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
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Figure 21–67 The complex organization of nerve cell connections. This
drawing depicts a section through a small part of a mammalian brain—the
olfactory bulb of a dog—stained by the Golgi technique. The black objects
are neurons; the thin lines are axons and dendrites, through which the various
sets of neurons are interconnected according to precise rules. (From
C. Golgi, Riv. sper. freniat. Reggio-Emilia 1:405–425, 1875.)
(Figure 21–68). At all stages, neurons are in intimate contact with various types of
non-neuronal supporting cells—the glial cells.
Neurons Are Assigned Different Characters According to the Time
and Place of Their Birth
We start our account here with the first phase of neural development: the generation
of neural progenitors and their differentiation into hundreds of different
neuronal subtypes, along with a much smaller number of glial types. Although the
nervous system is exceptional in the extent of cell diversity, the process depends
on the same principles that generate different cell types in other organs. We have
already discussed some of the underlying machinery in the developing Drosophila
nervous system. We turn now to vertebrates.
The vertebrate spinal cord, the brain, and the retina of the eye together constitute
the central nervous system (CNS). They all originate as parts of the neural
tube, whose formation was described earlier (see Figure 21–56). The brain and
eyes develop from the anterior neural tube and the spinal cord from the posterior.
The developmental anatomy is seen at its simplest in the spinal cord. As it
develops, the epithelium forming the walls of the posterior neural tube becomes
enormously thickened as the cells proliferate and differentiate, creating a highly
organized structure of neurons and glial cells, surrounding a small central channel.
Bands of neurons with different future functions—and expressing different
genes—are laid out along the dorsoventral axis of the tube. Motor neurons (those
that control the muscles) are located ventrally, whereas neurons that process sensory
information are found dorsally. This pattern is established by opposing gradients
of morphogens. These are secreted by specialized groups of cells that run the
length of the ventral and dorsal midlines of the neural tube (Figure 21–69). The
two morphogen gradients—consisting of Sonic hedgehog protein from the ventral
source and BMP and Wnt from the dorsal source—help induce different groups
of proliferating neural progenitor cells and differentiating neurons to express different
combinations of transcription regulators. These regulators in turn drive the
production of different combinations of neurotransmitters, receptors, cell–cell
adhesion proteins, and other molecules, creating terminally differentiated neurons
that will form synaptic connections selectively with the right partners and
exchange appropriate signals with them.
Figure 21–68 The four phases of neural
development.
genesis of neurons
outgrowth of axons and dendrites
synapse formation
refinement of synaptic connections
MBoC6 m22.94/22.67
neural progenitors
target cells