Molecular Biology of the Cell by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter by by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morg

1204 Chapter 21: Development of Multicellular Organisms
cones, preventing them from re-entering the midline territory. The responses of
the growth cone depend on the receptors that it expresses: as commissural neurons
approach the floor plate, the Slit receptors are kept inactive by an inhibitory
protein (Robo3.1) in the same membrane, allowing the commissural axons to
grow to the midline without being repelled. Robo3.1 is lost as the growth cones
cross the midline; now the growth cones become sensitive to repulsion by Slit and
are thereby prevented from crossing back to the other side. At the same time, signals
from the Slit receptors interfere with those from the Netrin receptors, making
the growth cones deaf to the signal that attracted them to the floor plate initially.
A similar mechanism, using similar proteins, seems to govern midline crossing of
commissural axons in other animals, including flies and worms.
The guidance of commissural axons illustrates how axons rarely navigate
directly to their targets. Instead, they use intermediate targets, or guideposts, and
switch their sensitivities as they move from one local guidepost to the next, steering
their way through a complex environment to a far-away destination.
The Formation of Orderly Neural Maps Depends on Neuronal
Specificity
In many cases, neurons of a similar type are laid out in a broad array of different
positions, but send out axons that come together for their journey and arrive at
the target region in a tight bundle. There the axons disperse again, to terminate
at different sites in the target territory. This they do in an orderly way, creating a
regular mapping from one territory to another—a neural map.
The axon projection from the eye to the brain provides an important example.
The neurons in the retina that convey visual information back to the brain
are called retinal ganglion cells (RGCs). There are more than a million of them in
humans, each one reporting on a different part of the visual field. Their axons converge
on the optic nerve head at the back of the eye and travel together along the
developing optic nerve toward the brain. Their main site of termination, in most
vertebrates other than mammals, is the optic tectum—a broad expanse of cells in
the midbrain. In connecting with tectal neurons, the RGC axons distribute themselves
in a predictable pattern according to the arrangement of their cell bodies in
the retina: RGCs that are neighbors in the retina connect with target cells that are
neighbors in the tectum. The orderly projection creates a retinotopic map of visual
space on the tectum (Figure 21–75).
Orderly maps of this sort are found in many brain regions. In the auditory system,
for example, the neurons that project from the ear to the brain form a tonotopic
map in which brain cells receiving information about sounds of different
pitch are ordered along a line, like the keys of a piano. And in the somatosensory
Figure 21–75 The neural map from
eye to brain in a young zebrafish.
(A) Diagrammatic view, looking down on
the top of the head. (B) Fluorescence
micrograph. Fluorescent tracer dyes have
been injected into each eye—red into the
anterior part, green into the posterior part.
The tracer molecules have been taken up
by the neurons in the retina and carried
along their axons, revealing the paths they
take to the optic tectum in the brain and
the map that they form there. (Courtesy
of Chi-Bin Chien, from D.H. Sanes,
T.A. Reh and W.A. Harris, Development
of the Nervous System. San Diego, CA:
Academic Press, 2000.)
eye
eye
tectum
(A)
larva head
(B)
100 µm