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Current Topics in Developmental Biology 2021The fertilized frog egg contains all the materials needed to initiate development of a new organism, including stored RNAs and proteins deposited during oogenesis, thus... (Review)
Review
The fertilized frog egg contains all the materials needed to initiate development of a new organism, including stored RNAs and proteins deposited during oogenesis, thus the earliest stages of development do not require transcription. The onset of transcription from the zygotic genome marks the first genetic switch activating the gene regulatory network that programs embryonic development. Zygotic genome activation occurs after an initial phase of transcriptional quiescence that continues until the midblastula stage, a period called the midblastula transition, which was first identified in Xenopus. Activation of transcription is programmed by maternally supplied factors and is regulated at multiple levels. A similar switch exists in most animals and is of great interest both to developmental biologists and to those interested in understanding nuclear reprogramming. Here we review in detail our knowledge on this major switch in transcription in Xenopus and place recent discoveries in the context of a decades old problem.
Topics: Animals; Genome; Oogenesis; Xenopus laevis; Zygote
PubMed: 34074529
DOI: 10.1016/bs.ctdb.2021.03.003 -
Nature Apr 2017Chromatin is reprogrammed after fertilization to produce a totipotent zygote with the potential to generate a new organism. The maternal genome inherited from the oocyte... (Comparative Study)
Comparative Study
Chromatin is reprogrammed after fertilization to produce a totipotent zygote with the potential to generate a new organism. The maternal genome inherited from the oocyte and the paternal genome provided by sperm coexist as separate haploid nuclei in the zygote. How these two epigenetically distinct genomes are spatially organized is poorly understood. Existing chromosome conformation capture-based methods are not applicable to oocytes and zygotes owing to a paucity of material. To study three-dimensional chromatin organization in rare cell types, we developed a single-nucleus Hi-C (high-resolution chromosome conformation capture) protocol that provides greater than tenfold more contacts per cell than the previous method. Here we show that chromatin architecture is uniquely reorganized during the oocyte-to-zygote transition in mice and is distinct in paternal and maternal nuclei within single-cell zygotes. Features of genomic organization including compartments, topologically associating domains (TADs) and loops are present in individual oocytes when averaged over the genome, but the presence of each feature at a locus varies between cells. At the sub-megabase level, we observed stochastic clusters of contacts that can occur across TAD boundaries but average into TADs. Notably, we found that TADs and loops, but not compartments, are present in zygotic maternal chromatin, suggesting that these are generated by different mechanisms. Our results demonstrate that the global chromatin organization of zygote nuclei is fundamentally different from that of other interphase cells. An understanding of this zygotic chromatin 'ground state' could potentially provide insights into reprogramming cells to a state of totipotency.
Topics: Animals; Cell Nucleus; Cell Transdifferentiation; Cellular Reprogramming; Chromatin; Chromosome Positioning; Female; Haploidy; Interphase; Maternal Inheritance; Mice; Nucleic Acid Conformation; Oocytes; Paternal Inheritance; Single-Cell Analysis; Stochastic Processes; Totipotent Stem Cells; Zygote
PubMed: 28355183
DOI: 10.1038/nature21711 -
PLoS Biology Jan 2021To ensure genome stability, sexually reproducing organisms require that mating brings together exactly 2 haploid gametes and that meiosis occurs only in diploid zygotes....
To ensure genome stability, sexually reproducing organisms require that mating brings together exactly 2 haploid gametes and that meiosis occurs only in diploid zygotes. In the fission yeast Schizosaccharomyces pombe, fertilization triggers the Mei3-Pat1-Mei2 signaling cascade, which represses subsequent mating and initiates meiosis. Here, we establish a degron system to specifically degrade proteins postfusion and demonstrate that mating blocks not only safeguard zygote ploidy but also prevent lysis caused by aberrant fusion attempts. Using long-term imaging and flow-cytometry approaches, we identify previously unrecognized and independent roles for Mei3 and Mei2 in zygotes. We show that Mei3 promotes premeiotic S-phase independently of Mei2 and that cell cycle progression is both necessary and sufficient to reduce zygotic mating behaviors. Mei2 not only imposes the meiotic program and promotes the meiotic cycle, but also blocks mating behaviors independently of Mei3 and cell cycle progression. Thus, we find that fungi preserve zygote ploidy and survival by at least 2 mechanisms where the zygotic fate imposed by Mei2 and the cell cycle reentry triggered by Mei3 synergize to prevent zygotic mating.
Topics: Cell Cycle; Cell Cycle Proteins; Fungal Proteins; Genes, Fungal; Mating Factor; Meiosis; Organisms, Genetically Modified; Ploidies; RNA-Binding Proteins; Recombination, Genetic; Schizosaccharomyces; Schizosaccharomyces pombe Proteins; Zygote
PubMed: 33406066
DOI: 10.1371/journal.pbio.3001067 -
Reproduction (Cambridge, England) Dec 2017Paternal contributions to the zygote are thought to extend beyond delivery of the genome and paternal RNAs have been linked to epigenetic transgenerational inheritance...
