Biology Reference
In-Depth Information
FIGURE 23.11
Orientated cell divisions in vertebrate neurulation.
directions (they still respect apico-basal polarity, in the z axis, but have lost planar, x-y,
polarity). The early embryo of zebrafish provides an example. Normally, cells divide so
that the line between the daughter nuclei is approximately parallel to the animal-vegetal
axis of the embryo. When Dishevelled is suppressed, this direction is lost in a cell-
autonomous manner.
30
The same loss of orientation happens when Strabismus (Vangl) is
suppressed. It is therefore clear that, for these systems at least, orientation of mitosis reads
the same cellular compass that can also be used to direct convergent extension (Chapter 16).
How might the complexes of proteins that encode planar cell polarity control the orienta-
tion of the mitotic spindle? The answer is not yet certain but biochemical data obtained in
insect cells provide a clue. Dishevelled associates biochemically with a complex of proteins
that includes Dynein, the microtubule's motor, and NuMA (also called 'Mud'); furthermore,
Dishevelled is incapable of orientating mitoses if NuMA is absent.
31
This suggests that acti-
vated Dishevelled, which will be present only at the North end of a cell, might recruit
a NuMA/Dynein complex and thus generate, through the motor activity of Dynein, a winch
that pulls microtubules towards this point, pulling one aster towards the North. At the South
end of the cell, Strabismus can be found. Strabismus recruits LGN (also called 'Partner of
Inscuteable') to the South end of the cell.
32
As has been described above, LGN forms
a complex with NuMa that can recruit and activate Dynein.
33
e
35
This would lead to the South
end of the cell also pulling an aster towards itself, so that the spindle will come to rest along
the North-South axis (
Figure 23.12
).
The planar mechanism suggested in
Figure 23.12
therefore converges on the same NuMA/
Dynein aster-pulling complexes that are thought to force the spindle to form across the cell, at
right angles to the apico-basal axis. This would, if true, be an economical method of giving
full three-dimensional control. The position of adherens junctions near the apical end of
