Embryonic stem cells (ESCs) are of great interest for tissue replacement therapies and for the study of mammalian development. Cultured ESCs as well as induced pluripotent stem cells (iPSCs), which are generated from somatic cells, can self-renew indefinitely and give rise to all cell types of the body including the germline. However, such stem cells are not totipotent because they are inefficient at generating extra embryonic tissues, such as the placenta (1). What underlies this developmental restriction? On page 596 of this issue, Choi et al. (2) report a mechanism that prevents totipotency. The barrier involves mobile genetic elements embedded throughout the genome and a small noncoding microRNA (miRNA).
Authors: Hidetoshi Hasuwa, Haruhiko Siomi