Cleavage-stage human embryo arrest, is it embryo genetic composition or others?

Embryo transfer is a crucial step in IVF cycle, with increasing trend during the last decade of transferring a single embryo, preferably at the blastocyst stage. Despite increasing evidence supporting Day 5 blastocyst-stage transfer, the optimal day of embryo transfer remains controversial. The crucial questions are therefore, whether the mechanisms responsible to embryos arrest are embryo aneuploidy or others, and whether those embryos arrested in-vitro between the cleavage to the blastocyst stage would survive in-vivo if transferred on the cleavage-stage. We therefore aim to explore whether aneuploidy can directly contribute to embryo development to the blastocyst stage. Thirty Day-5 embryos, that their Day-3 blastomere biopsy revealed a single-gene defect, were donated by 10 couples undergoing preimplantation genetic testing treatment at our center. Affected high quality Day-3 embryos were cultured to Day-5, and were classified to those that developed to the blastocyst-stage and those that were arrested. Each embryo underwent whole genome amplification. Eighteen (60%) embryos were arrested, did not develop to the blastocyst stage and 12 (40%) have developed to the blastocyst stage. Nineteen embryos (63.3%) were found to be euploid. Of them, 12 (66.6%) were arrested embryos and 7 (58.3%) were those that developed to the blastocyst-stage. These figures were not statistically different (p = 0.644). Our observation demonstrated that the mechanism responsible to embryos arrest in vitro is not embryo aneuploidy, but rather other, such as culture conditions. If further studies will confirm that Day-5 blastocyst transfer might cause losses of embryos that would have been survived in vivo, cleavage-stage embryo transfer would be the preferred timing. This might reduce the cycle cancellations due to failure of embryo to develop to the blastocyst stage and will provide the best cumulative live birth-rate per started cycle.

Constitutive Expression of TERT Enhances β-Klotho Expression and Improves Age-Related Deterioration in Early Bovine Embryos

Age-associated decline in oocyte quality is one of the dominant factors of low fertility. Aging alters several key processes, such as telomere lengthening, cell senescence, and cellular longevity of granulosa cells surrounding oocyte. To investigate the age-dependent molecular changes, we examined the expression, localization, and correlation of telomerase reverse transcriptase (TERT) and β-Klotho (KLB) in bovine granulosa cells, oocytes, and early embryos during the aging process. Herein, cumulus-oocyte complexes (COCs) obtained from aged cows (>120 months) via ovum pick-up (OPU) showed reduced expression of β-Klotho and its co-receptor fibroblast growth factor receptor 1 (FGFR1). TERT plasmid injection into pronuclear zygotes not only markedly enhanced day-8 blastocysts’ development competence (39.1 ± 0.8%) compared to the control (31.1 ± 0.5%) and D-galactose (17.9 ± 1.0%) treatment groups but also enhanced KLB and FGFR1 expression. In addition, plasmid-injected zygotes displayed a considerable enhancement in blastocyst quality and implantation potential. Cycloastragenol (CAG), an extract of saponins, stimulates telomerase enzymes and enhances KLB expression and alleviates age-related deterioration in cultured primary bovine granulosa cells. In conclusion, telomerase activation or constitutive expression will increase KLB expression and activate the FGFR1/β-Klotho pathway in bovine granulosa cells and early embryos, inhibiting age-related malfunctioning.

Analysis of Biochemical and Clinical Pregnancy Loss Between Frozen-Thawed Embryo Transfer of Blastocysts and Day 3 Cleavage Embryos in Young Women: A Comprehensive Comparison

BACKGROUND

To determine whether the embryo developmental stage affects biochemical or clinical pregnancy loss in young women undergoing frozen-thawed embryo transfer (FET) and to investigate the underlying mechanism.

METHODS

This was a retrospective study including a total of 18,34 β-HCG (human chorionic gonadotropin)-positive FET cycles. According to the morphological appearance (MA) of transferred blastocysts, FET cycles with blastocysts were divided into two groups: Group A: morphologically good (MG) blastocysts only, and Group B: at least one morphologically non-good (MNG) blastocyst. FET cycles with day 3 cleavage embryos were assigned as Group C. Biochemical and clinical pregnancy loss were the main outcome measures.

RESULTS

We predicted 78% in vivo-formed MG and 53.9% in vivo-formed day 5 blastocysts in Group C. (a) Including cases in Group A and Group B for binary logistic regression, we showed that Group B and day 6 blastocysts had significantly higher rates of BPL and CPL than Group A and day 5 blastocysts, respectively. (b) Including cases in Group A, Group B, and Group C for binary logistic regression, we showed that Group C had a significantly higher rate of BPL than Group A and day 5 blastocysts and a similar rate of BPL as Group B and day 6 blastocysts. Group C had a higher rate of CPL than Group A (p=0.071) and day 5 blastocysts (p=0.039), and a lower rate of CPL than Group B (p=0.199) and day 6 blastocysts (p=0.234).

CONCLUSIONS

(1) MA and days of usable blastocysts could serve as independent factors affecting the occurrence of BPL and CPL. (2) Transfer of day 3 cleavage embryos may produce "unusable blastocysts" in vivo, which significantly increased the rate of BPL. (3) The rate of CPL resulting from the transfer of day 3 embryos may depend on the rate of in vivo-formed MG or day 5 blastocysts. Our study indicated that the difference in the BPL or CPL between transfer of blastocysts and day 3 cleavage embryos may largely depend on the quality of embryos transferred.

High three pro-nuclei (3PN) zygotes proportion associated with normal embryo multinucleation at the two-cell stage: two cases report

Objective: Blastomere multinucleation at the two-cell stage (MN2) is a common nuclear abnormality observed in early human embryos and known to decrease the implantation rate. The aim of this study is to explore whether or not there is a link between high 3PN zygotes proportion and MN2 incidence.Methods: For embryo culture in the conventional incubator, the evaluation of nuclear status of two-cell stage is usually not performed. Therefore, the MN2 phenomenon is easily ignored. The time-lapse monitoring system (TLS) offers a promising new method to evaluate embryo development. We reported two cases who had single 2PN zygote and more than four 3PN zygotes in the conventional in-vitro fertilization (c-IVF) cycle.Results: We observed the MN2 incidence in the single normal embryo by TLS which suggested that it might be resulted from high 3PN zygotes proportion incidence. No available embryo was obtained in the first c-IVF cycle and the intracytoplasmic sperm injection (ICSI) treatment was performed in the second cycle. In subsequent ICSI cycles, we observed no 3PN zygotes incidence and transferred two day 3 embryos for the patients. Finally, the two couples successfully obtained healthy babies.Conclusions: High 3PN zygotes proportion might be associated with the MN2 incidence.

Pre-implantation genetic testing alters the sex ratio: an analysis of 91,805 embryo transfer cycles

PURPOSE

To determine if pre-implantation genetic testing (PGT) shifts the sex ratio (SER), the ratio of male to female births in a population normalized to 100 and typically stable at 105, following in vitro fertilization (IVF).

METHODS

Data from 2014 to 2016 was requested from the Society for Assisted Reproductive Technologies (SART) database including fresh and frozen transfer cycles. Women with a singleton live birth following a fresh or frozen autologous embryo transfer of a PGT blastocyst, non-PGT blastocyst, or non-PGT cleavage stage embryo were included. The SER between groups was compared using chi-square tests. Modified Poisson regression modeled the relative risk (RR) of having a male compared to a female among PGT blastocyst transfers versus non-PGT cleavage and blastocyst transfers adjusting for age, BMI, smoking status, race, parity, number of oocytes retrieved, and clinic region.

RESULTS

The SER was 110 among PGT blastocyst offspring, 107 among non-PGT blastocyst offspring (p = 0.005), and 99 among non-PGT cleavage offspring (p < 0.001). The risk of having a male infant was 2% higher among PGT blastocyst transfers compared to non-PGT blastocyst transfers (RR 1.02; 95% CI: 1.01, 1.04). The risk was 5% higher among PGT blastocyst transfers compared to non-PGT cleavage transfers (RR 1.05; 95% CI: 1.02, 1.07). The association between PGT and infant gender did not significantly differ by region (p = 0.57) or parity (p = 0.59).

CONCLUSION

Utilizing PGT shifts the SER in the IVF population from the standard of 105 to 110, increasing the probability of a male offspring.

Direct Single-Cell Analysis of Human Polar Bodies and Cleavage-Stage Embryos Reveals No Evidence of the Telomere Theory of Reproductive Ageing in Relation to Aneuploidy Generation

Reproductive ageing in women, particularly after the age of 35, is associated with an exponential increase in the proportion of chromosomally abnormal oocytes produced. Several hypotheses have attempted to explain this observation, including the 'limited oocyte pool' hypothesis and the 'two-hit' hypothesis, the latter explaining that a depletion in oocyte quality with age results from the multiple opportune stages for errors to occur in meiosis. Recently however, the telomere theory of reproductive ageing in women has been proposed. This suggests that shortened telomeres in oocytes of women of advanced maternal age render oocytes unable to support fertilization and embryogenesis. Despite a credible rationale for the telomere theory of reproductive ageing in women, very few studies have assessed telomere length directly in human oocytes or preimplantation embryos. Therefore, we directly assessed relative telomere length in first polar bodies and blastomeres from cleavage stage (day 3) embryos. In both cell types we tested the hypothesis that (1) older women have shorter telomeres and (2) chromosomally abnormal (aneuploid) gametes/embryos have shorter telomeres. In all cases, we found no evidence of altered telomere length associated with age-related aneuploidy.

