Abstract
Fertilization is a very complex biological process that requires the perfect cooperation between two highly specialized cells: the male and female gametes. The oocyte provides the physical space where this process takes place, most of the energetic need, and half of the genetic contribution. The spermatozoon mostly contributes the other half of the chromosomes and it is specialized to reach and to penetrate the oocyte. Notably, the mouse oocyte and early embryo are transcriptionally inactive. Hence, they fully depend on the maternal mRNAs and proteins stored during oocyte maturation to drive the onset of development. The new embryo develops autonomously around the four-cell stage, when maternal supplies are exhausted and the zygotic genome is activated in mice. This oocyte-to-embryo transition needs an efficient and tightly regulated translation of the maternally-inherited mRNAs, which likely contributes to embryonic genome activation. Full understanding of post-transcriptional regulation of gene expression in early embryos is crucial to understand the reprogramming of the embryonic genome, it might help driving reprogramming of stem cells in vitro and will likely improve in vitro culturing of mammalian embryos for assisted reproduction. Nevertheless, the knowledge of the mechanism(s) underlying this fundamental step in embryogenesis is still scarce, especially if compared to other model organisms. We will review here the current knowledge on the post-transcriptional control of gene expression in mouse early embryos and discuss some of the unanswered questions concerning this fascinating field of biology.
Highlights
IntroductionOocytes and spermatozoa are terminally differentiated haploid cells which can no longer divide
From the Gametes to the EmbryoOocytes and spermatozoa are terminally differentiated haploid cells which can no longer divide.Both gametes are shaped and programmed to fuse with each other as they touch
Chromatin remodelers might be attracted to specific loci in the early embryo by transcription factors, such as TIF1α, which translocates from the cytoplasm to the pronuclei and modulates the first wave of embryonic genome activation at zygotic genome activation (ZGA) [34]
Summary
Oocytes and spermatozoa are terminally differentiated haploid cells which can no longer divide. In just few hours after fertilization, the haploid parental pronuclei become clearly visible inside the mouse zygote (one-cell embryo) [1]. Before syngamy occurs, both pronuclei replicate their entire DNA, and nearly. Fertilization creates totipotent cells capable of generating all tissues of the new organism. Since the mouse zygote is essentially a transient state in which a diploid nucleus is never formed, the first totipotent cells originated by fertilization are the blastomeres of the two-cell embryo. Early mouse development from the zygote to the morula stage is not accompanied by overall growth in size of the embryo, suggesting that totipotency might be related to the maternally inherited cytoplasmic content [15]. We will discuss some intriguing new findings about the intracellular localization of maternal mRNAs in mouse
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