Utero-tubal transfer of mouse embryos

The History of Transgenesis

A transgenic mouse carries within its genome an artificial DNA construct (transgene) that is deliberately introduced by an experimentalist. These animals are widely used to understand gene function and protein function. When addressing the history of transgenic mouse technology, it is apparent that a number of basic science research areas laid the groundwork for success. These include reproductive science, genetics and molecular biology, and micromanipulation and microscopy equipment. From reproductive physiology came applications on how to optimize mouse breeding, how to superovulate mice to produce zygotes for DNA microinjection or preimplantation embryos for combination with embryonic stem (ES) cells, and how to return zygotes and embryos to a pseudopregnant surrogate dam for gestation and birth. From developmental biology, it was learned how to micromanipulate embryos for morula aggregation and blastocyst microinjection and how to establish germline competent ES cells. From genetics came the foundational principles governing the inheritance of genes, the interactions of gene products, and an understanding of the phenotypic consequences of genetic mutations. From molecular biology came a panoply of tools and reagents that are used to clone DNA transgenes, to detect the presence of transgenes, to assess gene expression by measuring transcription, and to detect proteins in cells and tissues. Technical advances in light microscopes, micromanipulators, micropipette pullers, and ancillary equipment made it possible for experimentalists to insert thin glass needles into zygotes or embryos under controlled conditions to inject DNA solutions or ES cells. To fully discuss the breadth of contributions of these numerous scientific disciplines to a comprehensive history of transgenic science is beyond the scope of this work. Examples will be used to illustrate scientific developments central to the foundation of transgenic technology and that are in use today.

Sperm selection by thermotaxis improves ICSI outcome in mice

The ejaculate is a heterogeneous pool of spermatozoa containing only a small physiologically adequate subpopulation for fertilization. As there is no method to isolate this subpopulation, its specific characteristics are unknown. This is one of the main reasons why we lack effective tools to identify male infertility and for the low efficiency of assisted reproductive technologies. The aim of this study was to improve ICSI outcome by sperm selection through thermotaxis. Here we show that a specific subpopulation of mouse and human spermatozoa can be selected in vitro by thermotaxis and that this subpopulation is the one that enters the fallopian tube in mice. Further, we confirm that these selected spermatozoa in mice and humans show a much higher DNA integrity and lower chromatin compaction than unselected sperm, and in mice, they give rise to more and better embryos through intracytoplasmic sperm injection, doubling the number of successful pregnancies. Collectively, our results indicate that a high quality sperm subpopulation is selected in vitro by thermotaxis and that this subpopulation is also selected in vivo within the fallopian tube possibly by thermotaxis.

Tauroursodeoxycholic acid improves the implantation and live-birth rates of mouse embryos

We previously demonstrated that tauroursodeoxycholic acid (TUDCA) improved the developmental competence of mouse embryos by attenuating endoplasmic reticulum (ER) stress-induced apoptosis during preimplantation development. Here, we present a follow-up study examining whether TUDCA enhances the implantation and live-birth rate of mouse embryos. Mouse 2-cell embryos were collected by oviduct flushing and cultured in the presence or absence of 50 μM TUDCA. After culture (52 h), blastocysts were transferred to 2.5-day pseudopregnant foster mothers. It was found that the rates of pregnancy and implantation as well as the number of live pups per surrogate mouse were significantly higher in the TUDCA-treated group compared to the control group, but there was no significant difference in the mean weights of the pups or placentae. Thus, we report for the first time that TUDCA supplementation of the embryo culture medium increased the implantation and livebirth rates of transferred mouse embryos.

Utero-tubal embryo transfer and vasectomy in the mouse model

The transfer of preimplantation embryos to a surrogate female is a required step for the production of genetically modified mice or to study the effects of epigenetic alterations originated during preimplantation development on subsequent fetal development and adult health. The use of an effective and consistent embryo transfer technique is crucial to enhance the generation of genetically modified animals and to determine the effect of different treatments on implantation rates and survival to term. Embryos at the blastocyst stage are usually transferred by uterine transfer, performing a puncture in the uterine wall to introduce the embryo manipulation pipette. The orifice performed in the uterus does not close after the pipette has been withdrawn, and the embryos can outflow to the abdominal cavity due to the positive pressure of the uterus. The puncture can also produce a hemorrhage that impairs implantation, blocks the transfer pipette and may affect embryo development, especially when embryos without zona are transferred. Consequently, this technique often results in very variable and overall low embryo survival rates. Avoiding these negative effects, utero-tubal embryo transfer take advantage of the utero-tubal junction as a natural barrier that impedes embryo outflow and avoid the puncture of the uterine wall. Vasectomized males are required for obtaining pseudopregnant recipients. A technique to perform vasectomy is described as a complement to the utero-tubal embryo transfer.

