Comparison of unilateral and bilateral embryo transfer in mice

Surgical embryo transfer in mice is a key technique in assisted reproduction and applied for different purposes in biomedical research. Due to its frequent application in rodent facilities across the world, further improvement of the procedure can substantially contribute to fulfil the principles of the 3Rs. Here, we investigated the effect of bilateral and unilateral left- or right-sided oviduct transfers on the success of embryo transfers. In total, we performed 223 embryo transfers (56 unilateral left, 56 unilateral right, 111 bilateral), in which we transferred 10-14 two-cell embryos each. We found that the type of transfer significantly influenced both the pregnancy rate of recipients and the survival rate of transferred embryos. Bilateral transfers yielded higher pregnancy and survival rates than left-sided unilateral transfers. Right-sided unilateral transfers yielded higher pregnancy rates than left-sided unilateral transfers and did not differ in embryo survival rates from bilateral transfers. We found no evidence that the number of transferred embryos affected the pregnancy rate. However, the number of born pups increased with the number of transferred embryos. In conclusion, unilateral embryo transfers into the right reproductive tract yield equally high pregnancy and embryo survival rates as bilateral transfers. Given that a second abdominal incision can be prevented and the time of surgery can be reduced, we recommend applying unilateral right-sided transfers, as this would reduce postoperative pain and lower the impact on recipients.

C57BL/6J and B6129F1 Embryo Transfer: Unilateral and Bilateral Transfer, Embryo Number and Recipient Female Background Control for the Optimization of Embryo Survival and Litter Size

Simple Summary

Embryo transfer is a common procedure in rodent facilities related to rederivation protocols, recovery of cryopreserved embryos and production of genetically engineered animals. This procedure consists of the transfer of mouse embryos into the oviduct of a pseudopregnant recipient female in order to obtain live pups. The aim of this study is to further characterize the optimal conditions to perform embryo transfer using wild type strains and particularly the bilateral transfer. C57BL/6J and B6129F1 embryos were freshly collected and transferred to recipient females, after overnight culture to a 2-cell stage and tested for different conditions (unilateral and bilateral surgical procedures, variable number of embryos and reciprocity between recipient mother and embryo’s genetic background). The results achieved show that C57BL/6J transfers with a low number of embryos provide higher success rates when using unilateral transfers, but for bilateral transfers a minimum number of embryos seems to be necessary. B6129F1 presented similar results, but bilateral transfers were more effective with low number of embryos. These results allow a better planning of the embryo transfer procedure, considering low number of embryos and the choice of unilateral transfers as the ideal condition for an optimal outcome. This optimization has a positive impact on the 3R’s application: it can help to reduce the number of recipient and donor females and to improve recipient female’s welfare through the use of a less invasive technique.

Abstract

Embryo transfer (ET) is a common procedure in rodent facilities. Optimizing this technique may help to reduce the number of animals, but little information is available regarding wild type strains and the conditions that affect embryo transfer. To explore this theme, 2-cell C57BL/6J embryos were transferred after overnight culture of freshly collected zygotes using different conditions: unilateral transfers using a total of 6, 8, 12, 15, 20 and 25 embryos were performed initially; then, this strain was also used for bilateral transfers using a total of 6, 12 and 20 embryos equally divided by the two oviducts. Groups of 25 embryos were not tested for the bilateral technique, since this condition produced the lower success rate when using the unilateral technique and 20 embryos would still represent a large number of embryos. A group of 2-cell B6129F1 embryos was also transferred using unilateral and bilateral ET with 6, 12 and 20 embryos. Crl:CD1(ICR) were used as recipient females for non-reciprocal transfers and C57BL/6J were used to test reciprocal transfers (only tested for six C57BL/6J unilateral transfers). Unilateral transfers using C57BL/6J mice produced higher success rates using six embryos, compared to the other groups transferred unilaterally (p-values between 0.0001 and 0.0267), but the mean number of pups per litter was not different among groups. Bilateral transfer produced higher number of pups when 20 embryos were divided by the two oviducts compared to six (p = 0.0012) or 12 (p = 0.0148) embryos, but with no differences in success rates. No statistical differences were found between the groups of B6129F1, but better results were obtained on bilateral transfers using a total of six embryos. For the strain tested (C57BL/6J), the uterine environment (Crl:CD1(ICR) or C57BL/6J recipient) does not impact the outcome of the technique. These results complement previous work published using genetically engineered mice strains and show that unilateral transfers using low number of embryos (6), produce better outcomes when compared to bilateral or unilateral transfers using more embryos. It also highlights differences between the outcome of bilateral transfers in the two strains tested. A set of historical data of genetically engineered mice at a C57BL/6J background was also included, confirming that lower embryo numbers are related to higher success rates. Together, the outcome of these experiments can be important to reduce the number of recipient and donor females, optimize embryo transfers and improve animal welfare discouraging the use of a more invasive technique.

