Transcription of rRNA in early mouse embryos promotes chromatin reorganization and expression of major satellite repeats

During the first cell cycles of the early development, the chromatin of the embryo is highly reprogrammed alongside that embryonic genome starts its own transcription. The spatial organization of the genome is a major process that contributes to regulating gene transcription in time and space, however, it is poorly studied in the context of early embryos. To study the cause and effect link between transcription and spatial organization in embryos, we focused on the ribosomal genes, that are first silent and begin to transcribe during the 2-cell stage in the mouse. We demonstrated that ribosomal sequences and early unprocessed rRNAs are spatially organized in a very peculiar manner from the 2-cell to the 16-cell. Using drugs interfering with ribosomal DNA transcription, we show that this organization, totally different from somatic cells, depends on an active transcription of ribosomal genes and induces a unique chromatin environment that favors transcription of major satellite sequences after the 4-cell stage.

Three-dimensional analysis of nuclear heterochromatin distribution during early development in the rabbit

Changes to the spatial organization of specific chromatin domains such as constitutive heterochromatin have been studied extensively in somatic cells. During early embryonic development, drastic epigenetic reprogramming of both the maternal and paternal genomes, followed by chromatin remodeling at the time of embryonic genome activation (EGA), have been observed in the mouse. Very few studies have been performed in other mammalian species (human, bovine, or rabbit) and the data are far from complete. During this work, we studied the three-dimensional organization of pericentromeric regions during the preimplantation period in the rabbit using specific techniques (3D-FISH) and tools (semi-automated image analysis). We observed that the pericentromeric regions (identified with specific probes for Rsat I and Rsat II genomic sequences) changed their shapes (from pearl necklaces to clusters), their nuclear localizations (from central to peripheral), as from the 4-cell stage. This reorganization goes along with histone modification changes and reduced amount of interactions with nucleolar precursor body surface. Altogether, our results suggest that the 4-cell stage may be a crucial window for events necessary before major EGA, which occurs during the 8-cell stage in the rabbit.

Contrasting epigenetic states of heterochromatin in the different types of mouse pluripotent stem cells

Mouse embryonic stem cells (ESCs) and epiblast stem cells (EpiSCs) represent naive and primed pluripotency states, respectively, and are maintained in vitro by specific signalling pathways. Furthermore, ESCs cultured in serum-free medium with two kinase inhibitors (2i-ESCs) are thought to be the ground naïve pluripotent state. Here, we present a comparative study of the epigenetic and transcriptional states of pericentromeric heterochromatin satellite sequences found in these pluripotent states. We show that 2i-ESCs are distinguished from other pluripotent cells by a prominent enrichment in H3K27me3 and low levels of DNA methylation at pericentromeric heterochromatin. In contrast, serum-containing ESCs exhibit higher levels of major satellite repeat transcription, which is lower in 2i-ESCs and even more repressed in primed EpiSCs. Removal of either DNA methylation or H3K9me3 at PCH in 2i-ESCs leads to enhanced deposition of H3K27me3 with few changes in satellite transcript levels. In contrast, their removal in EpiSCs does not lead to deposition of H3K27me3 but rather removes transcriptional repression. Altogether, our data show that the epigenetic state of PCH is modified during transition from naive to primed pluripotency states towards a more repressive state, which tightly represses the transcription of satellite repeats.

Prosurvival effect of cumulus prostaglandin G/H synthase 2/prostaglandin2 signaling on bovine blastocyst: impact on in vivo posthatching development

Apoptotic activity is a common physiological process which culminates at the blastocyst stage in the preimplantation embryo of many mammals. The degree of embryonic cell death can be influenced by the oocyte microenvironment. However, the prognostic significance of the incidence of apoptosis remains undefined. Prostaglandin E2 (PGE2) derived from prostaglandin G/H synthase-2 (PTGS2) activity is a well-known prosurvival factor that is mainly studied in oncology. PGE2 is the predominant PTGS2-derived prostaglandin present in the oocyte microenvironment during the periconceptional period. Using an in vitro model of bovine embryo production followed by transfer and collection procedures, we investigated the impact of periconceptional PGE2 on the occurrence of spontaneous apoptosis in embryos and on subsequent in vivo posthatching development. Different periconceptional PGE2 environments were obtained using NS-398, a specific inhibitor of PTGS2 activity, and exogenous PGE2. We assessed the level of embryonic cell death in blastocysts at day 8 postfertilization by counting total cell numbers, by the immunohistochemical staining of active caspase-3, and by quantifying terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling signals and apoptosis regulator (BCL-2/BAX) mRNA expression. Morphometric parameters were used to estimate the developmental stage of the embryonic disk and the extent of trophoblast elongation on day 15 conceptuses. Our findings indicate that periconceptional PGE2 signaling durably impacts oocytes, conferring increased resistance to spontaneous apoptosis in blastocysts and promoting embryonic disk development and the elongation process during preimplantation development.

