  Precise positioning of the mitotic spindle is important for specifying the
plane of cell division, which in turn determines how the cytoplasmic contents
are partitioned into the daughter cells, and how the daughters are positioned
within the tissue. During metaphase in the early C. elegans embryo, the spindle
is aligned and centered on the anterior-posterior axis by a
microtubule-dependent machinery that exerts restoring forces when the spindle
is displaced from the center. To investigate the accuracy and stability of
centering, we tracked the position and orientation of the mitotic spindle
during the first cell division with high temporal and spatial resolution. We
found that the precision is remarkably high: the cell-to-cell variation in the
transverse position of the center of the spindle during metaphase, as measured
by the standard deviation, was only 1.5% of the length of the short axis of the
cell. Spindle position is also very stable: the standard deviation of the
fluctuations in transverse spindle position during metaphase was only 0.5% of
the short axis of the cell. Assuming that stability is limited by fluctuations
in the number of independent motor elements such as microtubules or dyneins
underlying the centering machinery, we infer that the number is on the order of
one thousand, consistent with the several thousand of astral microtubules in
these cells. Astral microtubules grow out from the two spindle poles, make
contact with the cell cortex, and then shrink back shortly thereafter. The high
stability of centering can be accounted for quantitatively if, while making
contact with the cortex, the astral microtubules buckle as they exert
compressive, pushing forces. We thus propose that the large number of
microtubules in the asters provides a highly precise mechanism for positioning
the spindle during metaphase while assembly is completed prior to the onset of
anaphase.
