Epigenetic Control of Animal Development - Building the Most Complex Structure on Earth - page 149

fashion it. Sufficient empirical evidence shows that the nervous system is responsi-
ble. For the details of how this happens, we must wait until the nature of the com-
puting in the nervous system is fully comprehended, which we hope will come
eventually.
The role of the nervous system in animal communication with their environment
and in controlling and regulating their physiology is generally recognized and repre-
sents objects of various disciplines of biology. This epigenetic theory of development
and evolution recognizes an additional function for the nervous system: the control
and regulation of animal morphology. The role of the nervous system as a source of
epigenetic information for animal development is a new idea and already has been
covered in various forms in my previous works. Although the developmental role of
the nervous system has almost never been a special object of biological investigation,
the evidence to substantiate it, surprisingly or expectedly, is more than adequate.
Now, I will present succinctly the essential evidence on the role of the nervous
system in the postphylotypic development of vertebrates.
Apoptosis in Invertebrates
Apoptosis as a form of programmed cell death (PCD) was described first by Carl
Vogt in 1842 ( Seipp et al., 2001 ). During metamorphosis, animals have to rid them-
selves of cells, tissues, and organs that are no longer necessary for postlarval stages.
PCD is a well-coordinated process that includes morphological, physiological, and
behavioral changes. The initiation of apoptisis requires an acute sensibility of struc-
tural changes and the function of the larvae in order to determine when to stop grow-
ing, eliminate old structures, and develop new structures. There is no other way to
seek out the organs responsible for sensing those changes and determining the meta-
morphic transformations than to examine the molecular cascades that initiate met-
amorphosis. We have a clear picture today of the signal cascades that control and
regulate metamorphosis in invertebrates. The causal chain in all of them starts with
signals from the CNS, which trigger the activation of signal cascades that cause the
apoptosis of the obsolete organs and formation of postlarval structures. The facts that
the sensing organ is the CNS and that it starts signal cascades for metamorphosis
are not surprising. The nervous system is the only system that pervades thoughout
the animal body up to a cell level. The nervous system is well known for its ability
to “direct the destruction of obsolete larval tissues and their replacements by tissues
and structures that form the adult fly … via the precise stage- and tissue-specific
regulation of key death effector genes” ( Draizen et al., 1999 ).
In insects, the signal cascade begins in the brain. Responding to afferent input
on body size and other physiological changes, at a determined point in time, the
insect's brain releases the neuropeptide prothoracicotropic hormone (PTTH), which
activates secretion by the prothoracic glands of ecdysone, which in turn activates a
downstream gene regulatory network, leading to the PCD (apoptosis) of the salivary
gland, as represented in a simplified form in Figure 3.24 .