Biology Reference
In-Depth Information
FIGURE 26.6
Whether or not cells leave the margin depends on whether the forces they bear are dominated by
tension (a) or compression (b). (a) If the cells even at the margin carry strong tension, then if a cell (B) happens to
have a small marginal surface, its neighbours (A, C) will be forced to make acute angles with the margin. This
brings the tension vector they contribute nearly parallel with the margin, while the tension vector contributed by the
narrowing cell is more perpendicular: the resultant will therefore pull the cells' common vertex outwards, making
the narrowing cell broaden again. (b) If, on the other hand, the tensions are less so that internal pressures dominate,
cells making more acute angles with the margin (A, C) will advance, pushing the already narrowing cell B to be
even narrower until it leaves the margin altogether.
perpendicularly to the margin. The resultant force will therefore move the vertices together
(
Figure 26.6
b). When they are close enough, they will fuse to make a four-way junction,
which can then split to allow cell B to lose contact with the margin. As long as circumferential
tension is small compared with internal pressure, the margin is therefore unstable and it will
tend to lose cells.
Running the computer model (in this case, understandable without a computer, at least in
retrospect) was therefore useful in providing a possible mechanical explanation for what
seemed like paradoxical loss of cells from the region.