Analysis of STAT1 expression and biological activity reveals interferon-tau-dependent STAT1-regulated SOCS genes in the bovine endometrium

Signal transducer and activator of transcription (STAT) proteins are critical for the regulation of numerous biological processes. In cattle, microarray analyses identified STAT1 as a differentially expressed gene in the endometrium during the peri-implantation period. To gain new insights about STAT1 during the oestrous cycle and early pregnancy, we investigated STAT1 transcript and protein expression, as well as its biological activity in bovine tissue and cells of endometrial origin. Pregnancy increased STAT1 expression on Day 16, and protein and phosphorylation levels on Day 20. In cyclic and pregnant females, STAT1 was located in endometrial cells but not in the luminal epithelium at Day 20 of pregnancy. The expression of STAT1 during the oestrous cycle was not affected by progesterone supplementation. In vivo and in vitro, interferon-tau (IFNT) stimulated STAT1 mRNA expression, protein tyrosine phosphorylation and nuclear translocation. Using chromatin immunoprecipitation in IFNT-stimulated endometrial cells, we demonstrated an increase of STAT1 binding on interferon regulatory factor 1 (IRF1), cytokine-inducible SH2-containing protein (CISH), suppressor of cytokine signaling 1 and 3 (SOCS1, SOCS3) gene promoters consistent with the induction of their transcripts. Our data provide novel molecular insights into the biological functions of STAT1 in the various cells composing the endometrium during maternal pregnancy recognition and implantation.

Three-Dimensional Distribution of UBF and Nopp140 in Relationship to Ribosomal DNA Transcription During Mouse Preimplantation Development

The nucleolus is a dynamic nuclear compartment that is mostly involved in ribosome subunit biogenesis; however, it may also play a role in many other biological processes, such as stress response and the cell cycle. Mainly using electron microscopy, several studies have tried to decipher how active nucleoli are set up during early development in mice. In this study, we analyzed nucleologenesis during mouse early embryonic development using 3D-immunofluorescent detection of UBF and Nopp140, two proteins associated with different nucleolar compartments. UBF is a transcription factor that helps maintain the euchromatic state of ribosomal genes; Nopp140 is a phosphoprotein that has been implicated in pre-rRNA processing. First, using detailed image analyses and the in situ proximity ligation assay technique, we demonstrate that UBF and Nopp140 dynamic redistribution between the two-cell and blastocyst stages (time of implantation) is correlated with morphological and structural modifications that occur in embryonic nucleolar compartments. Our results also support the hypothesis that nucleoli develop at the periphery of nucleolar precursor bodies. Finally, we show that the RNA polymerase I inhibitor CX-5461: 1) disrupts transcriptional activity, 2) alters preimplantation development, and 3) leads to a complete reorganization of UBF and Nopp140 distribution. Altogether, our results underscore that highly dynamic changes are occurring in the nucleoli of embryos and confirm a close link between ribosomal gene transcription and nucleologenesis during the early stages of development.

