Maternal RNA content in oocytes of several mammalian and avian species

ULI-ssDRIP-seq revealed R-loop dynamics during vertebrate early embryogenesis

R-loop, a chromatin structure containing one RNA:DNA hybrid and one unpaired single-stranded DNA, plays multiple biological roles. However, due to technical limitations, the landscapes and potential functions of R-loops during embryogenesis remain elusive. Here, we developed a quantitative and high-resolution ultra-low input R-loop profiling method, named ULI-ssDRIP-seq, which can map global R-loops with as few as 1000 cells. By using ULI-ssDRIP-seq, we reveal the R-loop dynamics in the zebrafish from gametes to early embryos. In oocytes, the R-loop level is relatively low in most regions of the nuclear genome, except maternal-inherited rDNA and mitochondrial genome. The correlation between R-loop and CG methylation dynamics during early development is relatively weak. Furthermore, either up- or down-regulation of global R-loops by knockdown or overexpression of RNase H1 causes a delay of embryonic development with dramatic expression changes in zygotic and maternal genes. This study provides comprehensive R-loop landscapes during early vertebrate embryogenesis and demonstrates the implication of R-loops in embryonic development.

Why Is It So Difficult To Have Competent Oocytes from In vitro Cultured Preantral Follicles?

The developmental competence of oocytes is acquired gradually during follicular development, mainly through oocyte accumulation of RNA molecules and proteins that will be used during fertilization and early embryonic development. Several attempts to develop in vitro culture systems to support preantral follicle development up to maturation are reported in the literature, but oocyte competence has not yet been achieved in human and domestic animals. The difficulties to have fertilizable oocytes are related to thousands of mRNAs and proteins that need to be synthesized, long-term duration of follicular development, size of preovulatory follicles, composition of in vitro culture medium, and the need of multi-step culture systems. The development of a culture system that maintains bidirectional communication between the oocyte and granulosa cells and that meets the metabolic demands of each stage of follicle growth is the key to sustain an extended culture period. This review discusses the physiological and molecular mechanisms that determine acquisition of oocyte competence in vitro, like oocyte transcriptional activity, follicle and oocyte sizes, and length and regulation of follicular development in murine, human, and domestic animal species. The state of art of in vitro follicular development and the challenges to have complete follicular development in vitro are also highlighted.

DDX1 vesicles control calcium-dependent mitochondrial activity in mouse embryos

The DEAD box protein DDX1, previously associated with 3'-end RNA processing and DNA repair, forms large aggregates in the cytoplasm of early mouse embryos. Ddx1 knockout causes stalling of embryos at the 2-4 cell stages. Here, we identify a DDX1-containing membrane-bound calcium-containing organelle with a nucleic acid core. We show that aggregates of these organelles form ring-like structures in early-stage embryos which we have named Membrane Associated RNA-containing Vesicles. We present evidence that DDX1 is required for the formation of Membrane Associated RNA-containing Vesicles which in turn regulate the spatial distribution of calcium in embryos. We find that Ddx1 knockout in early embryos disrupts calcium distribution, and increases mitochondria membrane potential, mitochondrial activity, and reactive oxygen species. Sequencing analysis of embryos from Ddx1 heterozygote crosses reveals downregulation of a subset of RNAs involved in developmental and mitochondrial processes in the embryos with low Ddx1 RNA. We propose a role for Membrane Associated RNA-containing Vesicles in calcium-controlled mitochondrial functions that are essential for embryonic development.

Acrylamide modulates the mouse epididymal proteome to drive alterations in the sperm small non-coding RNA profile and dysregulate embryo development

Paternal exposure to environmental stressors elicits distinct changes to the sperm sncRNA profile, modifications that have significant post-fertilization consequences. Despite this knowledge, there remains limited mechanistic understanding of how paternal exposures modify the sperm sncRNA landscape. Here, we report the acute sensitivity of the sperm sncRNA profile to the reproductive toxicant acrylamide. Furthermore, we trace the differential accumulation of acrylamide-responsive sncRNAs to coincide with sperm transit of the proximal (caput) segment of the epididymis, wherein acrylamide exposure alters the abundance of several transcription factors implicated in the expression of acrylamide-sensitive sncRNAs. We also identify extracellular vesicles secreted from the caput epithelium in relaying altered sncRNA profiles to maturing spermatozoa and dysregulated gene expression during early embryonic development following fertilization by acrylamide-exposed spermatozoa. These data provide mechanistic links to account for how environmental insults can alter the sperm epigenome and compromise the transcriptomic profile of early embryos.