Effect of glucose concentration during in vitro culture of mouse embryos on development to blastocyst, success of embryo transfer, and litter sex ratio

A high-glucose concentration in the reproductive tract during early development may result in aberrant embryo or fetal development, with effects that could have a greater impact on one sex than the other. Here, we determine if a high-glucose concentration impacts embryo development and pregnancy outcomes in a sex-specific manner in the mouse. Zygotes were cultured in potassium simple optimized medium, which typically contains 0.2 mM D-glucose, with and without additional glucose supplementation to a concentration of 28 mM. Zygote cleavage and blastocyst rate did not differ between treatments, but total and trophectoderm cell counts were reduced in blastocysts cultured in a high glucose. No differences between sexes nor inner cell mass cell number were observed within each treatment. Blastocysts developed in both media were transferred to recipients. The percentage of blastocysts resulting in viable pups was significantly reduced when the blastocysts were cultured in 28 mM glucose (74 ± 4%, controls vs. 55.8 ± 7.1%, 28 mM glucose), but conceptus loss affected both sexes equally as litter sex ratio did not differ between treatments (52.7% and 52.2% males for controls and high glucose, respectively). Pup body weight at birth was higher for males than females, but was not affected by earlier culture in high glucose. In conclusion, in vitro culture in medium with a glucose concentration approximating that of diabetic serum reduces total and trophectoderm cell numbers at the blastocyst stage and conceptus development to term, but these detrimental effects are not sex-specific.

Comparison of male chimeric mice generated from microinjection of JM8.N4 embryonic stem cells into C57BL/6J and C57BL/6NTac blastocysts

To identify ways to improve the efficiency of generating chimeric mice via microinjection of blastocysts with ES cells, we compared production and performance of ES-cell derived chimeric mice using blastocysts from two closely related and commonly used sub-strains of C57BL/6. Chimeras were produced by injection of the same JM8.N4 (C57BL/6NTac) derived ES cell line into blastocysts of mixed sex from either C57BL/6J (B6J) or C57BL/6NTac (B6NTac) mice. Similar efficiency of production and sex-conversion of chimeric animals was observed with each strain of blastocyst. However, B6J chimeric males had fewer developmental abnormalities involving urogenital and reproductive tissues (1/12, 8%) compared with B6NTac chimeric males (7/9, 78%). The low sample size did not permit determination of statistical significance for many parameters. However, in each category analyzed the B6J-derived chimeric males performed as well, or better, than their B6NTac counterparts. Twelve of 14 (86%) B6J male chimeras were fertile compared with 6 of 11 (55%) B6NTac male chimeras. Ten of 12 (83%) B6J chimeric males sired more than 1 litter compared with only 3 of 6 (50%) B6NTac chimeras. B6J male chimeras produced more litters per productive mating (3.42 ± 1.73, n = 12) compared to B6NTac chimeras (2.17 ± 1.33, n = 6). Finally, a greater ratio of germline transmitting chimeric males was obtained using B6J blastocysts (9/14; 64%) compared with chimeras produced using B6NTac blastocysts (4/11; 36%). Use of B6J host blastocysts for microinjection of ES cells may offer improvements over blastocysts from B6NTac and possibly other sub-strains of C57BL/6 mice.

Pregnancy rates, prenatal and postnatal survival of offspring, and litter sizes after reciprocal embryo transfer in DBA/2JHd, C3H/HeNCrl and NMRI mice

Success of embryo transfer is often a limiting factor in transgenic procedures and rederivation efforts, and depends on the genetic background of the donor and recipient strains used. Here we show that embryo transfer to DBA/2J females is possible, and present data on pre- and postnatal success rates after reciprocal embryo transfer using the inbred DBA/2J and C3H/HeN, and outbred NMRI strains. The highest embryo yield was achieved in outbred NMRI females, but embryo yields were similar in DBA/2J and C3H/HeN mice following superovulation despite poor estrus cycle synchronization in DBA/2J females. In-strain transfer of DBA/2J blastocysts (transfer of embryos to recipients from the same strain) resulted in pregnancy rates (57.1%) similar to those obtained following in-strain transfer of C3H/HeN (60.0%) and NMRI mice (83.3%), although the prenatal survival rate of blastocysts was low. Moreover, from the pups born only half survived the postnatal period after transfer of DBA/2J and C3H/HeN blastocysts to DBA/2J recipients. These problems were not observed when transferring NMRI-blastocysts to C3H/HeN and DBA/2J mothers. The number of blastocysts transferred also had a positive effect on the success of embryo transfer. In conclusion, C3H/HeN and DBA/2J females can be used as recipients for embryo transfer procedures for certain donor strains like NMRI, as one major determinant seems to be the genetic background of the embryos transferred. We also recommend to increase the number of DBA/2J blastocysts transferred, and to foster the DBA/2J pups to other DBA/2J mothers postnatally for in-strain transfer of DBA/2J mice.

