Biomedical Engineering Reference
In-Depth Information
a reduced cytoplasmic catastrophe rate [39]. Forces may specifically enhance
catastrophes when contact is made with the cell ends, but this is apparently
not sucient when the spontaneous catastrophe rate is not high enough in
these mutants.
Figure 4.11.
Pushing and pulling
in vitro
. Microfabricated chambers can be used
to study positioning strategies in a simplified experimental environment. (a) Mi-
crotubule pushing combined with catastrophes is sucient to center microtubule
organizing centers in microfabricated chambers (left), but fails when catastrophes
occur too infrequently leading to long buckled microtubules that destabilize the
positioning process (right) [38]. Square chambers are 20 microns on the side. (b)
Modifying the chamber technology to study the competition between pushing and
pulling forces. A sandwiched layer of gold can be used to specifically functionalize
the side walls of chambers with (in this case) minus-end directed motor proteins. (c)
Preliminary data showing how “cortical” pulling forces generated by immobilized
dynein molecules appear to repeatedly pull a centrosome off center. Counteracting
pushing forces keep redirecting the centrosome to the chamber center.
In other systems, dynamic microtubules are observed to create pulling
force in contact with the cell cortex [6]. This pulling can be mediated by
minus-end directed motor proteins, such as dynein, or by the cortical attach-
ment of shrinking microtubules as discussed earlier. In fact, pulling forces
generated at the cortex seem to be the dominant mechanism for position-
ing spindles and microtubule organizing centers in larger eukaryotic cells. In
single cell-stage
Caenorhabditis elegans
(
Celegans
) embryos, the spindle gets
asymmetrically positioned towards the posterior end of the embryo by dynein-
related pulling forces generated at the cell cortex. Laser-cutting experiments
have not only demonstrated that pulling forces are generated, but also that
the unbalance between the two sides is probably caused by a higher number
of force-generating entities at the posterior side (as opposed to an increase in
the magnitude of the forces generated at each posterior interaction site) [40].
