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

1198 Chapter 21: Development of Multicellular Organisms
extracellular signals. The mystery is how these processes are regulated and coordinated
to produce and maintain the characteristic final size of the adult organ or
animal.
Some signals such as survival factors, growth factors, and mitogens stimulate
growth by promoting cell survival, cell growth, and cell division, respectively, while
other signal molecules do the opposite. Although most of these signals operate
locally to help sculpt the size and shape of the animal, its organs, and appendages,
others act as hormones to regulate the growth of the animal as a whole. Nutrients
can regulate growth through hormonal signals in the entire body.
Many animals and organs can, by unknown mechanisms, assess their total cell
mass and regulate it. If, for example, cell size is artificially increased or decreased in
these cases, cell numbers adjust to maintain a normal total cell mass. Conversely, if
cell numbers are artificially increased or decreased, cell size adjusts to compensate.
NEURAL DEVELOPMENT
The development of the nervous system poses problems that have little parallel
in other tissues. A typical nerve cell, or neuron, has a structure unlike that of any
other class of cells, with a long axon and branching dendrites, both of which make
many synaptic connections to other cells (Figure 21–66). The central challenge of
neural development is to explain how the axons and dendrites grow out, find their
right partners, and synapse with them selectively to create a neural network—an
electrical signaling system—that functions correctly to guide behavior (Figure
21–67). The problem is formidable: the human brain contains more than 10 11
neurons, each of which, on average, has to make connections with a thousand
others, according to a regular and predictable wiring plan. The precision required
is not so great as in a man-made computer, because the brain performs its computations
in a different way and is more tolerant of vagaries in individual components.
But the human brain nevertheless outstrips all other biological structures
in its organized complexity.
The components of a typical nervous system—the various classes of neurons,
glial cells, sensory cells, and muscles—originate in a number of widely separate
locations in the embryo. Thus, in the first phase of neural development, the different
parts of the nervous system develop according to their own local programs:
neurons are born and assigned specific characters according to the place and
time of their birth, under the control of inductive signals and transcription regulators,
by mechanisms of the types we have already discussed. In the next phase,
newborn neurons extend axons and dendrites along specific routes toward their
target cells, guided by extracellular signals that attract or repel them. In the third
phase, neurons form synapses with other neurons or muscle cells, setting up a
provisional but orderly network of connections. In the final phase, which continues
into adult life, the synaptic connections are adjusted and refined through
mechanisms that usually depend on synaptic signaling between the cells involved
dendrites
receive
synaptic
inputs
axon (less than 1 mm to
more than 1 m in length)
cell body
terminal branches of axon make
synapses on target cells
25 µm
Figure 21–66 A typical neuron of a
vertebrate. The arrows indicate the
direction in which signals are conveyed.
The neuron shown is a basket cell, a type
of neuron in the cerebellum. (Adapted from
S. Ramón y Cajal, Histologie du Système
Nerveux de l’Homme et des Vertébrés,
1909–1911. Paris: Maloine; reprinted,
Madrid: C.S.I.C., 1972.)