A single trophectoderm biopsy at blastocyst stage is mathematically unable to determine embryo ploidy accurately enough for clinical use

BACKGROUND

It has become increasingly apparent that the trophectoderm (TE) at blastocyst stage is much more mosaic than has been appreciated. Whether preimplantation genetic screening (PGS), utilizing a single TE biopsy (TEB), can reliably determine embryo ploidy has, therefore, increasingly been questioned in parallel.

METHODS

We for that reason here established 2 mathematical models to assess probabilities of false-negative and false-positive results of an on average 6-cell biopsy from an approximately 300-cell TE. This study was a collaborative effort between investigators at The Center for Human Reproduction in New York City and the Center for Studies in Physics and Biology and the Brivanlou Laboratory of Stem Cell Biology and Molecular Embryology, the latter two both at Rockefeller University in New York City.

RESULTS

Both models revealed that even under best case scenario, assuming even distribution of mosaicism in TE (since mosaicism is usually clonal, a highly unlikely scenario), a biopsy of at least 27 TE cells would be required to reach minimal diagnostic predictability from a single TEB.

CONCLUSIONS

As currently performed, a single TEB is, therefore, mathematically incapable of reliably determining whether an embryo can be transferred or should be discarded. Since a single TEB, as currently performed, apparently is not representative of the complete TE, this study, thus, raises additional concern about the clinical utilization of PGS.

Foxh1 Occupies cis-Regulatory Modules Prior to Dynamic Transcription Factor Interactions Controlling the Mesendoderm Gene Program

The interplay between transcription factors and chromatin dictates gene regulatory network activity. Germ layer specification is tightly coupled with zygotic gene activation and, in most metazoans, is dependent upon maternal factors. We explore the dynamic genome-wide interactions of Foxh1, a maternal transcription factor that mediates Nodal/TGF-β signaling, with cis-regulatory modules (CRMs) during mesendodermal specification. Foxh1 marks CRMs during cleavage stages and recruits the co-repressor Tle/Groucho in the early blastula. We highlight a population of CRMs that are continuously occupied by Foxh1 and show that they are marked by H3K4me1, Ep300, and Fox/Sox/Smad motifs, suggesting interplay between these factors in gene regulation. We also propose a molecular "hand-off" between maternal Foxh1 and zygotic Foxa at these CRMs to maintain enhancer activation. Our findings suggest that Foxh1 functions at the top of a hierarchy of interactions by marking developmental genes for activation, beginning with the onset of zygotic gene expression.

Vertebrate Embryonic Cleavage Pattern Determination

The pattern of the earliest cell divisions in a vertebrate embryo lays the groundwork for later developmental events such as gastrulation, organogenesis, and overall body plan establishment. Understanding these early cleavage patterns and the mechanisms that create them is thus crucial for the study of vertebrate development. This chapter describes the early cleavage stages for species representing ray-finned fish, amphibians, birds, reptiles, mammals, and proto-vertebrate ascidians and summarizes current understanding of the mechanisms that govern these patterns. The nearly universal influence of cell shape on orientation and positioning of spindles and cleavage furrows and the mechanisms that mediate this influence are discussed. We discuss in particular models of aster and spindle centering and orientation in large embryonic blastomeres that rely on asymmetric internal pulling forces generated by the cleavage furrow for the previous cell cycle. Also explored are mechanisms that integrate cell division given the limited supply of cellular building blocks in the egg and several-fold changes of cell size during early development, as well as cytoskeletal specializations specific to early blastomeres including processes leading to blastomere cohesion. Finally, we discuss evolutionary conclusions beginning to emerge from the contemporary analysis of the phylogenetic distributions of cleavage patterns. In sum, this chapter seeks to summarize our current understanding of vertebrate early embryonic cleavage patterns and their control and evolution.

Array comparative genomic hybridization analyses of all blastomeres of a cohort of embryos from young IVF patients revealed significant contribution of mitotic errors to embryo mosaicism at the cleavage stage

BACKGROUND

Embryos produced by in vitro fertilization (IVF) have a high level of aneuploidy, which is believed to be a major factor affecting the success of human assisted reproduction treatment. The aneuploidy rate of cleavage stage embryos based on 1-2 biopsied blastomeres has been well-reported, however, the true aneuploidy rate of whole embryos remain unclear because of embryo mosaicism. To study the prevalence of mosaicism in top quality IVF embryos, surplus embryos donated from young patients (aged 28-32) in the assisted reproduction program at Queen Mary Hospital, Hong Kong were used.

METHODS

Thirty-six good quality day 2 embryos were thawed. Out of the 135 blastomeres in these embryos, 121 (89.6%) survived thawing. Twelve of these embryos without lysed blastomeres and which cleaved to at least seven cells after a 24-h culture were dissembled into individual blastomeres, which were analysed by array comparative genomic hybridization and microsatellite marker analysis by fluorescent PCR.

RESULTS

Out of 12 day-3 embryos, 2 (16.7%) were normal, 3 (25%) were diploid/aneuploidy with <38% abnormality, 4 (33.3%) were diploid/aneuploidy mosaic with > =38% abnormality, and three (25%) were mosaic aneuploids. Conclusive chromosomal data were obtained from a high percentage of blastomeres (92.8%, 90/97). Microsatellite marker analysis performed on blastomeres in aneuploid embryos enabled us to reconstruct the chromosomal status of the blastomeres in each cleavage division. The results showed the occurrence of meiotic errors in 3 (25%) of the studied embryos. There were 16 mitotic errors (18.8%, 16/85) in the 85 mitotic divisions undertaken by the studied embryos. The observed mitotic errors were mainly contributed by endoreduplication (31.3%, 5/16), non-disjunction (25%, 4/16) and anaphase lagging (25%, 4/16). Chromosome breakages occurred in 6 divisions (7.1%, 6/85).

CONCLUSIONS

Mosaicism occurs in a high percentage of good-quality cleavage stage embryos and mitotic errors contribute significantly to the abnormality.

MicroRNA-34 family expression in bovine gametes and preimplantation embryos

BACKGROUND

Oocyte fertilization and successful embryo implantation are key events marking the onset of pregnancy. In sexually reproducing organisms, embryogenesis begins with the fusion of two haploid gametes, each of which has undergone progressive stages of maturation. In the final stages of oocyte maturation, minimal transcriptional activity is present and regulation of gene expression occurs primarily at the post-transcriptional level. MicroRNAs (miRNA) are potent effectors of post-transcriptional gene silencing and recent evidence demonstrates that the miR-34 family of miRNA are involved in both spermatogenesis and early events of embryogenesis.

METHODS

The profile of miR-34 miRNAs has not been characterized in gametes or embryos of Bos taurus. We therefore used quantitative reverse transcription PCR (qRT-PCR) to examine this family of miRNAs: miR-34a, -34b and -34c as well as their precursors in bovine gametes and in vitro produced embryos. Oocytes were aspirated from antral follicles of bovine ovaries, and sperm cells were isolated from semen samples of 10 bulls with unknown fertility status. Immature and in vitro matured oocytes, as well as cleaved embryos, were collected in pools. Gametes, embryos and ovarian and testis tissues were purified for RNA.

RESULTS

All members of the miR-34 family are present in bovine spermatozoa, while only miR-34a and -34c are present in oocytes and cleaved (2-cell) embryos. Mir-34c demonstrates variation among different bulls and is consistently expressed throughout oocyte maturation and in the embryo. The primary transcript of the miR-34b/c bicistron is abundant in the testes and present in ovarian tissue but undetectable in oocytes and in mature spermatozoa.

CONCLUSIONS

The combination of these findings suggest that miR-34 miRNAs may be required in developing bovine gametes of both sexes, as well as in embryos, and that primary miR-34b/c processing takes place before the completion of gametogenesis. Individual variation in sperm miR-34 family abundance may offer potential as a biomarker of male bovine fertility.

Temporal regulation of mRNAs for select bone morphogenetic proteins (BMP), BMP receptors and their associated SMAD proteins during bovine early embryonic development: effects of exogenous BMP2 on embryo developmental progression

BACKGROUND

We previously demonstrated embryotrophic actions of maternal (oocyte-derived) follistatin during bovine early embryogenesis. Classical actions of follistatin are attributed to inhibition of activity of growth factors including activins and bone morphogenetic proteins (BMP). However, temporal changes in BMP mRNA in early bovine embryos and the effects of exogenous BMP on embryo developmental progression are not understood. The objectives of present studies were to characterize mRNA abundance for select BMP, BMP receptors and BMP receptor associated SMADs during bovine oocyte maturation and early embryogenesis and determine effects of addition of exogenous BMP protein on early development.

METHODS

Relative abundance of mRNA for BMP2, BMP3, BMP7, BMP10, SMAD1, SMAD5, ALK3, ALK6, ALK2, BMPR2, ACVR2A and ACVR2B was determined by RT-qPCR analysis of germinal vesicle (GV) and in vitro matured metaphase II (MII) oocytes and in vitro produced embryos collected at pronuclear, 2-cell (C), 4C, 8C, 16C, morula and blastocyst stages. Effects of addition of recombinant human BMP2 (0, 1, 10 and 100 ng/ml) during initial 72 h of embryo culture on early cleavage (within 30 h post insemination), total cleavage, development to 8C-16C and blastocyst stages and blastocyst mRNA abundance for markers of inner cell mass (NANOG) and trophectoderm (CDX2) were also determined.

