A microtubule-dependent zone of active RhoA during cleavage plane specification

Cytokinesis in animal cells results from the assembly and constriction of a circumferential array of actin filaments and myosin-2. Microtubules of the mitotic apparatus determine the position at which the cytokinetic actomyosin array forms, but the molecular mechanisms by which they do so remain unknown. The small GTPase RhoA has previously been implicated in cytokinesis. Using four-dimensional microscopy and a probe for active RhoA, we show that active RhoA concentrates in a precisely bounded zone before cytokinesis and is independent of actin assembly. Cytokinetic RhoA activity zones are common to four echinoderm species, the vertebrate Xenopus laevis, and the highly asymmetric cytokinesis accompanying meiosis. Microtubules direct the formation and placement of the RhoA activity zone, and the zone is repositioned after physical spindle displacement. We conclude that microtubules specify the cytokinetic apparatus via a dynamic zone of local RhoA activity.

Influence of maturation culture period on the development of canine oocytes after in vitro maturation and fertilization

The objective of this study was to determine an optimum maturation period of canine oocytes for the development in vitro after in vitro fertilization (IVF). Canine oocytes larger than 110 micrometers in diameter, which were collected from ovaries at the follicular phase of the reproductive cycle, were cultured for each time (48, 72 and 96 h) in TCM 199 medium supplemented with 10% canine serum, fertilized, and then cultured in vitro for 8 days. Significantly more oocytes reached metaphase II (MII) in the 72-h culture group than in the 48-h culture group (25.6% vs. 41.0%). The percentages of oocytes that reached MII or beyond after maturation culture did not differ significantly between the 72- and 96-h culture groups, but the percentage of parthenogenetically activated oocytes in the 96-h culture group was significantly higher than that in the 72-h culture group. The percentages of cleaved embryos after IVF were significantly higher in the 48- and 72-h culture groups than in the 96-h culture group. In the 48-h culture group, 3.9% of fertilized oocytes developed to the 16-cell stage or beyond, but none of the cleaved embryos in the 72- and 96-h culture groups developed to the same stage. These results indicate that full nuclear maturation of oocytes collected from ovaries at the follicular phase occurs after 72 h of in vitro culture. However, an optimum maturation period (48 h) for the in vitro development of canine oocytes after IVF may be different from the period necessary to reach the maximal oocyte maturation rate, when based on the developmental stage of the cleaved embryos.

Dynamic CpG and Non-CpG Methylation of the Peg1/Mest Gene in the Mouse Oocyte and Preimplantation Embryo

In somatic tissues, the CpG island of the imprinted Peg1/Mest gene is methylated on the maternal allele. We have examined the methylation of CpG and non-CpG sites of this differentially methylated CpG island in freshly ovulated oocytes, in vitro aged oocytes, and preimplantation embryos. The CpG methylation pattern was heterogeneous in freshly ovulated oocytes, despite the fact that they all were arrested in metaphase II. After short in vitro culture, Peg1/Mest became hypermethylated, whereas prolonged in vitro culture resulted in demethylation in a fraction of oocytes. Non-CpG methylation also occurred in a stage-specific manner. On alleles that were fully methylated at CpG sites, this modification was found, and it became reduced in two-cell stage embryos and blastocysts. These observations suggest that the process of establishment of the methylation imprint at this locus is more dynamic than previously thought.

[Changes in the permeability of amphibian embryo envelope for fluorochromes under the action of ultrasound]

It was determined whether high-frequency ultra sound can change the permeability of gray toad Bufo bufo and grass frog Rana temporaria under the action of high-frequency ultrasound. The changes in the permeability of embryonic envelope were assessed by using slowly penetrating fluorochromes ANS, FDA, and fluorescein. It was found that the ultrasound of 0.88 MHz and 0.4 - 0.7 W/cm2 intensity increased the permeability of amphibian embryonic envelope for ANS and FDA, whereas the ultrasound of 2.64 MHz and the same intensity increased that for fluorescein with the retention of low permeability for FDA. Embryos continued the normal development after treatment with ultrasound under these conditions.

