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

1150 Chapter 21: Development of Multicellular Organisms
Small Numbers of Conserved Cell–Cell Signaling Pathways
Coordinate Spatial Patterning
Spatial patterning of a developing animal requires that cells become different
according to their positions in the embryo, which means that cells must respond
to extracellular signals produced by other cells, especially their neighbors. In what
is probably the commonest mode of spatial patterning, a group of cells starts out
with the same developmental potential, and a signal from cells outside the group
then induces one or more members of the group to change their character. This
process is called inductive signaling. Generally, the inductive signal is limited in
time and space so that only a subset of the cells capable of responding—the cells
close to the source of the signal—take on the induced character (Figure 21–6).
Some inductive signals depend on cell–cell contact; others act over a longer range
and are mediated by molecules that diffuse through the extracellular medium or
are transported in the bloodstream (see Figure 15–2).
Most of the known inductive events in animal development are governed by
a small number of highly conserved signaling pathways, including transforming
growth factor-β (TGFβ), Wnt, Hedgehog, Notch, and receptor tyrosine kinase
(RTK) pathways (discussed in Chapter 15). The discovery of the limited vocabulary
that developing cells use for intercellular communication has emerged over
the past 25 years as one of the great simplifying features of developmental biology.
inductive signal
cells directed to new
developmental pathway
Figure 21–6 Inductive signaling.
MBoC6 m22.10/22.06
Through Combinatorial Control and Cell Memory, Simple Signals
Can Generate Complex Patterns
But how can this small number of signaling pathways generate the huge diversity
of cells and patterns? Three kinds of mechanisms are responsible. First, through
gene duplication, the basic components of a pathway often come to be encoded
by small families of closely related homologous genes. This allows for diversity in
the operation of the pathway, according to which family member is employed in
a given situation. Notch signaling, for example, may be mediated by Notch1 in
one tissue, but by its homolog Notch4 in another. Second, the response of a cell
to a given signal protein depends on the other signals that the cell is receiving
concurrently (Figure 21–7A). As a result, different combinations of signals can
generate a large variety of different responses. Third, and most fundamental, the
effect of activating a signaling pathway depends on the previous experiences of
the responding cell: past influences leave a lasting mark, registered in the state
of the cell’s chromatin and the selection of transcription regulatory proteins and
RNA molecules that the cell contains. This cell memory enables cells with different
signal A
signal B
signal A
signal C
signal X
signal Y
(A)
COMBINATORIAL SIGNALING
(B)
signal C
CELL MEMORY
signal C
Figure 21–7 Two mechanisms for
generating different responses to the
same inductive signal. (A) In combinatorial
signaling, the effect of a signal depends on
the presence of other signals received at
the same time. (B) Through cell memory,
previous signals (or other events) can leave
a lasting trace that alters the response to
the current signal (see Figure 7–54). The
memory trace is represented here in the
coloring of the cell nucleus.