Epigenetic control of the early developmen
- From: CNCabej@xxxxxxx
- Date: Wed, 11 Feb 2009 06:23:33 -0800 (PST)
Epigenetic control of the early development
Leading investigators in the field of individual development consider
the control of early development by maternal/parental cytoplasmic
factors to be a scientific fact (Wolpert L, et al. 1998. Principles of
Development, Oxford University Press, 63-70 and 127-146; Hall BK,
1998a. Evolutionary Developmental Biology, 2nd edition. London:
Chapman & Hall,114-119; Gilbert SF, 2000. Developmental Biology, 6th
edition. Sunderland, MA, Sinauer Associates, Inc. Publishers, pp.
263-285), This is another way of saying that early embryonic
development is under epigenetic control. If this is true what is the
role of the genetic information in the early embryonic development?
To answer this question one has to look at the mechanism by which
epigenetic factors (cytoplasmic factors and paternal centrosome/
centrioles) perform the control of early development.
Parentally inherited epigenetic factors carry out their developmental
functions in two general ways:
1.Maternal cytoplasmic factors by inducing expression of zygotic genes
directly (maternal retinoic acid, e.g. regulation of expression of Hox
genes in bovines) or after being translated as proteins from maternal
cytoplasmic mRNAs.
2.Paternal centriole/centrosomes, by relocating at opposite poles,
organize the mitotic spindle apparatus, which is a prerequisite of
mitotic cell division. Additionally, the centrosome/MTOC by
determining formation of the cytoskeleton make an essential
contribution to the organization of zygotic structure.(Sathananthan,
A.H. 1997. Histology and Histopathology 12: 827-56).
From this point of view, the function of the epigenetic informationdeposited in gametes is to determine the early development of the
embryonic structure by using the species-specific genetic information.
Building on the overused metaphor of the “genetic tool kit”, the
epigenetic information contained in the gametes during the early
development is the user of the “genetic tool kit”. This fact has
important developmental and evolutionary implications. It enables us
to understand numerous cases of intragenerational and
transgenerational plasticity, of evolutionary changes and speciation,
when morphological (and phenotypical in general) changes involve no
functional changes in genes.
The CNS
Formation of the CNS is epigenetically determined by maternal
cytoplasmic factors deposited in gametes under control of the parental
nervous system(s):
“The development of the nervous system is a largely epigenetic
phenomenon in which events induce subsequent events in what is usually
a highly orderly sequence” (Tiemey, A.J. 1995. Behavioral Processes
35: 173-182).
Only a few years ago neural induction was believed to take place
during gastrulation but now we know that it begins during the
blastula stage, if not earlier:
“The cascade of inductive interactions leading to the formation of the
central nervous system starts in the uncleaved egg” (Müller, W.A.
1996. Developmental Biology. Springer, p. 90).
Maternal factors start the neural induction and specification of the
neural cells in Xenopus laevis as early as the blastula stage (Wilson,
S.I. and Edlund, T. 2001. Nature Neuroscience4: 1161-1168), i.e.
before the large-scale expression of zygotic genes (12th cell division
or 4096-cell embryonic stage). In the 32-cell blastula, maternal
factors determine formation of the Nieuwkoop center (Stennard F, et
al. 1996. Development 122: 4179-4188) and activation of downstream
zygotic genes (Yang et al. 2002), which stimulate cells dorsal to the
Nieuwkoop center to form the Spemann’s organizer (Schneider et al.
1996. Mechanisms of Development 57: 191-198.; Laurent M.N. et al.
1997. Development 124: 4905-4919), whose signals induce formation of
neuroepithelium.
This quick look at the processes of neural induction and the
development of the CNS reveals some thought-provoking facts:
First, the nervous system is the first organ system to develop,
although the conventional wisdom says that systems of blood
circulation and excretory system would be necessary to develop before
all else. The over-early embryonic development of the CNS suggests
that during that early stage it might serve something other than the
communication with the external world.
