Biology Reference
In-Depth Information
to be thought that the change in epithelial shape was driven by some external mechanism,
such as anisotropic tension in underlying extracellular matrix, and that the cells' ability to
exchange their old neighbours for new ones was an essentially passive process in response
to that mechanism. It is now becoming clear, however, that controlled neighbour exchange
may be the morphogenetic mechanism itself.
Convergent extension of epithelia driven by neighbour exchange is an important process
during the formation of an elongated axis in many animal phyla. It is particularly easy to
observe in ascidians due to the transparency of their embryos and the low numbers of cells
involved. In embryos of the ascidian Corella inlfata, an epithelial monolayer of 40 cells trans-
forms over six hours to produce a long stack of cells d the notochord d that is the main skel-
etal element in the forming tadpole. 1 e 3 Three-dimensional time-lapse recording of this
transformation in C. inflata shows that the cells undergo convergent extension so complete
that a patch of epithelium about six cells thick across the left-right axis becomes a single, elon-
gated row of cells 3 ( Figure 16.2 ). As convergent extension takes place, the area also invagi-
nates so that the developing notochord becomes internalized (invagination is discussed in
Chapter 18).
Convergent extension can also happen in epithelial tubes. For example, after the gut of the
sea urchin forms by invagination (Chapter 18), it elongates markedly. During this process, the
number of cells around its circumference decreases while the number along its length
increases, the change being brought about by cells changing their neighbour relationships 4
FIGURE 16.2 Convergent extension in C. inflata, based on detailed drawings made from three-dimensional
time-lapse images 3 . 'A' and 'P' refer to the antero-posterior axis. The schematic shows the epithelium flattened out
throughout and does not depict invagination that is happening at the same time. The drawing of the tadpole is to
indicate the eventual position of the notochord at a later stage of development.
Search WWH ::




Custom Search