Paternal contributions to the zygote are thought to extend beyond delivery of the genome and paternal RNAs have been linked to epigenetic transgenerational inheritance in different species. In addition, sperm-egg fusion activates several downstream processes that contribute to zygote formation, including PLC zeta-mediated egg activation and maternal RNA clearance. Since a third of the preimplantation developmental period in the mouse occurs prior to the first cleavage stage, there is ample time for paternal RNAs or their encoded proteins potentially to interact and participate in early zygotic activities. To investigate this possibility, a bespoke next-generation RNA sequencing pipeline was employed for the first time to characterise and compare transcripts obtained from isolated murine sperm, MII eggs and pre-cleavage stage zygotes. Gene network analysis was then employed to identify potential interactions between paternally and maternally derived factors during the murine egg-to-zygote transition involving RNA clearance, protein clearance and post-transcriptional regulation of gene expression. Our approach looked for factors in sperm, eggs and zygotes that could potentially interact co-operatively and synergistically during zygote formation. At least five sperm RNAs (, , , and ) met these requirements for a paternal contribution, which with complementary maternal co-factors suggest a wider potential for extra-genomic paternal involvement in the developing zygote.
Topics: Animals; Computational Biology; Computer Simulation; Databases, Genetic; Female; Gene Expression Profiling; Gene Expression Regulation, Developmental; Gene Regulatory Networks; Male; Mice; Mice, Inbred C57BL; Models, Genetic; Pregnancy; RNA Processing, Post-Transcriptional; RNA Stability; RNA, Messenger; RNA, Messenger, Stored; Signal Transduction; Sperm-Ovum Interactions; Spermatozoa; Transcription, Genetic; Zygote
PubMed: 28916718
DOI: 10.1530/REP-17-0097 -
Molecular Human Reproduction Feb 2021Human zygotes are difficult to obtain for research because of limited resources and ethical debates. Corrected human tripronuclear (ch3PN) zygotes obtained by removal of...
Human zygotes are difficult to obtain for research because of limited resources and ethical debates. Corrected human tripronuclear (ch3PN) zygotes obtained by removal of the extra pronucleus from abnormally fertilized tripronuclear (3PN) zygotes are considered an alternative resource for basic scientific research. In the present study, eight-cell and blastocyst formation efficiency were significantly lower in both 3PN and ch3PN embryos than in normal fertilized (2PN) embryos, while histone H3 lysine 9 trimethylation (H3K9me3) levels were much higher. It was speculated that the aberrant H3K9me3 level detected in ch3PN embryos may be related to low developmental competence. Microinjection of 1000 ng/µl lysine-specific demethylase 4A (KDM4A) mRNA effectively reduced the H3K9me3 level and significantly increased the developmental competence of ch3PN embryos. The quality of ch3PN zygotes improved as the grading criteria, cell number and pluripotent expression significantly increased in response to KDM4A mRNA injection. Developmental genes related to zygotic genome activation (ZGA) were also upregulated. These results indicate that KDM4A activates the transcription of the ZGA program by enhancing the expression of related genes, promoting epigenetic modifications and regulating the developmental potential of ch3PN embryos. The present study will facilitate future studies of ch3PN embryos and could provide additional options for infertile couples.
Topics: Blastocyst; Embryo Culture Techniques; Embryonic Development; Enzyme Induction; Female; Fertilization in Vitro; Gene Expression Regulation, Developmental; Histones; Humans; Jumonji Domain-Containing Histone Demethylases; Male; Methylation; Transcription, Genetic; Zygote
PubMed: 33599278
DOI: 10.1093/molehr/gaab012 -
Current Protocols in Molecular Biology Nov 2004This unit describes methods for the production of transgenic mice by injection of DNA into zygotes, including fertilized-egg isolation, zygote injection, and oviduct...
This unit describes methods for the production of transgenic mice by injection of DNA into zygotes, including fertilized-egg isolation, zygote injection, and oviduct reimplantation. Methods for the preparation of plasmid and BAC DNA suitable for microinjection are also presented.
Topics: Animals; DNA; Embryo Transfer; Female; Gene Transfer Techniques; Mice; Mice, Transgenic; Microinjections; Transgenes; Zygote
PubMed: 18265349
DOI: 10.1002/0471142727.mb2309s68 -
The Journal of Reproduction and... Apr 2022The zygotic genome is transcriptionally silent immediately after fertilization. In mice, initial activation of the zygotic genome occurs in the middle of the one-cell... (Review)
Review
The zygotic genome is transcriptionally silent immediately after fertilization. In mice, initial activation of the zygotic genome occurs in the middle of the one-cell stage. At the mid-to-late two-cell stage, a burst of gene activation occurs after the second round of DNA replication, and the profile of transcribed genes changes dramatically. These two phases of gene activation are called minor and major zygotic gene activation (ZGA), respectively. As they mark the beginning of the gene expression program, it is important to elucidate gene expression regulation during these stages. This article reviews the outcomes of studies that have clarified the profiles and regulatory mechanisms of ZGA.