IVF versus ICSI in sibling oocytes from patients with polycystic ovarian syndrome: a randomized controlled trial

BACKGROUND

This study compares the fertilization rate and embryonic development of oocytes randomly inseminated by conventional IVF or ICSI in patients with polycystic ovarian syndrome (PCOS) and normozoospermic semen during IVF cycles.

METHODS

Sibling oocytes were randomized to be inseminated either by ICSI or IVF. Fertilization rate (two pronuclei/COC), day 2 embryonic morphology and rate of development were assessed.

RESULTS

A total of 1089 cumulus-oocyte complexes (COC) were collected in 60 cycles (mean+/-SD, 18.2 +/- 7.2). Totals of 541 and 548 COC were inseminated by IVF and ICSI respectively, with a significantly higher fertilization rate in the ICSI group (ICSI versus IVF, 72.3 +/- 15.5 versus 44.8 +/- 25.1%). No fertilization failure occurred in the group of oocytes inseminated by ICSI, whereas the COC in nine patients (15%) inseminated by IVF had complete fertilization failure. The day 2 embryonic morphology and rate of development were not different regardless of the insemination method.

CONCLUSIONS

Our results suggested that another randomized controlled study, randomizing patients instead of sibling oocytes, should be undertaken to compare the pregnancy rate per started cycle and to see whether ICSI should be performed on all, or at least on a portion of, oocytes for patients with PCOS undergoing IVF cycles.

[Expression of soluble human leukocyte antigen G and its relationship to embryo development]

OBJECTIVE

To find out the expression of soluble human leukocyte antigen G (sHLA-G) and its relationship to the cleavage embryo development.

METHODS

One hundred and seventy-seven day 3 cleavage embryos were detected for sHLA-G by immunohistochemistry.

RESULTS

sHLA-G was detected in 57.1% cleavage embryos. The positive rate of sHLA-G in cleavage embryos developed from dipronucleate fertilized eggs was 66.2% (90/136), that developed from tripronucleate fertilized eggs was 26.8% (11/41). There was significant difference between these two groups (P < 0.01). The positive rate of sHLA-G in the grade 1 cleavage embryos developed from dipronucleate fertilized eggs was 64.3% (18/28), that in the grade 2 cleavage embryos developed from dipronucleate fertilized eggs was 91.7% (66/72), that in the grade 3 cleavage embryos developed from dipronucleate fertilized eggs was 16.7% (6/36), there were significant differences between these different embryo grades (P < 0.01), and the intensity of sHLA-G had negative relationship with the embryo grades (r = -0.503). The positive rate of sHLA-G in the first class cleavage embryos developed from tripronucleate fertilized eggs was 88.9% (32/36), that from dipronucleate fertilized eggs was 64.3% (18/28). There was significant difference in these two groups (P < 0.01). The intensity of sHLA-G in cleavage embryos developed from tripronucleate fertilized eggs was higher than that from dipronucleate fertilized eggs. The positive rate of sHLA-G in the cleavage embryos developed from dipronucleate fertilized eggs whose cell number was less than 4 was 56.7% (34/60), that from dipronucleate fertilized eggs whose cell number ranged from 5 to 6 was 67.9% (36/53), and that from dipronucleate fertilized eggs whose cell number ranged from 7 to 8 was 87.0% (20/23). There were no significant differences in these three groups. The intensity of sHLA-G had no significant difference between embryos with different cell number (P > 0.05), but it had relationship with the cell number (r = 0.267).

CONCLUSION

Cleavage embryos express sHLA-G which is related with the embryo development.