Transcriptome Profiling of the Ovarian Cells at the Single-Cell Resolution in Adult Asian Seabass

Single-cell RNA sequencing (scRNA-seq) is widely adopted for identifying the signature molecular markers or regulators in cells, as this would benefit defining or isolating various types of cells. Likewise, the signature transcriptome profile analysis at the single cell level would well illustrate the key regulators or networks involved in gametogenesis and gonad development in animals; however, there is limited scRNA-seq analysis on gonadal cells in lower vertebrates, especially in the sexual reversal fish species. In this study, we analyzed the molecular signature of several distinct cell populations of Asian seabass adult ovaries through scRNA-seq. We identified five cell types and also successfully validated some specific genes of germ cells and granulosa cells. Likewise, we found some key pathways involved in ovarian development that may concert germline-somatic interactions. Moreover, we compared the transcriptomic profiles across fruit fly, mammals, and fish, and thus uncovered the conservation and divergence in molecular mechanisms that might drive ovarian development. Our results provide a basis for studying the crucial features of germ cells and somatic cells, which will benefit the understandings of the molecular mechanisms behind gametogenesis and gonad development in fish.

A Comparative Analysis of Oocyte Development in Mammals

Sexual reproduction requires the fertilization of a female gamete after it has undergone optimal development. Various aspects of oocyte development and many molecular actors in this process are shared among mammals, but phylogeny and experimental data reveal species specificities. In this chapter, we will present these common and distinctive features with a focus on three points: the shaping of the oocyte transcriptome from evolutionarily conserved and rapidly evolving genes, the control of folliculogenesis and ovulation rate by oocyte-secreted Growth and Differentiation Factor 9 and Bone Morphogenetic Protein 15, and the importance of lipid metabolism.

Mother-Specific Signature in the Maternal Transcriptome Composition of Mature, Unfertilized Zebrafish Eggs

Maternal mRNA present in mature oocytes plays an important role in the proper development of the early embryo. As the composition of the maternal transcriptome in general has been studied with pooled mature eggs, potential differences between individual eggs are unknown. Here we present a transcriptome study on individual zebrafish eggs from clutches of five mothers in which we focus on the differences in maternal mRNA abundance per gene between and within clutches. To minimize technical interference, we used mature, unfertilized eggs from siblings. About half of the number of analyzed genes was found to be expressed as maternal RNA. The expressed and non-expressed genes showed that maternal mRNA accumulation is a non-random process, as it is related to specific biological pathways and processes relevant in early embryogenesis. Moreover, it turned out that overall the composition of the maternal transcriptome is tightly regulated as about half of the expressed genes display a less than twofold expression range between the observed minimum and maximum expression values of a gene in the experiment. Even more, the maximum gene-expression difference within clutches is for 88% of the expressed genes lower than twofold. This means that expression differences observed in maternally expressed genes are primarily caused by differences between mothers, with only limited variability between eggs from the same mother. This was underlined by the fact that 99% of the expressed genes were found to be differentially expressed between any of the mothers in an ANOVA test. Furthermore, linking chromosome location, transcription factor binding sites, and miRNA target sites of the genes in clusters of distinct and unique mother-specific gene-expression, suggest biological relevance of the mother-specific signatures in the maternal transcriptome composition. Altogether, the maternal transcriptome composition of mature zebrafish oocytes seems to be tightly regulated with a distinct mother-specific signature.

Single-cell transcriptome analyses reveal signals to activate dormant neural stem cells

The scarcity of tissue-specific stem cells and the complexity of their surrounding environment have made molecular characterization of these cells particularly challenging. Through single-cell transcriptome and weighted gene co-expression network analysis (WGCNA), we uncovered molecular properties of CD133(+)/GFAP(-) ependymal (E) cells in the adult mouse forebrain neurogenic zone. Surprisingly, prominent hub genes of the gene network unique to ependymal CD133(+)/GFAP(-) quiescent cells were enriched for immune-responsive genes, as well as genes encoding receptors for angiogenic factors. Administration of vascular endothelial growth factor (VEGF) activated CD133(+) ependymal neural stem cells (NSCs), lining not only the lateral but also the fourth ventricles and, together with basic fibroblast growth factor (bFGF), elicited subsequent neural lineage differentiation and migration. This study revealed the existence of dormant ependymal NSCs throughout the ventricular surface of the CNS, as well as signals abundant after injury for their activation.

