135dimensions, this combined mechanism results in changes in spindle orientation that
begin to transform a two-dimensional single-cell layer into a three-dimensional
blastula.
4.3.3.4 Cleavage Pattern Determination in Xenopus
The mechanistic model described above, which explains the early zebrafish cleav-
age pattern, is also consistent with the generation of cell cleavage pattern in Xenopus
laevis, another embryo with large blastomeres (Wühr et al. 2010 ; Fig. 4.6b). As in
zebrafish, blastomere cleavage planes exhibit an alternating 90o pattern (Nieuwkoop
Fig. 4.6 Cleavage pattern orientation in early zebrafish and Xenopus embryos. (a) Stereotypic
cleavage pattern in zebrafish. During the first five cell cycles, spindles (double arrows) remain
aligned to a single, horizontal (x–y) plane parallel to the blastodisc plane, alternating 90° every cell
cycle. During the sixth cell cycle, spindles tend to align in the vertical (z-axis) orientation, generat-
ing two tiers of blastula cells. Spindle orientation becomes random during the seventh cell cycle.
(b) Canonical cleavage pattern in Xenopus. The first two cell cycles have spindles aligned to the
x–y plane, also exhibiting an alternating 90° pattern, resulting in meridional cleavages. Spindle
tends to reorient along the z-axis during the third cell cycle to generate a cleavage plane parallel to
the equator. During the following cell cycles, the spindle aligns with the longest axis of the cell
(Strauss et al. 2006 ; Wühr et al. 2010 ). Graded shading represents asymmetric yolk distribution,
which is enriched in vegetal regions. Asymmetrically distributed yolk is hypothesized to interfere
with microtubule length and/or pulling forces to generate a force that places the spindle in an
eccentric position, biased toward the animal pole. In both (a) and (b), spindle orientation is indi-
cated with green double arrows (which appear as a dot when entering the plane of the page). The
order of furrow formation (corresponding to the cell cycle in which those furrows form) is indi-
cated by Roman numerals
4 Vertebrate Embryonic Cleavage Pattern Determination