Biology Reference
In-Depth Information
However, the tool is still conined to investigators who have access to powerful
computational resources.
16.5 CELLS
Dificulties in interpreting FD curves are also encountered with living cells,
which have highly anisotropic and inhomogeneous structures. Moreover, the
elastic modulus appears to vary on a region-speciic basis. This phenomenon
has been demonstrated by Hoffmann
for living chicken cardiocytes.
Mapping of these cells revealed stiff areas (Young's modulus: 100-200 kPa)
to be embedded within softer ones (Young's modulus: 5-30 kPa). Additional
dificulties are encountered during the AFM imaging of cells. These include
viability and mechanical ixation to the imaging substratum. Some cells, such
as erythrocytes, can be immobilized by attachment to a glass surface that has
been coated with poly-L-lysine. However, this treatment is not appropriate
for all cell types, since it can induce a reorganization of the membrane in
the contact area.
et al.
15
Spherical cells can sometimes be mechanically lodged
within the holes of millipore ilters
16
However, the
latter option requires the microfabrication of custom-built chambers and an
adaptation of the mathematical model of the FD curves. Another factor that
inluences the measurement of elastic modulus is cell thickness, which can
vary topographically. However, this factor can be neglected if the tip does not
indent more than 10% of the cell thickness.
17
or within microwells.
18
The processing of data that are
recorded at the cell periphery, or on lamellipodia or axons, can be particularly
taxing.
Despite these dificulties, the irst attempt to measure Young's modulus in
cells was made in 1994 by Hoh and Schoenenberger.
19
By recording successive
FD curves for canine kidney cells, these investigators monitored what occurs
after exposure to glutaraldehyde. This agent, which cross-links proteins, is
used as a chemical ixative in the preservation of biological samples. Two
years later, Radmacher
20
mapped the mechanical properties of human
platelets using the force-volume mode. This mode of AFM imaging involves
the recording of successive FD curves on a predeined area of the sample.
Each pixel represents an FD curve. The authors identiied areas of different
stiffness on the cell surface: the centrally located pseudonucleus was the
softest part (Young's modulus: 1.5-4 kPa), and the outer ilamentous zone,
which consists of actin ilaments and microtubules, was the stiffest (Young's
modulus: 10-40 kPa).
The cytoskeleton is an essential part of all eukaryotic cells. It is involved
in several basic cellular functions, such as division, vesicular transport and
et al.
21
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