183 CANINE EMBRYOS OBTAINED BY INTRACYTOPLASMIC SPERM INJECTION

In vitro fertilization encounters 2 specific difficulties in the canine species, with no puppies born to date: low penetration rates (10–50%) and high polyspermia (around 50% of fertilized oocytes; Saint-Dizier et al. 2001 J. Reprod. Fert. Suppl. 57, 147–150). The objectives of the study were to test whether intracytoplasmic sperm injection (ICSI), which overcomes these 2 obstacles, could allow production of canine embryos, using in vivo- or in vitro-matured oocytes. The time of ovulation was determined on 8 Beagle bitches from our experimental kennel by blood progesterone assay and transabdominal ultrasound examination. After ovariohysterectomy 82 to 100 h after ovulation, 58 metaphase II (MII) oocytes were collected by tubal flushing. In parallel, 88 oocytes from 6 anoestrus bitches were matured in vitro (M199 + 20% fetal calf serum for 72 h in 5% CO2 at 38°C). Sperm was collected from 1 Beagle dog with excellent fertility record at natural mating. The sperm was diluted 1 : 100 in PBS/BSA without any selection process. Intracytoplasmic sperm injection was performed at 38°C in M199 HEPES + 20% BSA (4-μm injection pipette; 120-μm holding pipette). One motile spermatozoon of normal morphology was injected per oocyte. Injected oocytes were cultured in vitro for 48 h after injection (M199 + 20% fetal calf serum in 5% CO2 at 38°C) in 4-well open dishes. Oocytes were then fixed and DNA and tubulin were stained for observation by confocal microscopy (Chebrout et al. 2012 Microsc. Microanal. 18, 483–492). Among the 58 MII oocytes recovered in vivo, 7.4% lysed at injection and 20% degenerated during the 48 h after injection. Among the 40 injected oocytes still alive, 6 fragmented (15%) and 4 developed as embryos [10%; 2-pronuclei (n = 2), 2-cell and 6-cell). None of the other oocytes showed decondensed female chromatin. Among the 88 oocytes incubated for in vitro maturation, 13 (14.8%) reached MII. These were successfully injected; 48 h after injection, 3 were embryos at the 2-cell stage and 10 were at the MII stage with a condensed sperm head. Fifty-one non-mature oocytes were injected; 31 were at the germinal vesicle (GV) stage and the stage of others was not determined. Of the GV oocytes, 71% degenerated during culture after injection. The 9 surviving oocytes were still at the GV stage with condensed sperm head 48 h after injection. In conclusion, canine embryos can be obtained through ICSI. Nevertheless, this procedure induced low activation rates. Development at later stages, especially after transfer into a recipient female, is to be evaluated, in particular for in vitro-produced MII oocytes, of lower cytoplasmic competence (Viaris et al. 2008 Reprod. Fert. Dev. 20, 626–639).

Genome organization and epigenetic marks in mouse germinal vesicle oocytes

During the final step of oogenesis, the oocyte nucleus is subject to large-scale modifications that correlate with transcriptional silencing. While oocytes with dense chromatin around the nucleolus are silent (SN, surrounded nucleolus), oocytes with uncondensed chromatin (NSN, non-surrounded nucleolus) are transcriptionally active. It is believed that epigenetic mechanisms that participate in gene expression regulation could play a role in this event. In this context, we examined the behaviour of heterochromatin and related histone modifications during the NSN to SN transition by immunostaining. Using fluorescent in situ hybridization on three dimensional-preserved nuclei (3D-FISH), we also studied the distribution of centromeric, pericentromeric and ribosomal (rDNA) sequences in relation to the nucleolus (also called the nucleolus-like body, NLB). We observed that in NSN-type oocytes, pericentromeric heterochromatin is aggregated within chromocenters. In SN-type oocytes, pericentromeric heterochromatin and centromeres form a discontinuous ring around the NLB. rDNA sequences, which initially present a pearl necklace structure, gather together in seven highly condensed foci at the NLB periphery. H3K9me3 and H4K20me3 heterochromatin marks clearly label chromocenters, whereas H3K4me3 and H4K5ac are totally excluded from heterochromatin regions, even in the very compact SN-nuclei. Remarkably, H3K27me3 displays an intermediate behavior. It appears that GV oocyte nuclei exhibit a specific epigenetic landscape. Histone modifications, related to both active and repressive chromatin structures, seem to follow the large-scale chromatin movements that occur during the NSN to SN transition. We also demonstrate that, while heterochromatin regions re-localize around the NLB, rDNA sequences adopt a highly compact structure in SN-type oocytes.

Chromatin patterns of immature canine oocytes after in vitro maturation

In canine species, in vitro maturation (IVM) rates of oocytes collected from anoestrous ovaries are low (<20%). Several IVM media have been tested without significant improvements. A critical step in the evaluation of culture conditions is the observation of the meiotic stage reached by the oocytes. The present study was designed to investigate the chromatin patterns of in vitro matured oocytes by visualizing Germinal Vesicle (GV) and Germinal Vesicle Breakdown (GVBD) structures at 72 h of IVM. Nuclear stages of 1678 oocytes were evaluated by confocal microscopy after IVM. 1204 oocytes were non-degenerated, and 94.4% were still immature and at GV stage. Five different patterns of chromatin configuration were observed. Higher percentages of oocytes with unmodified GV and with diffuse (58%; Type A) and filamentous chromatin (19%; Type B) were observed in comparison with those with modifications in the GV such as patched chromatin (12.5%; Type C), surrounded-nucleolus (3%; Type D) and in vivo type chromatin/fully grouped chromatin (2.5%; Type E). These results indicate that GVBD (absence of nucleolus, nucleus breakdown) is rarely observed in vitro. The percentage of type C-D-E GVs and MI (meiotic resumption) and of MII (completion of meiosis) can be used to evaluate meiotic resumption after IVM. Our results indicate that although a low number of in vitro matured oocytes exhibit the chromatin configurations observed in in vivo collected oocytes, chromatin changes in the GV can be induced during IVM.