Downregulation of Par3 and aPKC function directs cells towards the ICM in the preimplantation mouse embryo

Generation of inside cells that develop into inner cell mass (ICM) and outside cells that develop into trophectoderm is central to the development of the early mouse embryo. Critical to this decision is the development of cell polarity and the associated asymmetric (differentiative) divisions of the 8-cell-stage blastomeres. The underlying molecular mechanisms for these events are not understood. As the Par3/aPKC complex has a role in establishing cellular polarity and division orientation in other systems, we explored its potential function in the developing mouse embryo. We show that both Par3 and aPKC adopt a polarized localization from the 8-cell stage onwards and that manipulating their function re-directs cell positioning and consequently influences cell fate. Injection of dsRNA against Par3 or mRNA for a dominant negative form of aPKC into a random blastomere at the 4-cell stage directs progeny of the injected cell into the inside part of the embryo. This appears to result from both an increased frequency by which such cells undertake differentiative divisions and their decreased probability of retaining outside positions. Thus, the natural spatial allocation of blastomere progeny can be over-ridden by downregulation of Par3 or aPKC, leading to a deceased tendency for them to remain outside and so develop into trophectoderm. In addition, this experimental approach illustrates a powerful means of manipulating gene expression in a specific clonal population of cells in the preimplantation embryo.

Developmentally regulated cytoplasmic retention of the transcription factor XMI-ER1 requires sequence in the acidic activation domain

Xmi-er1 is a fibroblast growth factor regulated immediate-early gene that is activated during mesoderm induction in Xenopus embryonic explants. This gene encodes a nuclear protein with potent transcriptional regulator activity and overexpression of XMI-ER1 in Xenopus embryos inhibits mesoderm induction and leads to truncations along the anteroposterior axis. We showed previously that XMI-ER1 is retained in the cytoplasm during cleavage stages and only begins to appear in the nucleus at mid-blastula. Such developmentally regulated nuclear translocation may represent an important mechanism for regulating XMI-ER1 activity in the early embryo. Here, we investigate different mechanisms that might control nuclear translocation of XMI-ER1. Using alpha-amanitin to inhibit transcription, we show that nuclear localization is not dependent on zygotic transcription. Nor is it the result of a developmentally regulated import pathway, as the XMI-ER1 nuclear localization signal (NLS) fused to beta-galactosidase (betagal) was able to direct nuclear translocation prior to mid-blastula. Fusion of an additional, heterologous NLS to the N-terminus of XMI-ER1 was not sufficient to overcome cytoplasmic retention, indicating that retention does not involve NLS masking, but rather binding to a cytoplasmic anchor. The anchoring molecule is not an RNA, as microinjection of RNase A did not affect the timing of nuclear translocation. Western blot analysis using antibodies that recognize phosphorylated residues revealed that, while XMI-ER1 is not itself phosphorylated, it is associated with two differentially phosphorylated proteins, suggesting that the anchoring mechanism may involve interaction with a cytoplasmic protein(s). A series of XMI-ER1 deletion mutants was utilized to map the putative retention domain. Our analysis revealed that amino acids 144-175, containing the fourth acidic stretch of the acidic activation domain, are required for retention. These results suggest that XMI-ER1 is retained in the cytoplasm of the early embryo by interaction of the region containing amino acids 144-175 with a cytoplasmic anchor.

Intra-cytoplasmic sperm injection in a marsupial

Here we report the first use of intra-cytoplasmic sperm injection (ICSI) in a marsupial, the tammar wallaby (Macropus eugenii), to achieve in vitro fertilization and cleavage. A single epididymal spermatozoon was injected into the cytoplasm of each mature oocyte collected from Graafian follicles or from the oviduct within hours of ovulation. The day after sperm injection, oocytes were assessed for the presence of pronuclei and polar body extrusion and in vitro development was monitored for up to 4 days. After ICSI, three of four (75%) follicular and four of eight (50%) tubal oocytes underwent cleavage. The cleavage pattern was similar to that previously reported for in vivo fertilized oocytes placed in culture, where development also halted at the 4- to 8-cell stage. One-third of injected oocytes completed the second cleavage division, but only a single embryo reached the 8-cell stage. The success of ICSI in the tammar wallaby provided an opportunity to examine the influence of the mucoid coat that is deposited around oocytes passing through the oviduct after fertilization. The presence of a mucoid coat in tubal oocytes did not prevent fertilization by ICSI and the oocytes cleaved in vitro to a similar stage as follicular oocytes lacking a mucoid coat. Cell-zona and cell-cell adhesion occurred in embryos from follicular oocytes, suggesting that the mucoid coat is not essential for these processes. However, blastomeres were more closely apposed in embryos from tubal oocytes and cell-cell adhesion was more pronounced, indicating that the mucoid coat may be involved in maintaining the integrity of the conceptus during cleavage.

