The microenvironment created by non-blocking embryos in aggregates may rescue blocking embryos via cell-embryo adherent contacts

Effects of changing culture medium on preimplantation embryo development in rabbit

Many studies have focused on the optimization of the composition of embryo culture medium; however, there are few studies involving the effect of a culture medium changing procedure on the preimplantation development of embryos. In this study, three groups were designed: a non-renewal group, a renewal group and a half-renewal group. The levels of reactive oxygen species (ROS), apoptotic index, blastocyst ratio and blastocyst total cell number were analyzed in each group. The results showed that the ROS level and the apoptotic index of blastocyst in the non-renewal group were significantly higher than in the renewal group and the half-renewal group (P < 0.05). The blastocyst ratio and blastocyst total cell number were significantly higher in the half-renewal group than that in non-renewal group and the renewal group (P < 0.05). These results demonstrated that the procedure of changing the culture medium influenced ROS level, apoptotic index, blastocyst ratio and total cell number of blastocysts. In addition, the result suggested that changing the culture medium may lead to a loss of important regulatory factors for embryos, while not changing the culture medium may lead to the accumulation of toxic substances. Half-renewal can alleviate the defects of both no renewal and renewal, and benefit embryo development. This study will be of high value as a reference for the optimization of embryo culture in vitro, and is very significant for assisted reproduction.

Aggregation of bovine cloned embryos at the four-cell stage stimulated gene expression and in vitro embryo development

Pre-implantation embryos produced by somatic cell nuclear transfer (SCNT) have varied developmental potentials. The majority of SCNT blastocysts do not develop to term, and the mechanisms inhibiting development are still largely unknown. Aggregation of cloned embryos has been attempted to compensate for the developmental deficiency of individual cloned embryos. In this report, we investigated the impact of aggregation of bovine cloned embryos at the four-cell stage on in vitro development and gene expression of the embryos. Cell numbers and development rate of aggregated (NTagg) and non-aggregated (NT) blastocysts were characterized and compared. The blastocyst formation after aggregation was modeled using the binominal distribution. The results indicate that aggregation enhances the blastocyst formation but does not increase the overall blastocyst rate. Additionally, utilizing microarray gene chip analysis 8.8% of 8,059 genes analyzed were differentially expressed between NTagg and NT blastocysts, with more than 80% of the differentially expressed genes up-regulated in the NTagg blastocysts. Up-regulated genes include those involved in transcription, biosynthesis and signaling such as TDGF1, HNFA, CAV1, GLU5, and CD81. Our results indicate that aggregation of bovine cloned embryos at an early stage promotes the in vitro development of the resulting pre-implantation embryos.

In vitro fertilization in inbred BALB/c mice II: effects of lactate, osmolarity and calcium on in vitro capacitation

To elucidate requirements for in vitro sperm capacitation in inbred BALB/c mice, osmolarity, calcium and lactate were optimized using modified simplex optimization medium (mKSOM). Modified human tubal fluid (mHTF), a capacitation-supporting medium, was used as a control. In the first series of experiments, the effects of calcium and osmolarity were studied in the presence of lactate. Although preincubation with >or=5 mM CaCl2 improved fertilization after insemination significantly, it was still significantly lower than incubation with mHTF. To obtain fertilization at the equivalent levels to that of mHTF, isotonic osmolarity (305 mOsmol) was required. Trehalose, an osmotic reagent, could substitute for NaCl partially. In the second series of experiments, the effects of lactate were examined using a concentration of 5 mM calcium and isotonic osmolarity. Preincubation with <or=2.5 mM lactate increased fertilization significantly (>75%), as well as the percentages of B (capacitated) pattern sperm (>or=40%) in chlortetracycline (CTC) staining, as compared with incubation in mHTF (46% and 28%, respectively; p<0.05). In the third series of experiments, the effects of osmolarity and calcium in the absence of lactate were examined. An increase in osmolarity during sperm preincubation increased both fertilization and B-pattern sperm significantly in a dose-dependent manner. Trehalose, sucrose and choline chloride could substitute for NaCl. An increase in CaCl2 concentration during preincubation had no effect on fertilization, but this increase reduced the percentages of B-pattern sperm. In vitro capacitation of inbred BALB/c mice is sensitive to lactate and osmolarity, but that sensitivity for calcium varies depending on the presence or absence of lactate.