Expression of nuclear progesterone receptor and progesterone receptor membrane components 1 and 2 in the oviduct of cyclic and pregnant cows during the post-ovulation period

BACKGROUND

Progesterone (P4) may modulate oviductal functions to promote early embryo development in cattle. In addition to its nuclear receptor (PR), P4 may mediate its actions through P4 receptor membrane component 1 (PGRMC1) and its relative, PGRMC2. Two successive experiments were undertaken to characterise the expression of PR, PGRMC1 and PGRMC2 in the bovine oviduct during the post-ovulation period, and to relate their expression to the presence of an embryo, the proximity of the CL and to the region of the oviduct.

METHODS

In the first experiment (Exp. I), whole oviduct sections were collected from Holstein cows at Day 1.5, Day 4 and Day 5 post-ovulation (n = 2 cows per stage). The expression of PR, PGRMC1 and PGRMC2 was studied in the ampulla and isthmus by RT-PCR, western-blot and immunohistochemistry. In Exp. II, oviduct epithelial cells were collected from cyclic and pregnant Charolais cows (n = 4 cows per status) at Day 3.5 post-ovulation and mRNA expression of PR, PGRMC1 and PGRMC2 was examined in the ampulla and isthmus by real-time quantitative PCR.

RESULTS

In Exp. I, PR, PGRMC1 and PGRMC2 were expressed in all oviduct samples. PGRMC1 was mainly localised in the luminal epithelium whereas PR and PGRMC2 were localised in the epithelium as well as in the muscle and stroma layers of the oviduct. The expression was primarily nuclear for PR, primarily cytoplasmic for PGRMC1 and both nuclear and cytoplasmic for PGRMC2. In Exp. II, mRNA levels for PR, PGRMC1 and PGRMC2 were not affected by either the pregnancy status or the side relative to the CL. However, the expression of PR and PGRMC2 varied significantly with the region of the oviduct: PR was more highly expressed in the isthmus whereas PGRMC2 was more highly expressed in the ampulla.

CONCLUSIONS

This is the first evidence of PGRMC2 expression in the bovine oviduct. Our findings suggest that P4 regulates the functions of the bovine oviduct in a region-specific manner and through both classical and non classical pathways during the post-ovulation period.

Reliability of Hoechst 33342 staining under wide-field microscopy for evaluation of the nuclear status of living dog oocytes

Due to the marked cytoplasmic opacity of canine oocytes, the diagnosis of their nuclear status is difficult. The objective of the present study was to evaluate the accuracy of Hoechst staining observed under epifluorescence wide-field microscopy [living oocyte observation (LivOO)] by comparison to a reference technique [DNA staining with ethidium homodimer-2 under confocal microscopy; fixed oocyte observation (FixOO)]. Four Hoechst 33342 concentrations (200 ng, 500 ng, 1 μg, 2 μg/mL) were tested and 1 μg/mL was the lowest one with the lowest proportion of oocytes in which DNA was missed. At this concentration, LivOO procedure did not affect the degeneration rate. On 379 oocytes observed individually with the two techniques successively, diagnosis of meiosis resumption by LivOO was exact in 87.3% of the cases, but the meiosis resumption rate was underestimated (23.5% versus 34.3% with FixOO; p < 0.001). Diagnosis for metaphase II was exact in 80% of the cases, but LivOO detected only 72.7% of the metaphase II oocytes present. Metaphase rates did not differ between LivOO and FixOO. This study contributes to a better interpretation of in vitro maturation results. The developmental potential of metaphase II canine oocytes sorted after Hoechst staining is to be evaluated.

