Planar Cell Polarity in Coordinated and Directed Movements - Current Topics in Developmental Biology - page 80

Other than the mesoderm, the prospective hindbrain and spinal cord also
narrowmediolaterlly and extend along the body axis during gastrulation/neu-
rulation. The Xenopus neural plate consists of the two layers of cells: superficial
epithelial cells and deep mesenchyme-like cells. In an isolated neural plate, the
deep cells show mediolateral cell intercalation behavior that contributes to
elongation of the neural plate independently of the underlying mesoderm
( Elul et al., 1997 ). Although boundary capture is described as for monopolar
protrusive activity of neural cells away from the boundary to the prospective
floor plate ( Elul & Keller, 2000 ), it remains to be elucidated whether the
boundary capture is related to a force-generating process.
2.3. Convergent extension in zebrafish
Different to simultaneous movements of involution and convergence in
Xenopus , the cells located in the germ ring internalize separately from conver-
gence except within the shield. Indeed, local convergence-like movement
mediates the physical appearance of the shield ( Montero et al., 2005 ). Lateral
mesoderm cells do not initiate convergence movement until mid-gastrula
(75% epiboly). This is coincided with the geometry of the germ ring at this
stage of the zebrafish embryo, which is similar to that of the marginal zone at
the early gastrula in Xenopus . At the onset of mesoderm convergence, cells
appear only loosely associated yet to be visibly polarized and show only little
coordinated and directed movement ( Sepich et al., 2005 ). However, once
these cells get closer to the dorsal side and thus cell density increases, the cells
polarize along their mediolateral axis and exhibit highly coordinated and
directed convergence movements, which are reminiscent of collective migra-
tion ( Sepich et al., 2000 ). When arriving near the prospective notochord,
mesoderm cells initiate mediolateral intercalation, thereby contributing to
the elongation of the body axis. In contrast, notochord progenitors originated
from the shield start showing mediolateral cell intercalation behavior at
mid-gastrula stage and exclusively undergo extension movement. Thus, these
two different types of cell movements both mediate CE and contribute to the
elongation of body in zebrafish ( Fig. 4.3 ).
2.4. Collective migration of head mesendoderm/prechordal
plate progenitors
During gastrulation, head mesendoderm cells in Xenopus and prechordal
plate progenitors in zebrafish migrate as a cohesive sheet of cells toward
the animal pole. Cells located at the leadingedgeformdynamicprotrusions