Normal epigenetic inheritance in mice conceived by in vitro fertilization and embryo transfer

An association between assisted reproductive technology (ART) and neurobehavioral imprinting disorders has been reported in many studies, and it seems that ART may interfere with imprint reprogramming. However, it has never been explored whether epigenetic errors or imprinting disease susceptibility induced by ART can be inherited transgenerationally. Hence, the aim of this study was to determine the effect of in vitro fertilization and embryo transfer (IVF-ET) on transgenerational inheritance in an inbred mouse model. Mice derived from IVF-ET were outcrossed to wild-type C57BL/6J to obtain their female and male line F2 and F3 generations. Their behavior, morphology, histology, and DNA methylation status at several important differentially methylated regions (DMRs) were analyzed by Morris water maze, hematoxylin and eosin (H&E) staining, and bisulfite genomic sequencing. No significant differences in spatial learning or phenotypic abnormality were found in adults derived from IVF (F1) and female and male line F2 and F3 generations. A borderline trend of hypomethylation was found in H19 DMR CpG island 3 in the female line-derived F3 generation (0.40±0.118, P=0.086). Methylation status in H19/Igf2 DMR island 1, Igf2 DMR, KvDMR, and Snrpn DMR displayed normal patterns. Methylation percentage did not differ significantly from that of adults conceived naturally, and the expression of the genes they regulated was not disturbed. Transgenerational integrity, such as behavior, morphology, histology, and DNA methylation status, was maintained in these generations, which indicates that exposure of female germ cells to hormonal stimulation and gamete manipulation might not affect the individuals and their descendents.

Repeated use of surrogate mothers for embryo transfer in the mouse

Embryo transfer in mice is a crucial technique for generation of transgenic animals, rederivation of contaminated lines, and revitalization of cryopreserved strains, and it is a key component of assisted reproduction techniques. It is common practice to use females only once as surrogate mothers. However, their reuse for a second embryo transfer could provide hygienic and economic advantages and conform to the concept of the 3Rs (replace, reduce, refine). This investigation evaluated the potential for a second embryo transfer in terms of feasibility, reproductive results, and experimental burden for the animal. Virgin female ICR mice (age 8-16 wk) were used as recipients for the first embryo transfer. Immediately after weaning of the first litter, a second surgical embryo transfer was performed into the same oviduct. Virgin females of comparable age to the reused mothers served as controls and underwent the same procedure. The first surgery did not affect the success of the second embryo transfer. Histological sections showed excellent wound healing without relevant impairment of involved tissues. We observed no differences in pregnancy rates or litter sizes between the transfer groups. Most importantly, we found no change in behavior indicating reduced well-being and no increase of corticosterone metabolites in the feces of surrogate mothers reused for a second embryo transfer. We conclude that a second embryo transfer in mice is feasible with regard to reproductive and animal welfare aspects.

Genome-wide DNA methylation patterns in IVF-conceived mice and their progeny: a putative model for ART-conceived humans

The aim of this study was to use a mouse model to gain an understanding of the safety of reproduction between humans conceived through assisted reproductive technology (ART). Mice derived from in vitro fertilization and embryo transfer (IVF-ET) were crossed. Their behavior, morphology, histology and genome-wide DNA methylation status in the brain were examined by the Morris water maze, H&E staining and methylated DNA immunoprecipitation coupled with DNA methylation microarrays. Although no significant differences in behavior or morphology were observed, we did find small clusters of CpG islands and promoters that were aberrantly methylated. Hypermethylation was more common than hypomethylation in each of the two generations. Some of the aberrant methylated promoters were validated by bisulfite sequencing. Our results show that IVF may slightly modify the somatic methylation pattern and that some of this aberrant methylation might be inherited by the following generation.