68 TRANSCRIPTIONAL GENOME ACTIVATION IN CANINE EMBRYOS COLLECTED IN VIVO

Early embryonic stages are supported by maternal transcripts from the oocyte cytoplasm. Progressive transcription of embryonic genome is a key step for further embryonic development, especially during in vitro culture. To date, in vitro culture from fertilization to the blastocyst stage is inefficient in the canine species. The objective of this work was to identify minor and major activation in in vivo-produced dog embryos. Ovariectomies were performed in 31 Beagle bitches from 102 to 266 h after ovulation (post-ov), precisely timed by transabdominal ultrasonography. Embryos were collected by tubal flushing with M199-Hepes and immediately transferred into transcription buffer. Transcriptional activity was evaluated through 5-bromouridine 5′-triphosphate (BrUTP) incorporation in nascent RNA, without microinjection (Aoki et al. 1997). Oocytes from anoestrus ovaries were used as positive controls. 5-Bromouridine 5′-triphosphate incorporation was revealed by immunocytochemistry (anti-bromodeoxyuridine primary antibody) and embryonic DNA was stained by ethidium homodimer-2. Staining was quantified under laser scanning confocal microscopy. Transcriptional activity was calculated as (mean nuclear intensity – cytoplasmic mean intensity) × nuclear area and expressed in arbitrary units (AU). It was compared to 1 (similar intensity in nucleus and cytoplasm; i.e. no transcriptional activity) by t-test; levels of transcriptional activity were compared between stages by variance analysis. Seventy embryos (from 7 to 21 per stage) from 31 bitches were analysed, from 2 pronuclei to morula stage. Between 28 and 125 nuclei were quantified per stage. At each stage, transcriptional activity was calculated per embryo and per nucleus. A significant transcriptional activity was detected as early as the 2 pronuclei stage (102–132 h post-ov; 1.15 ± 0.05 AU). Transcriptional activity per embryo significantly increased between the 2- and the 4-cell stage and between the 8-cell and the morula stage. In early 8-cell embryos, staining intensity of the various nuclei was markedly heterogeneous within the same embryo, all nuclei being intensively stained from the late 8-cell stage onwards. Transcriptional activity per nucleus increased also from the 2- to the 4-cell stage (respectively, 120–161 h post-ov, 1.15 ± 0.02 AU and 133–154 h post-ov, 1.35 ± 0.04 AU) until the 8-cell stage (153–225 h post-ov, 5.12 ± 0.55 AU). Transcriptional levels at these 3 stages differed significantly. It decreased between the 8-cell and the morula stage (230–266 h post-ov, 3.06 ± 0.13 AU), probably reflecting the acquisition of a selectivity in gene expression at major activation, as in other species; Nothias et al. 1995). Addition of the transcriptional inhibitor α-amanitin during BrUTP incubation decreased the transcriptional activity by 60% (P < 0.05). Embryonic gene expression (minor activation) thus begins in the canine embryo as early as the 2 pronuclei stage, with major activation taking place during the 8-cell stage.

130 EXPRESSION OF STEROIDOGENIC ENZYMES IN THE CAT OVARY DURING FOLLICULAR GROWTH

This study was designed to describe the expression of steroidogenic enzymes in the various compartments of the feline ovary, from primordial to preovulatory follicles: P450 side-chain cleavage (P450scc), 17α-hydroxylase (17α-OH), 3β-hydroxysteroid dehydrogenase (3β-HSD). Nine female cats were ovariectomized during behavioural oestrus. After validation of their specificity by Western blot on feline tissue, 3 polyclonal antibodies obtained in the rabbit were used for immunohistochemistry (anti-bovine P450scc; anti-pig 17α-OH; anti-human 3β-HSD). Three successive ovarian cryosections were incubated with antibodies and 2 sections were used as controls (purified immunoglobulin G from a nonimmunized rabbit). Immunostaining was revealed by streptavidin-biotin system (LSAB kit, DAKO S.A., Trappes, France), with aminoethylcarbazole as a chromogen. The staining was evaluated according to its localization, intensity (from 1 to 4) and number of cell layers stained. Histological examination after haematoxylin-eosin-safran staining allowed follicle diameter measurement and evaluation of the atresia. Analysis of variance was used to compare follicles from various size classes. A total of 343 follicles from 140 to 3250 μm in diameter were observed (i.e. all follicles present on the slides), of which 15.2% were atretic. The observations were similar for the 3 enzymes. From the preantral stage onward (diameter >140 μm), more than 85% of the healthy follicles had the theca interna positive for P450scc, 3β-HSD and 17α-OH (respectively, 87.3, 86.0 and 99.0%). Approximately 20 to 25% were also positive in the theca externa. Granulosa were stained in 9.5% (P450scc), 0.8% (3β-HSD) and 1.8% (17α-OH) of the healthy follicles, but always with a low intensity (<2). The follicular size did not affect the proportion of stained follicles, but the staining intensity decreased progressively until 1200 μm and increased thereafter. The number of positive layers decreased continuously until 1500 μm in diameter. Between 90 and 95% of the preovulatory follicles (>2000 μm) were positive in the theca interna; the signal was more intense and the number of stained layers was higher than in smaller follicles. The 3 enzymes were detected in all the atretic follicles, with a similar intensity as in healthy ones; they were also present in the interstitial tissue, but with a higher intensity. The staining pattern in the various ovarian compartments suggests that the major steroidogenic tissues in the cat ovary are the theca interna of healthy follicles and the interstitial tissue. The lack of expression of the 3 key enzymes in the granulosa cells, even in preovulatory follicles, strongly suggests that these cells, shaping the future corpus luteum, are probably unable to produce progesterone.

