In vitro embryo development in the pig

Annadie Kidson Assisted reproductive technologies (ART), such as in vitro embryo production, cryopreservation and transfer, have become an integral part of commercially applied breeding techniques in a variety of animal species [1]. Whereas progress of genetics through ART has played a core role in food-producing domestic species, such as the cow and sheep, during the past two decades it has not shown the same progress in the pig. Its importance as a food-producing animal, as well as a biological model in human biomedical applications, such as transgenesis directed at the pig as donor of cells, tissues and organs (xenotransplantation) [2], has prompted increased efforts to improve the efficiency of reproductive technologies in the pig. Despite the establishment of several systems to generate embryos in vitro, the efficiency is still poor and the quality of resultant blastocysts inferior compared with their in vivo counterparts. This limits the improvement of other reproductive techniques, such as embryo transfer and embryonic stem cell technology, which are dependent on the blastocyst as source of material. Advanced reproductive technologies rely more on the basic techniques of oocyte maturation and fertilization, and the development and identification of baseline requirements for these processes are essential and still much in need of further refinement in the pig. In vitro embryo production (IVP) places major demands on oocytes and embryos with regard to developmental transitions, i.e. oocyte maturation, fertilization, compaction, blastocyst formation, all of which need to be traversed successfully in order for the embryo to develop to a new individual. These transitions are defined by exquisite remodelling of the gamete, zygote and embryo at intraand intercellular levels, which sustains the continuation of its developmental program. Oocytes and embryos need to summon many signal transduction mechanisms, cytoskeletal constituents and genes in order to successfully negotiate and complete all the necessary developmental processes. Support of all these functions and processes is of cardinal importance when applying in vitro embryo production techniques to produce embryos capable of generating functional cell lines or live healthy offspring. Therefore, IVP systems are generally comprised of three stage-specific culture environments, specially designed to supply in the needs of each stage of oocyte, zygote and embryo development: 1) in vitro oocyte maturation (IVM), 2) in vitro fertilization (IVF), and 3) in vitro embryo culture (IVC). Although culture media have been adapted to suit the specific individual needs of the pig oocyte and embryo (i.e. North Carolina State University-23 (NCSU-23) medium [3]), not much progress in the improvement of embryo viability has been made since the adoption of its use a decade ago. Chronic and cumulative stresses caused by sub-optimal culture conditions or insufficiencies in the constituents of the culture medium, can lead to oocyte or embryo anomalies at a genetic or metabolic level and severely jeopardise the viability of the resultant embryo [4]. Markers for the best developmental potential of pig oocytes and embryos are scarce, thus confronting us with a dilemma, as we cannot estimate the developmental outcome for the in vitro period or the subsequent remaining in vivo period. As we become more sophisticated in the in vitro production of pig embryos, so our battery of markers will increase and eventually improve our understanding and design of each separate phase of the IVP system for the pig.