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

CHAPTER 21 END-OF-CHAPTER PROBLEMS
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fundamental role in learning and memory. Clearly, for the nervous system as for
other parts of the body, developmental processes do not end at birth, as we discuss
in the next chapter.
Summary
The development of the nervous system proceeds in four phases. First, neurons and
glial cells are generated from dividing neural progenitor cells. Then, the newborn
neurons send out axons and dendrites toward their targets. Next, they make synaptic
connections with appropriate target cells so that communication can begin.
Finally, excessive neurons are eliminated by normal neuronal cell death, after
which the system of synaptic connections is refined and remodeled according to the
pattern of electrical and synaptic activity in the neural network.
Neurons born at different times and places are specialized to express different
sets of genes, and they have a cell memory that plays a major role in determining
the connections they will form. Their specialization depends not only on spatial
patterning by morphogens but also on intrinsic developmental programs that
unfold as the neural progenitors proliferate. Axons and dendrites grow out from
the neurons by means of growth cones, which follow specific pathways delineated
by attractive and repellant signals along the way, including cell-surface and extracellular
matrix molecules and soluble signal proteins to which growth cones from
different classes of neurons respond differently. In many parts of the nervous system,
neural maps are set up—orderly projections of one array of neurons onto another.
In the retinotopic system, the map is based on the matching of complementary systems
of position-specific cell-surface markers—ephrins and Eph receptors—possessed
by the two sets of cells. Other cell-surface molecules such as DSCAM proteins
in Drosophila and protocadherins in vertebrates mediate self-avoidance between
the branches arising from a single neuron, helping the cell spread out its processes.
The formation of synapses involves back-and-forth signaling between target
cells and the growth cone. After the growth cones have reached their targets and
initial connections have formed, individual synapses are eliminated in some places
and reinforced in others by mechanisms that depend on synaptic and electrical
activity. These mechanisms adjust the architecture of the neural network according
to the way in which it is used.
What we don't know
• What regulates the pace of
development? Why does a mouse
embryo develop faster than a human
embryo, for example?
• What are the mechanisms that
allow cell memory to be stored during
development, explaining how each
cell’s history determines its future
behavior?
• How do signals move through
tissues? What are the roles of the
extracellular matrix and of elongated
cell projections?
• How does a cell know exactly where
it is in a multicellular organism? How
does it know that its neighbors are the
correct ones and that, if not, it should
move or kill itself?
• How do cells respond to tiny
gradients of molecules in their
environment, as required for knowing
their positions? How are morphogen
gradients reliably interpreted?
• What are the genetic changes that
allow the repurposing of existing body
parts during evolution? For example,
how did bat wings evolve from arms?
• How do cells use genetic
instructions to form the shape of
something as complex as the human
nose?
Problems
Which statements are true? Explain why or why not.
21–1 In the early cleavage stages, when the embryo
cannot yet feed, the developmental program is driven and
controlled entirely by the material deposited in the egg by
the mother.
21–2 Because of the many later developmental transformations
that produce the elaborately structured organs,
the body plan set up during gastrulation bears little resemblance
to the body plan in the adult.
21–3 As development progresses, individual cells
become more and more restricted in the range of cell types
they can give rise to.
21–4 At different stages of embryonic development,
the same signals are used over and over again by different
cells, but with different biological outcomes.
21–5 Changes in the coding regions of genes involved in
development are primarily responsible for the differences
between species.
21–6 The cell cycle is the ticking clock that sets the
tempo of developmental processes, with maturational
changes in gene expression being dependent on cell-cycle
progression.
Discuss the following problems.
21–7 Name the four processes that are fundamental to
animal development, and describe each of them in a single
sentence.
21–8 What are the three germ layers formed during gastrulation,
and what are the principal structures each gives
rise to in the adult?