Nucleolus-like bodies of fully-grown mouse oocytes contain key nucleolar proteins but are impoverished for rRNA

It is well known that fully-grown mammalian oocytes, rather than typical nucleoli, contain prominent but structurally homogenous bodies called "nucleolus-like bodies" (NLBs). NLBs accumulate a vast amount of material, but their biochemical composition and functions remain uncertain. To clarify the composition of the NLB material in mouse GV oocytes, we devised an assay to detect internal oocyte proteins with fluorescein-5-isothiocyanate (FITC) and applied the fluorescent RNA-binding dye acridine orange to examine whether NLBs contain RNA. Our results unequivocally show that, similarly to typical nucleoli, proteins and RNA are major constituents of transcriptionally active (or non-surrounded) NLBs as well as of transcriptionally silent (or surrounded) NLBs. We also show, by exposing fixed oocytes to a mild proteinase K treatment, that the NLB mass in oocytes of both types contains nucleolar proteins that are involved in all major steps of ribosome biogenesis, including rDNA transcription (UBF), early rRNA processing (fibrillarin), and late rRNA processing (NPM1/nucleophosmin/B23, nucleolin/C23), but none of the nuclear proteins tested, including SC35, NOBOX, topoisomerase II beta, HP1α, and H3. The ribosomal RPL26 protein was detected within the NLBs of NSN-type oocytes but is virtually absent from NLBs of SN-type oocytes. Taking into account that the major class of nucleolar RNA is ribosomal RNA (rRNA), we applied fluorescence in situ hybridization with oligonucleotide probes targeting 18S and 28S rRNAs. The results show that, in contrast to active nucleoli, NLBs of fully-grown oocytes are impoverished for the rRNAs, which is consistent with the absence of transcribed ribosomal genes in the NLB mass. Overall, the results of this study suggest that NLBs of fully-grown mammalian oocytes serve for storing major nucleolar proteins but not rRNA.

DNA and RNA pattern of staining during oogenesis in zebrafish (Danio rerio): a confocal microscopy study

Oogenesis involves a sequence of cellular divisions and developmental changes leading to the formation of oocytes, whose role in development is to transfer genomic information to the next generation. During this process, the gene expression pattern changes considerably concomitant with genome remodeling, while genomic information is maintained. The development of the gonad in zebrafish is unique in that it goes through an initial ovarian phase and subsequently into either ovarian or testicular phases. How the germ cells choose to commit to an oogenic fate and enter meiosis or alternatively not to enter meiosis and commit to a spermatogenetic fate remains a key question in development. Lack of suitable markers has hampered the understanding of the principles controlling sex differentiation in zebrafish. The current study was aimed at finding substantive cytochemical markers to identify specific oocyte stages primarily focusing on the DNA and RNA component of cells, using fluorescent dyes: acridine orange and propidium iodide. The pattern of synthesis and appearance of nucleoli was stage specific and may be used to identify stages of oogenesis. A distinguishing and possibly diagnostic feature of the staining pattern observed was the low level of chromatin staining compared to other cellular structures. This may be related to the more diffuse state of chromatin that occurs prior to thickening of chromosomes from the pachytene stage onwards. Although the fluorescent dyes may be useful in determining the localization of nucleic acids in tissue sections, it was not possible to quantify the relative contribution of the DNA and RNA components of specific stages of oocyte growth.

Spermatozoal RNAs: what about their functions?

The profound architectural changes that transform spermatids into spermatozoa result in a high degree of DNA packaging within the sperm head. However, the mature sperm chromatin that harbors imprinted genes exhibits a dual nucleoprotamine/nucleohistone structure with DNase-sensitive regions, which could be implicated in the establishment of efficient epigenetic information in the developing embryo. Despite its apparent transcriptionally inert state, the sperm nucleus contains diverse RNA populations, mRNAs, antisense and miRNAs, that have been transcribed throughout spermatogenesis. There is also an endogenous reverse transcriptase that may be activated under certain circumstances. It is now commonly accepted that sperm can deliver some RNAs to the ovocyte at fertilization. This review presents potential links between male-specific genomic imprinting, chromatin organization, and the presence of diverse RNA populations within the sperm nucleus and discusses the functional significance of these RNAs in the spermatozoon itself and in the early embryo following fertilization. Some recent data are provided, supporting the view that analyzing the profile of spermatozoal RNAs could be useful for assessment of male fertility.