Cryopreservation of biopsied cleavage stage human embryos

The aim was to develop a method to optimize cryopreservation of biopsied multi-celled human embryos. Human day 3 embryos that were donated to research, along with those found to be chromosomally abnormal after blastomere biopsy and fluorescence in-situ hyridization (FISH), were cryopreserved using a slow-freezing protocol in either standard embryo cryopreservation solution [embryo transfer freezing medium (ETFM), a conventional sodium-based medium] or CJ3 (a choline-based, sodium-free medium). After thawing, the number of intact cells was recorded and the previously biopsied embryos were re-analysed using FISH. Biopsied embryos had a lower proportion of intact blastomeres after cryopreservation as compared with intact embryos. However, a significantly (P < 0.05) higher proportion of blastomeres from intact and biopsied embryos cryopreserved in CJ3 (84.1 and 80.1% respectively) survived after thaw than those in ETFM (73.6 and 50.5% respectively). The proportion of aneuploid and mosaic embryos was not statistically different between the two groups. In addition, the frequency of lost cells by aneuploid and mosaic embryos was similar. This study describes a new method that improves the survival of cryopreserved biopsied embryos, and shows that it may also be beneficial for the storage of intact human multi-celled embryos.

Efficient blastomere biopsy for mouse embryo splitting for future applications in human assisted reproduction

The objective of the current study was to establish a safe, efficient biopsy procedure for embryo splitting using the mouse model for future applications in human assisted reproduction. From mouse embryos at the 2-, 4-, 6- and 8-cell stage, half the number of blastomeres were microsurgically biopsied and transferred into empty mouse zonae pellucidae. Twin embryonic development was monitored during in-vitro culture. Blastocyst developmental rate using 2-, 4-, 6-, and 8-cell splitting was 74.4, 75.0, 66.7 and 38.4 respectively, with corresponding hatching rates of 94.9, 97.5, 92.7 and 83.8%. Blastocysts from 2-, 4-, and 6-cell splitting resulted in elevated hatching rates compared with non-operated blastocysts (87.5%), due to the Tyrode-assisted hatching effect. Blastocyst morphology was superior from 2- and 4-cell splitting when compared with 6- and 8-cell splitting. Furthermore, outgrowth of twin blastocysts from 2- and 4-cell splitting showed well-developed colonies with trophoblast cells and clusters of ICM cells, whereas those obtained from 6- and 8-cell splitting frequently formed small-sized colonies. Due to the high twinning success rate obtained under the experimental conditions employed in this study, it appears that with further modifications and proper safeguards, such embryo splitting efforts could have potential applications in humans.

Unbiased contribution of the first two blastomeres to mouse blastocyst development

Several recent studies have proposed a model that the organization of the mouse blastocyst is determined by the pattern of early cleavages: the plane of first cleavage divides the two-cell embryo into embryonic (Em) and abembryonic (Ab) halves, while the timing of the second cleavages specifies which blastomere becomes the Em half. This model is still controversial because of conflicting observations in various studies. Here, we investigated the possibility that the difference between mouse strains contributed to the discrepancy of the findings of different experiments regarding the relationship between the first two cleavages and the blastocyst axial pattern. First, we showed by using a lipophilic, fluorescent tracer that the plane of the first cleavage bears no consistent spatial relationship to the Em-Ab axis of the blastocyst regardless of the genotypic background. Secondly, the order of the second cleavage does not correlate with the Em-Ab polarity of the blastocyst. This was demonstrated by tracing the lineage of the early- and later-dividing two-cell stage blastomeres in the whole embryo as well as by comparing the developmental potential of isolated early- and later-dividing blastomeres and chimeras made entirely of early- or later-dividing blastomeres. These results suggest that contrary to recent studies, the differences between the early- and later-dividing blastomeres of the two-cell embryo are not functionally evident and do not define the Em-Ab polarity of the blastocyst. The significance of these findings is discussed in relation to human assisted reproduction and preimplantation genetic diagnosis.

