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show that the dorsal migration of the mutants is actually faster than that of wild-type cells
but the coherence of the migration is lost so that the mutant cells spread throughout the
dorsal midline. The actin cable system may not have evolved for optimum efficiency of
migration, therefore, but also to be a restraining mechanism that allows migration to take
place without cells' spatial relationships to one another becoming too randomized (orderli-
ness is important because the cells concerned have already been assigned a segmental iden-
tity, and segment identity needs to be conserved without stagger at the midline).
The forces that drive hole closure do not come only from the closing epidermis, but also
from the amnioserosa over which the epidermis moves. The cells of the amnioserosa are
under their own tension, which can again be revealed by their springing away from a site
of laser ablation. What is more, ablation of amnioserosa cells near the edge of the hole causes
nearby epidermis to spring away too (
Figure 17.3
c). If the epidermis were advancing solely
by means of its circumferential tension, there is no reason that it should spring back; indeed,
its progress may be expected to accelerate with less amnioserosa in the way. That it springs
back therefore indicates that it was being pulled in by forces in the amnioserosa. Shortly
before closure begins, the amnioserosa cells (which are also a simple epithelium) express
actin-myosin networks spread throughout their apical domains.
9
Careful time-lapse observa-
tion reveals these apical networks undergo rhythmic contractions, somewhat synchronized
within a group of cells
10
so that a wave of contraction starts at the edge and moves inwards.
11
The contractions reduce the apical area of the cells and therefore of the amnioserosa as
a whole. This pulls the surrounding epidermis inwards temporarily. Before the epidermal
actin cables form, the epidermis tends to relax when the amnioserosa relaxes but, once the
cables have formed, they act as a ratchet, shortening when the amnioserosa contracts and
pulls the hole closed but not relaxing again when the amnioserosa does.
11
In this way, the
epidermis 'parasitizes' the power of the amnioserosa.
9
As closure proceeds, amnioserosa
cells have to 'get out of the way' of the advancing epidermis, lest they become an obstruction.
They do so by contracting their apices so much that they squeeze out of the basal side of their
own epithelial layer: there, alone, they die by elective cell death (Chapter 24).
12
Repeated ablation of the amnioserosa does not abolish dorsal closure, so while the amnio-
serosa can assist the closure process, it seems not to be essential for it. Ablation of both the
amnioserosa and tension-bearing cells at the edge of the epidermis does block closure, sug-
gesting that while closure can be driven by either one of these cell types, it cannot take place
in the absence of both.
1
The possibility remains, however, that the double-ablation causes
such damage that closure may have been prevented by non-specific effects.
The closure of the dorsal hole presents a formidable problem of accurate navigation,
because the epithelium on each side of the hole has already established its segmental identity
(using Hox genes, and so on). When the two sides come together, it is essential that the stripe
of epithelium from the left side of the embryo representing abdominal segment 1 fuses with
the abdominal segment 1 stripe from the right side, and only with that stripe. The same
applies to all of the other segments
d
any other outcome would throw the identities of the
left and right sides of the dorsal body out of registration with each other, create unexpected
segment boundaries and make the creation of normal anatomical septa between segments
impossible. Relying on mechanical perfection in a complex and ever-changing embryo is
not enough, so before they meet and fuse the cells from approaching sides actively seek suit-
able partners. As the sides come close, leading edge cells become much more active in
