Effect of cumulus cell coculture and oxygen tension on the in vitro developmental competence of bovine zygotes cultured singly

The customary practice in bovine in vitro embryo production (IVP) is to handle oocytes and embryos in groups; although there are several reasons for establishing an IVP system for individual embryos that allows for following a single oocyte from retrieval through development to the blastocyst stage. To date, reports of individual IVP are inconsistent, and in most cases, resulted in unsatisfactory blastocyst rates. The objective of this study was to develop an efficient system for routine in vitro culture of individual bovine embryos. Single culture of zygotes in 2 different culture volumes (20 and 500 μL) yielded less than 3% blastocysts in experiment 1. In an attempt to improve these results, cumulus cells were added to the culture medium in experiment 2, after which blastocyst rates increased from 2.9 to 21.8% (P < 0.05). The third experiment revealed that an atmospheric oxygen tension, which is commonly used with somatic cell coculture, was not beneficial during individual embryo-cumulus cell coculture, because it resulted in lower blastocyst rates (Odds ratio 0.57, P < 0.001) and in lower blastocyst cell numbers (P < 0.05), when compared to culture in 5% oxygen. Grouped vs. single culture and reduced oxygen tension did not have a significant effect on cleavage and hatching rates. In experiment 4, three different cumulus cell coculture conditions during individual culture were tested and compared with the cleavage, blastocyst and hatching rates, and cell number of group culture (73.2%, 36.4%, 66.7% and, 155.1 ± 7.26, respectively). The outcome variables after individual embryo culture on a 5-day-old cumulus cell monolayer (74.1%, 38.2%, 71.9% and 133.4 ± 9.16, respectively), and single culture in the presence of added cumulus cells (69.9%, 31.9%, 66.7% and 137.3 ± 8.01, respectively) were not significantly different from those obtained after group culture (P < 0.05). Though, individual culture in a cumulus cell conditioned medium significantly reduced both the cleavage (59.0%) and blastocyst rates (6.3%). These results demonstrate that single culture of bovine zygotes can be fully sustained by coculture with cumulus cells in a low oxygen environment; implementation of these findings in our IVP system produced blastocysts comparable in quantity and quality to those obtained by group culture. These results were consistently achieved after acquiring experience and expertise in the handling of single zygotes.