Epigenetic Control of Animal Development - Building the Most Complex Structure on Earth - page 129

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Figure 3.9 Asymmetric division of mRNAs. 1 and 2. During prophase, certain mRNAs
bound to protein factors (blue) are transported to only one of the chromosomes (yellow circle)
via microtubules (red). The centrosome (green circle) lacks the competence to trap (bind
or catch) mRNAs. In the absence of microtubules, mRNA localization persists around the
pericentrosomal material. 3. Then mRNA-protein complexes, via the actin cytoskeleton, are
transported to a certain region of the cell cortex. 4. Localized correctly, mRNAs are bound
independently of actin. 5. During cell division, mRNAs go to only one daughter cell. As a
result, each daughter cell will have a different fate.

Source : From Giet and Prigent (2003) .

During telophase of a cell type in C. elegans , in response to extracellular Wnt sig-
nals, the spindle microtubule structure is restructured, thus inducing the asymmetric
localization of WRM-1 ( C. elegans ' β-catenin homologue) in the cell nucleus and
consequent asymmetric cell division ( Sugioka et al., 2011 ).
The reorganization of cytoskeleton microtubules is responsible for determining
the form and certain structural features of multinucleate myotubes during embryonic
muscle formation; instead of the radial array of myoblast microtubules originating
in the microtubule organizing center (MTOC), the fusion of myoblasts is associated
with the formation of microtubule longitudinal linear arrays originating from the
already diffuse MTOC and/or myotube extracentrosomal sites. An enzyme, LKB1,
destabilizes microtubules and facilitates their reorganization in the process of the
myotube development ( Mian et al., 2012 ) ( Figure 3.10 ). During muscle fiber differ-
entiation, several centrosomal proteins are redistributed to microtubule-nucleating
sites along nuclear membrane and through cytoplasm ( Bugnard et al., 2005 ).
A typical case of asymmetric division is observed in Caenorhabditis elegans
embryos; the sperm of this 1-mm-long roundworm provides the fertilizing egg with