Embryo cryopreservation in the presence of low concentration of vitrification solution with sealed pulled straws in liquid nitrogen slush

BACKGROUND: Vitrification is becoming the method of choice for embryo cryopreservation. Nevertheless, major problems are still associated with this process such as chemical toxicity and osmotic stress as well as risk of liquid nitrogen (LN) contamination. METHODS: An innovative vitrification method that combines LN slush and sealed pulled straws (SPS) was employed to achieve a high cooling rate, enabling a reduction in cryoprotectant concentration. Open pulled straws were sealed at both ends to prevent direct contact with LN. RESULTS: Ultrarapid cooling of murine embryos at 32 200°C/min in SPS with LN slush yielded a higher blastocyst survival rate (54 ± 3.5, 106/196) than cooling at 1700°C/min in 0.25 ml straws (10 ± 2.1, 21/197) (P < 0.05). Embryos at the 2-cell stage cryopreserved in 75 vitrification solution (VS) (100 VS contains ∼5 M ethylene glycol, 0.6 M trehalose and 6 w/v bovine serum albumin) in SPS formed blastocysts at a higher rate (79 ± 3.6, 99/126) than cryopreservation in 100 VS (31 ± 6.5, 16/51), however, this was not significantly different from the fresh control group (88 ± 4.6, 43/49). Early stage embryos at the 2 pronuclei- and 4-8-cell stage formed blastocysts at rates of 68 ± 4.5 and 60 ± 3.7, respectively, after vitrification in 87.5 VS. CONCLUSIONS: This method enables maintenance of high cooling rates as well as reduction of cryoprotectant concentration, despite the use of a sealed container that helps to reduce the potential risk of contamination.