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et al., 2000). Neural cells can intercalate using a bipolar mode, similar to that
used by the mesoderm, in absence of midline tissue and underlying mesoderm
(Elul et al., 1997) but normally when these tissues are present, neural deep cells
express the medially-biased (monopolar) mode of cell intercalation. This
medial-directed protrusive activity is dependent on a signal from the midline
notoplate/notochord tissues (Ezin et al., 2003).
Similar protrusive activity may underlie cell intercalation in other systems.
Cells near the dorsal midline in the fish embryo may intercalate using a
mechanism similar to the bipolar one seen in frogs. This notion is based on the
fact that the cells are similarly elongated and aligned mediolaterally (Sepich
et al., 2000; Glickman et al., 2003; Marlow et al., 2002; Topczewski et al.,
2001). However, the protrusive activity of these cells should be described in
order to confirm that the same mediolateral polarization underlies their
characteristic shape. Laterally, the cells of the germ ring show protrusive
activity and movement directed dorsally toward the embryonic shield
(Trinkaus et al., 1992; Trinkaus, 1998; Marlow et al., 2002). In this regard,
these cells resemble the neural cells of the amphibian. In the extending
notochord of ascidians, cells elongate, align and intercalate mediolaterally
much like their amphibian counterparts (Miyamoto and Crowther, 1985).
This cell intercalation involves transversely orientated protrusions at the
leading edges of the intercalating cells, suggesting that similar polarized
protrusive activity is a general mechanism underlying transversely orientated
cell intercalation in the chordates (Munro and Odell, 2002a). Notochord cell
intercalation in ascidians is dependent on interactions with surrounding
tissues (Munro and Odell, 2002b), which is similar to the dependency of
Xenopus notochord extension on the somitic mesoderm (Wilson et al., 1989).
Recent evidence shows that the vertebrate homologues of the genes in the
planar cell polarity pathway of Drosophila (Adler, 2002) are essential for
convergence and extension in frogs and fish. This pathway includes the
frizzled ligand, Wnt 11 (Tada and Smith, 2000; Heisenberg et al., 2000), its
serpentine receptor, Frizzled (Dijane et al., 2000), the cytoplasmic signalling
proteins Dishevelled (Sokol, 1996; Wallingford et al., 2000; Tada and Smith,
2000). Dishevelled signals downstream through DAAM1 (Dishevelled
Associated Activator of Morphogenesis) (Habas et al., 2002), and the small
GTPase and cytoskeletal regulator, Rho, and Rho kinase (Habas et al., 2002;
Marlow et al., 2002). Another member of the Rho family of small GTPases,
Cdc42, is also important in convergent extension (Choi and Han, 2002), as
well as maintaining separation between the mesoderm and overlying neural
tissue (Winklbauer et al., 2001). Both of these functions of Cdc42 appear to be
regulated not by the Dishevelled-dependent, planar cell polarity pathway, but
by the Wnt/Ca
++
pathway (Winklbauer et al., 2001; Choi and Han, 2002;
Sheldahl et al., 1999). Finally, Strabismus (also known as van Gogh), a
putative membrane protein regulating polarity in Drosophila (Adler et al.,
