180
8 Morphogenesis
of both the target and the ligand, dependent on the structure of the overall interaction scheme. If there is a single target gene and a single ligand, there are altogether
sixteen combinations of their expression in the presence or absence of the autocrine
signal. The number of combinations grows exponentially as 2 2(n+1) with the number
n of autocrine ligands, leading to a great variety of expression domains that may be
generated by the same intrinsic genetic scheme.
The autocrine ligand’s signal is assumed to be short-range, so that it affects
expression of the target only in the vicinity of the domain where the ligand is
expressed. In the three sketches in Fig. 8.9, the letters E and N show whether the
target is expressed or not in the absence of the ligand. The letter A denotes expression
of the ligand, which can affect expression in a neighboring domain, either promoting
it (P + , as in Fig. 8.9b) or repressing it (P − , as in Fig. 8.9c and d). An example
of expression patterns generated in this way by two versions of a simple interaction
scheme is shown in Fig. 8.10. Some expression schemes generate oscillatory patterns
that may lead to propagating waves (Pismen and Simakov, 2011). This approach has
been applied to locate a narrow strip destined to develop into Drosophila’s dorsal
appendages (Sect. 7.7), but it is too abstract to attract biologists’ attention, which is
concentrated on particular genes and proteins, even though they may be specific to
certain model animals.
A distinct example of the action of autocrine signaling is the development of
stripes in Drosophila (Nüsslein-Volhard, 2006). The area where these stripes emerge
is defined by gap gene expression induced by a gradient along the antero-posterior
axis. Each gap gene encodes a diffusive signaling protein which represses other
gap genes wherever its concentration is above some threshold. As its concentration decreases, activation of other gap genes becomes possible, creating a periodic
pattern in a way similar to Turing’s scenario illustrated in Fig. 8.1. As commonly
happens in studies of development, this mechanism was validated by observing the
effects of mutations (Nüsslein-Volhard and Wieschaus, 1980). Molecular details of
patterning (including oscillatory expression) via local cell–cell interactions in other
development processes have been reviewed recently by Boareto (2019).
8.4 Dynamic Patterning
In vertebrates, the axis is segmented in another way, by a propagating wave of
gene expression, which is closer (but not identical) to the dynamic mechanism by
Goodwin and Cohen (1969). Somites, precursors of vertebrae and skeletal muscles,
form successively, at regular time intervals, as regularly spaced subdivisions along
the antero-posterior axis, as it progressively elongates posteriorly. In view of the
inhomogeneities in living tissue, it is impossible to maintain phase coherence on the
long scale required to generate a fixed number of vertebrae in this sequence. Such
precision requires a mechanism whereby the number of cells per somite adjusts to
the overall size of the embryo. Several alternative oscillation-based models have
been proposed, but the winning mechanism is the clock and wavefront model by
8 Morphogenesis
of both the target and the ligand, dependent on the structure of the overall interaction scheme. If there is a single target gene and a single ligand, there are altogether
sixteen combinations of their expression in the presence or absence of the autocrine
signal. The number of combinations grows exponentially as 2 2(n+1) with the number
n of autocrine ligands, leading to a great variety of expression domains that may be
generated by the same intrinsic genetic scheme.
The autocrine ligand’s signal is assumed to be short-range, so that it affects
expression of the target only in the vicinity of the domain where the ligand is
expressed. In the three sketches in Fig. 8.9, the letters E and N show whether the
target is expressed or not in the absence of the ligand. The letter A denotes expression
of the ligand, which can affect expression in a neighboring domain, either promoting
it (P + , as in Fig. 8.9b) or repressing it (P − , as in Fig. 8.9c and d). An example
of expression patterns generated in this way by two versions of a simple interaction
scheme is shown in Fig. 8.10. Some expression schemes generate oscillatory patterns
that may lead to propagating waves (Pismen and Simakov, 2011). This approach has
been applied to locate a narrow strip destined to develop into Drosophila’s dorsal
appendages (Sect. 7.7), but it is too abstract to attract biologists’ attention, which is
concentrated on particular genes and proteins, even though they may be specific to
certain model animals.
A distinct example of the action of autocrine signaling is the development of
stripes in Drosophila (Nüsslein-Volhard, 2006). The area where these stripes emerge
is defined by gap gene expression induced by a gradient along the antero-posterior
axis. Each gap gene encodes a diffusive signaling protein which represses other
gap genes wherever its concentration is above some threshold. As its concentration decreases, activation of other gap genes becomes possible, creating a periodic
pattern in a way similar to Turing’s scenario illustrated in Fig. 8.1. As commonly
happens in studies of development, this mechanism was validated by observing the
effects of mutations (Nüsslein-Volhard and Wieschaus, 1980). Molecular details of
patterning (including oscillatory expression) via local cell–cell interactions in other
development processes have been reviewed recently by Boareto (2019).
8.4 Dynamic Patterning
In vertebrates, the axis is segmented in another way, by a propagating wave of
gene expression, which is closer (but not identical) to the dynamic mechanism by
Goodwin and Cohen (1969). Somites, precursors of vertebrae and skeletal muscles,
form successively, at regular time intervals, as regularly spaced subdivisions along
the antero-posterior axis, as it progressively elongates posteriorly. In view of the
inhomogeneities in living tissue, it is impossible to maintain phase coherence on the
long scale required to generate a fixed number of vertebrae in this sequence. Such
precision requires a mechanism whereby the number of cells per somite adjusts to
the overall size of the embryo. Several alternative oscillation-based models have
been proposed, but the winning mechanism is the clock and wavefront model by