RESULTS

Abundance of mRNA for BMP2, BMP10, SMAD1, SMAD5, ALK3, ALK2, BMPR2 and ACVR2B was elevated in MII oocytes and/or pronuclear stage embryos (relative to GV) and remained elevated through the 8C -16C stages, whereas BMP3, BMP7 and ALK2 mRNAs were transiently elevated. Culture of embryos to the 8C stage in the presence of α-amanitin resulted in increased abundance for all of above transcripts examined relative to untreated 8C embryos. Effects of addition of exogenous BMP2 on early cleavage rates and rates of development to 8C-16C and blastocyst stages were not observed, but BMP2 treatment increased blastocyst mRNA for CDX2 and NANOG.

CONCLUSIONS

Abundance of maternally derived mRNAs for above BMP system components are dynamically regulated during oocyte maturation and early embryogenesis. Exogenous BMP2 treatment does not influence progression to various developmental endpoints, but impacts characteristics of resulting blastocysts. Results support a potential role for BMPs in bovine early embryogenesis.

Insulin-like growth factor-1 (IGF-1) enhances developmental competence of cat embryos cultured singly by modulating the expression of its receptor (IGF-1R) and reducing developmental block

OBJECTIVE

The aim of this study is to determine the effects of insulin-like growth factor-1 (IGF-1) and the mRNA expression of IGF-1 receptor (IGF-1R) during the in vitro development of cat embryos cultured in groups versus singly.

METHODS

Cumulus-oocyte complexes (COCs) were matured and fertilized in vitro with frozen-thawed semen. Cleaved embryos (48h post-fertilization) were randomly assigned to one of the following treatments: 1) group embryo culture without IGF-1 (10 embryos per 50μl droplet), 2) single-embryo culture without IGF-1, and 3) to 6) single-embryo culture (50μl droplet per embryo) supplemented with different concentrations of IGF-1 (5, 25, 50 and 100ng/ml, respectively). During in vitro culture, the embryos were analyzed for development to the morula, blastocyst and hatching blastocyst stage. Relative mRNA expression of IGF-1R was also examined by qPCR at the morula and blastocyst stages. In addition, the mRNA expression of IGF-1R in morula-stage embryos treated with IGF-1 was determined. The influence of IGF-1 to preimplantation embryo development was then explored by co-incubation with 0.5μM IGF-1R inhibitor (Picropodophyllin; PPP).

RESULTS

Group embryo culture led to a significantly higher blastocyst development rate compared with single-embryo culture (P<0.05). The poor development of singly cultured embryos coincided with the significantly lower IGF-1R expression in morulae than in group-cultured morulae. IGF-1 (25 or 50ng/ml) supplementation significantly improved the blastocyst formation rate of single embryos to a level similar to group culture by promoting the morula-to-blastocyst transition. IGF-1 supplementation (25 or 50ng/ml) of singly cultured embryos upregulated the expression of IGF-1R mRNA in morula-stage embryos to the same level as that observed in group-cultured embryos (without IGF-1). The beneficial effects of IGF-1 on singly cultured embryo were (P<0.05) suppressed by PPP even in the group culture embryo without growth factor supplementation.

CONCLUSION

IGF-1 supplementation improves the developmental competence of feline embryos cultured individually and also increases IGF-1R gene expression to levels similar to group-cultured embryos.

Glucocorticoid receptor activation following elevated oocyte cortisol content is associated with zygote activation, early embryo cell division, and IGF system gene responses in rainbow trout

Increased in ovo cortisol content of rainbow trout oocytes from ~3·5 to ~5·0 ng.oocyte(-1) before fertilization enhances the growth of embryos and juveniles and changes the long-term expression pattern of IGF-related genes. This study used embryos reared from oocytes enriched with cortisol and the glucocorticoid receptor (GR) antagonist, RU486, to determine whether the growth-promoting actions of cortisol involve GR protein activation and modulation of gr expression. Whole-mount in situ immunohistofluorescence studies of zygotes showed that enhanced oocyte cortisol increased the immunofluorescent GR signal and activated the relocation of GR from a general distribution throughout the cytoplasm to an accumulation in the peri-nuclear cytoplasm. In ovo cortisol treatment increased the number of embryonic cells within 48-h post-fertilization, and RU486 partially suppressed this cortisol stimulation of cell duplication. In addition, there was complex interplay between the expression of gr and igf system-related genes spatiotemporally in the different treatment groups, suggesting a role for GR in the regulation of the expression of development. Taken together, these findings indicate an essential role for GR in the regulation of epigenomic events in very early embryos that promoted the long-term growth effects of the embryos and juvenile fish. Moreover, the pretreatment of the oocyte with RU486 had a significant suppressive effect on the maternal mRNA transcript number of gr and igf system-related genes in oocytes and very early stage embryos, suggesting an action of antagonist on the stability of the maternal transcriptome.

Evaluation of developmental changes in bovine in vitro produced embryos following exposure to bovine Herpesvirus type 5

BACKGROUND

Bovine Herpesvirus type-5 (BoHV-5) is a neurovirulent α-Herpesvirus which is potentially pathogenic for cows and suspected to be associated with reproductive disorders. Interestingly, natural transmission of BoHV-5 by contaminated semen was recently described in Australia. Additionally, BoHV-5 was also isolated from the semen of a healthy bull in the same country and incriminated in a natural outbreak of reproductive disease after artificial insemination. In contrast with BoHV-1, experimental exposure of in vitro produced bovine embryos to BoHV-5 does not affect embryo viability and seems to inhibit some pathways of apoptosis. However, the mechanisms responsible for these phenomena are poorly understood. In this study, we examined mitochondrial activity, antioxidant protection, stress response and developmental rates of in vitro produced bovine embryos that were exposed and unexposed to BoHV-5.

METHODS

For this purpose, bovine embryos produced in vitro were assayed for cell markers after experimental infection of oocytes (n = 30; five repetitions), in vitro fertilization and development. The indirect immunofluorescence was employed to measure the expression of superoxide dismutase 1 (SOD1), anti-oxidant like protein 1 (AOP-1), heat shock protein 70.1 (Hsp 70.1) and also viral antigens in embryos derived from BoHV-5 exposed and unexposed oocytes. The determination of gene transcripts of mitochondrial activity (SOD1), antioxidant protection (AOP-1) and stress response (Hsp70.1) were evaluated using the reverse transcriptase polymerase chain reaction (RT-PCR). MitoTracker Green FM, JC-1 and Hoechst 33342-staining were used to evaluate mitochondrial distribution, segregation patterns and embryos morphology. The intensity of labeling was graded semi-quantitatively and embryos considered intensively marked were used for statistical analysis.

RESULTS

The quality of the produced embryos was not affected by exposure to BoHV-5. Of the 357 collected oocytes, 313 (+/- 6.5; 87.7%) were cleaved and 195 (+/- 3.2; 54.6%) blastocysts were produced without virus exposure. After exposure, 388 oocytes were cleaved into 328 (+/- 8.9, 84.5%), and these embryos produced 193 (+/- 3.2, 49.7%) blastocysts. Viral DNA corresponding to the US9 gene was only detected in embryos at day 7 after in vitro culture, and confirmed by indirect immunofluorescence assay (IFA). These results revealed significant differences (p < 0.05) between exposed and unexposed oocytes fertilized, as MitoTracker Green FM staining Fluorescence intensity of Jc-1 staining was significantly higher (p < 0.005) among exposed embryos (143 +/- 8.2). There was no significant difference between the ratios of Hoechst 33342-stained nuclei and total cells in good-quality blastocysts (in both the exposed and unexposed groups). Using IFA and reverse transcriptase polymerase chain reaction (RT-PCR) for the set of target transcripts (SOD1, AOP-1 and Hsp 70.1), there were differences in the mRNA and respective proteins between the control and exposed embryos. Only the exposed embryos produced anti-oxidant protein-like 1 (AOP-1). However, neither the control nor the exposed embryos produced the heat shock protein Hsp 70.1. Interestingly, both the control and the exposed embryos produced superoxide dismutase (SOD1), revealing intense mitochondrial activity.

CONCLUSION

This is the first demonstration of SOD1 and AOP-1 production in bovine embryos exposed to BoHV-5. Intense mitochondrial activity was also observed during infection, and this occurred without interfering with the quality or number of produced embryos. These findings further our understanding on the ability of α-Herpesviruses to prevent apoptosis by modulating mitochondrial pathways.

Characteristics of chromosomal abnormalities diagnosed after spontaneous abortions in an infertile population

PURPOSE

To estimate the prevalence of chromosomally abnormal related miscarriages in an infertile population.

METHODS

Retrospective analysis of cytogenetics obtained by chorionic villi harvesting of the first miscarriage cycle of infertile patients at our center from 2001-2010 were reviewed. Abnormal results were characterized as trisomy, monosomy X, structural, or other. Age, # of eggs, #2PN, # embryos transferred, day of transfer, and performance of intracytoplasmic sperm injection (ICSI) were recorded.

RESULTS

In a study population of 299 patients with a mean age of 38.0 ± 4.5 y, 276(92 %) patients had some form of assisted reproductive technologies (ART), and 244(82 %) had IVF. Of all results, 71.6 % had an abnormal karyotype. Patients with abnormal cytogenetics were older (38.6 ± 4.1 vs. 36.3 ± 4.9, p < 0.001), and more likely to have a day 3 transfer (age < 38 ( 20.7 %) vs. age 38 (46.3 %), p = <0.001) with more embryos transferred (3.0 ± 1.2, vs. 2.3 ± 0.9, p < 0.001). The performance of ICSI did not affect the rate of cytogenetically abnormal products of conception (ICSI 68.3 % vs. no ICSI 70.7 %). In comparing patients, monosomy X was more common in <38 y. Rates of trisomy, although not statistically significant, were higher in older patients.

