Biology Reference
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this case the medial movement of the cells shows the progressive increase in
rate with distance from the midline (Glickman et al., 2003), characteristic of
intercalation by cell-on-cell traction model.
These differences in patterns of movements raise the issue of the definition
of convergence. On the one hand, convergence can be defined as the
movement of cells toward a midline, but that kind of movement does not
necessarily produce extension, for a number of reasons. If all move to the
midline at the same rate, no extension will be produced, at least not by virtue
of this movement. If there is a gradient in the rate of movement, those farther
away moving faster, intercalation should occur, but whether extension or
thickening will be produced, or how much of each, is dependent on additional
constraints on the intercalation process. These constraints would determine
whether intercalation occurred between radial neighbours to produce
thickening, or between mediolateral neighbours to produce extension. The
cell biological determinants of these alternatives of cell behaviour remain
unresolved and at once warn against oversimplifying the processes of
convergence and extension and assure that there is much more work to be
done on the cellular biomechanics of these processes.
Dynamics and stiffness: how does a dynamic tissue push?
The dorsal mesodermal tissues increase in stiffness during convergence and
extension and generate a pushing force of about a half micronewton (Moore
et al., 1995; Moore, 1992). How can cells shear between one another and yet
maintain a cohesive tissue suciently stiff to exert a pushing force? Our
mechanical model of convergent extension by cell intercalation holds that the
cells actively wedge between one another by mediolaterally orientated
traction, and as they do so, they push one another apart preferentially
along the anterior-posterior axis (Keller et al., 1992a, 2000). The wedging
forces generated by all of the individual cells are summed and generate the
total pushing force of the tissue. However, in order to push the cells must form
an array stiff enough to bear whatever compressive load the tissue will come
under as it pushes, without buckling. Mechanical measurements show that the
converging and extending mesodermal tissues become stiffer by a factor of
three or four, compared with pre-convergent extension tissues (Moore et al.,
1995), and that these tissues can push with about a half micronewton of force
(Moore, 1992).
Cell intercalation, the rearrangement of the structural elements of the tissue,
would seem to be at odds with forming a stiff structure. If a cell intercalates, it
must detach from old neighbours and reattach to new ones. If too many cells
did this at one time, the tissue would presumably become less stiff, because
external forces would be met with weaker intercellular adhesion, on average.
