ABSTRACT
The mammalian preimplantation/pre-attachment embryo undergoes considerable changes in its physiology and energy metabolism as it proceeds from the zygote to the blastocyst stage. Attempts to culture the mammalian zygote in the 1960s were restricted to a few strains of mice and their F1 hybrids, as the embryos of both sheep and cattle arrested in development at the 8- to 16-cell stage in culture. The introduction of co-culture techniques in the mid 1980s, whereby sheep and cattle embryos were incubated with somatic cells, helped to alleviate the in vitro induced arrest at the 8- to 16-cell stage and facilitated blastocyst development. However, such co-culture systems required the use of complex tissue culture media supplemented with serum. Serum has subsequently been shown to be associated with the production of offspring with significantly greater birth weights than normal, leading to both difficulties in managing such pregnancies and an unacceptable frequency of neonatal death. A recent resurgence of interest in mammalian embryo physiology has culminated in the formulation of defined embryo culture media, capable of supporting high levels of viable blastocyst development in vitro. Optimal embryo development in culture takes place not in one, but two or more media, each designed to cater for the changing requirements of the embryo as it develops. The ability to maintain the embryos of sheep and cattle in culture without compromising developmental potential will expedite the development and introduction of procedures such as transgenesis and cloning. This chapter reviews the physiology of the mammalian embryo and the subsequent development and application of new defined embryo culture systems for sheep and cattle.