Effect of stem cell activation, culture media of manipulated embryos, and site of embryo transfer in the production of F0 embryonic stem cell mice

Recently, F0 embryonic stem (ES) cell mice have been produced by injection of ES cells into eight-cell embryos using either laser- or piezo-assisted injection systems. To simplify the injection procedure, we have optimized the conventional blastocyst injection method, free of laser- or piezo-assisted micromanipulation systems, to produce F0 ES cell pups. To increase the efficiency of producing mice from ES cell injection into eight-cell and blastocyst stage embryos, we have tested: 1) the effect of activating ES cell before injection, 2) the effect of in vitro culture in medium optimized for the survival of both ES cells and embryos, and 3) the effect of transferring the micromanipulated embryos into the oviduct versus into the uterus of CD1 foster mice. Two B6D2 hybrid ES cell lines were used for injection in a multifactorial analysis to evaluate the efficiency of producing live chimeric and F0 ES cell mice. Our results demonstrate that the activation of ES cells and the appropriate culture conditions are crucial parameters influencing the generation of F0 ES cell offspring. Transfer of blastocysts injected with ES cells into the oviduct of 0.5-day postcoitum pseudopregnant females increased the number of live animals with higher chimera proportion. Under these conditions, injections into eight-cell embryos produce a high number of F0 ES mice, and the conventional blastocyst injection method produces a lower number of F0 ES cell pups; however, the efficiency of production of chimeric mice with germline transmission was high. We have developed an economical and efficient technique for producing fully ES cell-derived F0 mice with full germline transmission that can be applied in many laboratories without the use of piezo or laser instruments.

Pregnancy outcomes for mouse preimplantation embryos exposed in vitro to the estrogenic pesticide o,p′-DDT

Pregnancy outcomes were evaluated following uterine transfer of murine preimplantation embryos exposed in vitro to the estrogenic pesticide o,p'-dichlorodiphenyltrichloroethane (o,p'-DDT). Single-cell embryos were incubated 72 h in medium droplets containing 0.1% ethanol (control) or 0.1 microg/ml o,p'-DDT (pesticide). Morula and preblastocyst embryos were transferred in groups of eight to right uterine horns of pseudopregnant mice (n=111) and pups (n=132) were evaluated at Caesarean-section (C-section). In vitro exposure to o,p'-DDT reduced development to morula (P<0.001) and modestly increased blastomere apoptosis (P=0.05). However, treatment differences were not detected for implantation rates (35% versus 39%; P=0.64), pup numbers per dam (2.3 versus 1.9; P=0.36), transfer efficiencies (16% versus 14%; P=0.53), fetal weights (1.56 g versus 1.57 g; P=0.91), skeletal abnormalities (55% versus 66%; P=0.47), or male ratios (54.8% versus 53.8%; P=1.0). In vitro exposure of preimplantation embryos to 0.1 microg/ml o,p'-DDT for 72 h resulted in no measurable effects on subsequent implantation or pup characteristics at C-section.

Vitrification of Mouse Embryos at Various Stages by Open-Pulled Straw (OPS) Method

This study was performed to pursue the optimal condition for the cryopreservation of mouse morulae by a two-step OPS method and to investigate the feasibility of the optimal condition for vitrification of embryos at other developmental stages. First, the mouse morulae were vitrified in OPS using one-step procedure-that is, embryos were vitrified after direct exposure to EDFS30 (15% ethylene glycol (EG), 15% dimethyl sulfoxide (DMSO), Ficoll and sucrose), or two-step method-that is, embryos were first pretreated in 10%E + 10%D (10% EG and 10% DMSO in mPBS) for 30 sec, then exposed to EDFS30 for 15 to 60 sec, respectively. After vitrification and warming, the embryos were morphologically evaluated and assessed by their development to blastocysts, expanded/hatched blastocysts, or to term after transfer. The result showed that all the vitrified-warmed morulae had similar blastocyst rate compared to that of control (91.7% vs. 100%), and the highest developmental rate to expanded blastocysts (100%) or hatched blastocysts (62.3%) was observed when the morulae were pretreated with 10%E + 10%D for 0.5 min, exposed to EDFS30for 25 sec before vitrification and warming in 0.5 M sucrose for 5 min. After transfer, the survival rate (33.1%) in vivo of the vitrified morulae was higher (P > 0.05) than that of the fresh embryos (24.6%). Secondly, embryos at different stages were cryopreserved and thawed following the above program. Most (93.4 to 100%) of the embryos recovered after vitrification were morphologically normal at all the developmental stages. The blastocyst rates of the vitrified one-cell (52.5 to 66.7%) and the two-cell (63.3 to 68.9%) embryos were lower (P < 0.05) than those of the vitrified four-cell embryos (81.7 to 86.4%), the eight-cell embryos (90.0 to 93.3%), morulae (96.7 to 100%), and the expanded blastocysts rate (98.3 to 100.0%) of the vitrified early blastocysts. The highest survival rate in vivo of vitrified embryos were from the early blastocysts (40.4%), which was similar to that of fresh embryos (48.6%). The data demonstrate that the optimal protocol for the cryopreservation of morulae was suitable for the four-cell embryos to early blastocyst stages and that the early blastocyst stage is the most feasible stage for mouse embryo cryopreservation under our experimental conditions.