186 RELIABILITY OF HOECHST 33342 STAINING UNDER STANDARD EPIFLUORESCENCE MICROSCOPY FOR EVALUATION OF THE NUCLEAR STATUS OF LIVING DOG OOCYTES

Hoechst 33342 staining detected by standard epifluorescence microscopy (Epi) is widely used for nuclear stage determination of oocytes in numerous species. Because it maintains cell viability, observed oocytes can be further processed for assisted reproduction. This sorting would be of great interest in the canine, where oocytes resuming meiosis in vitro remain scarce. But because of the cytoplasmic opacity of the canine oocyte, the accuracy of this technique is questionable. The objective of this study was to evaluate the accuracy of Epi by comparison to a reference technique (confocal microscopy; Conf). Oocytes were obtained from anestrus ovaries of 46 pubertal bitches. In vitro maturation was conducted with 20 oocytes/well in 500 μL M199 with 20% fetal calf serum in a humidified atmosphere of 5% CO2 at 39°C for 72 h. After Hoechst 33342 staining (20 min at RT), oocytes were observed without fixation under Epi (IX70, Olympus, France) with the excitation beam attenuated by 150-fold and emission detected by a high sensitivity camera (CCD, Olympus). The mean time for capture was 300 ms. For Conf, oocytes were fixed, stained with Ethidium Homodimer-2 (2 μM for 20 min at RT) and examined with an Ion-Argon laser (LSM 310, Carl Zeiss, Germany). For each oocyte, the nuclear stage was successively determined by Epi and by Conf. The results of both observations were compared by chi-square analysis. The optimal concentration of Hoechst dye was first determined on 401 oocytes (200 ng, 500 ng, 1 μg, or 2 μg mL–1). The 1 μg mL–1 was the lowest concentration allowing the identification of the nuclear stage by Epi in the largest proportion of living oocytes seen with DNA by Conf. At this dye concentration, Epi did not significantly impact degeneration or meiosis resumption rates: 33.3 and 22.4%, respectively for 183 oocytes submitted to Epi before Conf, compared to 37.4 and 13.6% for 147 oocytes observed only under Conf. Finally, the concordance of the nuclear stage diagnosis was tested on 379 oocytes observed individually with Epi and Conf successively. On Epi, 149 oocytes were seen without DNA. Observed on Conf, 37.5% of these were also seen without DNA, 59.7% were in the germinal vesicle stage and 2% were in metaphase. On Epi, 89 oocytes were considered as having resumed meiosis. On Conf, meiosis resumption was confirmed for 88.7% of them, but Epi failed to detect 31.4% of the oocytes having resumed meiosis among the 379 cultured oocytes. Although 80% of the metaphase II oocytes identified by Epi were confirmed to have reached this stage, 23% of metaphase II oocytes present in the culture dishes were missed. In conclusion, probably because of the high density in lipids of the cytoplasm, Hoechst staining detected by Epi for canine oocytes sorting after in vitro maturation leads to erroneously discarding a high proportion of oocytes.

The canine oocyte: uncommon features of in vivo and in vitro maturation

The biology of the canine oocyte is unusual compared with that of other mammalian females. The present paper reviews both in vivo and in vitro specificities of canine oocytes. Final follicular growth in the bitch is characterised by an early appearance of LH binding sites in the granulosa, a high proportion of polyovular follicles and a preovulatory luteinisation, starting at the time of the LH surge. Through follicular fluid, preovulatory oocytes are thus exposed to high levels of progesterone, as high as 1000-fold plasma concentrations. The composition of the follicular fluid is affected by the size of the female. The more specific aspect of oocyte biology in the bitch is ovulation: oocytes are expelled immature, at the Prophase I stage. Ovulatory follicles are 6–8 mm in diameter, releasing oocytes from 110 µm, with dark cytoplasm. Resumption of meiosis occurs from 48 h postovulation, MII stages appearing 48–54 h after ovulation. The mechanisms controlling such a late meiotic resumption are still unknown. Granulosa cells seem to play a central role as in other mammalian species, but not with cAMP as the principal mediator. The importance of a transient reactivation of oocyte transcription a few hours before meiotic resumption is to be explored. These specific features may contribute to the low efficiency of IVM. Only 10–20% oocytes reach the metaphase stage and suffer from a poor cytoplasmic maturation. Moreover, in vitro culture of canine oocytes is associated with a high proportion of degeneration. To date, IVM of the oocytes is the main limiting factor for the development of assisted reproductive techniques in the canine. A better knowledge of the basic physiology of folliculogenesis and the molecular mechanisms controlling oocyte meiosis resumption in this species may allow us to overcome this obstacle.

