Biology Reference
In-Depth Information
As was reviewed in the beginning of this section, the issue of central vs. eccentric
positioning of the centrosome has been addressed in studies on flat cultured cells
and on biochemically reconstituted microtubule cytoskeletons in artificial cham-
bers. The Maly and Maly model predicts that in a flat arrangement, the deviation of
the centrosome from the center under the action of microtubule elasticity alone
should be equal approximately to the difference of the microtubule length and the
radius of the confining boundary, and that in three dimensions it should be, approxi-
mately, twice as large. Direct comparison of this theory with experiments has to date
remained impossible. Measurements that would provide the required information
are still lacking. Complicating the comparison in the case of the reconstituted sys-
tems (Holy et al.
1997
; Faivre-Moskalenko and Dogterom
2002
) is the small num-
ber of microtubules. Due to the smallness of this number, the specific distribution of
the few microtubules around the artificial centrosome, and their specific individual
lengths affect the positioning greatly. Further, central positioning in the terminology
of the reconstitution work at least in some cases described a degree of eccentricity
that is significant in the quantitative frame of reference set by the Maly and Maly
model. In the absence of measurements, it is unclear whether the terminology of
central vs. eccentric positioning is consistent between this quantitative framework
and the one that was used qualitatively to classify the positions of centrosomes in
the flat cultured cells. The one experimental report (Euteneuer and Schliwa
1992
)
that specifies the deviation from the center that was still considered as insignificant
sets this distance at 5 μm. While the microtubule length and the cell size measure-
ments were not at the same time reported, from the images one might judge the
above magnitude of deviation as potentially comparable with the eccentric equilib-
rium predicted by the Maly and Maly model. The notion that the centrosome is
positioned in the geometric center of flat cells, contrary to the quantitative model
prediction, should be re-examined by means of measurements done in the quantita-
tive framework set by the model. More generally, the very limited comparison that
is currently possible between the quantitative theory and experiment underscores
the need for a firm theoretical basis in future work that will address the fundamental
problem of the cell body positioning within the cell boundary.
Bipolar Cell Body
The unipolar cell body considered in the last section, which is centered around the
single interphase centrosome, becomes bipolar in preparation for division. The divi-
sion of the cell body is most commonly accompanied or followed by the division of
the cell boundary, as defined in the introduction, and together they comprise the
division of the cell. The bipolar microtubule cytoskeleton of the dividing cell body
(mitotic spindle) in characteristic instances consists essentially of two asters, each
assembled around its own centrosome (spindle pole). Positioning of the mitotic
spindle through the interaction of the astral microtubules with the cell boundary
often determines whether the cell division will be symmetric or asymmetric. This
process plays a crucial role in development.