Search for the genes involved in oocyte maturation and early embryo development in the hen

Background

The initial stages of development depend on mRNA and proteins accumulated in the oocyte, and during these stages, certain genes are essential for fertilization, first cleavage and embryonic genome activation. The aim of this study was first to search for avian oocyte-specific genes using an in silico and a microarray approaches, then to investigate the temporal and spatial dynamics of the expression of some of these genes during follicular maturation and early embryogenesis.

Results

The in silico approach allowed us to identify 18 chicken homologs of mouse potential oocyte genes found by digital differential display. Using the chicken Affymetrix microarray, we identified 461 genes overexpressed in granulosa cells (GCs) and 250 genes overexpressed in the germinal disc (GD) of the hen oocyte. Six genes were identified using both in silico and microarray approaches. Based on GO annotations, GC and GD genes were differentially involved in biological processes, reflecting different physiological destinations of these two cell layers. Finally we studied the spatial and temporal dynamics of the expression of 21 chicken genes. According to their expression patterns all these genes are involved in different stages of final follicular maturation and/or early embryogenesis in the chicken. Among them, 8 genes (btg4, chkmos, wee, zpA, dazL, cvh, zar1 and ktfn) were preferentially expressed in the maturing occyte and cvh, zar1 and ktfn were also highly expressed in the early embryo.

Conclusion

We showed that in silico and Affymetrix microarray approaches were relevant and complementary in order to find new avian genes potentially involved in oocyte maturation and/or early embryo development, and allowed the discovery of new potential chicken mature oocyte and chicken granulosa cell markers for future studies. Moreover, detailed study of the expression of some of these genes revealed promising candidates for maternal effect genes in the chicken. Finally, the finding concerning the different state of rRNA compared to that of mRNA during the postovulatory period shed light on some mechanisms through which oocyte to embryo transition occurs in the hen.

Differential regulation of abundance and deadenylation of maternal transcripts during bovine oocyte maturation in vitro and in vivo

Background

In bovine maturing oocytes and cleavage stage embryos, gene expression is mostly controlled at the post-transcriptional level, through degradation and deadenylation/polyadenylation. We have investigated how post transcriptional control of maternal transcripts was affected during in vitro and in vivo maturation, as a model of differential developmental competence.

Results

Using real time PCR, we have analyzed variation of maternal transcripts, in terms of abundance and polyadenylation, during in vitro or in vivo oocyte maturation and in vitro embryo development. Four genes are characterized here for the first time in bovine: ring finger protein 18 (RNF18) and breast cancer anti-estrogen resistance 4 (BCAR4), whose oocyte preferential expression was not previously reported in any species, as well as Maternal embryonic leucine zipper kinase (MELK) and STELLA. We included three known oocyte marker genes (Maternal antigen that embryos require (MATER), Zygote arrest 1 (ZAR1), NACHT, leucine rich repeat and PYD containing 9 (NALP9)). In addition, we selected transcripts previously identified as differentially regulated during maturation, peroxiredoxin 1 and 2 (PRDX1, PRDX2), inhibitor of DNA binding 2 and 3 (ID2, ID3), cyclin B1 (CCNB1), cell division cycle 2 (CDC2), as well as Aurora A (AURKA). Most transcripts underwent a moderate degradation during maturation. But they displayed sharply contrasted deadenylation patterns that account for variations observed previously by DNA array and correlated with the presence of a putative cytoplasmic polyadenylation element in their 3' untranslated region. Similar variations in abundance and polyadenylation status were observed during in vitro maturation or in vivo maturation, except for PRDX1, that appears as a marker of in vivo maturation. Throughout in vitro development, oocyte restricted transcripts were progressively degraded until the morula stage, except for MELK ; and the corresponding genes remained silent after major embryonic genome activation.

Conclusion

Altogether, our data emphasize the extent of post-transcriptional regulation during oocyte maturation. They do not evidence a general alteration of this phenomenon after in vitro maturation as compared to in vivo maturation, but indicate that some individual messenger RNA can be affected.

