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<p> <span style="font-size:10pt;font-family:Arial;">The paternal genome undergoes a remarkable transformation in the oocyte cytoplasm.<span> </span>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.<span> </span>The chromatin during spermatogenesis undergoes methylation and compacted with protamines (19) – which are crucial to facilitate normal fertilization.<span> </span>Whereas, genome of oocyte possesses oocyte-specific linker histones.<span> </span>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.<span> </span>Interestingly, the repetitive sequences (LINES, SINES, etc) are not uniformly methylated in gametes.</span> </p> <p> <span style="font-size:10pt;font-family:Arial;"> </span> </p> <p> <span style="font-size:10pt;font-family:Arial;">The somatic cell proliferation during organogenesis and histological differentiation follows mitosis which invariably results into two equal daughter somatic cells.<span> </span>They share equally both epigenetic factors and chromatin.<span> </span>Hence, the two daughter cells look similar in all respects.<span> </span>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).<span> </span>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.<span> </span>Soon after fertilization, the paternal genome is actively demethylated in contrast to maternal genome facilitating for paternal imprinting.<span> </span>The basic reprogramming events of paternal demethylation in zygote appear to be conserved in eutherian mammals.<span> </span>In mouse and bovine embryos, the <em>de novo</em> methylation occurs in the inner mass of cells of the blastocyst and in 8 to 16 cell stage respectively.<span> </span>Post-zygotic demethylation and remethylation bring about the modifications in epigenetic microenvironment.<span> </span>Thus, reprogramming of epigenetic factors is not only observed in germ cells and early embryos but also in embryonic stem cells (18).</span> </p> <p> </p> <p></p> <p> <span style="font-size:10pt;font-family:Arial;">In radial development, the third cleavage<span> </span>results into two tiers of unequal blastomeres viz., upper tier comprising<span> </span>of 4 micromeres and a lower tier with 4 macromeres indicating the influence imparted by epigenetic factors<span> </span>which are being shared unequally<span> </span>between them.<span> </span>Hence, their developmental fates vary profoundly.<span> </span>The micromeres tend to develop into epidermis.<span> </span>During involution<span> </span>of micromeres through the blastopore, there is a<span> </span>supplementation<span> </span>of microenvironmental factors (BMPs, Noggin, etc) from Nieuwkoop centre in frog gastrula and direct them to develop into neural derivatives causing tissue specificity (2).<span> </span>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. <span> </span>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.<span> </span>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).</span> </p> <p> <span style="font-size:10pt;font-family:Arial;"> </span> </p> <p> <span style="font-size:10pt;font-family:Arial;">The inbuilt 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.<span> </span>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. </span> </p> <p> </p>
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