Current Status of Porcine Embryo in Vitro Production

Systems for the in vitro production (IVP) of porcine embryos have advanced to the stage where large numbers of embryos can be reliably generated from the oocytes of abattoir-sourced ovaries in a single production run. These systems are extremely useful for studying various aspects of porcine gamete and embryo biology, developing associated reproductive technologies, such as embryo cryopreservation and somatic cell nuclear transfer (SCNT), and generating pigs that have been genetically engineered (GE) for agricultural or biomedical research purposes. However, while the efficiencies of these systems are generally considered to be acceptable, a number of issues remain. Following oocyte in vitro maturation, polyspermic fertilization continues to be a problem and the viability of embryos is poor compared with in vivo derived embryos. This is especially the case for oocytes recovered from small antral follicles. The molecular determinants of oocyte quality are still poorly understood, although studies examining the differences between oocytes from small and large follicles, and their corresponding follicular environments, are revealing interesting insights into the acquisition of oocyte quality. Also, despite recent advances in the in vitro culture of pig embryos, the conditions used are still considered to be sub-optimal. The use of completely defined media that support the development of viable embryos has been critical to further improvements in this area. However, the effectiveness of culture medium refinements may differ with embryo type (i.e. IVP embryo, parthenote, SCNT embryo), complicating the findings of such studies. The detailed annotation of the pig genome heralds the next era of GE pig production. In addition, new gene editing tools, such as zinc finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs) and clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9, have facilitated precise manipulation of the pig genome. The need for high quality porcine embryos for the generation of GE pigs will continue to drive improvements in the efficiency of in vitro production systems, as well as the procedures used to transfer embryos into recipient sows. Coupled with the improvements in embryo cryopreservation, the in vitro production of porcine embryos for breeding purposes may become commercially feasible in the not too distant future.