Identification of candidate mRNAs associated with gonadotropin-induced maturation of murine cumulus oocyte complexes using serial analysis of gene expression

In cultured cumulus oocyte complexes (COC), FSH induces gene transcription required for germinal vesicle breakdown (GVBD). Experiments were performed to determine the critical period when gene transcription is required for GVBD and to identify candidate mRNAs involved. Experiment I: murine COC were cultured 4 h in the presence of FSH with 5,6-dichloro-1-β-d-ribofuranosylbenzimidazole (DRB) added at different intervals after the start of culture. COC cultured with FSH underwent GVBD (82 ± 7%). When DRB was added at 0, 5, or 10 min after culture initiation, oocyte maturation was blocked (17 ± 7, 14 ± 6, and 21 ± 6% GVBD, respectively). When DRB was added after 15, 20, or 30 min, progressively more COC underwent GVBD (37 ± 6, 39 ± 6, and 66 ± 6%, respectively). The critical period of transcription required for GVBD occurred between 15 and 30 min after culture initiation. Experiment II: COC were cultured for 25 min in the presence (plusDRB) or absence (minusDRB) of DRB. SAGE libraries were generated from COC RNA of each treatment group. A total of 48,431 and 45,367 tags were sequenced for the plusDRB and minusDRB libraries, respectively. Criteria used to identify transcripts of interest included a total tag count of at least 10 across both libraries and a threefold or greater difference in expression between libraries. Using these criteria, 39 and 27 transcripts were identified as differentially expressed at the P ≤ 0.01 and P ≤ 0.001 levels, respectively. Differentially expressed transcripts were classed into major categories that included cell growth, development, and regulation of gene expression. Differentially expressed transcripts represent candidates potentially involved in regulating maturation of murine COC.

Gene expression and methylation patterns in cloned embryos

A considerable proportion of the offspring, in particular in ruminants and mouse, born from nuclear transfer (NT)-derived and in vitro-produced (IVP) embryos are affected by multiple abnormalities, of which a high birthweight and an extended gestation length are the predominant features; a phenomenon that has been termed "Large Offspring syndrome" (LOS). According to a current hypothesis, LOS is caused by persistent aberrations of expression patterns of developmentally important genes starting as early as at the preimplantation stages. The underlying mechanisms are widely unknown at present, but epigenetic modifications of embryonic and fetal gene expression patterns, primarily caused by alterations in DNA methylation are thought to be involved in this syndrome. Appropriate DNA methylation is essential for regular transcription during mammalian development and differentiation. Sensitive reverse transcription polymerase chain reaction assays allow the study of messenger RNA (mRNA) expression levels of specific genes in single embryos. The methylation status of a specific gene can be assessed by bisulfite sequencing. Studies to unravel mRNA expression patterns from IVP- and NT-derived embryos have revealed numerous aberrations ranging from suppression of expression to de novo overexpression or more frequently to a significant upregulation or downregulation of a specific gene. mRNA expression patterns from in vivo-derived embryos are essential as the "physiological standard" against which the findings for IVP and NT-derived embryos are to be compared. Unraveling the underlying molecular mechanisms will contribute to the production of viable embryos and aid to improve biotechnologies applied to early mammalian embryos.

Poly(A) RNA Is Reduced by Half During Bovine Oocyte Maturation but Increases when Meiotic Arrest Is Maintained with CDK Inhibitors1

Variations in the amount of different RNA species were investigated during in vitro maturation of bovine oocytes. Total RNA content was estimated to be 2 ng before meiosis, and after meiosis resumption, no decrease was observed. Ribosomal RNA did not appear to be degraded either, whereas poly(A) RNA was reduced by half after meiosis resumption, from 53 pg to 25 pg per oocyte. Real-time polymerase chain reaction was performed on growth and differentiation factor-9 (GDF-9), on cyclin B1, and on two genes implicated in the resistance to oxidative stress, glucose-6-phosphate-dehydrogenase (G6PD) and peroxiredoxin-6 (PRDX6). When these transcripts were reverse-transcribed with hexamers, the amplification results were not different before or after in vitro maturation. But when reverse transcription was performed with oligo(dT), amplification was dramatically reduced after maturation, except for cyclin B1 mRNA, implying deadenylation without degradation of three transcripts. Although calf oocytes have a lower developmental competence, their poly(A) RNA contents were not different from that of cow oocytes, nor were they differently affected during maturation. When bovine oocytes were maintained in vitro under meiotic arrest with CDK inhibitors, their poly(A) RNA amount increased, but this rise did not change the poly(A) RNA level once maturation was achieved. The increase could not be observed under transcription inhibition and, when impeding transcription and adenylation, the poly(A) RNA decreased to a level normally observed after maturation, in spite of the maintenance of meiotic arrest. These results demonstrate the importance of adenylation and deadenylation processes during in vitro maturation of bovine oocytes.