Global gene expression analysis of bovine blastocysts produced by multiple methods

Reproductive efficiency using somatic cell nuclear transfer (SCNT) technology remains suboptimal. Of the various efforts to improve the efficiency, chromatin transfer (CT) and clone-clone aggregation (NTagg) have been reported to produce live cloned animals. To better understand the molecular mechanisms of somatic cell reprogramming during SCNT and assess the various SCNT methods on the molecular level, we performed gene expression analysis on bovine blastocysts produced via standard nuclear transfer (NT), CT, NTagg, in vitro fertilization (IVF), and artificial insemination (AI), as well as on somatic donor cells, using bovine genome arrays. The expression profiles of SCNT (NT, CT, NTagg) embryos were compared with IVF and AI embryos as well as donor cells. NT and CT embryos have indistinguishable gene expression patterns. In comparison to IVF or AI embryos, the number of differentially expressed genes in NTagg embryos is significantly higher than in NT and CT embryos. Genes that were differentially expressed between all the SCNT embryos and IVF or AI embryos are identified. Compared to AI embryos, more than half of the genes found deregulated between SCNT and AI embryos appear to be the result of in vitro culture alone. The results indicate that although SCNT methods have altered differentiated somatic nuclei gene expression to more closely resemble that of embryonic nuclei, combination of insufficient reprogramming and in vitro culture condition compromise the developmental potential of SCNT embryos. This is the first set of comprehensive data for analyzing the molecular impact of various nuclear transfer methods on bovine pre-implantation embryos.

Mammalian embryo co-culture: Trials and tribulations of a misunderstood method

Embryo-somatic cell co-culture was devised over 40 years ago in an attempt to improve the development and viability of mammalian preimplantation embryos generated and cultured in vitro. While initial endeavours were successful in this respect, other studies soon highlighted a number of significant long-term detrimental impacts of this approach. Surprisingly little is known about the mechanisms underlying the beneficial effects of co-culture, although the production of embryotrophic compounds, modulation of nutrient profile, protection against culture-induced stress and/or toxin clearance are all contenders. The extent to which the inadvertent exposure of embryos to serum accounts for many of these effects remains open to question. Although the popularity of somatic cell co-culture has recently declined in favour of the use of sequential media due to concerns associated with its risk of disease transmission and long-term sequelae, we argue that complete dismissal of this technique is ill advised, given that our limited understanding of basic somatic cell interactions has prevented us from fully exploiting its potential. In this respect, there is some merit in focussing future research strategies based on reconstructed maternal tract tissue. Although the use of co-culture in clinical practice is unacceptable and its implementation in domestic species for commercial purposes should be viewed with diffidence, this technique can still provide a wealth of information on the development of novel, more physiological embryo in vitro culture systems. The proviso for acquiring such information is to gain a fuller understanding of the culture requirements/biochemistry of somatic cells and their interaction with the early conceptus.

Aggregating embryonic but not somatic nuclear transfer embryos increases cloning efficiency in cattle

Our objectives were to compare the cellular and molecular effects of aggregating bovine embryonic vs. somatic cell nuclear transfer (ECNT vs. SCNT) embryos and to determine whether aggregation can improve cattle cloning efficiency. We reconstructed cloned embryos from: 1) morula-derived blastomeres, 2) six adult male ear skin fibroblast lines, 3) one fetal female lung fibroblast line (BFF), and 4) two transgenic clonal strains derived from BFF. Embryos were cultured either singularly (1X) or as aggregates of three (3X). In vitro-fertilized (IVF) 1X and 3X embryos served as controls. After aggregation, the in vitro development of ECNT but not that of SCNT or IVF embryos was strongly compromised. The inner cell mass (ICM), total cell (TC) numbers, and ICM:TC ratios significantly increased for all the aggregates. The relative concentration of the key embryonic transcript POU5F1 (or OCT4) did not correlate with these increases, remaining unchanged in the ECNT and IVF aggregates and decreasing significantly in the SCNT aggregates. Overall, the IVF and 3X ECNT but not the 1X ECNT embryos had significantly higher relative POU5F1 levels than the SCNT embryos. High POU5F1 levels correlated with high in vivo survival, while no such correlation was noted for the ICM:TC ratios. Development to weaning was more than doubled in the ECNT aggregates (10/51 or 20% vs. 7/85 or 8% for 3X vs. 1X, respectively; P < 0.05). In contrast, the SCNT and IVF controls showed no improvement in survival. These data reveal striking biological differences between embryonic and somatic clones in response to aggregation.

Subzonal Older Adult Fibroblast Insertion in Both In Vivo–Fertilized and Nuclear Transfer Rabbit Zygotes and Embryos: Effects on Further In Vitro Embryo Development

In the present work, we evaluated the effect on further in vitro embryo development of inserting rabbit adult fibroblasts into in vivo-fertilized rabbit embryos. To this end, we inserted either 4 or 15-20 rabbit adult fibroblasts in two different early embryo stages of development, 1-cell stage and 4-8-cell stage embryos. We observed that fibroblast insertion not only did not negatively affect further embryo development, but also may have exerted a positive effect on development on it. Therefore, in forthcoming works were where we intend to study a possible cell helper role on early embryo development. The early embryo microenvironment may reprogram somatic cell gene expression of fibroblasts inserted within the embryo, making them suitable as nuclear donors.