[Changes in the distribution of mitochondria in mouse embryos blocked at the two-cell stage]

Changes in the distribution of mitochondria in the two-cell mouse embryos preceding the developmental arrest in vitro, caused by a genetically determined "two-cell block in vitro" or genisteine treatment, were examined vitally using the mitochondrial-specific probe rhodamine 123 and conventional fluorescence microscopy. In the former case, serious disturbances in the localization of mitochondria appeared already from the middle of two-cell stage, long before the time corresponding to the 2nd cleavage division. Comparison of the behavior of mitochondria in the embryos successfully developing between the one- and two-cell stages and that in the embryos that ceased to cleave suggests that the developmental arrest was accompanied by aggregation of the mitochondria into clusters. There are many such clusters unlike in the cytoplasm of normally developing embryos. Intracellular localization of clusters observed in the genisteine-treated embryos differed radically from that observed in the embryos blocked in vitro at the two-cell stage.

Similar Effects of Osmolarity, Glucose, and Phosphate on Cleavage past the 2-Cell Stage in Mouse Embryos from Outbred and F1 Hybrid Females1

One-cell-stage embryos derived from most random-bred and inbred female mice exhibit an in vitro developmental block at the two-cell stage in classical embryo culture media. However, embryos derived from many F1 hybrids develop easily past the two-cell stage under the same conditions. This has given rise to the commonly accepted idea that there exist blocking and nonblocking types of female mice, with only the former being prone to a two-cell block. Recently, culture media have been improved to the point that even embryos prone to the two-cell block will develop past the block in vitro, making it possible to study its etiology. Here, we show that either increased osmolarity or increased glucose/phosphate levels induced the expected two-cell block in random-bred CF1 embryos and the two-cell block at increased osmolarities could be rescued by the organic osmolyte glycine. Surprisingly, one-cell embryos from B6D2F1 (BDF1) F1 hybrid females, considered to be nonblocking, also became blocked at the two-cell stage when osmolarity or glucose/phosphate levels were increased. They were also similarly rescued by glycine from the osmolarity-induced block. The most evident difference was that the purportedly nonblocking embryos became blocked at a higher threshold of osmolarity or glucose/phosphate level than those considered prone to this developmental block. Thus, both blocking and nonblocking embryos actually exhibit a similar two-cell block to development.

Embryonic Stages from Cleavage to Gastrula in the Loach Misgurnus anguillicaudatus

Early developmental staging from the zygote stage to the gastrula is a basic step for studying embryonic development and biotechnology. We described the early embryonic development of the loach, Misgurnus anguillicaudatus, based on morphological features and gene expression. Synchronous cleavage was repeated for 9 cycles about every 27 min at 20 degrees C after the first cleavage. After the 10th synchronous cleavage, asynchronous cleavage was observed 5.5 h post-fertilization (hpf), indicating the mid-blastula transition. The yolk syncytial layer (YSL) was formed at this time. Expressions of goosecoid and no tail were detected by whole-mount in situ hybridization from 6 hpf. This time corresponded to the late-blastula period. Thereafter, epiboly started and a blastoderm covered over the yolk cell at 8 hpf. At 10 hpf, the germ ring and the embryonic shield were formed, indicating the stage of early gastrula. Afterward, the epiboly advanced at the rate of 10% of the yolk cell each hour. The blastoderm covered the yolk cell completely at 15 hpf. The embryonic development of the loach resembled that of the zebrafish in terms of morphological change and gene expression. Therefore, it is possible that knowledge of the developmental stages of the zebrafish might be applicable to the loach.

