Reproduction in mammals is a complex yet unique phenomenon of producing young ones. In Mammals, successful implantation of the embryo is possible with a number of unique and essential events occurring in the female body which lead to implantation and live birth. Considering the events of preimplantation embryo development occurs in four stages: fertilization, cell cleavage, morula, and blastocyst formation. These events are marked genetically and epigenetically. However, Epigenetic modification occurs differently in all four stages and leads to different patterns of gene expression. The primary purpose of epigenetic modifications and changes in gene expression is to activate the embryonic genome. These modifications and rearrangements are required for correct embryo development which further leads to accurate cell differentiation.
EPIGENETIC PROGRAMMING IN PREIMPLANTATION EMBRYO DEVELOPMENT
Epigenetics is a mechanism that leads to phenotypic changes of the cell without any change to its DNA sequence. After fertilization, critical epigenetic change occur to establish pluripotency. In mammals, zygote undergoes genome wide demethylation with exception of imprinted genes (whose expression is determined by the parent that contributed them). After gamete fusion, the spermatic genome undergoes dramatic remodelling, starting with evacuation of protamines and their substitution by acetylated histones. The male pronuclei undergoes selective demethylation which results into asymmetric methylated sister chromatids. The TET-protein family, TET3 is essentially expressed in the zygote, which binds specifically to the paternal genome. Therefore, TET-3 is responsible for these modifications. The female pronucleus of the zygote remains highly methylated at the stage. With first cleavage division, demethylation of maternal pronucleus starts by excluding the DNMT1. By this time the embryo is at the morula stage, the embryo under methylation followed by polarisation and compaction of blastomeres. The blastocyst stage of the embryo has fluid filled cavity and population of cells in the Inner Cell Mass (ICM) and Trophectoderm (TE). Following the implantation of the blastocyst (around 6-7 days post fertilization), the embryo forms the hypoblast and the epiblast, where latter at the 7-8 developmental day gives rise to a small population of cells, the precursors of primordial germ cells (PGCs). These PGC progenitors start to migrate through the handout endoderm to the genital ridge as they reach to the genital ridge, they undergo mitotic divisions and start proliferating. During expansion, the highly methylated PGC’s undergo a rapid genome-wide loss of methylation also including the majority of the parent of origin-specific DMRs of imprinted genes. Eventually, after global methylation loss in the PGC’s, the male and the female germ line enters the mitotic and the meiotic arrest. Following sexual differentiation of PGC’s, re-methylation takes place, while female and male PGCs reprogramming diverges. Reprogramming in germ cells is appropriate for resetting the imprints, so initially methylation of PGC’s obliterates and re-methylation takes place to facilitate the re-establishment of the germ line to ensure the proper inheritance of imprints for the new generation. Re- methylation occurred earlier in the male germ line, during the spermatogonia stage or before birth, while it is completed after birth and just before the end of the pachytene phase of meiosis. In contrast to male, female germ line starts to reprogram its genome postnatal, during oocyte growth and following the pachytene phase of meiosis. Eventually, by the end gametogenesis process, both gametes acquire new imprints and are fully methylated. Therefore, the differential epigenetic marking of the parental alleles that takes place during gametogenesis completes the genome methylation de-methylation cycle.
GENE EXPRESSION PATTERNS TO MAINTAIN PREIMPLANTATION DEVELOPMENT
A gene expression is basically as a series of differential accumulations of its products as the subsets of cells as its development progresses. Gene expression profiling of early embryos shows characteristic patterns of maternal RNA depletion. Embryonic Genome Activation (EGA) happens in two phases: the first phase is Zygotic Genome Activation (ZGA), and the second is Midpreimplantation Gene Activation (MGA), which undergoes the dynamic morphological and functional changes in the morula to the blastocyst stage. Two major groups of gene activities are defined as: oocytes and first early cleavage stages and the second consisting of 4-cell to blastocyst stages, which correlate with the transition from maternal to zygotic expression. Several transcripts of molecules involved in WNT or BMP signaling pathways were identified in the embryonic transcriptome, indicating that the critical regulators of cell fate and patterning are conserved and functional at these stages of pre-implantation development. The global patterns of gene expression in human embryos on day2 and 3 (n=8) by using DNA microarray analysis. The studies further revealed that the first few days of oocyte maturation and embryo development were characterized by a significant decrease in transcript levels, which results in the decay of RNA associated with gamete identity is integral to embryo development. Apparently, embryos with patterns of gene expression appropriate for their developmental stage may have superior viability to those displaying typical gene activities used a cDNA microarray containing 9,600 transcripts to investigate 631 differently expressed genes in oocytes, as well as the 4-cell and 8-cell human embryonic stages. These results indicate that the expression of some zygotic genes had already occurred at the 4-cell embryonic stage. In addition observed global gene expression changes during the hatching of mouse blastocysts, an essential process for implantation. Studies have reportedly observed that 85 genes which were unregulated in blastocysts at the hatching stage. The genes included cell adhesion molecules, epigenetic regulators, stress response regulators, and immune response regulators Furthermore, identified biomarker transcripts specific to Inner Cell Mass (ICM) such as (OCT4/ POU5F1, NANOG, HMGB1, and DPPA5) when comparing gene expression profiles of ICM and Trophectoderm Cells (TE) from human blastocysts. The emergence of pluripotent ICM lineages from the morula is controlled by metabolic and signaling pathways including WNT, Mitogen-Activated Protein Kinase (MAPK), Transforming Growth Factor-β (TGF-β), NOTCH, integrin-mediated cell adhesion, and apoptosis- signaling pathways.
Natural preimplantation of the embryo involves a complex mechanism. For natural development of the embryo expression of male and female is required. Epigenetic modification is said to be a major factor regulating gene expression for pre-implanting embryos. Also, various studies suggests that patterns of gene expressions vary differentially at each stage of development.