Coordinated morphogenetic cell movements during gastrulation are crucial for establishing embryonic axes in animals. Most recently, the non-canonical Wnt signaling

Most animals undergo gastrulation to establish germ layers and embryonic axes. The dynamic morphogenetic cell movements that take place during gastrulation are highly coordinated. In Xenopus embryos, these cell movements, which include involution and convergent extension, are driven predominantly by mesodermal cells (Keller, 1991; Keller et al., 1992). This is particularly obvious during convergent extension, when polarized axial mesodermal cells intercalate in radial and mediolateral directions to cause dramatic elongation of the dorsal marginal zone along the anteroposterior (AP) axis (Shih and Keller, 1992; Wilson and Keller, 1991). However, the molecular mechanisms by which mesodermal cells become polarized and drive these movements are poorly understood. Recently, it has been reported that a non-canonical Wnt signaling cascade, which is known to regulate planar cell polarity (PCP) in Drosophila (Adler, 1992; Mlodzik, 1999), also participates in the regulation of convergent extension movements in Xenopus, as well as in the zebrafish embryo (Djiane et al., 2000; Heisenberg et al., 2000; Tada and Smith, 2000; Wallingford et al., 2000). The zebrafish silberblick (slb) locus encodes Wnt11, and Slb/Wnt11 activity is required for cells to undergo correct convergent extension movements during gastrulation. The signal transducer Dishevelled (Dsh) acts downstream of Slb/Wnt11 through domains specific to the non-canonical Wnt/PCP signaling cascade, and directly regulates cell polarity within cells undergoing convergent extension (Heisenberg et al., 2000; Tada and Smith, 2000; Wallingford et al., 2000). In addition, the relocalization of Dsh to the cell membrane is required for convergent extension movements in Xenopus gastrulae, in the same way as recruitment of Dsh to the membrane through the Frizzled (Fz) receptor is required for the PCP pathway in Drosophila (Axelrod, 2001; Wallingford et al., 2000). Although many genetic experiments have demonstrated downstream effectors of the non-canonical PCP Fz/Dsh signal in Drosophila (Shulman et al., 1998; Tree et al., 2002), and some of these effectors (e.g. JNK) function to regulate the convergent extension movements (Darken et al., 2002; Yamanaka et al., 2002), the precise signaling mechanism that regulates the convergent extension movements remains unclear. In addition to the intracellular signaling mechanism, intercellular modulators are involved in regulating coordinated movements of large cell populations. Heparan sulfate proteoglycans (HSPGs) have been implicated in the modulation of intercellular signaling in vertebrates and in Drosophila, and have been shown to be required for gastrulation movements in the Xenopus embryo (Brickman and Gerhart, 1994; Itoh and Sokol, 1994). Glypican, a member of the membrane-associated 2129 Development 130, 2129-2138 © 2003 The Company of Biologists Ltd doi:10.1242/dev.00435