Biology Reference
In-Depth Information
One potential problem for a sheet of cells that is undergoing convergent extension (or that
maintains a planar polarity for any other reason) is accommodating mitosis. For any cell
with a division plane across the East e West axis (giving rise to daughters lined up North e
South), there is a risk that one daughter will inherit all of the South complexes and the other
will inherit all of the North complexes. This would both erase polarity from the daughters
and would distort the polarities of their interphase neighbours, just as patches of all-North
or all-South cells distorted wild-type neighbours in the experiments described in the para-
graph above. The problem seems to be solved by a mechanism that internalizes all of the
components of the North and South complexes during mitosis and shares them out so
that they are inherited equally by the daughter cells. These daughter cells then use the
signals from their interphase neighbours' maintained polarities to re-establish their own
sense of direction. 16
The instability of the unpolarized state in any cell that contains the core proteins involved
in the type of planar polarity described here means that they would be expected to polarize
spontaneously, polarity being propagated through the sheet from the first cells to achieve it. It
is assumed that signals from other tissues in the embryo can impose a robust bias on the
system when it becomes active, to ensure that the sheet of cells polarizes in the correct orien-
tation with respect to the rest of the embryo; for example, so that the North ends point to the
distal end of a fly wing. The association of proteins such as Frizzled and Dishevelled with
signalling by proteins of the Wnt family suggests that gradients of these molecules from other
parts of the embryo may be responsible for the initial bias, 17 but this has so far been difficult
to prove.
However, the initial bias of direction is set up, the different locations of the North and
South-type protein complexes polarize other activities in the cell as well. These other activ-
ities are generally considered 'effectors' of the planar polarity system; they are not needed
for the maintenance of polarity itself, but rather for the cell's ability to use the polarity infor-
mation to give direction to its morphogenetic activity. One important effector is ROCK: the
North complexes containing Frizzled and Dishevelled activate ROCK locally, and this in
turn activates myosin ( Figure 16.9 a) and assembly of contractile microfilament cables. This
contractile apparatus is therefore made most intensely along the Northern borders of each
cell, and contraction will therefore tend to shorten these borders, driving cell convergence
along the East e West axis and elongation along the North e South axis ( Figure 16.9 b).
ROCK also phosphorylates, and thereby inhibits, the protein Bazooka (Baz: called 'Par3' in
some animals), and the normal activity of Bazooka is to promote the assembly and stability
of cell-cell junctions. 18 The effect of ROCK activation is therefore to reduce the number of
junctions along the Northern borders, facilitating the shortening of these borders. While
the Northern border of each cell suffers a direct, internally generated contraction and loss
of junctions, the Southern border, which faces the Northern border of the next cell along,
suffers an equal but essentially passive contraction because one cell cannot maintain mutual
junctions if its partner refuses to do so. The Eastern and Western boundaries of the cells are
unaffected by all of this d they have no local ROCK activation and therefore enjoy stable junc-
tions and are able to form new ones when the opportunity arises to make a new boundary
running exactly parallel to the North e South axis ( Figure 16.9 b).
There are many other effectors of planar polarity, and it is in the effector systems that there
is most divergence between insects and mammals. In particular, it seems that primary cilia,
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