Re: Epigenetic Control of Development, Homeostasis and Reproduction

On Jan 30, 12:31 pm, rnorman <rnor...@xxxxxxxxx> wrote:
On Jan 30, 10:06 am, CNCa...@xxxxxxx wrote:





Epigenetic Control of Development, Homeostasis and Reproduction

I.

Control Systems in Metazoans

Any open system tends to lose its structural order and so do
living systems. But living systems could not have evolved without
evolving the capability for maintaining, within certain limits,
their characteristic structural order, underlying their vital
functions (growth, reproduction, evolution, etc).

Living systems, as highly improbable structures, are more
liable to entropic influences and are in every moment losing
their structural order at both the sub- and supracellular
level. At the molecular level, the genome performs a
great deal of work for maintaining a constant the
internal cell environment but epigenetic mechanisms
are also involved in the regulation of the specific
arrangement of supramolecular structures within the cell.

Metazoans, and unicellulars in general, are faced with an
aditional tasks: they have to coordinate the function of the
myriad of cells in order to perform  new supracellular,
systemic functions. The function of every cell is
subordinated to the function of the organism. The individual
cell in multicellulars is not as free as a unicellular is;
extracellular constraints have limited its freedom. The freedom
of the individual cells in metazoans has to be sacrificed for
the sake of the freedom of the multicellular organism. But
the function of the multicellular organism requires that it
is capable of maintaining its unavoidably degrading structure
at the cellular and supracellular level. Easy as it is to say,
this control of the the activity of individual cells was a
formidable challenge to evolution of multicellular life.

In order to function, even a simple device as a thermostat
needs a control mechanism that would be able to sense the
level of temperature and switch off/on a heat-producing
device for maintaining the internal temperature within limits
determined by a set point. If a control system is necessary
 for the function of a very simple device such as a thermostat,
the functioning and the very existence of an incomparably
more complex system, such as an unicellular
or multicellular organism is unimaginable without
a control system.

The control system would be capable of continually
monitoring  the state of the structure of the system at the
cell level, to compare it with the normal structure, to
determine deviations from the norm and send
instructions for restoring the normal structure. Such a
control system that would control the structure and
coordinate activities of the myriad of cells in the system
could not reside in a single cell (no cell could monitor
the status of the structure troughout the animal body
or send signals or restoring the normal state
throughout the body) but in a supracellular structure.

Tracing back the evolution of metazoan life in very simple
organisms of the type of cnidarians we see that this
control is function of their diffuse nervous system,
which has access to every part of the animal structure.
Due to its pervasive presence throughout the cnidarian body,
the capability of neural cells to receive, process, integrate
and transmit the information on external and internal
environment to other cells throughout the animal body,
the primitive nervous system of cnidarians was the only
system aparently meeting all the basic requirements of a
control system for maintaining the structure and functions
in these organisms. Indeed, the nervous system in these
simple animals controls all the vital functions.
The network of neurons in this diffuse nervous system
is specialized in receiving stimuli from external and internal
environment, in integrating them and sending
electrical/chemical signals throughout the body for coordinating
activities of different types of cells in cnidarians.Ultimately,
all the behavioral, reproductive and growth phenomena,
including metamorphosis, in Cnidaria are under neural
control via  neurohormones released by secretory  neurons
(Hartenstein, V. 2006. The neuroendocrine system in
inveretbrates: a developmental and evolutionary
perspective. Endocrinology 190: 555-570).

It may be not be purely by chance that the Cambrian
explosion coincided with the evolution of the neuron and
the nervous system. It may also not be a game of chance
the fact that sponges that have a comparable degree of
structural complexity (a comparable number
of differentiated cells) with cnidarians but evolved no
nervous system remained a "dead end" of evolution.

"The origin of differentiated nervous tissue must have
proceeded in a number of steps, of which the first
would obviously be the development of a specialized
receptor monitoring changes in the extrernal environment,
such as light. In order for the detected changes to influence
the organism, the receptor or primitive neuron would have
to communicate in some way with the rest of the organism....
That scenario defines a minimal endocrine structure,
in which a receptor becomes also an independent effector,
secreting a molecule that carries a message to all parts
of the organism." (Gorbman, A. and Davey, K.1991.
Endocrines. In: Neural and Integrative Animal Physiology
4th ed. C.L. Prosser ed., Wiley-Liss, p.744).

While capable of regulating processes of growth and
reproduction in simplest invertebrates like cnidarians, the
simple diffuse nervous system (in Hydra consisting of just
6700 neurons) obviously was not capable of controlling
and regulating these processes in more complex invertebrates
and vertebrates. This led to evolution of concentration of
neurons in structures like ganglia and more generally
centralization of he nervous system in cerebral structures.
This was associated with specialization of hormone-secreting
glands, which ultimately were under neural control,
in a process that led to what we know as neurohormonal
system.

In higher invertebrates, such as crustaceans and insects,
the processes of reproduction and growth, including
processes of metamorphosis and apoptosis, are
under hormonal control of specialized endocrine glands,
creating the impression that the endocrine system in
these invertebrate classes took the control of growth,
metamorphosis and reproduction. This impression, as
we know, is false: secretion of these hormones
(ecdysone and juvenile in insects, e.g.) is under strict
neural control, in the meaning that their production
and secretion is induced by brain chemical signals
(brain neuropeptides).

The further`process of centralization of the nervous
system in vertebrates was associated with addition
of another level in the hierarchy of neuro-hormonal
control of vital processes of growth and reproduction
and even evolution (remember numerous cases of
evolutionary changes that involved only changes in the
timing and amount of secretion/suppression of hormones).

How do tese these glands know when and how much of
each hormone have to secrete in order to regulate
growth, development and reproduction of animals?
Where the pertinent information comes from?

As a neurophysiologist, I certainly won't deny the importance of
nervous systems.  However I am under the impression (delusion,
perhaps?) that plants also have highly evolved, complexly coordinated
bodies that evolved completely without nervous systems.

There is no question that plants are not simple organisms (although
incomparably simpler than you and I, e.g.). But if you pay a
little more attention you are going to see that all I am suggesting
is that what a multicellular organism needs to maintain its normal
structure is a control system, not necessarily a "nervous system".
In this post, as shown by the title, I choose to write about the
control system of metazoans, the nervous system, which is known
best.


Nervous systems are useful in motile, active organisms.  Nervous
systems are not the key to understanding evolution and development.- Hide quoted text -


We all know that the nervous system is necessary for the
movement of animals; it determines our behavior, our thoughts
and ideas. However, a closer look at the individual development
of a bird, a reptile or a mammal, like us, from zygote to adulthood,
shows that

1. Signal cascades for the development of numerous animal organs
start in the CNS. No other organ or organ system but the central
nervous
system is observed to "engender a network of inductions that give rise
to
the different cells, tissues and organs of embryos and adults"
(Hall, B.K. 1998. Evolutionary Developmental Biology. Second edition.
p.134).

2. The nervous system is the first organ system that develops
and is operational in all these embryos, although these embryous
do not need to move or think while still in the womb (the common
sense would suggest that blood circulation and excretory system
would be first to develop).

3. The evidence that signal cascades for evolutionary changes
start in the CNS.

4. All the known signal for transgenerational plasticity come from
the central nervous system.

5. Numerous cases of the developmental plasticity where from eggs
of the same brood, i.e. of the same genotype, and reared in the same
environment, offspring with discrete different morphologies are
produced in particular proportions. Signal cascades for these
epigenetic changes in morphology come from the central nervous system


Does not all this suggests to you a possible key role of the nervous
system in individual development and evolution?

N.C.

.

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