Fitness selection in human pluripotent stem cells and interspecies chimeras: Implications for human development and regenerative medicine.

A small number of pluripotent cells within early embryo gives rise to all cells in the adult body, including germ cells. Hence, any mutations occurring in the pluripotent cell population are at risk of being propagated to their daughter cells and could lead to congenital defects or embryonic lethality and pose a risk of being transmitted to future generations. The observation that genetic errors are relatively common in preimplantation embryos, but their levels reduce as development progresses, suggests the existence of mechanisms for clearance of aberrant, unfit or damaged cells. Although early human embryogenesis is largely experimentally inaccessible, pluripotent stem cell (PSC) lines can be derived either from the inner cell mass (ICM) of a blastocyst or by reprogramming somatic cells into an embryonic stem cell-like state. PSCs retain the ability to differentiate into any cell type in vitro and, hence, they represent a unique and powerful tool for studying otherwise intractable stages of human development. The advent of PSCs has also opened up a possibility of developing regenerative medicine therapies, either through PSC differentiation in vitro or by creating interspecies chimeras for organ replacement. Here, we discuss the emerging evidence of cell selection in human PSC populations in vivo and in vitro and we highlight the implications of understanding this phenomenon for human development and regenerative medicine.