Hopes and Difficulties for Blastocyst Complementation.

CONTEXT: The clinical need for organ replacement therapies has inspired the idea of growing human organs in animal hosts. The injection of human pluripotent stem cells into animal blastocysts provides a possible strategy to accomplish this goal. Subject of Review: A recent study [Wu et al. Cell 2017;168:473-486.e415] tests the feasibility of this approach by creating chimeric embryos between humans and large domestic animals, including pigs and cattle. The study further examines the potential of combining CRISPR-Cas9 gene editing with blastocyst complementation to grow fully foreign organs in chimeric hosts. Second Opinion: Here, we consider what this report and related studies reveal about the likelihood of human-animal chimeras reaching the clinic and translating into therapies. A careful look suggests hope for eventual success in this area but also underscores important challenges that will require dedicated effort to resolve.