Epigenetic Control of Animal Development - Building the Most Complex Structure on Earth - page 146

Figure 3.21 The two major hypotheses about how developmental processes are conserved
against evolutionary changes. In both models, embryogenesis proceeds from bottom to top,
and the width represents the phylogenetic diversity of developmental processes, which are
deduced from morphological similarities. (A) The funnel-like model predicts conservation
at the earliest embryonic stage. During embryogenesis, diversity increases additively and
progressively. (B) The hourglass model predicts the conservation of the organogenesis stage.
During this stage, a highly intricate signaling network is established consisting of inductive
signals, including the Hox 11 genes, which leads to conservation of the animal body plan.
Source : From Irie and Kuratani (2011) , slightly simplified.
“neural crest effector genes” ( Muelemans and Bronner-Fraser, 2004 ), which deter-
mine the formation of mature NCCs in the neural plate.
NCCs from each region of the neural tube follow distinct itineraries of migration
to their target organs/tissues throughout the animal body. Before leaving the neural
tube, they are provided with epigenetic information on the following:
- Where to go and which are the relevant cues to follow?
- What to do in the specific organs/tssues they settle in?
On their journey, NCCs take their cues from certain receptors that express cells
that happen to be on the way to the target sites. Based on their origin along the
neural tube and to the site of destination in the animal body, different NCCs have
to follow different cues. It follows that different NCCs must be provided with dif-
ferent information on the cues they must follow. It is clear that “cues” that an NCC
follows on its path to the target site are not cues per se , in the sense that they are
meaningless signs to something other than cues. It is the NCC, and even a particu-
lar NCC type, that choose what chemical to take as a cue or a landmark on its way
to the target site. The destination site is the lodestar of the migration of NCCs, not
the chemicals that it may encounter on its travels through the labyrinthine path-
ways of the animal body.