Epigenetic Control of Animal Development - Building the Most Complex Structure on Earth - page 120

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Epigenetic Control of Animal
Development

In Chapter 2 (in Section “Epigenetic Programming of Gene Expression in the Egg”),
it was pointed out that the zygote's monopoly over procreation is the result of the
epigenetic information provided to gametes in the form of cytoplasmic factors and
epigenetic marks, rather than the genetic information in DNA, which is identical in
all animal cells. Using sophisticated epigenetic contrivances at the cell (cytoskele-
ton) and organismic (neural and neuroendocrine) levels, parents provided gametes
with the irreplaceable dowry of epigenetic information necessary for the zygote to
build up its incipient economy until the embryo reaches a stage when it becomes
informationally self-sufficient for erecting its complete adult structure.
Cloning experiments have shown that the epigenetic information in the egg cyto-
plasm, rather than the DNA or the cell nucleus, is what primarily makes cloning pos-
sible. This is why in all cloning experiments, biologists always use eggs (and eggs
of the same species, for that matter) alone. The transfer of a sheep somatic nucleus
into an ewe egg may produce a cloned sheep, but it is barely imaginable that it would
produce any viable organism if it were transferred into a somatic sheep cell or a
cow's egg. In cloning, there is no substitute for the epigenetic information contained
in the egg cytoplasm; it has the copyright on development.

Fertilization—Fusion of the Egg and Sperm Cell

Fertilization is an epigenetic process in which the genome is dormant. The develop-
ment of multicellular organisms from single-celled gametes occurs via zygotes (eggs
in parthenogenetic organisms), but since most animals are dioecious, herein I will
briefly deal with development via zygotes that form by the union of eggs and sperm
cells. Both are morphologically and physiologically prepared for the union.
No matter where the union of gametes takes place, whether in the environment
(external fertilization) or within the female organism (internal fertilization), the pro-
cess is generally conserved among the animal taxa and is similar in principle.
In the evolutionary division of labor, sperm is destined to seek and find the egg.
In its search for the egg, it is epigenetically guided by the gradient concentration of
chemoattractants released by the egg. In 1991, it was discovered that capacitated
spermatozoa are attracted to the egg by factors “released from the egg and its sur-
rounding cells” without identifying the factors ( Ralt et al., 1991 ). Later it was
reported that progesterone secreted by cumulus oophorus, a group of the ovarian fol-
licle cells surrounding the oocyte, serves as a chemoattractant, guiding sperm toward