Embryo biotechnology in the dog: a review

Canine embryos are a scarce biological material because of difficulties in collecting in vivo-produced embryos and the inability, to date, to produce canine embryos in vitro. The procedure for the transfer of in vivo-produced embryos has not been developed adequately, with only six attempts reported in the literature that have resulted in the birth of 45 puppies. In vitro, the fertilisation rate is particularly low (∼10%) and the incidence of polyspermy particularly high. So far, no puppy has been obtained from an in vitro-produced embryo. In contrast, cloning of somatic cells has been used successfully over the past 4 years, with the birth of 41 puppies reported in the literature, a yield that is comparable to that for other mammalian species. Over the same period, canine embryonic stem sells and transgenic cloned dogs have been obtained. Thus, the latest reproductive technologies are further advanced than in vitro embryo production. The lack of fundamental studies on the specific features of reproductive physiology and developmental biology in the canine is regrettable in view of the increasing role of dogs in our society and of the current demand for new biological models in biomedical technology.

Folliculogenesis and morphometry of oocyte and follicle growth in the feline ovary

This study was designed to describe, both quantitatively (morphometry) and qualitatively (histological differentiation), follicle and oocyte growth in the feline ovary. The ovaries of 43 cats were collected and processed for histology. The diameters of 832 follicle/oocyte pairs were measured, with and without zona pellucida (ZP), and a special emphasis was placed on the study of early folliculogenesis. Primordial, primary, secondary, pre-antral and early antral follicles were measured at 44.3, 86.2, 126.0, 155.6 and 223.8 microm in diameter respectively. A biphasic pattern of follicle and oocyte growth was observed. Before antrum formation, follicle (x) and oocyte (y) size were positively and linearly correlated (y = 0.500x + 20.01, r(2) = 0.89). Antrum formation occurred when the follicle reached 160-200 microm in diameter (when oocyte was at 102 microm). After antrum formation, a decoupling was observed, a minimal increase in oocyte size contrasting with a significant follicle development (y = 0.001x + 114.39, r(2) = 0.01). The pre-ovulatory follicle diameter was approximately 3500 microm and the maximal oocyte diameter was 115 microm. The ZP, absent in primordial and primary follicles, appeared at the secondary stage and reached almost 6 microm at the pre-ovulatory stage. These results suggest that (i) in feline ovary, follicle and oocyte growth pattern is similar to that observed in other mammals; (ii) the antrum forms in 160-200 microm follicles, which represents 5% of the pre-ovulatory diameter and (iii) the oocyte had achieved more than 90% of its maximal growth at the stage of antrum formation.

Ultrastructural evaluation of in vitro-matured canine oocytes

Cumulus–oocyte complexes (COCs) were recovered from ovaries of bitches during anoestrus. The ultrastructural organisation of COCs was determined before and after 72 h in vitro maturation (IVM) by transmission electron microscopy. The aim of the study was to determine the quality of oocytes used for IVM and to assess cytoplasmic maturation of IVM metaphase (M) II oocytes. In addition, we examined whether the oocytes that did not reach MII were engaged in an erratic maturation process or whether they were blocked during their progression through a normal maturation process. Before IVM, there were two populations of oocytes: (1) oocytes with a centrally located germinal vesicle, a transcriptionally active aspect and an immature cytoplasm; and (2) oocytes with an eccentric nucleus, a transcriptionally inactive aspect and a more mature cytoplasm. After IVM, most oocytes were still at the germinal vesicle stage with three different patterns and all showing a good synchronisation between nuclear and cytoplasmic maturation. MI oocytes had a similar cytoplasmic maturation to that observed in vivo, but failed to complete meiosis; however, IVM MII oocytes had a very poor cytoplasmic maturation. Ultrastructural analysis demonstrated that even when nuclear maturation is achieved, cytoplasmic maturation may not be obtained in vitro. Thus, all IVM systems should be evaluated on both criteria.

