Epigenetics of Reproduction in Animals - Building the Most Complex Structure on Earth - page 111

The ovaries and testicles possess an intrinsic catecholaminergic network of neu-
rons (Mayerhofer et al., 1997) and it is plausible that Ach released by these neurons
might modify the microtubule length, as illustrated later.
Recall that secretion of microtubule-modifying hormones is controlled by the CNS
via neuroendocrine signal cascades. No theoretical explanation or hypothesis has been
presented on the source of information necessary for the spatial patterns of the place-
ment of cytoplasmic determinants in the egg cell. We are clearly in the midst of an
explanatory conundrum, but the situation may not be as hopeless as it may seem.
Examination of the proximate mechanisms of modification of microtubules sug-
gests that the involvement of not only neurohormones and neurotransmitters, but also
of hormones of the target endocrine glands (which are secreted in response to neural
signals), suggests that the nervous system may play an important role in the dynam-
ics of microtubules in the oocyte, and, consequently, in the process of the ordered
placement of maternal cytoplasmic factors in the oocyte.
Can the nervous system be a source of epigenetic information that determines
the location of maternal factors in the oocyte? So far, we have no direct experimen-
tal evidence that neuroactive substances, neurohormones, and other hormones that
affect the length and dynamics of the cytoskeleton microtubules also regulate the
spatial patterns of deposition of maternal factors in the oocyte. Hence, I will restrict
my efforts to finding any empirical evidence on the adaptive neural regulation of the
length of microtubules. I chose to examine this possibility in studies on the adap-
tive coloration of fish and cephalopod species, an adaptive mechanism that in fish
evolved more than 400 million years ago ( Fujii, 2000 ).
The Nervous System Regulates “At Will” the Length
of Microtubules
A number of fish species can, in seconds and seemingly at will, change their body
color and pattern to match the perceived color of their background. Some can even
mimic body colors and patterns of heterospecific fishes to avoid detection within
the heterospecific crowd. This is the case with several fishes, such as the neon tetra
( Paracheirodon innesi ), blue damselfish ( Chrysiptera cyanea ), and the paradise
whiptail ( Pentapodus paradiseus ), as well as several cephalopods.
The cells responsible for the adaptive change in body color in these fishes are
iridophores, pigmentless light-reflecting cells of the skin that contain piles of thin
transparent guanine platelets, which form multilayer reflectors. The distance
between guanine plates is determined by the length of microtubules that connect
plates to each other ( Kasukawa et al., 1987; Mäthger et al., 2003 ). Any change in
the length of the microtubules between plates will produce a change in the color
reflected by iridophores, which is what the human eye perceives.
It was found that neurotransmitters like noradrenaline shift the light reflected
by iridophores toward longer wavelengths, while Ach and noradrenaline antago-
nists such as adenosine shift it toward shorter wavelengths ( Kasukawa, et al., 1987;