such as "Introduction", "Conclusion"..etc
paternal genome undergoes a remarkable transformation in the oocyte
cytoplasm. It constitutes the remodeling
of sperm chromatin through the removal of protamines and replacement by
acetylated histones followed by genome-wide demethylation (18). Typically, the
process of demethylation is intricately coupled to chromatin remodeling in the
zygote. The chromatin during
spermatogenesis undergoes methylation and compacted with protamines (19) –
which are crucial to facilitate normal fertilization. Whereas, genome of oocyte possesses oocyte-specific
linker histones. The chromatin of female
pro-nucleus is more repressive than male pro-nucleus, which possibly protects
the oocyte genome against extensive epigenetic modifications imposed on the
paternal genome in the zygote.
Interestingly, the repetitive sequences (LINES, SINES, etc) are not
uniformly methylated in gametes.
cell proliferation during organogenesis and histological differentiation
follows mitosis which invariably results into two equal daughter somatic
cells. They share equally both
epigenetic factors and chromatin. Hence,
the two daughter cells look similar in all respects. In contrast, the zygote undergoes cleavage
and yields two blastomeres: one of the two is left and another is right
blastomere whose developmental programme invariably is found to be different
(Fig.2). The possible implications for
the same are – 1) unequal sharing of epigenetic factors between the first two
blastomeres and 2) unequally conditioned chromatin material sharing equally
between them. Soon after fertilization,
the paternal genome is actively demethylated in contrast to maternal genome
facilitating for paternal imprinting.
The basic reprogramming events of paternal demethylation in zygote appear
to be conserved in eutherian mammals.
In mouse and bovine embryos, the de
novo methylation occurs in the inner mass of cells of the blastocyst and in
8 to 16 cell stage respectively. Post-zygotic
demethylation and remethylation bring about the modifications in epigenetic
microenvironment. Thus, reprogramming
of epigenetic factors is not only observed in germ cells and early embryos but
also in embryonic stem cells (18).
development, the third cleavage results
into two tiers of unequal blastomeres viz., upper tier comprising of 4 micromeres and a lower tier with 4
macromeres indicating the influence imparted by epigenetic factors which are being shared unequally between them.
Hence, their developmental fates vary profoundly. The micromeres tend to develop into
epidermis. During involution of micromeres through the blastopore, there
is a supplementation of microenvironmental factors (BMPs, Noggin,
etc) from Nieuwkoop centre in frog gastrula and direct them to develop into
neural derivatives causing tissue specificity (2). Another classical example is one of the
mesodermal derivatives namely somites and their differentiation. After 96 hours
of incubation, the somites of chick embryo undergo differentiation as dermatome,
sclerotome and myotome by sharing cellular microenvironmental factors (FGFs)
among them resulting into fully differentiated states viz., dermis, skeleton
and muscles respectively. These
observations reveal that in advanced embryos, genome methylation patterns are
stable and heritable, whereas in early embryos (zygote, cleavage and gastrula)
the methylation patterns are reprogrammable to manifest a broad developmental
potential. The imposed methylation
patterns among blastomeres are irreversibly continued by maintenance DNA
methyltransferase (DNMT1) and this would make the germ layers to progress in
predetermined programmes of gene expression (18).
genome-wide developmental reprogrammes in the life-history of an organism
provide several avenues to design epigenetic modifications of chromatin
structure to simulate for somatic nuclear cloning experiments to succeed. Because epigenetic conformations of any
somatic nuclei markedly vary from that of the nuclei of mature gametes and it
is unique that the cytoplasm of oocyte can reverse the epigenetic modifications
to reestablish a state of totipotency.
Enter the code exactly as it appears. All letters are case insensitive, there is no zero.