Survival and developmental potential of stored human early cleavage stage embryos

Human early cleavage stage embryos which survive cryopreservation and thawing fully intact demonstrate similar developmental potential to equivalent non frozen embryos when returned to the in vivo environment, whereas blastomere loss is directly related to the loss of potential for subsequent implantation in thawed embryos. This suggests that blastomere lysis during freezing and thawing does not occur preferentially in non viable blastomeres. Prefreeze growth rate rather than prefreeze blastomere number per se correlates with the developmental potential of stored embryos. When blastomere loss occurs as a consequence of cryopreservation, development of thawed early cleavage stage embryos to the blastocyst stage in vitro is impaired and the resultant blastocysts have a reduced total cell content. Blastomere loss is more prevalent in embryos which have been biopsied for preimplantation genetic diagnosis but this increased sensitivity can be circumvented by modification of the standard cryopreservation protocol.

Apoptosis in Parthenogenetic Preimplantation Porcine Embryos1

Parthenogenesis (PA) of the oocyte is essential to a number of oocyte- or embryo-related technologies such as intracytoplasmic sperm injection and cloning by nuclear transfer. This study investigated the onset and frequency of apoptosis in PA- porcine embryos and the morphological changes that conform to the general criteria of apoptotic cell death by using a terminal deoxynucleatidyl transferase-mediated deoxyuridine 5-triphosphate nick-end labeling (TUNEL) assay. PA embryos had a higher degree of apoptotic cell death during in vitro culture, a lower cleavage rate (45% vs. 71%), and a lower development rate to the blastocyst stage (16% vs. 29%), relative to in vitro fertilization (IVF). The earliest positive TUNEL signal in the PA embryos was detected on Day 6, 1 day later than that in IVF embryos. Apoptosis in PA embryos increased from 15% of the embryos on Day 6 to 29% on Day 8. The mean level of apoptosis of the PA embryos was statistically higher than that of IVF embryos, except on Day 5. In particular, apoptosis in PA embryos was twice that of IVF embryos on Day 6 (15% vs. 6.7%) and Day 8 (29% vs. 13%). The mean cell number in PA blastocysts was significantly lower than that of IVF blastocysts, whereas the percentage of apoptosis in PA blastocysts was significantly higher than that of IVF blastocysts. There was a high percentage of haploid (62.5%) PA blastocysts. The ploidy may contribute to a high level of apoptosis. These results may help to explain the mechanism of parthenogenetic developmental failure and may lead to methods that will improve parthenogenetic development.

Serial bull cloning by somatic cell nuclear transfer

Although the list of species successfully cloned continues to grow, serial cloning has not been reported in species other than the mouse. Here we describe two live births of second-generation clones of a bull. Clones of the first and second generations appear healthy and have normal telomere lengths. Our attempts to produce the third generation of clones were unsuccessful.

Comparative analysis of pregnancy rates after the transfer of early dividing embryos versus slower dividing embryos

BACKGROUND

We compared retrospectively the pregnancy outcome in two subgroups of ICSI patients, using early division (26 h post injection) to the 2-cell stage as a criterion for embryo quality and viability (ability to produce a pregnancy).

