Cryopreservation of Porcine Embryos: Recent Updates and Progress

Cryopreservation of embryos is important for long-distance embryo transfer and conservation of genetic resources. Porcine research is important for animal husbandry and biomedical research. However, porcine embryos are difficult to cryopreserve because of their high cytoplasmic lipid content and sensitivity to chilling stress. Vitrification is more efficient than slow freezing, and vitrification is mostly used in embryo cryopreservation. So far, the vitrification process of porcine embryos has been continuously improved, resulting in improved survival rates of warmed embryos and farrowing rates after the transplant procedure. It is worth noting that automatic vitrification has made great progress, which is expected to promote the standardization and application of vitrification. In this article, the vitrification process of porcine embryos at the blastula stage and early development stages is reviewed in detail. In addition, the efficiency of different vitrification systems was compared. In addition, we summarize technology that can improve the survival rate of cryopreserved porcine embryos, such as delipidation methods (including physical delipidation and chemical delipidation) and medium improvements (including chemically defined media and adding antioxidants). Meanwhile, gene expression changes during cryopreservation are also elaborated.

Micromanipulation of equine blastocysts to allow vitrification

Embryo cryopreservation presents an essential method for banking of valuable genetics. However, in equine species the cryopreservation of embryos is complicated by three interacting factors: (1) the late entry of the embryo into the uterus (~6 days after ovulation); (2) the rapid expansion of the blastocyst; and (3) the formation of the equine embryonic capsule, a glycoprotein membrane that forms between the embryo and zona. Efforts to freeze or vitrify equine expanded blastocysts were initially met with little success. In addition, it was thought that breaching the capsule led to loss of embryo viability. We found that micromanipulation with the Piezo drill to puncture the capsule and collapse the blastocyst before vitrification provided a means for successful cryopreservation of equine expanded blastocysts, and that this can be done successfully using a standard sperm injection pipette. Modification of cryoprotectants and methods for vitrification and warming resulted in a technique that allowed successful vitrification of expanded equine blastocysts up to 650 µm diameter, with pregnancy rates approaching those for fresh embryos. After blastocyst collapse, vitrification is performed with ethylene glycol and galactose as cryoprotectants, and the embryo is cooled in a low-volume micropipette tip. Vitrification of expanded equine blastocysts provides a valuable tool for use in exotic equids to preserve genetics.

A comparison of different vitrification devices and the effect of blastocoele collapse on the cryosurvival of in vitro produced porcine embryos

The aim of this study was to determine the optimum conditions for vitrifying in vitro produced day 7 porcine embryos using different vitrification devices and blastocoele collapse methods. Firstly embryos were collapsed by micro-pipetting, needle puncture and sucrose with and without conducting vitrification. In the next experiment, non-collapsed embryos were vitrified in an open device using either superfine open-pulled straws (SOPS) or the CryoLoop^TM system, or vitrified in a closed device using either the CryoTip^TM or Cryo Bio^TM’s high security vitrification system (HSV). The post-thaw survival of embryos vitrified in the open devices did not differ significantly (SOPS: 37.3%; CryoLoop^TM: 37.3%) nor did the post-thaw survival of embryos vitrified in the closed devices (CryoTip™: 38.5%; HSV: 42.5%). The re-expansion rate of embryos that were collapsed via micro-pipetting (76.0%) did not differ from those that were punctured (75.0%) or collapsed via sucrose (79.6%) when vitrification was not performed. However, embryos collapsed via sucrose solutions (24.5%) and needle puncture (16.0%) prior to vitrification were significantly less likely to survive vitrification than the control (non-collapsed) embryos (53.6%, P < 0.05). The findings show that both open and closed vitrification devices were equally effective for the vitrification of porcine blastocysts. Collapsing blastocysts prior to vitrification did not improve survival, which is inconsistent with the findings of studies in other species. This may be due to the extremely sensitive nature of porcine embryos, and/or the invasiveness of the collapsing procedures.

Assisted reproduction techniques in the horse

This paper reviews current equine assisted reproduction techniques. Embryo transfer is the most common equine ART, but is still limited by the inability to superovulate mares effectively. Immature oocytes may be recovered by transvaginal ultrasound-guided aspiration of immature follicles, or from ovaries postmortem, and can be effectively matured in vitro. Notably, the in vivo-matured oocyte may be easily recovered from the stimulated preovulatory follicle. Standard IVF is still not repeatable in the horse; however, embryos and foals can be produced by surgical transfer of mature oocytes to the oviducts of inseminated recipient mares or via intracytoplasmic sperm injection (ICSI). Currently, ICSI and in vitro embryo culture are routinely performed by only a few laboratories, but reported blastocyst development rates approach those found after bovine IVF (i.e. 25%–35%). Nuclear transfer can be relatively efficient (up to 26% live foal rate per transferred embryo), but few laboratories are working in this area. Equine blastocysts may be biopsied via micromanipulation, with normal pregnancy rates after biopsy, and accurate genetic analysis. Equine expanded blastocysts may be vitrified after collapsing them via micromanipulation, with normal pregnancy rates after warming and transfer. Many of these recently developed techniques are now in clinical use.

Effect of dehydration prior to cryopreservation of large equine embryos

Cryopreservation of equine embryos>300microm in diameter results in low survival rates using protocols that work well for smaller equine embryos. These experiments tested the potential benefit of incorporating a dehydration step prior to standard cryopreservation procedures. Forty-six, day 7-8, grade 1, equine embryos 300-1350microm in diameter were subjected to one of the following treatments: (A) 2 min in 0.6M galactose, 10min in 1.5M glycerol, slow freeze (n=21); (B) 10min in 1.5M glycerol, slow freeze (n=15); (C) 2min in 0.6M galactose, 10min in 1.5M glycerol, followed by exposure to thaw solutions, then culture medium (n=5); (D) transferred directly to culture medium (n=5). Frozen embryos were thawed and subjected to a three-step cryoprotectant removal. Five embryos from each treatment were evaluated morphologically after 24 and 48h culture (1=excellent, 5=degenerate/dead). All treatments had at least 4/5 embryos with a quality score >or=3 at these time points except treatment B (2/5 at 24h, 1/5 at 48h). Subsequent embryos from treatment A (n=16) or B (n=10) were matched in sets of two for size and treatment, thawed, and immediately transferred in pairs to 13 recipients. Only two recipient mares were pregnant; one received two 400microm embryos from treatment A, and the other one 400 and one 415microm embryo from treatment B. There was no advantage of incorporating a 2min dehydration step into the cryopreservation protocol for large equine embryos.