CONCLUSIONS

The classic associations between advancing age and chromosomal abnormalities, and younger age and monosomy X, are affirmed in our infertile population. There was no increase in chromosomal abnormalities in cycles where ICSI was performed. Older patients are more likely to have day 3 transfers and more embryos transferred. Our chromosomal abnormality rates are higher than classic estimates but comparable to recent studies. The limitation of this study was a lack in uniformity among practitioners in recommending all patients have a Dilation and Curettage (D&C) at time of diagnosis. Such information may serve to improve the counseling of patients after miscarriage.

[A comparative study of fluorescent patterns in the nuclei of early mouse embryos using antibodies against different domains of the actin molecule]

In this work, new data on the peculiarities of actin immunofluorescent detection in 2-cell mouse embryos using antibodies against C- and N-terminal domains are presented. We studied the distribution of nuclear actin after artificial suppression of transcription and after enzymatic digestion of DNA. The visual observations were supplemented with morphometric analyses of confocal images. In both experimental groups (treated by transcription inhibitors or DNase) the reliable increasing of fluorescence intensity is revealed when antibody against C-, but not against N-terminal domain of actin was used. These finding allows to suppose that antibody against C-terminal domain of actin is suitable with a high efficiency for immunofluorescent studies on the nuclei of cleavage mouse embryo.

The echinoid mitotic gradient: effect of cell size on the micromere cleavage cycle

Like other euechinoids, the fertilized eggs of the sand dollar Dendraster excentricus proceed through cleavages that produce a pattern of macromeres, mesomeres, and micromeres at the 4th division. The 8 cells of the macro-mesomere lineage proceed through 6 additional cleavages before hatching. At the fifth overall division, the 4 micromeres produce a lineage of large micromeres that will divide 3 additional times, and a lineage of small micromeres that will divide once more before hatching. Irrespective of lineage, the length of the cell cycles is closely related to the size of the blastomere; cells of the same size have the same cell cycle time. A consequence is that at the fourth cleavage, there is a gradient of mitotic activity from the fastest dividers at the animal pole and the slowest cleaving micromeres at the vegetal pole. By the time of hatching, which is the 10th division of meso-macromeres, all cells are the same small size, the metachronic pattern of division gives way to asynchrony, and the mitotic gradient along the polar axis is lost. Experimental pre-exposure to sodium dodecyl sulfate (SDS), however, blocks the appearance of the gradients in cell size, the mitotic gradient, and the differential in cell cycle times. It is proposed that the mitotic gradients, cell cycle times, and attainment of a state of asynchrony are functions of cell size. Developmental consequences of the transition are large, and include coordinated activation of transcriptions, synthesis of new patterns of proteins, alterations of metabolism, and onset of morphogenesis.

A spatio-temporal model of Notch signalling in the zebrafish segmentation clock: conditions for synchronised oscillatory dynamics

In the vertebrate embryo, tissue blocks called somites are laid down in head-to-tail succession, a process known as somitogenesis. Research into somitogenesis has been both experimental and mathematical. For zebrafish, there is experimental evidence for oscillatory gene expression in cells in the presomitic mesoderm (PSM) as well as evidence that Notch signalling synchronises the oscillations in neighbouring PSM cells. A biological mechanism has previously been proposed to explain these phenomena. Here we have converted this mechanism into a mathematical model of partial differential equations in which the nuclear and cytoplasmic diffusion of protein and mRNA molecules is explicitly considered. By performing simulations, we have found ranges of values for the model parameters (such as diffusion and degradation rates) that yield oscillatory dynamics within PSM cells and that enable Notch signalling to synchronise the oscillations in two touching cells. Our model contains a Hill coefficient that measures the co-operativity between two proteins (Her1, Her7) and three genes (her1, her7, deltaC) which they inhibit. This coefficient appears to be bounded below by the requirement for oscillations in individual cells and bounded above by the requirement for synchronisation. Consistent with experimental data and a previous spatially non-explicit mathematical model, we have found that signalling can increase the average level of Her1 protein. Biological pattern formation would be impossible without a certain robustness to variety in cell shape and size; our results possess such robustness. Our spatially-explicit modelling approach, together with new imaging technologies that can measure intracellular protein diffusion rates, is likely to yield significant new insight into somitogenesis and other biological processes.

Fresh MESA improved embryo fertilization, cleavage, blastula formation and implantation rates after failed TESA in couples with obstructive azoospermia

PURPOSE

To determine whether our use of fresh MESA cycles improved outcomes in patients with obstructive azoospermia who had failed IVF with TESA.

METHODS

A prospective observational trial of couples undergoing IVF for obstructive azoospermia was performed at an academic IVF center.

RESULTS

When TESA resulted in poor embryo cleavage, implantation and ongoing pregnancy rates, subsequent fresh MESA cycles in these same couples, demonstrated dramatic improvement in cleavage, blastulation, implantation and live birth rates.

CONCLUSIONS

In patients undergoing IVF-TESA-ICSI with obstructive azoospermia resulting in poor cleavage rates, blastulation rates and cycle failure, a repeat cycle with MESA may result in marked improvement in outcome.

Early embryonic development in the spider Achaearanea tepidariorum: Microinjection verifies that cellularization is complete before the blastoderm stage

The spider Achaearanea tepidariorum is emerging as a non-insect model for studying developmental biology. However, the availability of microinjection into early embryos of this spider has not been reported. We defined the early embryonic stages in A. tepidariorum and applied microinjection to its embryos. During the preblastoderm 16- and 32-nucleus stages, the energids were moving toward the egg periphery. When fluorochrome-conjugated dextran was microinjected into the peripheral region of 16-nucleus stage embryos, it was often incorporated into a single energid and inherited in the progeny without leaking out to surrounding energids. This suggested that 16-nucleus stage embryos consisted of compartments, each containing a single energid. These compartments were considered to be separate cells. Fluorochrome-conjugated dextran could be introduced into single cells of 16- to 128-nucleus stage embryos, allowing us to track cell fate and movement. Injection with mRNA encoding a nuclear localization signal/green fluorescent protein fusion construct demonstrated exogenous expression of the protein in live spider embryos. We propose that use of microinjection will facilitate studies of spider development. Furthermore, these data imply that in contrast to the Drosophila syncytial blastoderm embryo, the cell-based structure of the Achaearanea blastoderm embryo restricts diffusion of cytoplasmic gene products.

Characterization of a diverse secretome generated by the mouse preimplantation embryo in vitro

This study investigates the suitability of surface-enhanced laser desorption and ionization time-of-flight (SELDI-TOF) and electrospray ionization (ESI) mass spectrometry for analysis of the proteins released by the mouse preimplantation embryo in vitro. SELDI-TOF analysis with CM10 or IMAC30 (but not Q10) protein chips detected a protein peak at m/z approximately 8570 released by both C57BL6 and hybrid embryos. No other peaks unique to the presence of the embryo were identified with this method. ESI mass spectrometry of tryptic digests of embryo-conditioned media identified a total of 20 proteins released during development from the zygote to blastocyst stage. Four proteins were expressed in at least 7 out of 8 cultures tested, one of these (lactate dehydrogenase B) was in all cultures. A further five proteins were in at least half of the cultures and 11 more proteins were in at least one culture. The expression of two of these proteins is essential for preimplantation embryo development (NLR family, pyrin domain containing 5 and peptidyl arginine deiminase, type VI). A further four proteins detected have roles in redox regulation of cells, and three others are capable of inducing post-translational modifications of proteins. This study shows the feasibility of ESI mass spectrometry for identifying the proteins secreted by the preimplantation embryo in vitro. This analysis identifies a range of targets that now require detailed functional analysis to assess whether their release by the embryo is an important property of early embryo development.

A comparative genome analysis of gene expression reveals different regulatory mechanisms between mouse and human embryo pre-implantation development

BACKGROUND

Pre-implantation development is a crucial step in successful implantation and pregnancy in mammals. It has been studied in depth, but mostly in laboratory animal models. Less is known about the regulatory mechanism involved in the pre-implantation development in humans and about the comparative aspects.

METHODS

Here, we employed the microarray datasets from the public database library of GEO and applied comparative analysis of genome wide temporal gene expression data based on statistical analysis and functional annotation for both mouse and human, demonstrating the discordance between the regulatory mechanisms of both mouse and human pre-implantation development.

RESULTS

There were differences between mouse and human pre-implantation development both in the global gene expression pattern and in the expression changes of individual genes at each stage, including different major transient waves of transcription profiles and some stage-specific genes and significantly related pathways. There also appeared to be different functional changes from one stage to another between mouse and human.

CONCLUSIONS

The analysis presented here lead to interesting and complementary conclusions that the regulatory mechanism of human pre-implantation development is not completely the same as the mouse. Not as the fact that 1-cell to 2-cell stage is important for mouse pre-implantation development, the 4-cell stage and 8-cell stage are both essential for human. Unlike in mouse, of which most of pathways found were related to energy, RNA and protein metabolism, the identified pathways in human were mostly disease-related and associated with human pre-implantation embryonic development. All of these suggest that a further comparative analysis should be required for applying the result of mouse expression data to human research or therapy, particularly in pre-implantation developments. Our study provides several potential targets of genes and pathways for studying the regulatory mechanism of human pre-implantation development using mouse model.

