14 4
general understanding of cell cleavage patterning in early embryos. The mechanism
appears to involve symmetric cleavages in alternating orientations, consistent with
shape-sensing spindle orientation mechanisms acting in large embryonic cells. This
underlying cell division pattern is further modified in a subset of cells by a special-
ized structure capable of influencing the orientation and position of the spindle
through multiple cell cycles (at least cycles 3–6), adding asymmetric details to the
blastula, which begin to sculpt the embryo. This modified structure, capable of
influencing spindle placement and cell pattern arrangement, also contains factors
that function in cell shape specification. Thus, basal mechanisms of cell division
interact with a specialized structure to coordinate cell division cues and cell fate
signals to generate the basic body plan (Fig. 4.9).
The first two cleavage planes in ascidian embryos are meridional, passing
through the animal and vegetal poles, with the second cleavage oriented perpen-
dicular to the first to generate four equal-sized blastomeres. The third cleavage
plane is also perpendicular to the first and second cleavage but equatorial, divid-
Fig. 4.9 Early embryonic cleavage pattern in ascidians. Early cleavages in general follow an alter-
nating perpendicular pattern, consistent with a dependence on cell shape. Spindle orientation in the
posterior-vegetal cell is influenced by the attraction of one of the spindle poles to the CAB (blue),
which becomes condensed and attached to the posterior-vegetal cortex in the eight-cell and 16-cell
embryo. This causes the eccentric placement of the spindle at the posterior end of the posterior-
vegetal blastomere during these stages, resulting in consecutive asymmetric cell divisions that
generate a larger blastomere anteriorly and a smaller blastomere posteriorly. Spindle orientation is
also influenced by the condensing CAB during formation of the two-cell embryo, resulting in a
shift of the cleavage plane axis with respect to the polar bodies at the embryo animal pole and
animal–vegetal axis (orange arrow) of the embryo, as well as during formation of the four-cell
embryo, resulting in the observed protrusion of posterior-vegetal blastomeres (B4.1) at this stage.
Spindle centering in anterior- and posterior-animal blastomeres, which do not contain the CAB,
occurs normally resulting in symmetric cell division. Attraction of the spindle pole by the CAB is
dependent on the function of PEM, a maternal product localized to the CAB. Reproduced, with
permission, from Negishi et al. ( 2007 )
A. Hasley et al.