The evolutionarily conserved piRNA-producing locus pi6 is required for male mouse fertility

Pachytene PIWI-interacting RNAs (piRNAs), which comprise >80% of small RNAs in the adult mouse testis, have been proposed to bind and regulate target RNAs like microRNAs, cleave targets like short interfering RNAs or lack biological function altogether. Although piRNA pathway protein mutants are male sterile, no biological function has been identified for any mammalian piRNA-producing locus. Here, we report that males lacking piRNAs from a conserved mouse pachytene piRNA locus on chromosome 6 (pi6) produce sperm with defects in capacitation and egg fertilization. Moreover, heterozygous embryos sired by pi6^-/- fathers show reduced viability in utero. Molecular analyses suggest that pi6 piRNAs repress gene expression by cleaving messenger RNAs encoding proteins required for sperm function. pi6 also participates in a network of piRNA-piRNA precursor interactions that initiate piRNA production from a second piRNA locus on chromosome 10, as well as pi6 itself. Our data establish a direct role for pachytene piRNAs in spermiogenesis and embryo viability.

Extracellular vesicles derived from donor oviduct fluid improved birth rates after embryo transfer in mice

Embryo transfer (ET) is an important procedure for assisted reproduction. However, the relatively lower success rate of ET hampers its application potential. In this study we aimed to elucidate the effects of extracellular vesicles derived from donor oviduct fluid (EDOF) on embryo development after ET. Extracellular vesicles from the oviduct were isolated and purified using ultracentrifugation and identified using transmission electron microscopy, NanoSight, bicinchoninic acid (BCA) protein assay and western blotting. The results revealed that extracellular vesicles were present in donor oviduct fluid in higher concentrations (P<0.05) and contained more proteins (P<0.05) than extracellular vesicles derived from recipient oviduct fluid (EROF). EDOF or EROF were supplemented in an ET medium (ETM) and the results showed that EDOF significantly improved birth rate via resisting apoptosis and promoting differentiation. In conclusion, our study indicated that there are differences in EDOF and EROF and that supplementing EDOF to ETM can improve the efficiency of ET; improved ET efficiency promotes the use of gene editing and benefits assisted reproductive technology and animal welfare.

Success of embryo transfer in mice with freshly collected and cryopreserved two-cell embryos with different genetic backgrounds correlated with the number of transferred embryos: A 5-year retrospective analysis

Embryo transfer of pre-implantation embryos to surrogate dams is a key technique for the hygienic sanitation of strains, cryopreservation, in vitro fertilization, genetic modification and engineering. However, the effects of several parameters, such as the number of transferred embryos, on the success of embryo transfer are not well studied. In this retrospective study, we reanalysed 1320 embryo transfers of two-cell embryos originating from genetically altered donors, which were performed under routine conditions in our facility over a period of 5 years. Of them, 453 embryo transfers were done with freshly collected embryos and 867 transfers were performed with cryopreserved embryos. Despite the fact that the genetic background of the embryo donors was quite heterogeneous, we found that the transfer of ≥ 21 embryos reduced the success of embryo transfers for freshly collected embryos in correlation with the number of pregnancies and born pups, whereas this was not the case for transfer in the cryopreservation group. Most pregnancies were achieved after embryo transfer of 10-20 freshly collected embryos (90.4%), which dropped to 37.5% if more embryos were transferred. The highest pregnancy rates in the cryopreservation group were achieved if 15-17 embryos were transferred (62.9%). Despite the fact that the precise substrains were only rarely defined, we confirmed that beside the number of transferred embryos, the genetic background of the donors had an influence on the success of embryo transfer. Significantly more embryos in a C57BL/6 background developed to term than embryos on a BALB/c background.