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shape changes at different time points (Wühr et al. 2010 ). By applying pressure to
Xenopus blastomeres to artificially impose a cleavage plane bisecting the elongated
cell axis, the authors found that in prophase, prior to nuclear breakdown, the form-
ing spindle (as determined by the inter-centrosomal axis) was already aligned with
the long axis of the cell, within about 5o (compared to 45o if randomly oriented).
Between anaphase and cytokinesis, when the telophase aster would be expected to
have an effect, this alignment improved slightly yet significantly, to 1–2o. These
experiments clearly show that (1) the metaphase spindle largely acquires its final
orientation at a time when astral microtubules do not reach the cortex and (2) ana-
phase asters contribute to the fine-tuning of spindle alignment.
The second of these structures—anaphase astral microtubules—appears to pro-
vide two functions to the ongoing cell cycle. As in the case for the sperm-aster (see
above), anaphase astral microtubules experience dramatic growth due to continuous
nucleation at internal sites, resulting in microtubules becoming nucleated as they
expand outward toward the cortex (Ishihara et al. 2014 ). This internal priming
mechanism may allow astral tips to reach the cortex in spite of the large size of the
Fig. 4.4 Spindle alignment occurs early in the cell cycle and conforms to an alternating perpen-
dicular pattern. Shown are microtubules (green) and DNA (blue, arrowhead in insert) in a frog
embryo shortly before the first cleavage. Alignment of the axis of the incipient spindle is concur-
rent with telophase for the previous (in this embryo, first) mitotic spindle. Arrow in insert points to
the nascent second mitotic spindle indicating the spindle axis. The forming spindle is parallel to
the microtubule interaction zone, which indicates the location of the first cleavage furrow (thick
arrow in the main panel; Wühr et al. 2010 ). The second spindle is therefore oriented perpendicular
to the spindle of the previous cell cycle. Scale bar corresponds to 500 μm. Figure adapted from
Wühr et al. ( 2010 )
A. Hasley et al.