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 e nergy metabolism as it proceeds from the zygote to the blastocyst stage. Co mplete development of the mammalian zygote in vitro was restricted to a few strains of mice and their Fl hybrids for many years, as the ruminant embry o arrested development at the 8- to 16-cell stage. The introduction of co-c ulture of ruminant embryos with somatic cells in the mid-1980s helped to al leviate this in vitro induced arrest. However, such culture systems require d the use of complex tissue culture media and serum. Serum has subsequently been shown to induce several abnormalities during embryo development in cu lture and has been associated with the production of offspring with signifi cantly greater birth weights than normal, leading to both difficulties in p regnancy management and an unacceptable frequency of neonatal death. Resurg ence of interest in mammalian embryo physiology has culminated in the formu lation of defined embryo culture media, capable of supporting a high percen tage 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 thos e embryos with the highest developmental potential will further increase th e efficiency of embryo transfer procedures. Assays based upon nutrient upta ke and subsequent utilization make promising candidates.