Biology Reference
In-Depth Information
CHAPTER
12
Waypoint
Navigation in th
e Embryo
The foregoing chapters in this section concentrated on the basic mechanisms of locomo-
tion and how cells direct that motion in response to local cues. Most of the experimental
evidence came from simple culture systems in which cells were presented with just one
potential guidance cue and in which a response was continuous migration in one direction.
Cells migrating in a developing embryo generally face a much more complicated task; they
have to make journeys that are seldom straight lines and have to respond to, or ignore,
many potential guidance cues. The ultimate destination may not exist at the time that
a migration is initiated and, even when it does, a beeline straight to it may not be the
optimum route, as it may conflict with other events that need to take place in the inter-
vening space. For this reason, it is perhaps not surprising that complex embryos have
evolved a multi-stage process of direction finding analogous to what sailors and aviators
call 'waypoint navigation'. In waypoint navigation, a route is split into several phases
that run from one distinct location
d
a 'waypoint'
d
to the next. For each phase of the
journey, navigational effort is concentrated on reaching the next waypoint rather than on
the ultimate destination: once a particular waypoint has been reached, it is forgotten about
and all effort is now invested in the next waypoint in the series. The 'turn left at the Rose and
Crown, turn right after the pillar box and right again at the old watermill' style of navigation,
familiar to motorists, is a homely example of waypoint navigation.
WAYPOINT NAVIGATION BY GERM CELLS IN
DROSOPHILA
MELANOGASTER
All of the gametes produced in the life of a D. melanogaster fly derive ultimately from
30
e
40 primordial germ cells that arise very early in embryogenesis. These cells arise at the
very posterior of the early embryo, far from the eventual location of the gonads and, like
the vertebrate germ cells described in Chapter 7, they have to undertake a complex migra-
tion. The first part of their journey appears to be entirely passive; they are caught up in
a general flow of cells that takes them and their neighbouring endodermal cells, which
will form the posterior midgut, towards the dorsal surface of the embryo quite near its
middle (
Figure 12.1
). From that point, they begin active migration.
During the time that it and the primordial germ cells are being swept along the embryo,
the posterior midgut primordium undergoes morphogenesis to form a typical gut-like tube
