Self-renewal of teratocarcinoma and embryonic stem cells

Pluripotent stem cells derived from preimplantation embryos, primordial germ cells or teratocarcinomas are currently unique in undergoing prolonged symmetrical self-renewal in culture. For mouse embryonic stem (ES) cells, self-renewal is dependent on signals from the cytokine leukaemia inhibitory factor (LIF) and from either serum or bone morphogenetic proteins (BMPs). In addition to the extrinsic regulation of gene expression, intrinsic transcriptional determinants are also required for maintenance of the undifferentiated state. These include Oct4, a member of the POU family of homeodomain proteins and a second recently identified homeodomain protein, Nanog. When overexpressed, Nanog allows ES cells to self-renew in the absence of the otherwise obligatory LIF and BMP signals. Although Nanog can act independent of the LIF signal, a contribution of both pathways provides maximal self-renewal efficiency. Nanog function also requires Oct4. Here, we review recent progress in ES cell self-renewal, relate this to the biology of teratocarcinomas and offer testable hypotheses to expose the mechanics of ES cell self-renewal.