Biology Reference
In-Depth Information
fold downwards, creating the folded structure of the mature epithelium. The process is
dependent on actin, and will not take place in the presence of cytochalasin D.
35
Although
Shroom is known to be expressed in the gut during early development, no studies of its
expression during late development
d
around the time of colon folding
d
have, at the time
of writing, been published so it is not clear whether folding in the colon and the neural
tube are driven by exactly the same mechanisms.
EVAGINATION OF IMAGINAL DISCS
Evagination is the morphological reverse of invagination and results in the outpushing of
a dome or a tube. One of the most striking and best-studied examples of evagination is
responsible for the limb imaginal discs of D. melanogaster transforming from almost flat plates
of epithelia to limb-shaped tubes. Imaginal discs are epithelial sheets that develop inside the
body during embryogenesis and remain hidden inside during the larval stages of the life
cycle. When the hormone ecdysone reaches a high enough concentration to trigger metamor-
phosis, the imaginal discs undergo spectacular morphogenetic movements to form structures
such as wings and legs, characteristic of the adult body plan.
The leg imaginal disc is a typical epithelium in that it consists of a sheet of cells linked to
each other by adherens junctions towards the apical end of the cell and linked to a basement
membrane by integrin-containing junctions on the basal surface. Both the adherens cell-cell
junctions and the integrin cell-matrix junctions are associated with large arrays of actin
microfilaments.
39
Early in the development of a disc, the morphology of the epithelial cells
is unremarkable. As the disc grows, however, the cells change shape, particularly in the
regions that will give rise to the proximal and dorsal regions of the leg. The previously
columnar cells become highly anisotropic, showing a pronounced elongation along an axis
tangential to the edge of the disc and shortening along the radial axis (
Figure 18.12
). Further-
more, this anisotropy is more extreme at each cell's apical end than at its basal end, which has
the result that the set of cells with which the apical end of a given cell makes contact overlaps
but is not identical with the set of cells with which its basal end makes contact.
39
As these
shape changes take place, the epithelium begins to fold although it does not yet evert.
At the onset of metamorphosis, the shape of the anisotropic cells changes once more and
the circumferential elongation disappears so that the cells are again isotropic. This has the
effect of elongating the tissue in a radial direction and, because the disc has already begun
to evert so that its centre is now at the tip of what will be the leg, elongation in a radial direc-
tion with respect to the disc becomes elongated along the axis of what is now an emerging leg
(
Figure 18.12
). There must also presumably be a degree of neighbour exchange, because
without it the number of cells around the circumference of the leg at different points of its
length would vary with the number around the corresponding radius of the original disc,
which would be proportional to the radius itself.
As in the examples of invagination described earlier in this chapter, evagination in the leg
disc depends on myosin-mediated contraction of actin fibre bundles. Actin is expressed
strongly in cables under the adherens junctions of the disc and it is required for morphogen-
esis; evagination fails if discs are first treated with the actin-depolymerizing drug cytocha-
lasin B.
37
Myosin is also required, and this has been demonstrated by an ingenious