METHODS AND RESULTS

In the early dividing embryo (EDE) group, at least one of the transferred embryos was early dividing. In the late dividing embryo (LDE) group, no early dividing embryo was transferred. Additionally, tubal and uterine transfer in the two groups was also evaluated. Clinical pregnancy rates in the EDE group were significantly increased when compared with that in the LDE group (41.3 versus 20.0%). This was also true for ongoing pregnancy rates (33.3 versus 16.3%). The tubal transfer route showed increased (but not significant) ongoing pregnancy rates when compared with uterine transfer in both EDE (38.5 versus 25.0%) and LDE (22.7 versus 8.3%) groups respectively. In uterine transfer cycles, however, clinical pregnancy rates for EDE were significantly increased compared to LDE (37.5 and 11.1% respectively). The baby rate (number of live babies/embryos transferred) was also significantly increased in the EDE group and the tubal transfer group. Statistical analysis of pregnancy outcome, adjusted for the total number of embryos transferred (expressed as percentage risk difference - %RD), resulted significantly in favour of EDE compared to LDE (RD = 18%, P = 0.02). When adjusted for the combined factors: total number of embryos transferred, EDE and LDE, the pregnancy outcome result was significantly in favour of tubal transfer compared to uterine transfer (RD = 15%, P = 0.05). Pregnancy results of the LDE group only were significantly better in the tube compared to the uterus (RD = 19%, P = 0.04) but not significantly so for the EDE group (RD = 10%, P = 0.4).

CONCLUSION

Early division is associated with embryo quality and a very easy and successful embryo transfer selection method. Our results also suggest that when EDE are available, both tubal and uterine embryo transfer can be considered. When only LDE are available, however, tubal transfer should be the preferred transfer route.

Impact of oxygen concentration on embryonic development of mouse zygotes

The aim of the present study was to examine the effect of culture under 5 and 20% oxygen on the development, differentiation and viability of zygotes and in-vivo-produced embryos at the 2-cell and 8-cell stages of development. First, zygotes collected in a common pool were cultured in 20% O2 for 0, 23, 46 and 95 h. Zygotes and in-vivo-produced embryos at the 2-cell and 8-cell stages of development were then cultured in 5 or 20% O2. The proportion of embryos reaching the compaction and blastocyst stages of development did not differ between groups regardless of the period of time embryos were cultured in 20% O2 or the stage at beginning of culture. Duration of culture under 20% O2 had a significant effect on total number of blastocyst cells. A stage-specific effect was observed on total and trophectoderm cell numbers in blastocysts resulting from the culture of zygotes and in-vivo-produced embryos under 20% O2. ICM and percent ICM development was significantly decreased by culture in 20% O2 at all stages examined. Oxygen concentration had no effect on implantation rate and fetal weights upon embryo transfer. However, transfer of zygotes grown to the blastocyst stage in 20% O2 resulted in a dramatic decrease in fetal development per blastocyst and fetal development per implantation. These results demonstrate that culture of F1 mouse zygotes in 20% O2 compromises the developmental potential of resultant blastocysts, which appear to be normal on morphological assessment.

The glycine neurotransmitter transporter GLYT1 is an organic osmolyte transporter regulating cell volume in cleavage-stage embryos

Cells subjected to sustained high osmolarity almost universally respond by accumulating compatible organic osmolytes that, in contrast to inorganic ions, are not deleterious even at high intracellular concentrations. Their accumulation from the external environment by known organic osmolyte transporters, such as the four identified in mammals, occurs only slowly in response to sustained high osmolarity, by synthesis of new transporter proteins. Most cells, however, are not subject to high or varying osmolarity, and it is not clear whether organic osmolytes are generally required at normal osmolarities or how they are regulated. The fertilized egg of the mouse is protected in the oviduct from perturbations in osmolarity. However, deleterious effects of osmotic stress were evident in vitro even at normal oviductal osmolarity. Glycine was found to protect development, indicating that early mouse embryos may use glycine as an organic osmolyte at physiological osmolarity. We have now found that GLYT1, a glycine transporter of the neurotransmitter transporter gene family, functions as the organic osmolyte transporter that mediates the osmotically regulated accumulation of glycine and regulates cell volume in early embryos. Furthermore, osmotic stimulation of GLYT1 transport was immediate, without a requirement for protein synthesis, implying regulation different from known organic osmolyte transporters. Thus, GLYT1 appears to have a previously unidentified role as an organic osmolyte transporter that functions in acute organic osmolyte and volume homeostasis near normal osmolarity.

