Biology Reference
In-Depth Information
FIGURE 26.5
Epiboly in
F. heteroclitus
. (a) An epithelial cap advances down over the egg in an animal-vegetal
direction until the whole egg is covered. (b) As the cap advances, the number of cells around its margin reduces as
cells reduce their marginal surfaces and allow neighbours to meet.
still increasing. The number of cells reduces because cells within the margin leave it by
reducing the length of their exposed membrane until their neighbours have met
(
Figure 26.5
b). The puzzle was why this happens, given that 'common sense' would suggest
a mechanism that adds cells while the circumference is expanding and removes them only
when it has passed the equator and is contracting.
In the computer model,
16
the epithelium was modelled using the system in
Figure 26.4
.
The cells were given initial conditions in which all margins (springs) in the bulk epithelium
had the same starting tension, but those at the lower margin of the epithelium could be given
a lower tension (as if, for example, they had less actin-myosin contractile activity along this
free margin). The cells all began with the same internal pressure. The activity of the yolk
syncytial layer was not modelled explicitly; instead its effect was simulated by a steadily
accelerating advance of the position of the epithelium's margin, vegetally. In other words,
the margin was 'pulled down by computer'.
As the simulation ran and the margin began to advance, the cells bordering it experienced
stronger forces than those behind them. How much stronger depended on the speed of
advance, relative to the speed at which cells behind could rearrange to share the stresses:
for a given maximum speed of cell rearrangements, slow advance of the margin led to
only modest differences in stress whereas rapid advance led to the margin experiencing
very much more mechanical force. The behaviour of the cells at the edge depended critically
on the balance between circumferential stretch forces at the margin (for example, actin-
myosin activity there) and internal pressure. Consider a section of the margin, specifically
just three cells, which happen to have the shapes drawn in
Figure 26.6
. The critical point
about these cells is that the boundaries of A and C subtend acute angles with the margin,
and B subtends obtuse angles. Under conditions in which there is strong circumferential
tension (compared with pressure), the two vertices marked with an asterisk will experience
a net force away from one another (
Figure 26.6
a), and will therefore move away. Only when
the angle is 90 degrees will there be no net sideways force. Under these conditions, therefore,
the system is self-stabilizing: any departure from the 'upright' cell boundaries shown in
Figure 26.6
a (lower) will be opposed and the system will return to its starting state. No cells
will leave the boundary.
Under conditions in which circumferential tension is weak compared with internal pres-
sure, this stability disappears. At the asterisked vertices, the pressure pushing from the
outside cells will be running at a shallow angle to (more nearly parallel to) the margin, while
the opposing pressure from the inside cell middle cells (B) will be pushing more
