The maternal to zygotic transition in mammals

Abstract

Prior to activation of the embryonic genome, the initiating events of mammalian development are under maternal control and include fertilization, the block to polyspermy and processing sperm DNA. Following gamete union, the transcriptionally inert sperm DNA is repackaged into the male pronucleus which fuses with the female pronucleus to form a 1-cell zygote. Embryonic transcription begins during the maternal to zygotic transfer of control in directing development. This transition occurs at species-specific times after one or several rounds of blastomere cleavage and is essential for normal development. However, even after activation of the embryonic genome, successful development relies on stored maternal components without which embryos fail to progress beyond initial cell divisions. Better understanding of the molecular basis of maternal to zygotic transition including fertilization, the activation of the embryonic genome and cleavage-stage development will provide insight into early human development that should translate into clinical applications for regenerative medicine and assisted reproductive technologies.

Role for PADI6 in securing the mRNA-MSY2 complex to the oocyte cytoplasmic lattices

“…Although the exact identity of most of these factors remains a mystery, they are nevertheless instrumental in preparing the oocyte for the egg to embryo transition. [1][2][3][4][5] The presence and correct storage of these maternal factors is crucial for embryonic genome activation (EGA) which needs to occur for proper embryonic development. 6,7 During this transition, mRNAs are released, translated, and the resulting proteins, along with other stored maternal factors, are targeted to the nucleus in a regulated fashion where they then orchestrate the nuclear reprogramming events leading to the EGA.…”

“…6,7 During this transition, mRNAs are released, translated, and the resulting proteins, along with other stored maternal factors, are targeted to the nucleus in a regulated fashion where they then orchestrate the nuclear reprogramming events leading to the EGA. 2,[8][9][10][11] It has been reported that 75-80% of ribosomes in the mouse oocytes are not incorporated into polysomes at ovulation and do not engage in protein synthesis in vitro. 12,13 Interestingly, previous studies have suggested that these inactive ribosomes are embedded in the oocyte cytoplasmic lattices (CPLs), a highly abundant structure that has been well-described in the literature but remains poorly understood.…”

Positional changes of a pluripotency marker gene during structural reorganization of fibroblast nuclei in cloned early bovine embryos

“…5,6,10 For this purpose, one has to take into account that mammalian species differ in the timing of major embryonic gene activation. 2 In rabbit embryos this event apparently starts, like in bovine embryos, at the 8-cell stage. In a previous study of in vivo derived and cloned rabbit embryos from our group, only occasional nuclei recorded at this stage showed the peripheral chromatin arrangement phenotype.…”

“…1 This critical event occurs at species-specific times after one or several rounds of blastomere cleavage. 2 In bovine embryos major embryonic genome activation takes place at the 8-cell stage, where the largest proportion of genes becomes active during preimplantation development. 3 Some genes, however, are already activated earlier during minor genome activation.…”

Comparison of the Absolute Level of Epigenetic Marks 5-Methylcytosine, 5-Hydroxymethylcytosine, and 5-Hydroxymethyluracil Between Human Leukocytes and Sperm1

“…Sperm passes its paternal genetic and epigenetic information to the embryo via the male pronucleus. After gamete fusion, the protamines must be removed, which is linked with a widespread and rapid DNA demethylation process, which suggests that the paternal epigenome has an important role during embryonic development [1,2]. This DNA demethylation takes place before the first round of replication, which indicates an active process [3].…”