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be described as follows (Fig. 4.5). During anaphase, at a time when nuclear enve-
lope membranes are being reformed, a microtubule interaction zone forms at the
plane along which the furrow will cleave. Because of the rapid cycling of early
blastomeres, this time period also coincides with the separation of centrosomes and
start of formation of the spindle for the following cell cycle. The nascent spindle is
located in relatively close proximity to the microtubule interaction zone at the fur-
row corresponding to the previous cell cycle. The close proximity of astral
microtubules to this aster interaction zone limits the length of astral microtubules
directed toward the furrow. Under these circumstances, pulling forces on these
shorter astral microtubules (extending between the interaction zone and the aster
center) are weaker than forces on the opposite side of the aster center (extending
toward the central and larger portion of the blastomere). This pulling force differen-
tial results in a net force on the spindle that moves it away from the furrow plane
toward the center of the blastomere, resulting in spindle centering. Asymmetric
pulling forces on the radially structured metaphase astral microtubules can also
Fig. 4.5 Mechanism of spindle centering and alignment in large embryonic cells. Rapid cell
cycling results in temporal overlap between processes corresponding to different cell cycles (see
also Fig. 4.4). Anaphase astral microtubules for a given cycle (depicted in red for the first spindle
in the top panel) expand all the way to the cortex. The asters form a microtubule exclusion region,
the microtubule interaction zone, at the site of aster overlap (left insert). Formation of an incipient
spindle for the following cell cycle (depicted in green for the second spindle in the bottom panel
and right insert) occurs simultaneous with events associated with the previous cell cycle, including
microtubule furrow array (FMA) formation and reorganization (see also Fig. 4.10). The influence
of the microtubule interaction zone on the forming spindle for the following cycle results in asym-
metric microtubule growth. Coupled to pulling from internal sources, the resulting astral microtu-
bule length asymmetry generates a centering force (blue arrow) for the forming spindle apparatus
within the forming blastomere. This same asymmetric influence, coupled to spindle elongation and
a hypothesized stress force within the spindle, results in spindle alignment in an orientation paral-
lel to the furrow (and perpendicular to the spindle) for the previous cell cycle (green arrows)
A. Hasley et al.