Ectopic expression of prelamin A in early Xenopus embryos induces apoptosis

Lamin proteins are components of metazoan cell nuclei. During evolution, two classes of lamin proteins evolved, A- and B-type lamins. B-type lamins are expressed in nearly all cell types and in all developmental stages and are thought to be indispensable for cellular survival. In contrast, A-type lamins have a more restricted expression pattern. They are expressed in differentiated cells and appear late in embryogenesis. In the earliest steps of mammalian development, A-type lamins are present in oocytes, pronuclei and during the first cleavage stages of the developing embryo. But latest after the 16-cell stage, A-type lamin proteins are not any longer detectable in embryonic cells. Amphibian oocytes and early embryos do not express lamin A. Moreover, extracts of Xenopus oocytes and eggs have the ability to selectively remove A-type lamins from somatic nuclei. This observation and the restricted expression pattern suggest that the presence of lamin A might interfere with developmental processes in the early phase of embryogenesis. To test this, we ectopically expressed lamin A during early embryonic development of Xenopus laevis by microinjection of synthetic mRNA. Here, we show that introducing mature lamin A does not interfere with normal development. However, expression of prelamin A or lamin A variants that cannot be fully processed cause severe disturbances and lead to apoptosis during gastrulation. The toxic effect is due to lack of the conversion of prenylated prelamin A to its mature form. Remarkably, even a cytoplasmic prelamin A variant that is excluded from the nucleus drives embryos into apoptosis.

Molecular determinants of oocyte competence: potential functional role for maternal (oocyte-derived) follistatin in promoting bovine early embryogenesis

Previous studies established a positive relationship between oocyte competence and follistatin mRNA abundance. Herein, we used the bovine model to test the hypothesis that follistatin plays a functional role in regulation of early embryogenesis. Treatment of early embryos with follistatin during in vitro culture (before embryonic genome activation) resulted in a dose-dependent decrease in time to first cleavage, increased numbers of blastocysts, and increased blastocyst total and trophectoderm cell numbers. To determine the requirement of endogenous follistatin for early embryogenesis, follistatin ablation/replacement studies were performed. Microinjection of follistatin small interfering RNA into zygotes reduced follistatin mRNA and protein and was accompanied by a reduction in number of embryos developing to eight- to 16-cell and blastocyst stages and reduced blastocyst total and trophectoderm cell numbers. Effects of follistatin ablation were rescued by culture of follistatin small interfering RNA-injected embryos in the presence of exogenous follistatin. To investigate whether follistatin regulation of early embryogenesis is potentially mediated via inhibition of endogenous activin activity, the effects of treatment of embryos with exogenous activin, SB-431542 (inhibitor of activin, TGF-beta, and nodal type I receptor signaling) and follistatin plus SB-431542 were investigated. Activin treatment mimicked positive effects of follistatin on time to first cleavage and blastocyst development, whereas negative effects of SB-431542 treatment were observed. Stimulatory effects of follistatin on embryogenesis were not blocked by SB-431542 treatment. Results support a functional role for oocyte-derived follistatin in bovine early embryogenesis and suggest that observed effects of follistatin are likely not mediated by classical inhibition of activin activity.

DAZL protein expression in mouse preimplantation embryo

OBJECTIVE

To evaluate the expression pattern of Dazl (deleted in azoospermia-like) protein in the mouse preimplantation embryo.

DESIGN

Experimental study.

SETTING

Medical research laboratory in a university hospital.

ANIMAL(S)

Twenty female 28- to 35-day-old FVB mice.

INTERVENTION(S)

Embryo collection at 1.5, 2.5, and 3.5 days postcoitus (plug date, 0.5 d postcoitus) to examine the Dazl protein expression from the two-cell embryo to the blastocyst.

MAIN OUTCOME MEASURE(S)

Dazl protein expression was analyzed by immunofluorescent staining.

RESULT(S)

There is abundant expression of Dazl protein in the cytoplasm of the blastomere. Strong fluorescent signals of Dazl protein expression were found in preimplantation embryo cytoplasm, including two-cell, eight-cell, morula, and blastocyst.

CONCLUSION(S)

By using an antibody raised against mouse Daz-like protein (Dazl), we showed that Dazl protein is present in all cleaving stages of the preimplantation embryo. This is the first report on the protein expression of a Dazl gene during embryogenesis in mice. However, further study is needed to evaluate the molecular functional role of Dazl.

Vascular endothelial growth factor level changes during human embryo development in culture medium

OBJECTIVE

Implantation is a complex phenomenon consisting of the first strong contact between embryo and endometrium. Recent studies have demonstrated that this process is dependent not only on the 'readiness' of the endometrium, but also on complex interactions between endometrial and embryonic tissues that cross-talk by means of different molecules (growth factors, cytokines, vasoactive factors). Investigations performed on human blastocysts indicate a role for vascular endothelial growth factor (VEGF) in these processes. The aim of the present study was to investigate VEGF levels at different stages in human embryo culture medium.

STUDY DESIGN

We selected 20 women among patients undergoing assisted reproduction with the in vitro fertilization-blastocyst transfer protocol. The oocytes were inseminated by intracytoplasmic sperm injection. For each patient, approximately two cultures of four microinjected oocytes (and then of four embryos) were performed. Each culture of four oocytes/embryos was placed in one dish to increase the probability to detect small VEGF concentrations.

RESULTS

Results showed significantly higher VEGF levels in the medium at blastocyst stage (12.16 +/- 2.80 pg/ml) compared with embryos at pronuclear stage (13.58 +/- 2.32 pg/ml) and microinjected oocytes (12.80 +/- 3.45 pg/ml).

CONCLUSIONS

An important VEGF synthesis by blastocysts occurs during human embryo development.

Isolation and characterization of a novel Xenopus gene (xVAP019) encoding a DUF1208 domain containing protein

We have identified a novel Xenopus gene (xVAP019) encoding a DUF1208 domain containing protein. Using whole-mount in situ hybridization and RT-PCR, we found abundant xVAP019 maternal transcripts in the animal hemisphere during the cleavage stages and blastula stages. During gastrulation xVAP019 is differentially expressed with higher levels in the animal helf and the highest in marginal zone, then further expressed widely at neuronal stages with strongest signals in the prospective CNS regions and the epidermal ectoderm. Subsequently xVAP019 was expressed predominantly in the head, the eyes, the otic vesicle, branchial arches, spinal cord, notochord, somites, and tailbud. It is absent or very weak in the endoderm. Injecting a morpholino oligo complementary to xVAP019 mRNA or injecting a caped xVAP019 mRNA caused most of embryos to die during gastrulation and neurulation. Overexpression of xVAP019 mRNA also led to eye defect, shorten interocular distance, small body size and abnormal pigment formation in parts of the survival embryos. Similar effects were induced by injecting the xVAP019 human homologous gene FAM92A1. Our results suggest that xVAP019 is essential for the normal ectoderm and axis mesoderm differentiation and embryos survival. This investigation is for the first time in vivo study examining the role of this novel gene and reveals an important role of xVAP019 in embryonic development.

Ca2+SIGNALLING AND EARLY EMBRYONIC PATTERNING DURING ZEBRAFISH DEVELOPMENT

1. It has been proposed that Ca2+ signalling, in the form of pulses, waves and steady gradients, may play a crucial role in key pattern-forming events during early vertebrate development. 2. With reference to the embryo of the zebrafish (Danio rerio), herein we review the Ca2+ transients reported from the cleavage to segmentation periods. This time-window includes most of the major pattern-forming events of early development, which transform a single-cell zygote into a complex multicellular embryo with established primary germ layers and body axes. 3. Data are presented to support our proposal that intracellular Ca2+ waves are an essential feature of embryonic cytokinesis and that propagating intercellular Ca2+ waves (both long and short range) may play a crucial role in: (i) the establishment of the embryonic periderm and the coordination of cell movements during epiboly, convergence and extension; (ii) the establishment of the basic embryonic axes and germ layers; and (iii) definition of the morphological boundaries of specific tissue domains and embryonic structures, including future organ anlagen. 4. The potential downstream targets of these Ca2+ transients are also discussed, as well as how they may integrate with other pattern-forming signalling pathways known to modulate early developmental events.

The Initiation and Propagation of Hes7 Oscillation Are Cooperatively Regulated by Fgf and Notch Signaling in the Somite Segmentation Clock

Periodic formation of somites is controlled by the segmentation clock, where the oscillator Hes7 regulates cyclic expression of the Notch modulator Lunatic fringe. Here, we show that Hes7 also regulates cyclic expression of the Fgf signaling inhibitor Dusp4 and links Notch and Fgf oscillations in phase. Strikingly, inactivation of Notch signaling abolishes the propagation but allows the initiation of Hes7 oscillation. By contrast, transient inactivation of Fgf signaling abolishes the initiation, whereas sustained inactivation abolishes both the initiation and propagation of Hes7 oscillation. We thus propose that Hes7 oscillation is initiated by Fgf signaling and propagated/maintained anteriorly by Notch signaling.

Murine pre-embryo oxygen consumption and developmental competence

PURPOSE

In search for a new marker of preimplantation embryo viability the present study investigated oxygen consumption of individual cleavage stage murine embryos, and evaluated the predictive value regarding subsequent development to expanded blastocysts.

METHODS

In all, 248 embryos were investigated from 2 cell stage until blastocyst stage with individual measurement of oxygen consumption and recording of developmental stage. Cleavage stage embryos and morula were divided in groups according to their oxygen consumption, and odds ratios (OR) for subsequent development to expanded blastocyst were calculated.

