Biology Reference
In-Depth Information
chemoattractant, thermal noise will create a temporary random bias in one direction or
another. As soon as the cell moves in that direction, the balance of attractions will be broken
and a clear environmental cue will be present.
INTEGR
ATION OF CHEMOTAXIS AND CONTACT GUI
DANCE
In real embryos, chemotaxis is seldom the sole source of navigational information and cells
migrating in response to chemotaxis generally cross substrates that also provide navigational
information of some sort, even if only of a low-grade 'you may crawl across this area/you
may not crawl across that area' sort. Generally, chemotaxis is better adapted to provide
long-range navigational information and contact-guidance short-range information, and
the cell has to integrate both types of cue. Generally, since both cues tend to converge on
the Rac/Rho/cdc42 signalling systems, integration of directional information is 'automatic'.
Other 'branches' of intracellular pathways can also determine the ways in which signals are
integrated; for example, the binding of a particular component of the extracellular matrix
might result in a change in gene transcription to cause expression of a receptor for an already
present chemoattractant for the first time. In this way, cells can use local information about
their current location to 'decide' which long-range cues to follow. The ability of a cell to focus
on different navigational cues depending on where it finds itself is the basis of the 'waypoint
navigation' discussed in more detail in Chapter 12.
This chapter has presented an account of chemotaxis using just a few examples, chosen to
be those that are known in most detail. There are many other cell types that are presumed to
respond by chemotaxis to a range of other signalling molecules. The striking similarity of the
systems studied so far and their convergence on the same intracellular mediators of cytoskel-
etal activity suggests that all other examples will work by the same general mechanisms.
Nevertheless, it is worth keeping an open mind on that issue at least until more examples
have been studied in detail.
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