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The elongation of the cells is correlated with the onset of the polarized
protrusive activity. The cells initially show rapid filo-lamelliform protrusive
activity in all directions, and they are rotund in the planar aspect of the tissue.
As they restrict their protrusive activity to the medial and lateral ends of the
cells, they also become elongated in this axis and begin to intercalate along
this axis (Shih and Keller, 1992a) (Figure 18.3). Manipulation of components
of the planar cell polarity pathway, such as Dishevelled (Wallingford et al.,
2000) and Strabismus (Goto and Keller, 2002), result in random protrusive
activity, and manipulation of other components, such as Rho kinase and
Strabismus in fish (Marlow et al., 2002; Jessen et al., 2002) show loss of the
mediolaterally elongated phenotype. These results argue that the polarized
protrusive activity is, in fact, the basis of the elongate, fusiform morphology
and the intercalation parallel to the mediolateral axis.
Do the intercalating cells actually 'migrate' on one another?
One of the most intriguing aspects of convergence and extension is that these
movements can occur in sandwich explants that are unattached to any
external substratum, and in open-faced explants (ones exposing the actively
intercalating deep cell cells) cultured on non-adhesive substrates, such as
agarose (Keller and Danilchik, 1988; Shih and Keller, 1992a,b). However,
there may be an internal substrate, other than the surfaces of adjacent cells,
that might be used by the cells to move between one another. A web of matrix
could surround all of the cells. The fibronectin-containing matrix separating
the neural and mesodermal tissue could serve this function. These potential
substrates could be carried along in cultured explants. Both the neural and the
mesodermal tissues have protrusions in the vicinity of this matrix (Keller and
Schoenwolf, 1977). However, the pattern of cell movements argues that the
intercalating cells, in fact, use one another as substrates. If cells intercalate
mediolaterally at a locally constant rate of overlap by using one another as
substrates, regardless of whether it is by the bipolar (Figure 18.5A) or the
monopolar mode (Figure 18.5B), the cumulative effect is progressively faster
displacement of cells toward the centre of the array at distances farther from
the centre (Figure 18.5A,B). In both mesodermal (Shih and Keller, 1992a) and
neural tissues (Keller et al., 1992b) this is what is observed. In contrast, if cells
translocate at a constant rate toward the midline on an external substrate, no
additive effect is realized and the cells farther from the midline approach it at
an equal rate (Figure 18.5C), which is inconsistent with what is observed. In
order to intercalate, the cells farther away on the axis of convergence would
have to translocate progressively faster on an external substrate in order to
overtake and get between their medial counterparts. There is no evidence of
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