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2000), is involved in some unknown way in convergence and extension
(Darken et al., 2002; Goto and Keller, 2002; Jessen et al., 2002; Park and
Moon, 2002) (Figure 18.2).
Convergent extension has been reviewed several times recently with a focus
on various aspects, including the involvement of the planar cell polarity (PCP)
pathway (Wallingford et al., 2002; Mlodzik, 2002), the cell biological and
biomechanical aspects (Keller et al., 2000), a focus on these movements in
teleosts (Myers et al., 2002), and the relation of polarized cell behaviour to
organismal polarity (Keller, 2002). The cell-traction/cell substrate model that
we proposed for convergence and extension by cell intercalation in Xenopus
some years ago (Keller et al., 1991, 1992a), and revisited recently (Keller et al.,
2000; Keller, 2002) (Figure 18.1E,F), has been widely interpreted and
reinterpreted in the context of many experimental results on amphibians
and other organisms. Here we will revisit some of the key issues about
convergence and extension by cell intercalation with a focus on the mechanism
by which cell behaviour and cell motility generates active, force-producing cell
intercalation.
The diversity and complexity of convergence and extension
It is a mistake to assume that convergence and extension are universal,
uniform processes. Convergence and extension simply means narrowing and
lengthening, without an implication of mechanism. Some examples of these
movements occur passively in response to forces generated elsewhere, and
others are active, and driven by forces generated within the tissues. Both the
mesodermal and neural tissues in Xenopus actively extend when isolated as
explants, and they are capable of exerting a pushing force (Keller and
Danilchik, 1988; Moore, 1992; Moore et al., 1995). However, the deep
mesenchymal cells of these regions appear to be the active force-generating
elements of these tissues, whereas the epithelial component seems to converge
and extend passively by virtue of its attachment to the underlying deep cells
(Keller and Danilchik, 1988). The superficial epithelial cells initially respond
to being stretched in the axis of extension by elongating and narrowing, but
then they slide by one another and intercalate to form a longer, narrower
array and become less elongated (Keller, 1978). However, in most cases of
convergence and extension that have been described, it is not known whether
the movements observed are active or passive, nor has the relative importance
of internally and externally generated forces been evaluated.
Secondly, although convergence and extension are often referred to
collectively as 'convergent extension', this convenient somewhat misleading
term oversimplifies the fundamental nature of an underlying machine, of
which the paired movements of convergence and extension are only one
