Evaluating totipotency using criteria of increasing stringency.

Totipotency is the ability of a single cell to give rise to all of the differentiated cell types that build the conceptus, yet how to capture this property in vitro remains incompletely understood. Defining totipotency relies on a variety of assays of variable stringency. Here, we describe criteria to define totipotency. We explain how distinct criteria of increasing stringency can be used to judge totipotency by evaluating candidate totipotent cell types in mice, including early blastomeres and expanded or extended pluripotent stem cells. Our data challenge the notion that expanded or extended pluripotent states harbour increased totipotent potential relative to conventional embryonic stem cells under in vitro and in vivo conditions.

Placental cell death patterns exhibit differences throughout gestation in two strains of laboratory mice.

Cell death is an essential physiological process required for the proper development and function of the human placenta. Although the mouse is a commonly used animal model for development studies, little is known about the extent and distribution of cell death in the mouse placenta throughout development and its physiological relevance. In the present study, we report the results of a systematic and quantitative assessment of cell death patterns in the placentae of two strains of laboratory mice commonly used for developmental studies-ICR and C57Bl/6. TUNEL staining revealed that ICR and C57Bl/6 placentae exhibited similar cell death patterns to those reported in human placentae during pregnancy, with comparatively infrequent death observed during early gestation, which increased and became more organized towards term. Interestingly, when comparing strain differences, increased cell death was observed in almost all regions of the inbred C57Bl/6 placentae compared to the outbred ICR strain. Finally, since Bcl-2 ovarian killer (Bok) has been reported to be a key player in human placental cell death, we examined its expression in murine placentae throughout gestation. Bok protein expression was observed in all placental regions and increased towards term in both strains. The results of this study indicate that although strain-specific differences in placental cell death exist, the overall rates and patterns of cell death during murine placentation parallel those previously described in humans. Thus, the murine placenta is a useful model to investigate molecular pathways involved in cell death signaling during human placentation.

The mammalian embryo's first agenda: making trophectoderm.

One of the bottlenecks for a successful pregnancy in mammalian species is the implantation of the early embryo into the wall of the mother's uterus. The first cell lineage the embryo sets aside following fertilization is the trophectoderm - a specialized cell type that establishes contact with the mother and mediates embryo implantation. We summarize the events that lead to the formation of the trophectoderm lineage in the preimplantation embryo and highlight key features of this cell type, which could be useful in the clinical setting for prediction of implantation outcomes.