Rise of the Animal Kingdom and Epigenetic Mechanisms of Evolution - Building the Most Complex Structure on Earth - page 284

growth of the muscle. These experiments demonstrated that the choice by the moto-
neuron of the type of ecdysone receptor that will be expressed determines whether
the ecdysone will play its muscle growth-inhibiting action (when EcRA is expressed)
or not (when the local innervation induces expression of EcR-B1).
[B]oth exposure to ecdysteroids and local influences from the motoneuron are
required for the upregulation and the maintenance of this high level of EcR-Bl
expression that is associated with muscle regrowth.
Hegstrom and Truman (1996)
Innervation determines which of EcR isoforms expresses the growing muscle.
Hegstrom et al. (1998)
Results of these experiments were corroborated by other in vitro and in vivo
experiments, which showed that local innervation performs its myogenetic func-
tion by releasing diffusible substances or by transmitting electrical signals in tissues
( Launay, 2001 ).
As mentioned earlier, muscle growth in insects is cerebrally regulated by the
types of neurohormones, the growth-promoting Ilps, and the growth-inhibiting
PTTH, with ecdysone serving as the mediator of the muscle-inhibiting action of the
PTTH. The above experiment shows that muscle growth is under a dual neural con-
trol, a central neural control via the circulating hormones and neurohormones, and
an adjacent control via local innervation. This is a binary neural mechanism of con-
trol of gene expression. It is indispensable for the development of animal structure
and must have evolved no later than the evolution of the centralized nervous system
in planarian-like animals during the Cambrian period.
The role of the ICS in regulating gene expression in animals, both globally and
locally, may be illustrated through the experimental evidence on brain surveillance
and the regulation of bone homeostasis.
The brain integrates and processes internal stimuli to generate and provide
instructions for bone remodeling to the bone skeleton ( Elefteriou, 2008 ).
Neurons in the central nervous system integrate clues from the internal and exter-
nal milieux, such as energy homeostasis, glycemia, or reproductive signals, with the
regulation of bone remodeling.
Elefteriou (2008)
It sends these instructions in the form of chemical signals to increase/decrease
production of osteoblasts and osteoclasts via two complementary pathways.
The hypothalamus regulates bone homeostasis remotely, via four signal cascades,
and locally, via sympathetic innervation. The integrated regulation of bone homeo-
stasis is depicted in Figure 5.17 .
The local regulation of bone homeostasis is related to the action of leptin, a hor-
mone that acts surprisingly in a nonhormonal way (i.e., not directly in bones, but in the
brain by stimulating the sympathetic innervation), which induces noradrenaline release
on osteoblasts, thus inhibiting osteoblast proliferation and bone loss ( Figure 5.18 ).