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Figure 18.2 Convergence and extension of Xenopus mesodermal and neural tissues occurs
by two processes of cell intercalation. During radial intercalation (A), several layers of deep
mesenchymal cells intercalate between one another along the radius of the embryo to form
a tissue that is thinner and longer (A). In this case, radial intercalation produces thinning
and extension. During mediolateral intercalation (B), cells intercalate between one another
along the mediolateral axis to form an array that is narrower (convergence) and longer
(extension), and usually somewhat thicker as well. These intercalating deep cells are
normally associated with an overlying epithelial layer that forms the outside of the embryo
(not shown) and does not appear actively to converge and extend but does so passively by
virtue of its attachment to the underlying deep cells. Modified from Keller et al., 1992a
(Figure 18.3C-D). Macroscopically, the expression of polarized protrusive
activity is patterned in progressively developing arc-like patterns, which
consist of a contiguous array of intercalating cells, anchored at both ends
(Figure 18.3E). The arcs are shortened as the cells pull between one another,
resulting in some cells being displaced to join other similarly displaced cells to
form new arcs (Figure 18.3E). For example, cells 1-4 initially form a tensile
unit that is shortening by virtue of mutual traction (Figure 18.3C). But as cells
2 and 4 intercalate between cells 1 and 3 (Figure 18.3C), and begin to exert
traction on one another (Figure 18.3D), cells 1 and 3 are displaced anteriorly
