Biology Reference
In-Depth Information
architecture of the living cell in real time, in response to locally applied
extracellular perturbations, together with quantiied measurements of
changes in cell elasticity, can provide insights into the immediate effect of
stress on the behaviour of the cell and on the mechanism in which forces are
transmitted through the cell.
11,18-20
The cellular cytoskeleton and organelles are some of the major elements
responsible for modulating and controlling the mechanical properties
of the cell. Moreover, internal remodelling and deformation of this
complex network is highly dependent of the mechanics, topography and
biochemistry of the microenvironment.
1-13
The cytoskeleton is an elaborate
network of ilamentous protein ibres spread throughout the cytoplasm. The
cytoskeleton provides mechanical stability and often regulates controlled
and dynamic mechanical processes such as migration, chromosome
separation during mitosis and muscle contractions. The cytoskeleton also
forms a network of tracks on which cargos, both membrane-bound such as
the Golgi and mitochondria and non-membrane-bound such as mRNA and
protein, can be transported.
21,22
Three major types of ilaments that make
up the cytoskeleton include the actin ilaments, intermediate ilaments (IFs)
and microtubules (MTs).
23
Actin ilaments (
Fig. 18.1a
) are typically located below the plasma
membrane and are cross-linked by a variety of proteins, including motor
proteins such as myosin, which can generate forces and perform mechanical
work. They are assembled from subunits called G-actin and are roughly 8
nm thick in diameter. The ilaments are also linked to the plasma membrane
through the Ezrin-Radixin-Moesin (ERM) proteins and membrane-
spanning integrins, allowing signals from the ECM to be transmitted to the
cytoskeleton, and vice versa.
24-27
MTs (
Fig. 18.1b
) are hollow, cylindrical
ilaments of approximately 25 nm in diameter, which are formed by the
assembly of tubulin monomers. Individual MTs originate from a centrosome
near the nucleus and can span the entire cell. They play an important role
in organelle transport and organization, in cell division and chromosome
distribution, and in mechanical stabilisation of the cell.
28
IFs (
Fig. 18.1c
),
unlike actin ilaments and MTs, are not polarised and are made of elongated
polypeptide rods that are arranged in a coiled-coil structure of about 8-
10 nm in diameter. They are located in two separate systems, one in the
nucleus and one in the cytoplasm. Their main role is believed to be that
of a passive mechanical absorber to provide structural reinforcement,
particularly in cells that need to withstand strong mechanical stress such as
epithelial cells.
Apart from the structural contribution, IFs also have cell-
type-speciic physiological roles and contribute to some gene-expression
programmes.
29
29,30
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