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Figure 18.5 The relationship between the rates of local cell intercalation or migration and
tissue-level displacement argues that the cells crawl on one another rather than on an
external substratum. Assume that the cells exercise a constant rate of translocation on one
another's surfaces in the mediolateral axis, either by the bipolar, mediolaterally orientated
mode (A) or the monopolar, medially-directed mode (B). Under this assumption, the rates
of approach to the midline will be progressively greater with distance from the midline of
the array (arrows, A, B). Intercalation would occur throughout the array, because the cells,
using one another as substrates, would move between one another uniformly. However, if
the cells exercise a uniform, medially-directed rate of translocation on an external
substrate, the rate of approach to the midline of the array will be the same with distance
from the midline (C). Intercalation would occur only at the row of cells next to the midline,
assuming that the second row would continue to move until it reached the midline and then
such progressively increasing rates of local activity with distance from the
midline in converging mesodermal or neural tissues of Xenopus.
Interestingly, the convergence of the cells of the teleost germ ring toward the
embryonic shield has the reverse pattern from what is expected by the cell-
traction/cell substrate model of cell intercalation: they move faster as they
approach the shield (Trinkaus et al., 1992; Trinkaus, 1998; Marlow et al.,
2002). This fact suggests that they are migrating on an 'external' substrate, in
this case, either the overlying enveloping layer or the underlying yolk syncytial
layer, rather than one another, although they do interact with one another and
even intercalate between one another (Trinkaus et al., 1992). However, they
do not undergo the organized intercalation associated with convergence-
driven extension, and these cells do not produce extension of the germ ring as
they converge. This movement has all the hallmarks of a directed migration,
and the fact that the cells appear to be more persistent in their movement as
they approach the midline suggests that they are responding to a midline-
generated chemotactic signal. In contrast, in the dorsal sector of the
blastoderm, convergence and extension occurs by cell intercalation and in
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