Bovine embryos were produced by in vitro maturation and fertilization of abattoir oocytes. The embryos were randomly allocated either to coculture with bovine oviduct cells in Menezo-B2 medium (control group), or to culture in the defined HECM-6 medium. At Day 5 after insemination the HECM-6 embryos were transferred to Menezo-B2 medium with (HECM-B2/BOEC) or without (HECM-B2) oviduct cells for further culture. The proportion of cleaved embryos and blastocysts, the morphology and the speed of development were compared for the control and HECM groups. Significantly more HECM-6 embryos than control embryos cleaved (88+/-3% vs 76+/-5% (+/-SD)). Significantly fewer blastocysts developed in the HECM-B2 than in the control group (28+/-2% vs 35+/-3%), in addition the speed of development was delayed and the morphology was impaired. In the HECM-B2/BOEC group no differences in neither morphology, blastocyst rates (31+/-8%) nor speed of development could be demonstrated, when compared with the control group. A portion of the control and HECM-B2 embryos were vitrified at Days 7-8, but no differences were noted in survival or morphology at 48 and 72 h post thawing. It can be concluded, that the defined medium HECM-6 can support bovine embryonic development through the 8-16 cell in vitro block stage without the use of coculture in a reliable way. In our system it was however necessary to transfer the embryos at Day 5 to coculture in Menezo-B2 medium to ensure optimal continuation of development.
