137and vegetally located yolk. The result of this z-axis shift in spindle orientation is cell
furrowing along a plane parallel to the equator (above the equator due to the spin-
dles being located closer to the animal pole), generating four smaller animal blasto-
meres and four larger vegetal blastomeres. Subsequent cleavage planes generally
follow a rule for alternating shifts in spindle orientation albeit showing increasing
variation: cleavage in cycle 4 again tends to generate longitudinal furrows, reflect-
ing a realignment of the spindle with the x–y plane, whereas cleavage in cycle 5
tends to generate furrows parallel to the equator, reflecting a second z-axis reorien-
tation. The alternating alignment of the spindle along the x–y plane and the z-axis
may reflect, as in zebrafish, a cell shape-sensing mechanism influenced by changing
dimensions of the blastomeres as they undergo cell division (Strauss et al. 2006 ;
Wühr et al. 2010 ).
Thus, and in spite of exhibiting an entirely different type of cleavage pattern
(meroblastic compared to holoblastic), the global pattern of cleavage orientation in
zebrafish and Xenopus embryos can be explained spatially and temporally through
the cleavage stages using the same simple mechanistic model (Wühr et al. 2010 ).
This model initially relies on microtubule length-dependent forces and the influence
of the furrow from the previous cell cycle, together with additional intracellular
modulation, such as the distribution of yolk. Over time cells acquire a smaller size
and the patterning system may transition to the spindle being able to directly sense
the cortex (Strauss et al. 2006 ; Wühr et al. 2010 ; Xiong et al. 2014 ). Together, these
influences generate a three-dimensional blastula.
The fact that species as phylogenetically distant as teleosts and amphibians
appear to obey a conserved set of cell-biological mechanisms, which generate
manifestly different cleavage patterns from different initial starting conditions,
suggests that a common set of rules may provide the basis for the various cell
arrangements observed in many early vertebrate embryos. During evolution, such
unifying rules may be all that is necessary to accommodate limited changes in start-
ing blastodisc dimensions and/or the influence of modifying factors (i.e., embryo
size, affinity of internal anchors, amount or nature of yolk particles) to generate
cleavage pattern variation. It will be interesting to test this hypothesis through fur-
ther studies in additional species.
4.3.4 Cell Cleavage Orientation in Other Vertebrate and Proto-
vertebrate Systems
Studies in amphibians and teleosts have, until now, contributed the most to our
mechanistic understanding of cleavage plane determination in vertebrate embryos.
However, a full comparative picture will involve patterns in other vertebrate systems
such as Aves, reptiles, and mammals. The chordates include invertebrates such as
tunicates in addition to vertebrates, and mechanisms involved in cell cleavage pat-
tern have been well described in ascidian (sea squirt) tunicate embryos. We sum-
marize our current knowledge in these systems below.
4 Vertebrate Embryonic Cleavage Pattern Determination