Bovine embryo culture in vitro: new developments and post-transfer consequences.
The past decade has seen a significant shift away from co-culture systems for cattle blastocyst production. In particular, recent adoption of sequential media systems has increased performance. However, wholly defined systems, such as the replacement of albumin with nonbiological macromolecules, fail to reproduce the nutritive role that this molecule has during development. Cattle blastocysts developed in protein-free medium are metabolically compromised. A further new concept is the use of metabolic inhibitors to stimulate embryo development in vitro. Non-toxic levels of NaN3, 2,4-dinitrophenol or very low oxygen atmospheres (approximately 2%) significantly increase both the yield (by approximately 10-20%) and the quality of blastocysts when these treatments are applied during the peri-compaction period in vitro. Nevertheless, there are also negative consequences of cattle embryo culture, such as fetal oversize and/or significant post-day 35 fetal loss. We have recently found that much of this loss is due to failure of normal allantoic development within the conceptus. Early fetal development is supported by vascularization 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 development has not been identified as yet, but may share a common 'cause-effect' mechanism with the fetal oversize syndrome.