Cryopreservation of Bovine IVM/IVF/IVC Blastocysts by Vitrification

체외성숙, 체외수정 및 체외배양에서 생산된 소 배반포기배의 초자화 동결

The objective of this study was to examine the effect of developmental stage and embryo age of in vitro produced bovine blastocysts after vitrification and thawing. In vitro cultured day 8 blastocysts after IVF were equilibrated 20% ethylene glycol (EG) for 3 min. and were vitrified using EFS40, which is consisted of 40% EG, 18% ficoll, 0.3M sucrose and 10% FBS added in mDPBS for 30 sec. before being plunged into $LN_2$. Also, survival in vitro was assessed by re-expansion and hatching or hatched at 24 hand 48 h postwarming, respectively. The results obtained in these experiments were summarized as follows; 1) When the embryos were cultured for 8 day after IVF, 41.0% of the cleaved embryos developed to the blastocysts (early; 7.6%, expanded; 22.9%, hatching; 4.6% and hatched; 5.9%). 2) When the embryos were exposed or vitrified to the freezing solution, the re-expansion of vitrified embryos (73.3%) was significantly lower than that of control and exposed embryos (100, 97.0%) (p<0.05). But the formation rate of hatching or hatched blastocysts of vitrified embryos (66.7, 46.7%) at 48h after thawing was similar to that of exposed embryos (66.7, 39.4%) but not control (100, 100%) (p<0.01). However, in the total cell numbers of those developed hatched blastocysts, there were not significantly different among the treatment groups. 3) When the embryo survival rates by different developmental stage were examined, the re-expansion was not different among the groups $(64.5{\sim}75.6%)$. After warming 48 h, the hatching and hatched formation of early blastocysts (25.8, 9.7%) was significantly lower than those of expanded (69.7, 39.4%) and hatching blastocysts (53.3, 43.3%) (p<0.05). 4) In addition, when the expanded blastocysts at day 7, 8 and 9 were vitrified, the re-expansion of day 8 and 9 embryos was significantly lower than that of day 7 (day 7; 93.9%, day 8; 75.8% and day 9; 87.5%) (p<0.05). However, the rates of development to hatched blastocysts were no difference among the groups (day 7; 36.4%, day 8; 36.4% and day 9; 31.3%). These results suggested that in vitro produced expanded or hatching blastocysts can be efficiently cryopreserved by the two-step vitrification method using EFS40.

본 연구는 체외생산된 소 배반포기배를 발달 단계 및 배양일에 따라 구별하여 초자화 동결 및 융해하였을 때, 그 발달능을 유지하는지 확인하고자 실시하였다. 체외수정 후 8일간 배양된 배반포기 배는 20% ethylene glycol에 3분 동안 평형시키고, EFS40 (40% ethylene glycol, 18% ficoll, 0.3M sucrose 그리고 10% FBS가 함유된 mDPBS) 동결액에 30초 동안 노출한 후, 액체질소에 침지하여 초자화 동결되었다. 체외 생존 여부는 응해 24시간 및 48시간에 재팽창 및 탈출 또는 완전탈출로써 평가하였다. 그 결과를 요약하면 다음과 같다. 1) 체외수정 후 8일간 배 양하였을 때, 난할된 배의 배반포기 배로의 발달율은 41.0%였다 (초기 ; 7.6%, 팽창; 22.9%, 탈출; 4.6%, 완전탈출; 5.9%). 2) 배반포기배를 동결액에 노출 또는 초자화 동결하였을 때, 초자화 동결된 배반포기배의 재팽창율 (73.3%)은 대조군 및 동결액에 노출된 경우 (100, 97.0%)보다 낮았다 (p<0.05). 그러나 융해 48시간 후 탈출 또는 완전탈출 배반포기배 형성율은 초자화 동결된 경우 (66.7, 46.7%)와 노출된 경우 (66.7, 39.4%)는 유의한 차이를 나타내지 않았으나, 대조군(100, 100%)과는 차이를 보였다 (p<0.01). 그러나, 완전탈출까지 발달한 배반포기배의 총 세포수를 조사하였을 때, 각 처리군간의 유의한 차이는 없었다. 3) 배반포기배의 발달 단계에 따른 체외 생존율을 비교하였을 때, 재팽창율은 실험군간에 유의한 차이를 보이지 않았다 $(64.5{\sim}75.6%)$. 그러나 융해 48시간 후, 탈출 또는 완전탈출로써 평가된 초기 배반포기배의 발달율 (25.8, 9.7%)은 팽창 (69.7, 39.4%)및 탈출 배반포기배 (53.3, 43.3%)의 발달율보다 낮게 나타났다 (p<0.05). 4) 또한, 배양 7, 8 그리고 9일의 팽창 배반포기배를 초자화 동결하였을 때, 8일 및 9일간 배양된 배반포기배의 재팽창율은 7일간 배양된 경우보다 낮게 나타났다 (7일; 93.9%, 8일; 75.8%, 9일; 87.5%) (p<0.05). 그러나 완전탈출 배반포기배로의 발달율에서는 처리군간에 유의한 차이를 보이지 않았다 (7일; 36.4%, 8일; 36.4%, 9일; 31.3%). 이러한 결과는 EFS40을 이용한 2단계 초자화 동결 방법이 체외생산 된 팽창 및 탈출 배반포기배의 동결에 유용하게 이용될 수 있음을 시사한다.