133explain the alignment of the spindle in the same direction as the furrow from the
previous cycle, if the sister centrosomes are connected and exert a stress force on
each other. Because aster microtubules oriented in the same orientation as the aster
interaction zone (for the previous cell cycle) do not experience a microtubule length
limit, internal pulling along the same in the direction of the previous furrow would
not be restricted. Together, these forces result in alignment of the spindle for the
new cell cycle along the same axis as the furrow plane for the previous cell cycle.
This in turn results in the orientation of the new furrow, for any given cell cycle, in
a plane perpendicular to that of the previous cell cycle. In zebrafish, where this
sequence of events has been studied in more detail, the process is repeated to gener-
ate the embryonic cleavage pattern at least until about the sixth cell cycle, when the
blastomeres become small enough to be contacted by the metaphase asters. As
described in the following sections, this mechanism has been proposed to mediate
the stereotypical cleavage patterns in zebrafish and Xenopus early embryos.
4.3.3.3 Cleavage Pattern Determination in Zebrafish
In the zebrafish, embryonic cleavage is known to be not only highly synchronous, a
characteristic of pre-MBT cleavage patterns (see Chap. 9 ), but also stereotypical in
terms of the orientation of the furrow plane during each cell cycle (Kimmel et al.
1995 ). As mentioned above, zebrafish embryos have meroblastic cleavage. This
cleavage pattern is presaged by the structure of the oocyte, which has two primary
regions, a wedge-shaped region at the animal pole of the egg where the oocyte
nucleus is arrested in metaphase II of meiosis and the remaining region that contains
a mixture of ooplasm and yolk granules (Selman et al. 1993 ). In zebrafish, the sperm
site of entry is found at a specific location within the oocyte animal pole region
which corresponds to the approximate center of the blastodisc that forms after egg
activation (Hart and Donovan 1983 ; Hart et al. 1992 ). This likely facilitates sperm-
aster centering with respect to a plane parallel to that of the blastodisc itself (which
we refer to as the x–y plane). Although not yet shown, centering in a perpendicular
dimension (in the z-direction, along the height of the forming blastodisc and the
animal–vegetal axis of the embryo) may still occur. Similar to sperm-aster centering
in Xenopus (Wühr et al. 2010 ), sperm-aster centering along the z-axis in zebrafish
could occur through asymmetric forces on the sperm-aster due to microtubule
length restriction on the side of the cortex (see below).
The earliest embryonic cleavage cycles exhibit a largely stereotypic pattern
(Kimmel et al. 1995 ; Fig. 4.6a). During the first five cycles, blastomeres divide
along the x–y plane (with x being the dimension along the first cell division and y
along the second cell division) and in an orientation that alternates 90o every cell
cycle. This generates, in subsequent cell cycles, a pattern of one-tiered blastomere
arrays of 2 × 1 (two-cell embryo), 2 × 2 (four-cell), 4 × 2 (eight-cell), 4 × 4 (16-cell),
and 8 × 4 (32-cell). Furrow positioning for the sixth cell cycle bisects the blasto-
meres along a z-plane, generating a two-tiered 8 × 4 blastomere array (64-cell
embryo). This cleavage pattern is remarkably constant, although detailed live imag-
4 Vertebrate Embryonic Cleavage Pattern Determination