Planar Cell Polarity in Coordinated and Directed Movements - Current Topics in Developmental Biology - page 77

on the roles for the PCP pathway in other contexts to recent excellent
reviews ( Gray et al., 2011; Simons & Mlodzik, 2008; Wallingford &
Mitchell, 2011 ).

2. COORDINATED AND DIRECTED MOVEMENTS DURING
VERTEBRATE GASTRULATION
2.1. Different types of cell movements — Collective cell

migration and cell intercalation
During vertebrate gastrulation, coordinated and directed movements of both
epithelial and mesenchymal cells shape the body axis of the embryo. Basically,
there are two different types of movements: collective cell migration and cell
intercalation. In collectivemigration, cells move either individually ( Fig. 4.1A ;
e.g., neural crest cells) or as a cohesive clump ( Fig. 4.1B ; e.g., prechordal plate
progenitors) without neighbor exchanges. During cell intercalation, cells ex-
change neighbors in a directed manner, thereby allowing directed movement
of epithelial cells ( Fig. 4.1 C; e.g., the Drosophila germband, which is mediated
bymulticellular rosettes, and similarly themouse AVE) or elongation of a mes-
enchymal tissue ( Fig. 4.1D ; e.g., notochord progenitors, which is mediated by
mediolateral intercalation). However, different species utilize these twomodes
to different degrees in both epithelial and mesenchymal tissues.

2.2. Convergent extension in Xenopus

At the onset of gastrulation, soon after involution, chordamesoderm
cells (the presumptive notochord and somites) are highly polarized, which
elongate and orient in their mediolateral axis with bipolar lamellipodia
being stabilized at both ends, and undergo mediolateral cell intercalation
( Shih & Keller, 1992 ). These polarized cells intercalate between one an-
other to redistribute their positions along the anteroposterior (AP)
axis, thereby contributing to the extension of the forming body axis.
Convergence and extension movements simultaneously occur in Xenopus ,
and therefore, this process is called CE. Isolated chordamesoderm tissues,
called Keller explants, can undergo CE movements in the absence of
external substrates, and thus CE is a cell-autonomous force-generating
process ( Keller & Danilchik, 1988 ). Likewise, CE behaviors associated
with the elongation of tissues can be observed in isolated na¨ve
ecotodermal tissues, called animal cap explants, when treated with the me-
soderm inducer Activin (e.g., Tada & Smith, 2000 )( Fig. 4.2 ).