The unique transcriptome through day 3 of human preimplantation development

Successful human development is dependent upon a cascade of events following fertilization. Unfortunately, knowledge of these critical events in humans is remarkably incomplete. Although hundreds of thousands of human embryos are cultured yearly at infertility centers worldwide, the vast majority fail to develop in culture or following transfer to the uterus. In this study, we sought to characterize global patterns of gene expression in individual, normal embryos during the first three days of embryonic life using microarrays; we then compared gene expression between normally growing and growth-arrested embryos using quantitative PCR. Our results documented several novel findings. First, we found that a complex pattern of gene expression exists; most genes that are transcriptionally modulated during the first three days following fertilization are not upregulated, as was previously thought, but are downregulated. Second, we observed that the majority of genes exhibiting differential expression during preimplantation development are of unknown identity and/or function. Third, we show that embryonic transcriptional programs are clearly established by day 3 following fertilization, even in embryos that arrested prematurely with 2-, 3- or 4-cells. This indicates that failure to activate transcription is not associated with the majority of human preimplantation embryo loss. Finally, taken together, these results provide the first global analysis of the human preimplantation embryo transcriptome, and demonstrate that RNA can be amplified from single oocytes and embryos for analysis by cDNA microarray technology, thus lending credence to additional studies of genetic regulation in these cell types, as well as in other small biological samples.

p34cdc2 expression and meiotic competence in growing goat oocytes

The expression of the catalytic subunit of the maturation promoting factor (MPF), p34cdc2, was analyzed during meiosis and final growth of goat oocytes. Western blot analysis revealed the presence of p34cdc2 in fully grown oocytes (follicles > 3 mm in diameter) prior to and during meiotic maturation. p34cdc2 was present in partially competent oocytes at the germinal vesicle stage (follicles 0.5 to 0.8 mm and 1 to 1.8 mm in diameter). In contrast, p34cdc2 was not expressed in meiotically incompetent oocytes from small antral follicles (follicles < 0.5 mm in diameter). The amount of p34cdc2 increased with oocyte growth and acquistion of meiotic competence. Moreover, p34cdc2 accumulated in partially competent and incompetent oocytes within 27 hr of culture, but the level of p34cdc2 in incompetent oocytes remained very low and was not sufficient to allow spontaneous resumption of meiosis. The expression of the cdc2 gene was analyzed by polymerase-chain-reaction (PCR) on reverse transcribed mRNA. The presence of the cdc2 transcript was evidenced in both competent and incompetent oocytes at the germinal vesicle stage. These data indicate that a deficiency in the expression of p34cdc2 that could be regulated at the translational level, may be a limiting factor for meiotic competence acquistion in goat oocytes. We further investigated the mechanisms of MPF activation in competent and incompetent oocytes by using okadaic acid, a protein phosphatase inhibitor. Okadaic acid induced the premature resumption of meiosis associated with MPF activation in competent oocytes. In partially competent oocytes, okadaic acid induced premature meiosis reinitiation and MPF activation, but only after pre-culture for 10 hr. Acquisition of sensitivity to okadaic acid treatment was dependent on protein synthesis since it failed to occur when cycloheximide was added during the pre-culture period. Incompetent oocytes responded to okadaic acid treatment only after 27 hr of culture, when they had accumulated small amounts of p34cdc2. These data suggest that okadaic acid may bypass the subthreshold level of p34cdc2, provided the oocytes have synthesized some additional factors that remain to be identified.

Glucose metabolism during bovine preimplantation development: analysis of gene expression in single oocytes and embryos