In vivo meiotic resumption, fertilization and early embryonic development in the bitch

Early development in canine species follows a very specific pattern. Oocytes are ovulated at the germinal vesicle stage and meiotic resumption occurs in the oviduct. However, because of difficulties in the accurate determination of ovulation time and in the observation of oocyte nuclear stage by light microscopy, these early events have not been fully described. Moreover, the oocyte stage at which sperm penetration occurs is still uncertain since fertilization of immature oocytes has been reported in vivo and in vitro. The aim of this study was to establish the exact timing of in vivo meiotic resumption, fertilization and early embryo development in the bitch with reference to ovulation. Ovulation was first determined by ultrasonography, artificial inseminations were performed daily and oocytes/embryos were collected between 17 and 138 h after ovulation. After fixation and DNA/tubulin staining, the nuclear stage was observed by confocal microscopy. Of the 195 oocytes/embryos collected from 50 bitches, the germinal vesicle stage was the only one present until 44 h post-ovulation, and the first metaphase II stage was observed for the first time at 54 h. Sperm penetration of immature oocytes appeared to be exceptional (three out of 112 immature oocytes). In most cases, fertilization occurred from 90 h post-ovulation in metaphase II oocytes. Embryonic development was observed up to the eight-cell stage. No significant influence of bitch breed and age on ovulation rate, maturation and developmental kinetics was observed. However, some heterogeneity in the maturation/development process was observed within the cohort of oocytes/embryos collected from one bitch. In conclusion, the most peculiar aspect of the canine species remains oocyte meiotic maturation whereas fertilization follows the same pattern as in other mammals.

Presence of permanently activated signal transducers and activators of transcription in nuclear interchromatin granules of unstimulated mouse oocytes and preimplantation embryos

We previously described that mouse oocytes and preimplantation embryos express the two subunits of interferon-gamma receptor. We now report that, despite the presence of STAT1 (signal transducer and activator of transcription 1) at both the mRNA and protein levels, interferon gamma (IFNgamma) as well as IFNalpha are unable to trigger massive nuclear translocation of STAT1 in these cells, even at high cytokine concentrations. Conversely, nuclear accumulation of STAT1 was readily observed in murine L929 somatic cells under the same conditions. However, in the absence of any stimulation, both tyrosine (Y701p) and serine (S727p) phosphorylated forms of STAT1 were already detected in the nuclei of oocytes and early embryos. Phosphorylated STAT1 appeared concentrated in large nuclear dots, which were identified by indirect immunofluorescence and electron microscopy as clusters of interchromatin granules (IGCs or speckles). A similar distribution was also observed for the serine (S727p) phosphorylated form of STAT3 as well as for tyrosine (Y689p) phosphorylated STAT2. Western blot analysis confirmed that STAT factors present in mouse oocytes are predominantly phosphorylated. In parallel, we showed that the transcription of two IFNgamma-target genes, namely interferon regulatory factor-1 (IRF-1) and suppressor of cytokine signaling-1 (SOCS-1) is indeed increased in two-cell embryos in response to IFNgamma. Altogether, our results suggest that, despite the lack of massive nuclear accumulation of STAT1 in response to exogenous IFNs and the permanent presence of phosphorylated STATs in the nucleus, JAK/ STAT pathways are functional during early development.

The step-wise assembly of a functional nucleolus in preimplantation mouse embryos involves the cajal (coiled) body

After fertilization, ribosomal RNA synthesis is silenced during a period which depends on the species. Data concerning the reassembly of a functional nucleolus remain scarce. We have examined by immunocytochemistry, Western blots, and BrUTP microinjection the dynamics of major nucleolar proteins during the first cycles of mouse embryogenesis, in relation to rDNA transcription sites and coilin, a marker of Cajal bodies. We show that: (1) the reinitiation of rDNA transcription occurs at the two-cell stage, 44-45 h after hCG injection (hphCG), at the surface of the nucleolar precursor bodies (NPBs), where the RNA polymerase I (pol I) transcription complex is recruited 4-5 h before; (2) the NPBs are not equal in their ability to support recruitment of pol I and rDNA transcription; (3) maternally inherited fibrillarin undergoes a dynamic redistribution during the second cell stage, together with coilin, leading to the assembly of the Cajal body around 40 hphCG; and (4) the pol I complex is first recruited to the Cajal body before reaching its rDNA template. We also find that fibrillarin and B23 are both directly assembled around NPBs prior to ongoing pre-rRNA synthesis. Altogether, our results reveal a role of the Cajal bodies in the building of a functional nucleolus.