185 ASSESSMENT OF VIABILITY OF IN VITRO PRODUCED BOVINE EMBRYOS BY TRIPLE AND SINGLE TRANSFER

Abstract

Factors that affect the viability of in vitro-produced (IVP) embryos are usually evaluated by comparing pregnancy rates of a treatment and a control group. The ‘er’ model of embryo survival (McMillan WH et al., 1998 Theriogenology 50, 1053–1070) utilizes twin embryo transfer to estimate embryo (‘e’) and recipient (‘r’) contributions to embryo survival, and allows the comparison of treatment effects without using a control group, when treatment is the only change in operations. Application of the model to data of contemporaneous single and twin transfer indicates that ‘e’ and ‘r’ are independent of the number of embryos transferred. Thus, twin transfers enable the efficient use of costly recipients while providing meaningful estimates of single embryo survival rates. The objective of this study was to assess the embryo survival rates of fresh IVP embryos of a newly established IVP lab by applying the model to triple transfers and comparing the expected embryo survival rates with those achieved for single transfers. Cumulus-oocyte complexes (COCs) were aspirated from abattoir-derived ovaries of cows of unknown breeds or by ovum pick-up (OPU) from Holstein-Friesian 2- or 3-yr-old donor cows. COCs were matured in 500 μL of TCM199 + 10% FCS (Life Technologies, Auckland, NZ), 10 μg mL⁻¹ FSH and LH (ICPBio, Auckland, NZ), 1 μg mL⁻¹ estradiol (Sigma, Auckland, NZ), 100 μM cysteamine (Sigma) for 24 h under 5% CO₂ and then fertilized with 1 × 10⁶ percoll-separated sperm mL⁻¹ from a single bull (Tervit HR and Pugh PA, 2000 14th ICAR 18, 37(abst)). Twenty-four h after insemination, presumptive zygotes were transferred into 500 μL mSOF (Pugh A et al., 2001 Theriogenology 55, 314 (abst)) and cultured for 4 days under humidified 5% CO₂, 7% O₂ and 88% N₂. On Day 4, cleaved embryos were transferred into fresh culture medium and culture continued for a further 3 days under the same conditions. Embryo stage and grade were evaluated on Day 7 of culture. Grades 1, 2 and 3 (IETS manual, 2002) compact morulae and blastocysts produced from abattoir-derived COCs were transferred in triplets, while grades 1 and 2 compact morulae and blastocysts from OPU-derived COCs were transferred singly, in 0.25 mL insemination straws into synchronized Holstein-Friesian heifers. Recipients received a CIDR (CIDR Cattle Insert, Pharmacia, Auckland, NZ) at Day −12 followed by a prostaglandin (Estroplan, Parnell Laboratories, Auckland, NZ ) injection at Day −6. CIDRs were removed at Day −2, followed by estrus at Day 0 (= day of IVF). Embryos were transferred on Day 7 and recipients received a CIDR after transfer (ET). CIDRs were removed at Day 19 to synchronize any returns. Two experienced practitioners performed all the transfers. Pregnancies (single transfers) and number of live fetuses (triple transfers) were confirmed at Days 60 and 42, respectively. Pregnancies were terminated between Days 62 and 65 by two prostaglandin injections 48 h apart. A total of 76 single transfers resulted in 36 pregnancies (47.4%, binomial SD 5.7%). A total of 75 triple transfers (225 embryos) resulted in 98 viable fetuses (44%) and 58 pregnant recipients (77.3%). For triple transfers, the estimates for ‘e’ and ‘r’ were 0.50 and 0.89, respectively, with the product yielding an expected triple embryo survival rate of 44.1%. The actual distribution of 17, 23, 30 and 5 recipients carrying 0, 1, 2, or 3 fetuses, respectively, was not significantly different from the expected values of 16, 25, 25 and 8 estimated from the model (chi-square = 2.49, NS). Estimates for ‘e’ and ‘r’ were not significantly different when combined single and triple data were included in the model (‘e’ = 0.55 and ‘r’ = 0.90), indicating that embryo survival is independent of the number of embryos transferred. Results indicate that multiple transfers do increase pregnancy rate (from 47.4 to 77.3%), but not embryo survival posttransfer (44.1 v. 47.4%). Although single ET was done with OPU-derived embryos and triple with slaughterhouse-derived embryos and results are not strictly comparable, the similarity of estimates for ‘e’ suggests that using the same in vitro-embryo assessment criteria resulted in embryos of similar intrinsic viability from the two sources. In the near future, we will perform triple transfers of cryopreserved IVP embryos and use the model to estimate embryo and recipient contributions to embryo survival of frozen IVP embryos, without using a fresh control. We will continue to build a dataset based on triple and single transfers to further assess the effect on embryo survival rates of triple and single transfers.