RESULTS

Cleavage stage (2-8 cell) individual oxygen consumption was 0.16-0.20 nl O(2) h(-1), with a significant increase to 0.21-0.23 nl O(2) h(-1) at the morula stage followed by a more than twofold increase for the expanded blastocyst 0.47 nl O(2) h(-1). A significantly higher chance of reaching the expanded blastocyst stage was found in 4-cell embryos with high oxygen consumption, than embryos with low consumption (OR 2.25, 95% CI 1.04-4.90). Among 2-cell embryos the chance of low and high consumers was not significantly different. The method used in the present study somewhat compromised embryo development (51% blastocyst rate) compared to controls (80% blastocystrate) which could make our results less robust.

CONCLUSION

Preliminary data from the present study suggest that oxygen consumption in cleavage stage embryos may be an indicator, but a not a strong predictor, of subsequent development to expanded blastocysts.

Amphioxus AmphiDelta: evolution of Delta protein structure, segmentation, and neurogenesis

The amphioxus genome has a single Delta gene (AmphiDelta) encoding a protein 766 amino acids long. Comparison of Delta proteins of amphioxus and other animals indicates that AmphiDelta retains features of a basal bilaterian Delta protein--in having nine epidermal growth factor (EGF) repeats and also in having characteristic numbers of amino acids separating successive cysteines between and within EGF repeats. During development, AmphiDelta is expressed in the forming somites, in some regions of pharyngeal endoderm, and in cells (presumably differentiating neurons) scattered in both the neural plate and ectoderm. Expression is strongly associated with cells initiating movements to separate themselves from parent epithelia, either en masse by evagination (endoderm and mesoderm) or by delamination as isolated cells (ectoderm). The AmphiDelta-expressing cells delaminating from the ectoderm apparently migrate beneath it as they begin differentiating into probable sensory neurons, suggesting a scenario for the evolutionary origin of the placode-derived neurons of vertebrate cranial ganglia.

Zscan4: a novel gene expressed exclusively in late 2-cell embryos and embryonic stem cells

The first wave of transcription, called zygotic genome activation (ZGA), begins during the 2-cell stage in mouse preimplantation development and marks a vital transition from the maternal genetic to the embryonic genetic program. Utilizing DNA microarray data, we looked for genes that are expressed only during ZGA and found Zscan4, whose expression is restricted to late 2-cell stage embryos. Sequence analysis of genomic DNA and cDNA clones revealed nine paralogous genes tightly clustered in 0.85 Mb on mouse chromosome 7. Three genes are not transcribed and are thus considered pseudogenes. Among the six expressed genes named Zscan4a-Zscan4f, three - Zscan4c, Zscan4d, and Zscan4f - encode full-length ORFs with 506 amino acids. Zscan4d is a predominant transcript at the late 2-cell stage, whereas Zscan4c is a predominant transcript in embryonic stem (ES) cells. No transcripts of any Zscan4 genes are detected in any other cell types. Reduction of Zscan4 transcript levels by siRNAs delays the progression from the 2-cell to the 4-cell stage and produces blastocysts that fail to implant or proliferate in blastocyst outgrowth culture. Zscan4 thus seems to be essential for preimplantation development.

Hermes RNA-binding protein targets RNAs-encoding proteins involved in meiotic maturation, early cleavage, and germline development

The early development of metazoans is mainly regulated by differential translation and localization of maternal mRNAs in the embryo. In general, these processes are orchestrated by RNA-binding proteins interacting with specific sequence motifs in the 3'-untranslated region (UTR) of their target RNAs. Hermes is an RNA-binding protein, which contains a single RNA recognition motif (RRM) and is found in various vertebrate species from fish to human. In Xenopus laevis, Hermes mRNA and protein are localized in the vegetal region of oocytes. A subpopulation of Hermes protein is concentrated in a specific structure in the vegetal cortex, called the germ plasm (believed to contain determinants of the germ cell fate) where Hermes protein co-localizes with Xcat2 and RINGO/Spy mRNAs. The level of total Hermes protein decreases during maturation. The precocious depletion of Hermes protein by injection of Hermes antisense morpholino oligonucleotide (HE-MO) accelerates the process of maturation and results in cleavage defects in vegetal blastomeres of the embryo. It is known that several maternal mRNAs including RINGO/Spy and Mos are regulated at the translational level during meiotic maturation and early cleavage in Xenopus. The ectopic expression of RINGO/Spy or Mos causes resumption of meiotic maturation and cleavage arrests, which resemble the loss of Hermes phenotypes. We found that the injection of HE-MO enhances the acceleration of maturation caused by the injection of RINGO/Spy mRNA, and that Hermes protein is present as mRNP complex containing RINGO/Spy, Mos, and Xcat2 mRNAs in vivo. We propose that as an RNA-binding protein, Hermes may be involved in maturation, cleavage events at the vegetal pole and germ cell development by negatively regulating the expression of RINGO/Spy, Mos, and Xcat2 mRNAs.

Geminin is essential for the development of preimplantation mouse embryos

Replication of DNA is strictly controlled to ensure that it occurs only once per cell cycle. Geminin has been thought to serve as a central mediator of this licensing mechanism by binding to and antagonizing the function of Cdt1 and thereby preventing re-replication during S and G2 phases. We have now generated mice deficient in geminin to elucidate the physiologic role of this protein during development. Lack of geminin was shown to result in preimplantation mortality. A delay in the development of homozygous mutant embryos was first apparent at the transition from the four- to eight-cell stages, concomitant with the disappearance of maternal geminin protein, and development was arrested at the eight-cell stage. The mutant embryos manifest morphological abnormalities such as dispersed blastomeres with nuclei that are irregular both in size and shape as well as impaired cell-cell adhesion. DNA replication occurs but mitosis was not detected in the mutant embryos. The abnormal blastomeres contain damaged DNA and undergo apoptosis, likely as a consequence of the deregulation of DNA replication. Our results suggest that geminin is essential for cooperative progression of the cell cycle through S phase to M phase during the preimplantation stage of mouse development.

Analysis of specific factors generating 2-cell block in AKR mouse embryos

The phenomenon of the developmental arrest at the 2-cell stage of 1-cell embryos from some mouse strains during in vitro culture is known as the 2-cell block. We investigated the specific factors involved in the 2-cell block of AKR embryos by means of a modified culture system, the production of reconstructed embryos by pronuclear exchange and a cross experiment. In a culture medium with phosphate, 94.6% of 1-cell embryos from the C57BL mouse strain developed to the blastocyst stage, but 95.7% of embryos from the AKR mouse strain showed 2-cell block. Phosphate-free culture medium rescued the 2-cell block of AKR embryos and accelerated the first cell cycle of the embryos. Co-culture with BRL cells and a BRL-conditioned medium fractionated below 30 kDa also rescued the 2-cell block of AKR embryos. Examinations of in vitro development of reconstructed embryos and of embryos from F1 females between AKR and C57BL strains clearly demonstrated that the AKR cytoplast caused the 2-cell block. In the backcrossed female progeny between (AKR x C57BL) F1 males and AKR females, about three-quarters of the embryos were of the 2-cell blocking phenotype and about one-quarter were of the non-blocking phenotype. These results suggest that two genes are responsible for the 2-cell block of AKR embryos.

Developmental arrest induced in cleavage stage porcine embryos following microinjection of mRNA encodingBrahma (Smarca 2), a chromatin remodeling protein

Smarca 2 (Brahma) and Smarca 4 (Brahma related gene 1, BRG1) alternatively occupy the catalytic site of SWI/SNF chromatin remodeling complexes. Mammalian embryos undergo a dramatic amount of epigenetic remodeling during cleavage development, which plays key roles in regulating both gene transcription and the developmental potential of the embryo. In order to understand how the epigenetic state of cleavage stage embryos is regulated, it is important to identify the factors that mediate epigenetic changes during cleavage development. In this study we hypothesized that altered expression of Smarca 2 would have profound effects on embryo development. The objectives of this study were to determine the expression pattern of Smarca 2 and determine the effects of Smarca 2 overexpression in cleavage stage parthenogenetic porcine embryos. Smarca 2 transcripts are most abundant in germinal vesicle (GV) stage oocytes and decline progressively during cleavage development. At the blastocyst stage, Smarca 2 transcripts are reduced by 18-fold (GV stage oocyte vs. blastocyst stage embryo, P < 0.05). Parthenogenetic porcine embryos injected with mRNA encoding wild type human Smarca 2 exhibited a dramatic developmental arrest as compared to noninjected embryos, embryos injected with GFP mRNA, or mRNA encoding a dominant negative version of human Smarca 2 (P < 0.01). This work demonstrates the importance of Smarca 2 containing SWI/SNF chromatin remodeling complexes in preimplantation porcine embryos and how perturbing the amount of Smarca 2 in porcine embryos disrupts cleavage development.

Profiling of maternal and developmental-stage specific mRNA transcripts in Atlantic halibut Hippoglossus hippoglossus

cDNA libraries were constructed from the following developmental stages (tissues) of the Atlantic halibut (Hippoglossus hippoglossus): 2-cell stage (embryos), 1 day-old yolk sac larvae (trunk) and juvenile (fast skeletal muscle). A total of 4249 high quality expressed sequence tags from the three libraries were clustered into a partial transcriptome of 2124 putative genes. A large proportion of the gene clusters (48.3%) had no significant matches against known proteins. The most abundant ESTs of nuclear transcripts in the 2-cell library included sequences with high identity to zebrafish H1M, a linker histone-like protein involved in primordial germ cell specification, zinc finger protein, rRNA external transcribed spacer, thymosin beta-4, cyclin B1 and several predicted peptides from the Tetraodon nigroviridis genome assembly with unknown functions. 170 and 123 ESTs represented ribosomal proteins in the larval and juvenile libraries respectively, compared with only two sequences in the 2-cell library, which may reflect an abundance of maternally inherited pre-formed ribosomes in the yolk. Even though some clusters were common to all three libraries, most putative genes showed a developmental-stage specific distribution with 72% (2-cell embryo), 59% (larval) and 57% (juvenile) sequences having no significant matches against the 8400 adult halibut sequences in the EMBL nucleotide database. Comparison between the predicted halibut peptide data set and the human, zebrafish, and pufferfishes (T. nigroviridis and Takifugu rubripes) proteomes revealed that, as expected, the halibut sequences were more similar to the other two fish species than to human proteins. However, no clear bias towards the pufferfishes was observed, suggesting significant sequence variation between orthologues within the clade Acanthomorpha. The sequence information generated in the present study will represent a significant new resource for future studies on normal and abnormal development in Atlantic halibut.