The maternal-effect gene futile cycle is essential for pronuclear congression and mitotic spindle assembly in the zebrafish zygote

Embryos have been successfully used for the general study of the cell cycle. Although there are significant differences between the early embryonic and the somatic cell cycle in vertebrates, the existence of specialised factors that play a role during the early cell cycles has remained elusive. We analysed a lethal recessive maternal-effect mutant, futile cycle (fue), isolated in a maternal-effect screen for nuclear division defects in the zebrafish (Danio rerio). The pronuclei fail to congress in zygotes derived from homozygous fue mothers. In addition, a defect in the formation of chromosomal microtubules prevents mitotic spindle assembly and thus chromosome segregation in fue zygotes. However, centrosomal functions do not appear to be affected in fue embryos, suggesting this mutant blocks a subset of microtubule functions. Cleavage occurs normally for several divisions resulting in many anucleate cells, thus showing that nuclear- and cell division can be uncoupled genetically. Therefore, we propose that in mitotic spindle assembly chromosome-dependent microtubule nucleation is essential for the coupling of nuclear and cell division.

Opposing FGF and Retinoid Pathways Control Ventral Neural Pattern, Neuronal Differentiation, and Segmentation during Body Axis Extension

Vertebrate body axis extension involves progressive generation and subsequent differentiation of new cells derived from a caudal stem zone; however, molecular mechanisms that preserve caudal progenitors and coordinate differentiation are poorly understood. FGF maintains caudal progenitors and its attenuation is required for neuronal and mesodermal differentiation and to position segment boundaries. Furthermore, somitic mesoderm promotes neuronal differentiation in part by downregulating Fgf8. Here we identify retinoic acid (RA) as this somitic signal and show that retinoid and FGF pathways have opposing actions. FGF is a general repressor of differentiation, including ventral neural patterning, while RA attenuates Fgf8 in neuroepithelium and paraxial mesoderm, where it controls somite boundary position. RA is further required for neuronal differentiation and expression of key ventral neural patterning genes. Our data demonstrate that FGF and RA pathways are mutually inhibitory and suggest that their opposing actions provide a global mechanism that controls differentiation during axis extension.

Mitogenic and anti-apoptotic activity of insulin on bovine embryos produced in vitro

Insulin improves development of mammalian preimplantation embryos and, in addition to the regulation of glucose transport, it exerts mitogenic and anti-apoptotic activities. The expression of glucose transporters (Glut) mediating the uptake of this essential energy substrate is critical for embryo survival. An impaired expression of Glut leads to an increase in apoptosis at the blastocyst stage and involves Bax. The various effects of insulin were unravelled by supplementing the in vitro culture medium with insulin (1.7 micromol l(-1)) and (i) the rates of cleavage and blastocyst development were recorded; (ii) mitogenic activity was studied by determining the total number of blastocyst cells and the ratio between trophectoderm and inner cell mass (ICM) cells; (iii) the frequency of apoptosis in blastocysts was determined by the TdT-mediated duTP nick-end labelling (TUNEL) assay and by quantification of the relative amounts of mRNA for Bax and Bcl-XL; and (iv) expression for Glut1, Glut3 and Glut8 transcripts was compared between embryos cultured in the presence or absence of insulin. Insulin increased rates of cleavage (81.2+/-2.2 (control) to 86.0+/-2.5) and blastocyst development (24.7+/-1.9 to 31.3+/-1.2), and number of blastocyst cells (123.7+/-6.0 to 146.3+/-6.6); the increase in the number of blastocyst cells was due to a significantly higher number of trophectoderm cells (82.3+/-5.0 versus 100.3+/-5.5). Blastocysts derived from cultures supplemented with insulin showed a significant decrease in apoptosis as determined by the TUNEL assay (14.8+/-0.9 to 12.2+/-0.7). No effects of insulin on the mRNA expression of Glut isoforms and Bax and Bcl-XL were found. These results demonstrate that the mitogenic and anti-apoptotic effects of insulin on bovine preimplantation embryos did not correlate with changes in the amounts of mRNA for the glucose transporter isoforms Glut1, -3 and -8, or transcripts for Bax and Bcl-XL.