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

NEURAL DEVELOPMENT
1201
embryonic
cerebral
cortex
outer surface of
developing neural tube
inner surface of
developing neural tube
radial glial
cell process
migrating
neuron
nucleus
cell body of
radial glial cell
ventricular
proliferative
zone
The Growth Cone Pilots Axons Along Specific Routes Toward
Their Targets
10 µm
According to the character assigned to it during its early development, a neuron
will proceed to make connections with specific partners. This phase of neural
development involves a type of morphogenesis unique to the nervous system, in
which axons and dendrites extend along specific routes toward their target cells.
A typical neuron sends out one long axon and many dendrites, which are usually
shorter. The axon projects to distant target cells to which the neuron will eventually
send signals. The dendrites will receive incoming signals from axon terminals
MBoC6 m22.98/22.71
of other neurons. Axons and dendrites extend by growth at their tip, where one
sees an irregular, spiky enlargement called a growth cone (Figure 21–72 and
Movie 21.6). The growth cone is both the engine that produces the crawling movement
and the steering apparatus that directs the tip along the proper path. Cytoskeletal
machinery in the growth cone creates active protrusions, in the form of
filopodia and lamellipodia (see Chapter 16 for details): when such a protrusion
Figure 21–70 Migration of immature
neurons. Before sending out axons
and dendrites, newborn neurons often
migrate from their birthplace and settle in
another location. The diagrams are based
on reconstructions from sections of the
cerebral cortex (part of the neural tube) of
a monkey and rely on a staining technique
that picks out at random a small subset of
the whole dense mass of neuroepithelial
cells. The neurons go through their final
cell division close to the inner, lumenal
face of the neural tube (in the ventricular
proliferative zone) and then migrate
outward by crawling along radial glial cells
that form a scaffold. Each of these latter
cells extends from the inner to the outer
surface of the tube, a distance that may be
as long as 2 cm in the cerebral cortex of
the developing brain of a primate.
The radial glial cells can be considered
as persisting cells of the original columnar
epithelium of the neural tube that become
extraordinarily stretched as the wall of
the tube thickens. They also serve as
neural stem cells: depending on stage
and region, the newborn neurons can
be generated from radial glial cells that
undergo mitosis while their nuclei are close
to the inner surface of the tube, or they
can be generated from a nearby class of
specialized progenitors in the ventricular
proliferative zone. (After P. Rakic, J. Comp.
Neurol. 145:61–84, 1972. With permission
from John Wiley & Sons, Inc.)
neurons
dividing progenitor cell
radial glial cell
etc
layers of cortical
neurons
last-born
neurons
first-born
neurons
Figure 21–71 Programmed production
of different types of neurons at different
times from dividing progenitors in
the cerebral cortex of the brain of a
mammal. Close to one face of the cortical
neuroepithelium, progenitor cells divide, in
stem-cell fashion, to produce successive
generations of neurons (colored here
blue, green, red, orange, and black). The
neurons migrate out toward the opposite
face of the epithelium by crawling along the
surfaces of radial glial cells, as shown in
Figure 21–70. The first-born neurons settle
closest to their birthplace, while neurons
born later crawl past them to settle farther
out. Successive generations of neurons
thus occupy different layers in the cortex
and have different intrinsic characters
according to their birth dates.