Functional genomics studies of oocyte competence: evidence that reduced transcript abundance for follistatin is associated with poor developmental competence of bovine oocytes

Poor oocyte competence contributes to infertility in humans and livestock species. The molecular characteristics of such oocytes are generally unknown. Objectives of the present studies were to identify differences in RNA transcript abundance in oocytes and early embryos associated with reduced oocyte competence and development to the blastocyst stage. Microarray experiments were conducted using RNA isolated from germinal vesicle stage oocytes collected from adult versus prepubertal animals (model of poor oocyte competence). A total of 193 genes displaying greater mRNA abundance in adult oocytes and 223 genes displaying greater mRNA abundance in prepubertal oocytes were detected. Subsequent gene ontology analysis of microarray data revealed significant overrepresentation of transcripts encoding for genes in hormone secretion classification within adult oocytes and such genes were selected for further analysis. Real-time PCR experiments revealed greater abundance of mRNA for βA and βB subunits of inhibin/activin and follistatin, but not the α subunit in germinal vesicle stage oocytes collected from adult versus prepubertal animals. Cumulus cell follistatin and βB subunit mRNA abundance were similar in samples collected from prepubertal versus adult animals. A positive association between time of first cleavage (oocyte competence) and follistatin mRNA abundance was noted. Follistatin, βB, and α subunit mRNAs were temporally regulated during early bovine embryogenesis and peaked at the 16-cell stage. Collectively, results demonstrate a positive association of follistatin mRNA abundance with oocyte competence in two distinct models and dynamic regulation of follistatin, βB, and α subunit mRNAs in early embryos after initiation of transcription from the embryonic genome.

[Cytochalasin B induced changes in cytoplasmic Na+/K+ balance in 2-cell mouse embryo]

This work was performed to study changes in intracellular elemental (Na/K) concentrations caused by Cytochalasin B in two-cell mouse embryo using Electron Probe Microanalysis. The presence of Cytochalasin B is required to transfer a somatic cell nuclear into an early embryo cell. The direct effect of this chemical is cytoskeleton transformation, which would be able to cause the increase of potassium channel activity resulting in cytoplasmic Na/K imbalance. In our study Cytochalasin B was shown to decrease the intracellular sodium concentration. The Na/K balance in the cytoplasm of mouse embryos reverted to its intact level after treatment them with Cytochalasin B free Dulbecco's solution. Possible mechanisms responsible for the changes in the intracellular sodium concentration observed in the embryo cells are discussed.

Spatio-temporal localization of membrane lipid rafts in mouse oocytes and cleaving preimplantation embryos

We report for the first time the detection of membrane lipid rafts in mouse oocytes and cleaving preimplantation embryos. Cholera toxin beta (CTbeta), which binds to the raft-enriched ganglioside GM1, was selected to label rafts. In a novel application a Qdot reagent was used to detect CTbeta labeling. This is the first reported use of nanocrystals in mammalian embryo imaging. Comparative membrane labeling with CTbeta and lipophilic membrane dyes containing saturated or unsaturated aliphatic tails showed that the detection of GM1 in mouse oocytes and embryo membranes was consistent with the identification of cholesterol- and sphingolipid-enriched rafts in the cell membrane. Distribution of the GM1 was compared with the known distribution of non-raft membrane components, and disruption of membrane rafts with detergents confirmed the cholesterol dependence of GM1 on lipid raft labeling. Complementary functional studies showed that cholesterol depletion using methyl-beta-cyclodextrin inhibited preimplantation development in culture. Our results show that the membranes of the mouse oocyte and zygote are rich in lipid rafts, with heterogeneous and stage-dependent distribution. In dividing embryos, the rafts were clearly associated with the cleavage furrow. At the morula stage, rafts were also apically enriched in each blastomere. In blastocysts, rafts were detectable in the trophectoderm layer, but could not be detected in the inner cell mass without prior fixation and permeabilization of the embryo. Lipid rafts and their associated proteins are, therefore, spatio-temporally positioned to a play a critical role in preimplantation developmental events.

The influence of follicle size, FSH-enriched maturation medium, and early cleavage on bovine oocyte maternal mRNA levels

Transcription is arrested in the bovine oocyte within the first few hours of in vitro maturation, thus the stored maternal mRNAs accumulated in the oocyte are essential to sustain development until the Maternal-Zygotic Transition. In vivo matured oocytes have superior blastocyst formation rates than in vitro matured oocytes, suggesting that the mRNA content of these oocytes is of higher quality. To determine which transcripts may be associated with developmental competence, a Suppressive Subtractive Hybridization was performed between oocytes collected by ovariectomy at 6 hr post-LH surge and oocytes from slaughterhouse collected after 6 hr of maturation, resulting in a library enriched in these functionally important mRNAs. The clones were spotted onto a cDNA microarray and transcripts potentially associated with developmental competence were hybridized onto these slides. Hybridizations were performed with transcripts up-regulated in oocytes cultured for 6 hr in the presence or absence of rFSH in vitro, and secondly with transcripts up regulated in early-cleaving embryos versus those at the one-cell stage at 36 hr postfertilization. From these hybridizations, 13 candidates were selected. Their functional association with embryonic competence was validated by measuring their relative transcript levels by quantitative real-time PCR in eight different conditions: oocytes cultured with or without rFSH, early--versus late-cleaving embryos, and oocytes from different follicle sizes (1-3, 3-5, 5-8, and >8 mm of diameter). The gene candidates CCNB2, PTTG1, H2A, CKS1, PSMB2, SKIIP, CDC5L, RGS16, and PRDX1 showed a significant quantitative association with competence compared to BMP15, GDF9, CCNB1, and STK6.

Zebrafish Angiotensin II Receptor-like 1a (agtrl1a) is expressed in migrating hypoblast, vasculature, and in multiple embryonic epithelia

The human gene AGTRL1 is an angiotensin II receptor-like gene expressed in vasculature, which acts as the receptor for the small peptide APELIN, and a co-receptor for Human Immunodeficiency Virus. Mammalian AGTRL1 has been shown to modulate cardiac contractility, venous and arterial dilation, and endothelial cell migration in vitro, but no role in the development of the vasculature, or other tissues, has been described. We report the identification and expression of the zebrafish ortholog of the human gene AGTRL1. Zebrafish agtrl1a is first expressed before epiboly in dorsal precursors. During epiboly it is expressed in the enveloping layer, yolk syncytial layer and migrating mesendoderm. During segmentation stages, expression is observed in epithelial structures such as adaxial cells, border cells of the newly formed somites, developing lens, otic vesicles and venous vasculature.

Glutathione content and glutathione peroxidase expression in in vivo and in vitro matured equine oocytes

The in vitro developmental competence of equine oocytes is still low in comparison with other domestic mammals. A major factor affecting the viability of cells during in vitro culture is the increased oxidative stress. Oxidative modifications could be responsible for oocyte incompetence for in vitro maturation (IVM). Cysteamine, a glutathione (GSH) synthesis enhancer, has been shown to increase intracellular GSH content and to improve embryo development when added during IVM of bovine, porcine, and ovine oocytes. The aim of the present study was (1) to determine whether equine cumulus-oocyte complexes (COCs) benefit from the addition of cysteamine during IVM, (2) to compare the GSH content of oocytes after in vivo maturation and IVM, (3) to assess whether cysteamine administration during IVM of equine oocyte enhances early embryonic developmental capability following ICSI, (4) to study the glutathione peroxidase (GPX) mRNA level in COCs. In vivo matured COCs were collected by aspiration from preovulatory follicles, and analyzed at collection. Immature COCs were collected in vivo or from slaughterhouse ovaries and matured in culture media supplemented or not with 100 microM cysteamine. After nuclear stage assessment, oocytes were analyzed for GSH concentration and both oocytes and cumulus cells were analyzed for GPX and GAPDH mRNA. Our data showed that the maturation capability was similar in both in vivo aspirated oocytes and in those isolated from slaughterhouse ovaries. Moreover, the addition of cysteamine during IVM affected neither GSH content nor maturation rate. At the time of collection, intra-oocyte GSH content was not influenced by the chromatin status. GSH concentration was similar in in vivo and in vitro matured metaphase II (MII) stage oocytes, and was significantly higher in MII than immature germinal vesicle stage oocytes. Moreover, the presence of serum inhibited whereas its absence stimulated the accumulation of GSH within oocytes during IVM. After ICSI, a similar proportion of zygotes in each group developed beyond the two-cell stage after 72 hr of culture. Cumulus cells expressed GPX mRNA, while GPX transcript was absent in both immature and mature oocytes. Cumulus expression of GPX mRNA was significantly higher when analyzed at collection than after IVM. Taken together, our results demonstrate that in equine oocytes, GSH increases during IVM but the relative intra-oocyte content of this thiol does not affect maturation and early development efficiency after fertilization. We hypothesize that factor(s) other than GSH/GPX are responsible for the limited in vitro early developmental capability of equine oocytes.

