Biology Reference
In-Depth Information
Figure 18.4 Active, force-producing convergence and extension in early Xenopus
development is thought to involve convergence of the mediolateral aspect of the tissue
by virtue of the mediolaterally (in the mesoderm) or medially (in the neural tissue)
polarized protrusive activity (A, top). Volume is conserved, and therefore, if thickening
does not occur, convergence would result in a proportional extension (A, left). In fact,
convergence results in some extension and some thickening (A, right). Thickening can be
explained as follows. Although the protrusive activity driving convergence is orientated in
the mediolateral axis (or directed medially in the case of the neural tissue), as the cells
wedge between one another, they come under compression in the anterior-posterior axis.
Such compression would tend to squeeze the cells into a taller shape, or squeeze them out
of the layer to form a second layer (B). This tendency toward multilayering may be resisted
by continued radial intercalation (C)
