Culture medium for bovine oocyte maturation affects resulting blastocyst metabolism

Abstract

Two experiments were designed to determine the effects of embryonic sex on the rate of development of in vitro-produced ovine embryos and the effects of transferring early developing ovine blastocysts on the sex ratio of the lambs born. In Experiment 1, in vitro matured and fertilized (IVM/IVF) ovine oocytes were cultured in synthetic oviduct fluid (SOF) supplemented with BSA and amino acids, and assessed for blastocyst formation daily from 6 to 9 d post insemination. The embryos were assigned to 5 developmental stages: early blastocyst (EB), blastocyst (B), expanding or expanded blastocyst (ExB), hatching blastocyst (HgB) and hatched blastocyst (HB). A total of 99 blastocysts were produced, 76 of which were successfully sexed by polymerase chain reaction (PCR) and 80 assessed for cell number. Overall the male:female sex ratio (45:31) of in vitro produced blastocysts was not different from the expected 1:1 ratio. Twenty-six of the 50 embryos (52%) first observed as blastocysts on Day 6 were at the advanced stages (ExB, HgB and HB) and were designated Early Day 6, of these, 18 were sexed and 83 3% were male (significantly different from the expected 1:1 sex ratio; P < 0.01). Slower developing Day 6 blastocysts (B and EB; Late Day 6), and all blastocyst stages first observed on Days 7, 8 and 9 did not differ from the 1:1 sex ratio. The mean cell number of blastocysts from the Early Day 6 group was higher than that of blastocysts first seen on Day 7 (P < 0.05) or Days 8 and 9 (P < 0.01). Overall, male blastocysts had a higher mean cell number than the female blastocysts (85.7 ± 8.54 and 54.5 ± 4.67, mean ± SEM, respectively, P < 0.05). In Experiment 2, 90 blastocysts first seen on Day 6 or 7 were transferred in pairs to recipient sheep and their in vivo survival and the resultant sex of lambs recorded. Of the 48 blastocysts derived from the Early Day 6 group that were transferred, 22 (45.8%) survived to birth, 8 (36.4%) of which were males. Twelve lambs were born derived from transfer of 31 Late Day 6 blastocysts (38.9% survival), 4 of which were males (33.3%). One female lamb was born from 11 (9.1% survival) embryos derived from blastocysts first seen on Day 7. In all 3 groups, more females than males were born, but the ratios were not significantly different from the expected 1:1 ratio. It would appear from these findings that our in vitro culture system enhances the growth rate in male compared with female ovine embryos. However, the transfer of early developing in vitro-produced embryos did not alter the sex ratio of offspring in favor of the males.