Epigenetic Control of Animal Development - Building the Most Complex Structure on Earth - page 128

(A) (B)

Figure 3.7 Possible models by which an unequal pulling force could be generated. (A) The
force generated by the pushing apart of the overlapping midzone microtubules is unequal.
(B) The force of the pulling astral microtubules is unequal.
Source : From Kaltschmidt and Brand (2002) .

Figure 3.8 Generation of an eccentrically placed cleavage plane in the Drosophila
neuroblast. The spindle forms symmetrically between the two spindle poles. Apical is up and
basal is down. At the onset of anaphase, the microtubules appear to shorten on the basal side
of the cell and elongate/enrich on the apical side. At telophase (gray ovals represent telophase
DNA), the centrosome of the basal aster is smaller than that of the apical aster.
Source : From Kaltschmidt and Brand (2002) .

major aster. Then one of them loses the pericentriolar material and migrates to the
basal cortex, where it acquires pericentriolar material and forms the second aster
( Figure 3.8 ). Microtubules elongate on the apical side and shorten on the basal side,
leading to the formation of the furrow closer to one of the spindle poles and to the
asymmetric division of cells ( Kaltschmidt and Brand, 2002; Rebollo et al., 2007 ).
The fact that “[t]he direction of division and the ability of a cell to divide sym-
metrically or asymmetrically in size is brought about by rearrangement of the
cytoskeleton” ( Kaltschmidt and Brand, 2002 ) unambiguously indicates the epige-
netic nature of the cell differentiation in animals. Messenger RNAs (mRNAs) dis-
tributed throughout the cytoplasm are localized, in a microtubule-dependent mode,
in the pericentriolar material, as proved by the fact that experimental depolymeri-
zation of microtubules prevents accumulation of these mRNAs in the centrosome.
Then, during the cleavage division, they dissociate from the centrosome and, via
actin filaments, are transported to particular sites of the presumptive daughter cell
cortex, leading to asymmetric division of mRNAs in daughter cells ( Lambert and
Nagy, 2002 ) ( Figure 3.9 ). Asymmetric behavior of centrosomes seems to play an
important role in the asymmetric division of cells ( Yamashita and Fuller, 2008 ).