Gene expression profiling of the pre-implantation mouse embryo by microarray analysis: comparison of the two-cell stage and two-cell block

To improve our understanding of the molecular mechanisms underlying early embryo development, further characterization of gene activity in oocytes and embryos is urgently required. The transition from the two-cell to four-cell stage is particularly important in pre-implantation embryonic development, as it involves transcriptional reprogramming and cellular differentiation. In this study, we used a 7.4 K cDNA microarray to screen mRNA transcript levels in the pre-implantation mouse embryo. Real-time PCR was used to confirm microarray data. We profiled 7,410 genes and identified 4,562 genes that were differentially expressed in the pre-implantation embryo. We selected a total of 248 genes with significant expression changes that are functionally involved in the two-cell and two-cell block embryo. Of these genes, 114 were down-regulated and the remainder (n=134) were up-regulated in the two-cell embryo. This study provides a developmental map of a large number of genes in the pre-implantation mouse embryo with particular emphasis on gene expression in the two-cell embryo and two-cell block embryo. Further investigations based on this data will provide a better understanding of the effects of various external conditions and may facilitate comparative analysis of pre-implantation development in other mammalian species, including human.

Involvement of slingshot in the Rho-mediated dephosphorylation of ADF/cofilin during Xenopus cleavage

ADF/cofilin is a key regulator for actin dynamics during cytokinesis. Its activity is suppressed by phosphorylation and reactivated by dephosphorylation. Little is known, however, about regulatory mechanisms of ADF/cofilin function during formation of contractile ring actin filaments. Using Xenopus cycling extracts, we found that ADF/cofilin was dephosphorylated at prophase and telophase. In addition, constitutively active Rho GTPase induced dephosphorylation of ADF/cofilin in the egg extracts. This dephosphorylation was inhibited by Na(3)VO (4) but not by other conventional phosphatase-inhibitors. We cloned a Xenopus homologue of Slingshot phosphatase (XSSH), originally identified in Drosophila and human as an ADF/cofilin phosphatase, and raised antibody specific for the catalytic domain of XSSH. This inhibitory antibody significantly suppressed the Rho-induced dephosphorylation of ADF/cofilin in extracts, suggesting that the dephosphorylation at telophase is dependent on XSSH. XSSH bound to actin filaments with a dissociation constant of 0.4 microM, and the ADF/cofilin phosphatase activity was increased in the presence of F-actin. When latrunculin A, a G-actin-sequestering drug, was added to extracts, both Rho-induced actin polymerization and dephosphorylation of ADF/cofilin were markedly inhibited. Jasplakinolide, an actin-stabilizing drug, alone induced actin polymerization in the extracts and lead to dephosphorylation of ADF/cofilin. These results suggest that Rho-induced dephosphorylation of ADF/cofilin is dependent on the XSSH activation that is caused by increase in the amount of F-actin induced by Rho signaling. XSSH colocalized with both actin filaments and ADF/cofilin in the actin patches formed on the surface of the early cleavage furrow. Injection of inhibitory antibody blocked cleavage of blastomeres. Thus, XSSH may reorganize actin filaments through dephosphorylation and reactivation of ADF/cofilin at early stage of contractile ring formation.

Requirement for a localized, IP3R-generated Ca2+transient during the furrow positioning process in zebrafish zygotes

We report that the first localized Ca2+transient visualized in the blastodisc cortex of post-mitotic zebrafish zygotes has unique features. We confirm that this initial ‘furrow positioning’ Ca2+transient precedes the physical appearance of the first cleavage furrow at the blastodisc surface and that it has unique dynamics, which distinguish it from the subsequent furrow propagation transients that develop from it. This initial transient displays a distinct rising phase that peaks prior to the initiation of the two linear, subsurface, self-propagating Ca2+waves that constitute the subsequent furrow propagation transient. Through the carefully timed introduction of the Ca2+buffer, dibromo-BAPTA, we also demonstrate the absolute requirement of this initial rising phase Ca2+transient in positioning the furrow at the blastodisc surface: no rising phase transient, no cleavage furrow. Likewise, the introduction of the inositol 1,4,5-trisphosphate receptor (IP3R) antagonist, 2-aminoethoxydiphenyl borate, eliminates both the rising phase transient and the appearance of the furrow at the cell surface. On the other hand, antagonists of the ryanodine receptor and NAADP-sensitive channels, or simply bathing the zygote in Ca2+-free medium, have no effect on the generation of the rising phase positioning transient or the appearance of the furrow at the surface. This suggests that like the subsequent propagation and deepening/zipping Ca2+transients, the rising phase furrow positioning transient is also generated specifically by Ca2+released via IP3Rs. We propose, however, that despite being generated by a similar Ca2+release mechanism, the unique features of this initial transient suggest that it might be a distinct signal with a specific function associated with positioning the cleavage furrow at the blastodisc surface.

Gonadotropin stimulation of steroid synthesis and metabolism in the Rana pipiens ovarian follicle: sequential changes in endogenous steroids during ovulation, fertilization and cleavage stages

Steroid synthesis and metabolism have been followed in Rana pipiens ovarian follicles, denuded oocytes and eggs during ovulation, fertilization and cleavage stages (blastula formation). Under physiological conditions, gonadotropin stimulation of the fully grown follicle leads to progesterone synthesis from [(3)H]acetate as well as formation of much smaller amounts of 17alpha-hydroxyprogesterone, androstenedione, pregnanedione and pregnanediol. Progesterone levels increase during completion of the first meiotic division, but by ovulation progesterone disappears from the egg. Plasma membrane-bound progesterone is taken up into the oocyte cortical granules and is largely metabolized to 5alpha-pregnane-3alphaol,20-one and 5beta-pregnane-3alpha,17alpha,20beta-triol coincident with internalization of 60% of the oocyte surface (and >90% of bound progesterone) by the end of the hormone-dependent period. The principal steroid in the ovulated egg is 5beta-pregnane-3alpha,17alpha,20beta-triol. There is a rapid efflux of 5beta-pregnane-3alpha,17alpha,20beta-triol into the medium immediately following fertilization and residual steroid levels remain low in the developing blastula. Dissociated blastulae cells prepared from stage 9 1/2 embryos concentrate both pregnenolone and progesterone from the medium with minimal metabolism. The results indicate that the ovarian follicle has the ability to synthesize and metabolize progesterone but that this ability disappears in the ovulated egg. The progesterone metabolites formed during meiosis are largely released at fertilization.

Differences in regulation of the first two M-phases in Xenopus laevis embryo cell-free extracts

The first embryonic M-phase is special, being the time when paternal and maternal chromosomes mix together for the first time. Reports from a variety of species suggest that the regulation of first M-phase has many particularities; however, no systematic comparative study of the biochemical aspects of first and the following M-phases has been previously undertaken. Here, we ask whether the regulation of the first embryonic M-phase is modified, using Xenopus cell-free extracts. We developed new types of extract specific for the first and the second M-phase obtained either from parthenogenetic or from in vitro fertilized embryos. Analyses of these extracts confirmed that the amplitude of histone H1 kinase activity reflecting CDK1/cyclin B (or MPF for M-phase Promoting Factor) activity is higher and persists longer than during the second M-phase, and that levels of cyclins B1 and B2 are correspondingly higher during the first than the second embryonic M-phase. Inhibition of protein synthesis shortly before M-phase entry reduced mitotic histone H1 kinase amplitude, shortened the period of mitotic phosphorylation of chosen marker proteins, and reduced cyclin B1 and B2 levels, suggesting a role of B-type cyclins in regulating the duration of mitotic events. Moreover, addition of exogenous cyclin B to the extract prior the second mitosis brought forward the activation of mitotic histone H1 kinase but prolonged the duration of this activity. We also confirmed that the inhibitory phosphorylation of CDK1 on tyrosine 15 oscillates between the first two embryonic M-phases, but is clearly more pronounced before the first than the second mitosis, while the MAP kinase ERK2 tended to show greater activation during the first embryonic M-phase but with a similar duration of activation. We conclude that discrete differences exist between the first two M-phases in Xenopus embryo and that higher CDK1/cyclin B activity and B-type cyclin levels could account for the different characteristics of these M-phases.

Separate pathways of RNA recruitment lead to the compartmentalization of the zebrafish germ plasm

The maternal RNAs vasa, dead end, nanos1, and daz-like all become localized to the peripheral ends of the first and second cleavage furrows, where they form part of the zebrafish germ plasm. We show that aggregates of a first class of germ plasm components, which include dead end, nanos1, and vasa RNAs, are initially present in a wide cortical band at the animal pole. Aggregates containing these three RNAs appear to be associated with f-actin, which during the first cell cycle undergoes a microtubule-dependent movement towards the periphery as well as circumferential alignment. These cytoskeletal rearrangements lead to the further aggregation of particles containing these RNAs and their concomitant recruitment to the forming furrow. Aggregates containing a second class of germ plasm RNA components, which include the transcript for daz-like, translocate along the plane of the cortex towards the animal pole, where they are recruited to the germ plasm. After recruitment to the furrow, these two classes of RNAs occupy overlapping yet distinct regions of the germ plasm, and this arrangement is maintained during the early cleavage stages. Our observations suggest that separate pathways of RNA recruitment facilitate the compartmentalization of the zebrafish germ plasm.