Surface-expressed E-cadherin, and mitochondrial and microtubule distribution in rescue of mouse embryos from 2-cell block by aggregation

E-cadherin (uvomorulin)-mediated cell interactions are essential for preimplantation development in mammals. We observed that E-cadherin is expressed at contact sites between blastomeres of 2-cell mouse embryos of non-blocking genotype (CBA x C57BL F1) explanted at 32 h post human chorionic gonadotrophin (HCG) and cultured in vitro, while blastomere rounding and reduced zones of contact and E-cadherin-staining were observed in embryos of a blocking strain (MF1) arrested at the 2-cell stage. Embryos of MF1 strain can be rescued by aggregation with four 2-cell embryos of the non-blocking genotype. An early event in rescue is E-cadherin expression at contact zones between adjacent embryos of different genotype in aggregation chimeras. E-cadherin-mediated signalling appears important for the rescue (including formation of adherens-like contacts, cell polarization and morphogenetic processes) since there is no rescue when E-cadherin-specific antibodies are present during phytohaemagglutinin-mediated aggregation and subsequent culture. In blocked embryos, the distribution of microtubules is disturbed and concomitantly mitochondria cluster around the nucleus. Rescue by aggregation retains normal mitochondrial distribution in the presence of a dense microtubular lattice in all blastomeres. Therefore, E-cadherin-mediated signalling and its downstream effects on cytoskeletal organization are essential in the rescue of blocking embryos by aggregation. Normal preimplantation development appears to be dependent on the polarized expression of surface E-cadherin and the microtubule-mediated dispersal of mitochondria.

Blastomeres from Somatic Cell Nuclear Transfer Embryos Are Not Allocated Randomly in Chimeric Blastocysts

A marker has been developed to allow detection of blastomeres that originate from embryos produced by nuclear transfer (NT) of genetically engineered fetal fibroblasts. A plasmid (phEFnGFP) was constructed with a G418 resistance cassette for selection in fibroblasts and a nuclear localized green fluorescent protein (nGFP) expression cassette that expresses in every cell of day-6, -7, and -8 bovine embryos. This construct was utilized to follow the blastomere distribution in aggregation chimeras produced from fertilized embryos (in vitro produced, IVP) or parthenotes and NT embryos. Fluorescent and nonfluorescent NT embryos were aggregated early on day 4 and evaluated on day 8. Nuclei of blastomeres that carried the transgene were fluorescent under both UV epifluorescence (Hoechst 33342) and blue epifluorescence (nGFP). There was no bias in the distribution of green fluorescent blastomeres in the inner cell mass (ICM) or trophectoderm in NT<>NT chimeras. However, there was a strong bias for NT blastomeres to populate the ICM when aggregated with IVP embryos or parthenotes. There was also a strong bias against NT blastomeres in the trophectoderm when aggregated to IVP embryos. However, the bias against NT blastomeres in the trophectoderm was significantly less (p < 0.05) when aggregated with parthenotes as compared to aggregation with IVP embryos. In NT<>NT aggregates, no chimeric embryos were produced that had an ICM composed of blastomeres from a single origin. However, in NT<>Parthenote aggregates, 67% of the blastocysts had an ICM composed exclusively of NT origin. The remaining blastocysts ranged from 0% to 83% of the ICM that expressed nGFP. Similarly, in NT<>IVP aggregates 50% of the blastocysts had an ICM composed exclusively of NT origin. The remaining blastocysts ranged from 19% to 71% of the ICM being of NT origin. We conclude that production of divaricated chimeras from NT origin is feasible. Other applications of this technology are discussed.

The behaviour of mitochondria and cell integration during somatic hybridisation of sister blastomeres of the 2-cell mouse embryo

The capacity of sister blastomeres of mouse embryos for induced fusion changed during the 2-cell stage. It was at low level (24%) at the early 2-cell stage, increased and reached 98.5% at the middle 2-cell stage and fell sharply to 31% at the late 2-cell stage. At the time corresponding to the G2/M-phase of the cell cycle the blastomeres fused in only 8% of cases. Vital staining of 2-cell embryos by rhodamine 123 showed that the mitochondria were dispersed throughout the cytoplasm with a ring-like (around the nucleus) or spot-like (over the metaphase plate) concentration in the centre of each blastomere. At the periphery of blastomeres the mitochondrial content was low. The behaviour of the mitochondria reflected the subsequent events of structural and functional integration of the sister blastomeres under induced fusion: a discernible boundary between partners during 30 min after electrofusion or 1 h after fusion with polyethylene glycol; movement of the two 'rings' to the centre of the blastomere fusion products (BFP) to form one large bright 'spot' over the common metaphase plate; mitochondria outlining the shape of the spindle and connection between sister blastomeres until completion of the first mitosis of BFP. The data obtained suggest that fusion of the blastomeres does not lead to extensive changes in the hybrid cytoplasm and integration of nuclear material is taking place only at metaphase stage. Cytogenetic examination of BFP at the 2-cell stage confirmed reconstruction of the tetraploid embryos and found that sister blastomeres of such embryos could asynchronously enter the next cleavage division similarly to normal diploid 2-cell embryos.