22 Vitrification of bovine embryo using antifreeze polyamino acid

Abstract

Carboxylated poly-l-lysine (CPLL) is an ampholytic polymer compound and a polyamino acid with a known functional resemblance to antifreeze proteins. We previously reported that CPLL is an effective cryoprotectant for bovine cells, sperm, and slow-frozen embryos. In this study, we investigated CPLL as a cryoprotectant for vitrified bovine embryos. We developed bovine embryos in vitro and vitrified them at the blastocyst stage. Embryos were equilibrated (3 min) and vitrified (1 min). Vitrified embryos were cryopreserved in LN (Cryotop® device; Kitazato Corp., Tokyo, Japan) for at least 1 week, thawed with a 0.3 M sucrose warming solution, and then cultured in a basal medium (Gibco® medium 199, Grand Island, NY, USA; supplemented with 100 µM 2-mercaptoethanol, 10% fetal bovine serum, and antibiotics) at 38.5°C in a humidified atmosphere (5% CO₂, 5% O₂, 90% N₂). We evaluated the embryos morphologically for survival and hatched rate at 0, 24, 48, and 72 h post-thawing. In control, the equilibration solution (ES) consisted of 7.5% (vol/vol) dimethyl sulfoxide (DMSO) and 7.5% (vol/vol) ethylene glycol, and the vitrification solution (VS) consisted of 16.5% (vol/vol) DMSO and 16.5% (vol/vol) ethylene glycol and 0.5 M sucrose. In this study, CPLL was added to ES and VS at various concentrations instead of DMSO. The CPLL was added at 16.5, 11.0, 5.5, and 2.2% (wt/vol) to VS; respectively, these solutions were named P16.5, P11.0, P5.5, and P2.2. The ES was used 45% CPLL of VS each. Embryos underwent the above procedure concurrently, with testing replicated at least 3 times. We evaluated 88, 34, 38, 44, and 28 embryos with each solution (control, P16.5, P11.0, P5.5, and P2.2, respectively). Results were analysed statistically with a chi-square test and residual analysis, regarding P < 0.05 as significant. Survival rates were significantly greater in P11.0 at 24 h post-thawing (55.7% v. 89.5%; P < 0.05) and in P11.0 and P5.5 at 48 h post-thawing (47.7% v. 78.9% and 47.7% v. 79.5%, respectively; P < 0.05) relative to controls but showed no significant differences at 0 h post-thawing. Hatched rates were significantly greater in P11.0 and P5.5 through 72 h post-thawing relative to controls (44.7% v. 22.7% and 52.3% v. 22.7%, respectively; P < 0.05). The CPLL improved post-thawing embryo survival and hatched rates when applied during vitrification, thus demonstrating cryoprotective effectiveness. We conclude that CPLL acts as a low-toxicity cryoprotectant for vitrified bovine embryos, and our results are consistent with previous reports of protective CPLL effects for cells and cell membranes.