Topics: Animals; DNA Replication; Embryonic Development; Gene Expression Regulation, Developmental; Genome; Mice; Transcriptional Activation; Zygote
PubMed: 35034936
DOI: 10.1262/jrd.2021-129 -
Zygote (Cambridge, England) Nov 2011A little studied aspect of developmental arrest (DA) in ART is zygote arrest (ZA). Etiologically, blockage at the first cleavage stage includes molecular and chromosomal...
A little studied aspect of developmental arrest (DA) in ART is zygote arrest (ZA). Etiologically, blockage at the first cleavage stage includes molecular and chromosomal anomalies, some of which manifest morphologically. Given considerations on embryo culture, transfer and cryopreservation, optimal zygote selection is very important. The aim of this study was to ascertain whether zygote morphological features were indicators of increased ZA. In this study we performed a prospective, observational study of 2105 zygotes obtained from consecutive patients who were undergoing IVF/ICSI treatment, of which 43 (2%) suffered ZA. Morphological features observed under the inverted microscope were qualitatively categorized: pronuclear size, nucleolar precursor bodies (NPB) alignment, light and dark halos, polar body placement and fragmentation observed at 16-18 h post-insemination. We compared these features in blocked versus cleaved zygotes at 48 h and found significant correlations (p < 0.05) between ZA and three features: the absence of a light halo (p = 0.001), the absence of a dark halo (p < 0.005), and non-aligned NPB (p < 0.05). We can say that certain morphological features are indicators of significantly increased zygote arrest. These findings may be of utility for optimal zygote selection and culture strategies, especially in countries under restrictive conditions.
Topics: Cell Nucleolus; Cleavage Stage, Ovum; Embryo, Mammalian; Humans; Morphogenesis; Prospective Studies; Reproductive Techniques, Assisted; Zygote
PubMed: 20663238
DOI: 10.1017/S0967199410000407 -
Theriogenology Nov 2006Tests were made of the effects of altering nitrogen metabolism in zygote donor ewes on fetal development and expression of the gene encoding the type II insulin-like...
Tests were made of the effects of altering nitrogen metabolism in zygote donor ewes on fetal development and expression of the gene encoding the type II insulin-like growth factor receptor (IGF2R) following the transfer of ovine embryos cultured from these zygotes, either in the absence or presence of serum. Zygotes, recovered from superovulated ewes (32 on a urea supplemented (30 g urea/kg) diet (high N) and 32 on a control diet (low N)) 36 h after intrauterine AI using semen from a single sire, were cultured for 5 days in synthetic oviductal fluid (SOF) media either with BSA and amino acids (SOF-) or with 10% (v/v) steer serum (SOF+). In total, 166 embryos, including 30 in vivo controls, were transferred singly at day 6 post-AI to synchronous recipients and the products of conception recovered at day 125 of gestation. Elevated plasma urea concentrations in zygote donors were associated with accelerated early embryo development, low pregnancy rates (16%) for embryos from the high N, SOF+ treatment, and significantly influenced fetal development and the expression of IGF2R in the fetal heart at day 125 of gestation. Importantly, the culture of sheep zygotes under serum-free conditions led to a high incidence of aberrant conceptus development and IGF2R expression. Consequently, maternal nitrogen metabolism prior to zygote recovery and in vitro culture can influence fetal development and the expression of an imprinted gene following embryo transfer, and these data support the notion that environmental effects on the follicle-enclosed oocyte may contribute to the etiology of the Large Offspring Syndrome.
Topics: Animals; Blood Urea Nitrogen; Culture Media; Embryo Culture Techniques; Embryo Transfer; Embryo, Mammalian; Embryonic Development; Female; Nitrogen; Pregnancy; Pregnancy Rate; Receptor, IGF Type 2; Sheep; Zygote
PubMed: 16777210
DOI: 10.1016/j.theriogenology.2006.05.008 -
Human Reproduction (Oxford, England) Dec 2000One of the main problems concerning IVF units is the need to decrease the occurrence of multiple pregnancies in their practice without affecting the overall success...
One of the main problems concerning IVF units is the need to decrease the occurrence of multiple pregnancies in their practice without affecting the overall success rate. Different embryological parameters concerning every step of the early embryo development are known to have some predictive value for implantation potential. In this prospective study, a pronuclear scoring system was used to classify zygotes into six patterns from 0 to 5. Cleaved, day 3 embryos developed from pattern 0 zygotes, which was described as the normal pattern, were transferred when available. For each zygote pattern, the subsequent embryological development was analysed. Pattern 0 zygotes led to significantly more 'good quality' embryos with higher implantation potential than embryos developing from the other zygote patterns (P < 0.01). Embryo transfers including at least one pattern 0 resulted in significantly more pregnancies than transfers without any pattern 0 zygotes (39.3 versus 19.7%, P < 0. 01). No relationship between clinical parameters (age of female partner, infertility cause) and zygote pattern distribution was demonstrated.
Topics: Aging; Cell Nucleus; Embryo Implantation; Embryo Transfer; Female; Fertilization in Vitro; Humans; Infertility; Male; Pregnancy; Pregnancy Outcome; Prospective Studies; Reproductive Techniques; Zygote
PubMed: 11098032
DOI: 10.1093/humrep/15.12.2591