174
8 Morphogenesis
Fig. 8.3 Two models of the development of sea-shell pattern during growth (Meinhard, 1995).
In the left panel, new regions are activated when the inhibitor level becomes too low to suppress
autocatalytic action in the growing space between the maxima. On the right, existing activator
maxima are shifted towards higher substrate concentrations, which leads to their splitting
model, combining activation with inhibition, but operates in an entirely different
way. It presumes asymmetry sustained by chemical gradients, which have long been
known to play a crucial role in development (Sander, 1997). In Wolpert’s scheme the
activating and repressing actions are tied into the feed-forward motif, S → P, S → T,
P T (Fig. 8.4), which includes two activating (→) links with different thresholds
initiated by the same signal S induced by a morphogen diffusing from a signaling
source, and an inhibiting () link from the intermediate protein P to the target T.
The inhibiting link has a higher activation threshold, so that, as the morphogen level
decreases, it is switched off. This generates the classical “French flag” pattern shown
in the lower part of Fig. 8.4, with the target T expressed in the middle (“white”)
interval, where the signal level is below the higher threshold of the link to the protein
P and above the lower threshold of the direct link to the target.
Fig. 8.4 The feed-forward
motif and Wolpert’s French
flag
This basic scheme has been further extended, and its abstract links filled by specific interactions of specific chemicals, as will be elaborated in Sect. 8.3. Very readable expositions of the development processes, including molecular details of genes and signaling proteins (avoided here),
were published by some of the most prominent figures in
the field: Wolpert (2002) and Nüsslein-Volhard (2006).
Murray (1989) criticized the notion of positional information for its allegedly static character, but this limitation
fades in realistic complex networks involving more than
just a single morphogen. The early dynamic hypothesis
(Goodwin and Cohen, 1969) related the patterning to the
phase difference of two morphogenetic waves, propagating with different velocities. This opens rich possibilities
8 Morphogenesis
Fig. 8.3 Two models of the development of sea-shell pattern during growth (Meinhard, 1995).
In the left panel, new regions are activated when the inhibitor level becomes too low to suppress
autocatalytic action in the growing space between the maxima. On the right, existing activator
maxima are shifted towards higher substrate concentrations, which leads to their splitting
model, combining activation with inhibition, but operates in an entirely different
way. It presumes asymmetry sustained by chemical gradients, which have long been
known to play a crucial role in development (Sander, 1997). In Wolpert’s scheme the
activating and repressing actions are tied into the feed-forward motif, S → P, S → T,
P T (Fig. 8.4), which includes two activating (→) links with different thresholds
initiated by the same signal S induced by a morphogen diffusing from a signaling
source, and an inhibiting () link from the intermediate protein P to the target T.
The inhibiting link has a higher activation threshold, so that, as the morphogen level
decreases, it is switched off. This generates the classical “French flag” pattern shown
in the lower part of Fig. 8.4, with the target T expressed in the middle (“white”)
interval, where the signal level is below the higher threshold of the link to the protein
P and above the lower threshold of the direct link to the target.
Fig. 8.4 The feed-forward
motif and Wolpert’s French
flag
This basic scheme has been further extended, and its abstract links filled by specific interactions of specific chemicals, as will be elaborated in Sect. 8.3. Very readable expositions of the development processes, including molecular details of genes and signaling proteins (avoided here),
were published by some of the most prominent figures in
the field: Wolpert (2002) and Nüsslein-Volhard (2006).
Murray (1989) criticized the notion of positional information for its allegedly static character, but this limitation
fades in realistic complex networks involving more than
just a single morphogen. The early dynamic hypothesis
(Goodwin and Cohen, 1969) related the patterning to the
phase difference of two morphogenetic waves, propagating with different velocities. This opens rich possibilities