Second, initially the CNS is excessively large (in some cases
representing a quarter of the overall embryonic mass) what again
cannot be related to the almost nonexistent communication with the
external environment.
Third, the exhaustion of the reserve of epigenetic information
(maternal cytoplasmic factors), which regulate the early development,
coincides with the formation of a functioning CNS at the phylotypic
stage.
Fourth, the incipient CNS immediately engenders
“a network of inductions that give rise to the different cells,
tissues and organs of embryos and adults.” (Hall 1998. p. 131) and
“further structures arise in relation to this central axis. This is
especially evident in the development of paired elements such as the
somites that presage the vertebrae, and paired organ rudiments such as
left and right limb buds and the rpimorfia of the gonads, kidney, lung
heart” (Hall, B.K. 1998. Evolutionary Developmental Biology, 2nd
edition. London: Chapman & Hall,114-119. p. 163).
While all this points in the direction of an essential role of the
embryonic CNS in the post-phylotypic development, adequate empirical
evidence also shows that the CNS is the source of signals for starting
signal cascades for the postphylotypic development (to be shortly
dealt in the next post).
Formation of primordial germ cells
Among the first types of embryonic cells that enter the process of
differentiation are the primordial germ cells. Their fate as gamete
precursors is epigenetically determined by maternal factor(s)
(Weidinger, G. et al., 2003. Current Biology 13: 1429-1434). In most
species studied so far, a maternal factor, vasa mRNA, deposited in the
vegetal pole of the egg is the most important factor for gamete
differentiation (primordial germ cells) during the early development.
In insects
Cell division during the cleavage is regulated epigenetically by
maternal cyclins and the maternal String protein, whose transcripts
are present in the zygote and are among the earliest mRNAs to be
translated. Because of the uniform distribution of these factors, the
early cycles of nuclear divisions are uniformly executed but no cell
divisions occur. The resulting naked nuclei move to the outer edge of
the embryo, thus leading to formation of a syncytial blastoderm. This
lasts until the 14th cell cycle, when normal cell divisions begin to
occur and the syncytial blastoderm is transformed into a cellularized
blastoderm. This change is related to the fact that at this point in
time the maternal reserve of the String protein is exhausted and
differential expression of the gene for the String protein in various
parts of the embryo begins.
"After the 17th or 18th cycle, cells in the epidermis and mesoderm
stop dividing, and differentiate. This cessation of proliferation is
caused by the exhaustion of maternal cyclin E, originally laid down in
the egg, which is required for progression through the cell
cycle." (Wolpert et al. 1998. Principles of Development. Oxford
University Press)
In vertebrates
The first divisions of the zygote are stimulated by the synthesis of
the cyclin protein Cdc6 by the maternal Cdc6 mRNA that is stored in
the egg cell during the maturation of the oocyte shortly after the
GVBD (germinal vesicle breakdown). The maternal Cdc6 mRNA makes
possible the replication of zygotic chromosomes by activating the MCM
(minichromosome maintenance) helicase complex (Lemaitre, J-M et al.,
2002. Nature 419: 718-722).
Upon each cell division, cyclins are destroyed and new cyclins are
synthesized from the reserve of maternal cyclin mRNAs. They combine
with a cyclin-dependent kinase to form the MPF (maturation promoting
factor), a phosphoprotein that is responsible for cell division since
the early stages of the embryonic development.
“In most species (mammals being the chief exception), the rate of cell
division and the placement of the blastomeres with respect to one
another is completely under the control of the proteins and mRNAs
stored in the oocyte by the mother” (Gilbert, S.F. 2000. Developmental
Biology 6th edition, p. 223))
In mammals
The physical continuity of the mother and the developing embryo makes
it possible for mammals to depend less on the deposition of maternal
factors and rely more on direct maternal control, via the
neuroendocrine system, a "real time" control and regulation of
embryonic development. Maternal hormones, growth factors, and other
secreted proteins that reach the embryo transplacental, are
essentially involved in mammal embryogenesis from its beginning.
N.C.
.
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