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tion in Xenopus eggs (Wühr et al. 2010 ). These studies suggest that, in zebrafish and
Xenopus, dynein-dependent pulling forces are required for the centering of astral
microtubules during pronuclear fusion and blastomere divisions. Because in these
species the centering and experimentally induced movements occur prior to astral
microtubules contact with the cortex, the pulling force is unlikely to be generated at
the cortex, but is instead generated along presently unknown internal elements of
the cytoplasm. Knockdown experiments in C. elegans suggest that, in this system,
vesicles are likely the anchor for cytoplasmic dynein to generate pulling forces on
microtubules (Kimura and Kimura 2011 ). Distribution of such internal anchors
through the cytoplasm could result in a length-dependent force, such that longer
microtubules contribute larger pulling forces than shorter ones, resulting in net aster
movement. A similar model for microtubule length-dependent pulling depending on
cell volume (i.e., internal stores) as opposed to cell surface has been derived through
cell shape manipulations and mathematical modeling (Minc et al. 2011 ). Thus,
although pulling from cortical anchors has been documented to orient the spindle in
smaller differentiated cells (McNally 2013 ), large embryonic blastomeres appear to
use a tug-of-war pulling mechanism from internal sites, whose consequences for
embryonic cleavage patterning are described below.
4.3.3 Mechanisms Underlying Spindle Orientation in Large
Embryonic Cells in Fish and Amphibians
Two model systems, zebrafish and Xenopus, provide insight on the mechanisms that
drive cleavage patterning in early vertebrate embryos with large blastomeres.
Embryos from these species exhibit similar behaviors with regard to the spatial
arrangement of the spindle and associated DNA in relation to the blastomere center.
Due to the small size of the spindle relative to the large size of the blastomere, each
of the resulting nuclear masses is at the end of anaphase in a location relatively close
to the previous furrow, off-center with respect to the newly formed daughter blasto-
meres. Thus, in preparation for the next cell division, an important initial require-
ment is the centering of the forming spindle within the daughter blastomeres.
Additionally, the cell cleavage plane in these embryos is known to alternate with
each cell division, with each cleavage plane at a 90o angle relative to that of the
previous one.
4.3.3.1 Spindle Orientation Based on Cell Geometry Can Be Overridden
by Molecular Cues
As described above, aster-mediated forces influence the position of both sperm-
asters and amphiasters emanating from bipolar spindles. As will be discussed
shortly, in the case of bipolar spindles, stress forces and/or coordinate centering of
A. Hasley et al.