Biology Reference
In-Depth Information
FIGURE 7.6
Netrin-1 attracts commissural neurons but repels trochlear motor axons during development of the
spinal cord/hindbrain. The source of netrin is known, but the existence of a gradient is assumed rather than proven.
the embryo to set up the 'wiring pattern' of neural connections. The principle processes of
neuronal cells, axons, are laid down behind a quasi-autonomous migratory system called
a 'growth cone'. Growth cones, which will be described in more detail in Chapters 8 and
11, move in a manner similar to complete cells and they have been described as 'fibroblasts
on a leash'. They use both local, contact-mediated cues and diffusible molecules to navigate
to targets that may be at a considerable distance from the cell body (for example, the muscles
at the end of the limb). What is more, navigation of axons has to be fairly precise in order for
the correct neural connections to be made, although mechanisms do exist to deal with occa-
sional wrong connections (see Chapter 24). Different neurons can also navigate accurately in
quite opposite directions within the same tissue. For example, within the spinal cord, axons
from commissural neurons, the cell bodies of which are located in the dorsal spinal cord,
grow ventrally towards the floor plate of the cord, being lured there by the locally produced
chemoattractant, netrin 1, and also by netrin 2 that is produced in the areas adjacent to the
floorplate.
19
e
21
Some motor axons, for example those of trochlear motor neurons, are repelled
by netrin 1 and migrate in the opposite direction,
22
although their apparently normal navi-
gation in netrin1
/
mice suggests that other factors must also be involved
23
(
Figure 7.6
).
The foregoing examples of migration were chosen to illustrate typical developing events
that make use of directed cell movement, and this short chapter is in no way intended to be
a comprehensive list. Many more examples, with more details on pathways and anatomy, can
be found in general textbooks of embryology and in specialist texts on the development of
specific systems such as the brain. Having established that cell migration is an important
morphogenetic mechanism, this section of the topic will go on to focus on the processes by
which cells can move, and then on the systems that allow them to navigate accurately
through the complex environment of a growing embryo.
Reference List
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oocyte interactions in Caenorhabditis elegans. BMC Dev Biol 2003;
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:10
4.
e
3. Wilson EB. The cell in development and inheritance. Macmillan; 1896.
