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and Faber 1967 ; Sawai and Yomota 1990 ). Also similar to zebrafish, spindles
acquire their final orientation early in the mitotic cycle, before astral microtubules
are long enough to reach the cortex (Wühr et al. 2010 ).
In contrast to zebrafish, fertilization of anuran amphibian eggs, such as Xenopus,
does not occur through a spatially defined sperm entry point, but instead occurs at
random locations in the animal hemisphere (Elinson 1975 ; Schatten 2012 ). This
generates in the Xenopus embryo an immediate need for sperm-aster centering
along the x–y plane. This centering likely occurs by internal pulling forces acting on
the sperm-aster, formed immediately after fertilization and generated by reconstitu-
tion of a centrosome around the sperm-derived centrioles, which act as an MTOC
for the sperm-aster. The location of the sperm centrosome immediately below the
surface will automatically result in centering: the membrane generates a restriction
on the astral microtubule lengths, and pulling forces from the opposite (internal)
side generate an overall asymmetric force that centers the sperm-aster (Wühr et al.
2010 ). The aster’s closeness to the cortex induces an asymmetry generating a long
sperm-aster axis that is roughly parallel to the tangent of the cortex at the sperm
entry point. The aster seems to sense this long axis and transfer it to the nascent first
mitotic spindle (see below). This might explain the old observation that the first
cleavage plane typically cuts through the sperm entry point (Roux 1903 ).
During the first division cycles, Xenopus MTOCs tend to move toward the
animal- most third of the embryo (Wühr et al. 2008 ). This movement in the animal
direction may also be explained if, as in zebrafish, yolk granules present in more
vegetal regions limit astral microtubule attachments. More yolk could inhibit micro-
tubule growth or result in fewer cytoplasmic structures for dynein to pull on, thus
again providing a force differential that pulls the spindles animally until reaching an
equilibrium point at the observed location. However, the proposed mechanism by
which asters and yolk interact is not understood. Induction of furrows in Xenopus
initiates at the animal region and only gradually moves vegetally (Danilchik et al.
1998 ), consistent with asymmetric location of the spindles, which can act to induce
a furrow first on the more closely located animal cortex. The first two divisions
occur with spindles aligned along an x–y plane parallel to the equator, alternating 90
°C to generate four cells whose furrows span meridians along the animal–vegetal
axis. This arrangement is similar to the cleavage pattern in zebrafish and, even with
the obvious contrast that cleavage in Xenopus is holoblastic, is easily explained by
the microtubule exclusion model described above. As in zebrafish, maintenance of
the spindles in the x–y plane may rely on a differential force that precludes z-axis
tilting during these cycles. The frog egg is not perfectly spherical but slightly oblate,
and vegetal yolk likely weakens pulling forces on asters in the z-direction, two con-
ditions that would promote spindle orientation along the x–y plane.
Xenopus embryos, also like zebrafish, exhibit a transition of spindle alignment
from the x–y plane to a z-axis orientation (as in zebrafish, the latter also corresponds
to the animal–vegetal axis of the embryo). However, this transition normally occurs
in the third cycle in Xenopus, compared to the sixth cycle in zebrafish. This earlier
transition in Xenopus may reflect a differing balance between microtubule pulling
lengths along the x–y axis (limited by membranes laid out between blastomeres)
A. Hasley et al.