Bovine embryo culture in vitro: new developments and post-transfer consequences

The past decade has seen a significant shift away from co-culture systems f or cattle blastocyst production, In particular, recent adoption of sequenti al media systems has increased performance. However, wholly defined systems , such as the replacement of albumin with nonbiological macromolecules, fai l to reproduce the nutritive role that this molecule has during development . Cattle blastocysts developed in protein-free medium are metabolically com promised, A further new concept is the use of metabolic inhibitors to stimu late embryo development in vitro. Non-toxic levels of NaN3, 2,4-dinitrophen ol or very low oxygen atmospheres (approximate to2%) significantly increase both the yield (by similar to 10-20%) and the quality of blastocysts when these treatments are applied during the peri-compaction period in vitro, Ne vertheless, there are also negative consequences of cattle embryo culture, such as fetal oversize and/or significant post-day 35 fetal loss. We have r ecently found that much of this loss is due to failure of normal allantoic development within the conceptus. Early fetal development is supported by v ascularization within the yolk sac, but from day 35 to day 110, loss occurs through poor nutrient supply and an inability to remove nitrogenous wastes , leading to fetal death around day 35, The cause of disrupted allantois de velopment has not been identified as yet, but may share a common 'cause-eff ect' mechanism with the fetal oversize syndrome.