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Fig. 2. Formation of the microtubule (MT) array. (A-C) Vegetal views of microtubule
array formation during the first cell cycle. Times shown are normalized times
(NT), with 0.0 NT representing fertilization and 1.0 NT representing the first
cleavage division. (A) At 0.4 NT, short disorganized MT polymers have started
to appear in the vegetal shear zone. (B) By 0.5 NT, more MTs are present, but
are not yet aligned. (C) By 0.7 NT, during peak cortical rotation, the vegetal
shear zone is populated by a parallel array of MT bundles that are aligned
along the axis of rotation. (D-F) MTs of the vegetal array arise from several
sources. (D) The sperm centriole introduces polarity by acting as a minus-end
MT-organizing center (-). The resulting radial array of MTs is called the
sperm aster. (E) MTs from the sperm aster grow toward the periphery of the
egg, as do additional MTs from unknown sources in the core cytoplasm. In
addition, short disorganized MT polymers arise in the vegetal shear zone. (F)
During rotation, MTs from deep in the cytoplasm bend into the vegetal shear
zone and align with peripheral MTs to form the parallel array, with the
plus-ends (+) of the growing MTs pointing towards the future dorsal (D) side
of the embryo. V, ventral. (A-C) Reproduced, with permission, from Cha and
Gard (Cha and Gard, 1999).
(D-F) Adapted, with permission, from Houliston and Elinson
(Houliston and Elinson,
1991b).
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