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during gastrulation of other amphibians (Vogt, 1929; Shook et al., 2002),
teleost fish (Glickman et al., 2003; Trinkaus et al., 1992; Trinkaus, 1998),
ascidians (Miyamoto and Crowther, 1985; Munro and Odell, 2002a,b), birds
(Schoenwolf and Alvarez, 1989; Schoenwolf et al., 1992; Sausedo and
Schoenwolf, 1993; Lawson and Schoenwolf, 2001a,b), and mammals (Sausedo
and Schoenwolf, 1994). However, the biomechanical consequences and the
morphogenic function of these movements vary among these groups of
organisms.
During gastrulation and neurulation of amphibians studied thus far, these
movements are active processes, driven by internal forces rather than passive
responses to forces developed elsewhere in the embryo. Convergence and
extension occur in cultured explants of the anuran (tail-less) amphibians,
Xenopus laevis (Keller and Danilchik, 1988; Elul et al., 1997) and Hyla regilla
(Schechtman, 1942), and in the urodele (tailed) amphibians, Taricha torosus,
Ambystoma mexicanum and Ambystoma maculatum (Shook et al., 2002)
(Figure 18.1C-D). In Xenopus, both the mesodermal and neural regions
actively converge and extend in explants (Keller and Danilchik, 1988; Elul
et al., 1997; Elul and Keller, 2000) and in vivo (Wallingford and Harland,
2001). It is not known whether the neural region actively extends in other
species.
Convergence and extension in Xenopus occur by active movement of cells
between one another, a process initially referred to as 'interdigitation' (Keller,
1984) but later changed to the more appropriate 'intercalation' (Keller et al.,
1985; Keller, 1986). Two types of cell intercalation occur. First, radial
intercalation occurs, a process in which several layers of deep cells intercalate
between one another along the radius of the embryo to produce fewer layers
of greater length (Figure 18.2A). Then at the midgastrula stage, mediolateral
intercalation begins. Mediolateral intercalation is a process in which cells
intercalate mediolaterally to produce a narrower, longer array (Figure 18.2B).
Mediolateral intercalation is driven by polarized cell motility. Before
convergence and extension begin, the protrusive activity of the cells is not
orientated within the plane of the tissue (Figure 18.3A). At the onset of
convergence and extension and cell intercalation, the protrusive activity of the
deep mesodermal cells becomes polarized. Large lamelliform protrusions form
at the medial and lateral ends of the cells, and numerous, small filiform
protrusions form at their anterior and posterior surfaces (Figure 18.3D). The
large medial and lateral protrusions appear to exert traction on adjacent cells,
and the cells become mediolaterally elongated, and orientated parallel to one
another (Figure 18.3B). They then move between one another along the
mediolateral axis, producing a longer, narrower array (Figure 18.3C-D) (Shih
and Keller, 1992a,b; Keller et al., 1991, 1992a). The cells appear to use one
another as movable substrates. As they exert traction on one another's
surfaces (pointers, Figure 18.3B-D), they pull themselves between one another
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