtubules, and its sliding
of axonemal microtubules underlies the beating of cilia and
flagella.

Cilia from an epithelial cell in cross section

Intermediate filaments
• In the cytoplasm of cells that are subject to mechanical
stress and are generally not found in animals that have
rigid exoskeletons, such as arthropods and
echinoderms. It seems that intermediate filaments
impart mechanical strength to tissues for the squishier
animals.
• Intermediate filaments range in diameter from 8–12
nanometers, larger than microfilaments but smaller
than microtubules
• They support cell shape and fix organelles in place
• Intermediate filaments are more permanent
cytoskeleton fixtures than the other two classes

Intermediate filaments
Types of
Intermediate filament
Component polypeptides Location
Nuclear Lamins A, B, and C Nuclear lamina (inner
lining of
nuclear envelope)
Vimentin-like Vimentin
Desmin
Glial fibrillary acidic protein
Peripherin
Many cells of mesenchymal
origin
Muscle
Glial cells
Some neurons
Epithelial Type I keratins
Type II keratins
Epithelial cells and their
derivatives
(e.g., hair and nails)
Molecular biology of the cell 6th edition

• Keratins are the most
diverse intermediate
filament family: there are
about 20 found in different
types of human epithelial
cells and about 10 more
that are specific to hair and
nails.
• Keratin filaments impart
mechanical strength to
epithelial tissues in part by
anchoring the intermediate
filaments at sites of cell–cell
contact, called
desmosomes, or cell-matrix
contact, called
hemidesmosomes.
• .
Keratin filaments (blue) in epithelial cells
Molecular biology of the cell 6th edition

• Neurofilaments
• Members of another family
of intermediate filaments,
called neurofilaments, are
found in high
concentrations along the
axons of vertebrate
neurons.
• Neurofilaments provide
strength and stability to the
long cell processes of
neurons
Molecular biology of the cell 6th Edition

• The vimentin-like filaments are a third family
of intermediate filaments.
• Desmin, a member of this family, is expressed
in skeletal, cardiac, and smooth muscle.
• Desmin play important role in stabilizing
• muscle fibers.
• one class of lamins, the A-type, together with
many proteins of the nuclear envelope, are
scaffolds for proteins that control myriad
cellular processes including transcription,
chromatin organization, and signal
transduction.

• Microtubules – function in cell division and
serve as a "temporary scaffolding" for other
organelles.
• Actin microfilaments are thin threads that
function in cell division and cell motility.
• Intermediate filaments are between the size
of the microtubules and the actin filaments.
Reference used: Molecular biology of the cell 6th Edition