Biology Reference
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et al.
31
have studied the mechanical properties of normal and
cancerous epithelial bladder cells. Young's modulus for the normal cells was
found to be about one order of magnitude higher than that for the cancerous
ones. The changes in the elastic properties of the cancerous cells were
attributed to a reorganization of the cytoskeleton.
Lekka
16.6 CELLULAR STRUCTURES
The mechanical properties of several different isolated cell structures have
been studied using the AFM. The relevance of such studies lies in the fact that
subcellular structures, such as the nucleus or components of the cytoskeleton,
inluence the mechanical properties of the cell as a whole. The cytoskeleton,
in particular, has an important inluence on the properties of a cell. A study
of this structural component is of particular interest in the pharmacological
industry, since several anti-mitotic drugs that are used in cancer therapy
interfere with components of the cytoskeleton.
Among the cytoskeletal components, microtubules are the largest, with a
diameter of about 20 nm. During cell division, they guide in the segregation of
the chromosomes within the mitotic and meiotic spindles. They also serve as
tracks for the intracellular movement of several components, such as vesicles
and mitochondria. Their mechanical properties were irst measured by AFM
imaging in 1995.
For these experiments, the microtubules were deposited
upon a substratum of mica, and the FD curves were recorded in the absence
or presence of increasing concentrations of glutaraldehyde. Young's modulus
was calculated to lie between 1 and 12 MPa. However, it later appeared that
shear rather than Young's modulus had been calculated. In 2002, Young's
modulus for suspended microtubules was measured for the irst time in the
AFM.
32
In these experiments, the shear modulus was found to be about 1.4
MPa. Subsequently, inite-element modelling of AFM measurements revealed
the Young's modulus of microtubules to be about 0.8 GPa.
33
Measurements
made by the same authors using the plastic regime (irreversible deformation)
yielded a crude estimate of the rupture force, which was in the order of several
hundred piconewtons.
Intermediate ilaments are another important component of the
cytoskeleton. They play an essential role in maintaining the shape and
the mechanical stability of the cells. Using AFM, their bending and sliding
properties have been measured by the deformation of single vimentin
ilaments that were suspended over a porous membrane;
34
35
Young's modulus
was determined to be at least 0.9 GPa.
The mechanical properties of chromosomes have also been measured by
AFM.
36,37
In the irst study,
36
Young's modulus was found to vary between 0.05
and 0.1 MPa, depending on the nature of the imaging buffer: chromosomes
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