Biology Reference
In-Depth Information
FIGURE 12.1
A schematic representation of primordial germ cell migration in Drosophila melanogaster. The
depiction of the gut and mesenchyme anatomy in the lower figures is simplified a little to make the germ cell
movements more clear.
(see Chapter 18). The topology of this process presents the primordial germ cells with
a problem: they find themselves on the luminal surface of the gut wall, the wrong side to
be topologically within the body. The first step of active germ cell migration is therefore to
migrate across the gut wall itself. This epithelium is specialized to open up spaces between
its cells to allow germ cell migration
1,2
and it opens up even in mutant flies in which germ
cells fail to form. This unusual ability of the posterior midgut to allow the passage of cells is
essential, and germ cell migration fails in fate-specification mutants (for example, huckebein,
serpent), in which the region of the gut that is supposed to be posterior midgut instead
develops hindgut character and will not open up.
3,4
The timing of germ cell migration seems
to be set by the gut wall itself. The crossing must also require active participation by the germ
cells because other cells transplanted to the same location will not cross. Activation of the
wall-crossing phase of germ cell migration requires activation of a specific G-protein
coupled receptor of the chemokine receptor family, called Tre (from the gene trapped in endo-
derm).
5
Tre acts autonomously in the germ cells themselves and, although its ligand has not
yet been identified, it is probably significant that a related molecule, CXCR4, is the receptor
for the gonad-derived chemoattractant CXCL12 in mice.
6,7
The precise pathway of Tre
signalling has not yet been elucidated, but it seems to require Rho1 and therefore may affect
actin organization.
5
Once through, and on the correct side of the gut wall, the germ cells have to migrate along
its surface towards the dorsal side of the embryo and thence to the mesoderm in which
gonads will form (
Figure 12.1
). Their accurate migration on the gut surface depends on
expression of the closely linked genes wunen1 and wunen2 by precisely those gut cells that
do NOT form a suitable pathway for the germ cells.
8,9
In the absence of any wunen expression
(in other words, a double mutant), germ cells migrate randomly and scatter over the gut
surface so that few find their ultimate mesenchymal targets, even though these targets are
