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Chromosomes therefore can typically be separated equally between the two daugh-
ter cells. Analyses of various cell types have shown furrow-inducing activity in
midzone microtubules of the spindle (Martineau et al. 1995 ; Cao and Wang 1996 ).
Studies in amphibians show that neighboring asters induce a cleavage furrow, but
only if chromatin is located between them (Brachet 1910 ), showing a key role for a
chromatin-derived factor in furrow induction. However, experimental manipula-
tions of sand dollar embryos by Rappaport, which created a barrier between the
poles of a forming spindle to force overlap of normally nonadjacent asters in other
regions of the blastomere, showed the induction of an ectopic furrow in the region
of overlap in the absence of nearby chromatin, confirming a role for astral microtu-
bules in furrow induction (Rappaport 1961 , 1996 ). These experiments indicate that
signals present in both the midzone and astral microtubules contribute to furrow
induction (Rappaport 1996 ; Mishima 2016 ). The degree to which one of these two
mechanisms alone is able to induce a cleavage furrow varies between different spe-
cies (Brachet 1910 ; Rappaport 1961 ; Rappaport and Rappaport 1974 ; Su et al.
2014 ; Field et al. 2015 ). Because the location of the midzone and that of astral
microtubule overlap typically occur in the same position, both of these structures act
together to establish a robust positioning mechanism to place the furrow at a plane
halfway between spindle poles.
The large cells of some embryos like fish or amphibians require some special
adaptations related to spindle structure and function. A major adaptation involves
aster morphology. In smaller cells, nearly all microtubules’ minus ends are believed
to be close to the centrosome. However, such an arrangement, due to its radial nature
within the volume of the blastomere, would lead to severely reduced microtubule
densities near the cortex in very large cells. Instead, large cells such as the zebrafish
and Xenopus embryonic blastomeres implement an alternative strategy, in which
sites of microtubule nucleation are evenly distributed throughout these large asters
(Figs. 4.2 and 4.4). The implementation of these internal microtubule nucleation
sites results in microtubule density remaining near constant, independent of dis-
tance from the aster center (Wühr et al. 2009 ). The induction of microtubule nucle-
ation sites within the aster can be explained with a chemical trigger wave that relies
on microtubule-dependent nucleation (Ishihara et al. 2014 ).
Another major adaptation of asters which is particularly apparent in very large
embryos is the formation of an aster–aster interaction zone, a region depleted of
microtubules at the site of overlap between adjacent asters (Wühr et al. 2010 ;
Nguyen et al. 2014 ; Figs. 4.2 and 4.4). As described below, this interaction zone
seems to enable to communicate the proper plane for cell division from the mitotic
spindle apparatus to the cell cortex, which can be separated by several hundred
micrometers. Furthermore, the interaction zone preempts the barrier of the future
cleavage plane allowing the aster to center and orient along the longest axis of the
future daughter cell before it actually exists.
Analysis of the function of the Chromosomal Passenger Complex (CPC) compo-
nent Aurora kinase B (Aur B) in zebrafish embryos provides additional insight into
this redundancy as it applies to the large embryonic cells (Yabe et al. 2009 ). Embryos
from females homozygous for a maternal-effect mutant allele in the gene cellular
A. Hasley et al.