Abstract
Precise positioning of the mitotic spindle is important for specifying the plane of cell division, which in turn determines how the cytoplasmic contents of the mother cell are partitioned into the daughter cells, and how the daughters are positioned within the tissue. During metaphase in the early Caenorhabditis 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 ∼1000, 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 before the onset of anaphase.
Highlights
During cell division, the correct positioning and orientation of the mitotic spindle are important for the developmental fate of the daughter cells
This is because the cleavage furrow usually bisects the spindle [1,2] and thereby determines, in part, how the cytoplasmic contents are distributed to the two daughter cells [3,4,5]
We focused our attention on the transverse position because the axial position of the spindle is not stationary: it moves along the A-P axis toward the posterior pole during metaphase (Fig. 1 E, SDs give the axial (Sx))
Summary
The correct positioning and orientation of the mitotic spindle are important for the developmental fate of the daughter cells. This is because the cleavage furrow usually bisects the spindle [1,2] and thereby determines, in part, how the cytoplasmic contents are distributed to the two daughter cells [3,4,5]. We have asked: after the initial positioning of the spindle at the cell center early in mitosis, how accurately, precisely, and stably is the position maintained during metaphase? We mean how much variability is there from cell to cell, and by stability, we mean how well do individual cells maintain their spindle position and orientation during metaphase
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