Glucose metabolism of the bovine embryo is low during the first cleavages and increases sharply after the major resumption of the genome (8-16 cells). The mRNA level for genes involved in glucose metabolism was tested by RT-PCR on individual oocytes and embryos at different stages of development. These genes were: glucose transport GLUT-1, hexokinase (HK), glucose-6-phosphatase-dehydrogenase (G6PDH), and glucose-phosphate-isomerase (GPI); actin was used as a reference transcript. RT-PCR results revealed three types of oocytes or embryos: positive with a PCR signal for each transcript considered, nul with no signal for any transcript, and heterogeneous with a PCR signal for some transcripts and none for others. The number of nul and heterogeneous samples was higher for slow than for fast-cleaving embryos (81% vs. 36%), and the proportion of positive embryos increased significantly at the 16-cell and morula stages (P < 0.002), suggesting a correlation between mRNA content and developmental capacity. In positive embryos, GLUT-1 level was reduced by half during maturation and fertilization. Actin and hexokinase mRNA levels decreased during the first cleavages, but significantly increased at the 16-cell and morula stages, respectively. GPI transcript remained stable throughout development, whereas there was a significant rise for G6PDH at the 4-cell stage, perhaps due to a polyadenylation process. Finally, the absence or decrease in intensity of several transcripts at the blastocyst stage suggests suboptimal culture conditions.

Maternal age effect on early human embryonic development and blastocyst formation

In humans, age-related decline in female fertility can be explained by a reduction in quality either of the older uterus or of the embryos arising from aging oocytes. The aim of this study was to examine the latter hypothesis, using in vitro fertilization (I.V.F.) and coculture of embryos until the blastocyst stage. We determined the blastocyst formation rate ([blastocysts/embryos on day 2]* 100) and the blastocyst expansion rate ([expanded blastocysts/blastocysts]* 100) according to the patient's age the day of I.V.F. With increase in age, the number of retrieved oocytes decreased, without alteration of the cleavage rate. In patients above age 30 years, preimplantation development to blastocysts declined due to an increase in embryo arrest at the morula stage. If blastocyst stage was reached, a negative linear relationship between blastocyst expansion rate and patient age was observed. Drops in gamete production and embryo development with increasing age led to a drastic decrease in patients having at least one expanded blastocyst (< 30 years, 82%; > or = 40 years, 36%). A high delivery rate per oocyte retrieval (25.8%) was observed in patients above age 40 years after embryo transfer at the blastocyst stage. These results give a clear indication of decline in the quality of human embryos arising from aging oocytes. The origin of this alteration is discussed in terms of chromosome abnormalities, role of maternally-inherited products from the oocyte, timing of genomic activation, and temporal pattern of gene expression during initial development of the human embryo.

Characteristics of poly(A)-degrading factor present in the avian oocytes and early embryos

The presence of poly(A)-degrading activity was studied in vitro in the quail and mouse oocytes and early embryos using 3H-poly(A) as a substrate. The activity was measured by adsorption of the undegraded substrate to DE-81 filter paper discs, by chromatographic separation on Sephadex G-50 column and by agarose gel electrophoresis followed by transfer onto a Zeta-probe membrane (BioRad, Richmond, CA) and autoradiography. High poly(A)-degrading activity was found in the quail previtellogenic and vitellogenic oocytes and lower activity in the early embryos from cleavage stage to gastrulation. This activity is localized predominantly in the nucleus and, to a lesser degree, in the cytoplasm and in the vitellus of vitellogenic oocytes. The length of the poly(A) degradation product was estimated to be of about (A)10. Optimum activity was at pH 8.7 and at Mn2+ concentration of 0.5 mM. This makes the deadenylating enzyme from the quail oocytes similar to endoribonuclease IV from the chick and quail oviducts (Müller [1976] Eur. J. Biochem., 70:241-248; Müller [1976], Eur. J. Biochem., 70:249-258). We suggest that the poly(A)-degrading enzyme, similar to endoribonuclease IV found in the quail oocytes, might be the "deadenylating factor" reported in Xenopus oocytes (Varnum et al. [1992] Dev. Biol., 153:283-290). Such poly(A)-degrading activity is undetectable in unfertilized mouse eggs; however, a slight, statistically insignificant tendency for poly(A) degradation was seen in two-cell embryos.

RNase A activity in Japanese quail oocytes

The presence of RNase A activity was studied in vitro in homogenates of quail oocytes and early embryos using [3H]poly(U) as a substrate. The activity was measured by adsorption of the undegraded substrate onto DE-81 filter paper discs and by chromatographic separation on a Sephadex G-50 column. RNase A activity examined by these methods was almost undetectable in quail previtellogenic, vitellogenic and ovulated oocytes as well as in the embryos from laid eggs. It is estimated to be about 1.1 x 10(-5) Kunitz units per ovulated oocyte. Higher activity starts to appear in gastrulating embryos. These findings are discussed in relation to other studies demonstrating the high stability of maternal RNA during early development, especially in growing oocytes.