Development of serum-free culture systems for the ruminant embryo and subsequent assessment of embryo viability.

The mammalian embryo undergoes considerable changes in its physiology and energy metabolism as it proceeds from the zygote to the blastocyst stage. Complete development of the mammalian zygote in vitro was restricted to a few strains of mice and their F1 hybrids for many years, as the ruminant embryo arrested development at the 8- to 16-cell stage. The introduction of co-culture of ruminant embryos with somatic cells in the mid-1980s helped to alleviate this in vitro induced arrest. However, such culture systems required the use of complex tissue culture media and serum. Serum has subsequently been shown to induce several abnormalities during embryo development in culture and has been associated with the production of offspring with significantly greater birth weights than normal, leading to both difficulties in pregnancy management and an unacceptable frequency of neonatal death. Resurgence of interest in mammalian embryo physiology has culminated in the formulation of defined embryo culture media, capable of supporting a high percentage of viable blastocyst development in vitro. Optimum embryo development in culture has been shown to take place not in one, but two or more media, each designed to cater for the changing requirements and metabolism of the embryo as it develops. The development of viability assays to identify those embryos with the highest developmental potential will further increase the efficiency of embryo transfer procedures. Assays based upon nutrient uptake and subsequent utilization make promising candidates.