Initial embryology and pluripotent stem cells in the pig—The quest for establishing the pig as a model for cell therapy

The quest for porcine pluripotent stem cells (PSCs) was initiated in the early 90s. Initially, it was the intention to benefit from these cells for production of genetically modified pigs using homologous recombination followed by derivation of chimeric offspring; a technology that has been used to produce genetically modified mice since the mid-80s. However, no convincing reports on the generation of bona fide porcine embryonic stem cells or embryonic germ cells resulted from these activities, and with the advent of somatic cell nuclear transfer during the late 90s, alternative methods for creating genetically modified pigs emerged. Over the past years, renewed interest in porcine PSCs has sparked activities in deriving in particular porcine induced pluripotent stem cells to develop the pig as a faithful model for studying the potentials and risks associated with induced pluripotent stem cell–based human therapy. Here, we review the recent data on establishment of porcine PSCs and the differences in embryonic development between pig and mouse, which may be underlying factors for the continuing challenge to culture and maintain porcine PSCs.