Biology Reference
In-Depth Information
FIGURE 23.12
Diagram of the hypothetical mechanism (supported by some biochemical data) by which North
and South complexes grab aster microtubules and pull on them to orientate the spindle.
the cell would specify a planar ring across which the spindle would have to lie, and then the
additional activities of plane-polarized Dishevelled and Strabismus would pull the spindle to
one unique orientation across that plane (
Figure 23.13
).
Cell division may be orientated by planar polarity signals but the process of division
would seem to threaten the maintenance of such signals, which depend on every cell having
both 'South'-and 'North'-type membrane complexes. If nothing took place to prevent this
outcome, simple division of a cell along the North-South axis of the tissue would leave
one daughter inheriting only the South-type complexes and the other only the North-type.
This problem is avoided by cells internalizing their complexes into separate endosomes at
the onset of mitosis. Once in the cytoplasm, the endosomes containing South-end proteins,
such as Vangl2, and the endosomes containing North-end proteins, such as Frizzled, spread
throughout the cell so that both types of endosome are inherited by each daughter.
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It is important to note that not all orientated cell division is under the control of the planar
cell polarity pathway described above. Germband extension in D. melanogaster, for example,
involves clear planar polarity but it is unaffected by mutation of the dsh gene;
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the polarity
must therefore be encoded by a different system.
ADAPTIVE SELF-ORGANIZATION OF MITOTIC ORIENTATION
AND HERTWIG'S RULE
So far, this chapter has concentrated on 'programmed' orientation of cell division to
achieve specific morphogenetic change. For much of embryogenesis, the orientation of
mitosis is a reaction to other processes rather than a driving force, and is important in
