Many mammals can control the timing of gestation and birth by pausing embryonic development at the blastocyst stage. It is unknown whether the capacity to pause development is conserved, in general across mammals, and more specifically in humans. Activity of the growth regulating mTOR pathway governs developmental pausing in the mouse. Here we show a stage-specific capacity to delay the progression of human development via mTOR inhibition. In this context, human blastoids and pluripotent stem cells in naive and naive-like, but not primed, states can be induced to enter a dormant state, which is reversible at the functional and molecular level. Comparative analysis of mouse and human longitudinal response to mTORi revealed distinct temporal dynamics and metabolic requirements of dormancy in each species. Mouse and human blastocysts show similar tissue-specific patterns of mTOR pathway activity, suggesting that the mTOR pathway may be a conserved regulator of blastocyst development and timing in both species. Our results raise the possibility that the developmental timing of the human embryo may be controllable, with implications for reproductive therapies.

The quest to model and modulate embryonic development became a recent cornerstone of stem cell and developmental biology. Mammalian developmental timing is adjustable in vivo by preserving preimplantation embryos in a dormant state called diapause. Inhibition of the growth regulator mTOR (mTORi) pauses mouse development in vitro, yet constraints to pause duration are unrecognized. By comparing the response of embryonic and extraembryonic stem cells to mTORi-induced pausing, we identified lipid usage as a bottleneck to developmental pausing. Enhancing fatty acid oxidation (FAO) boosts embryo longevity, while blocking it reduces the pausing capacity. Genomic and metabolic analyses of single embryos point toward a deeper dormant state in FAO-enhanced pausing and reveal a link between lipid metabolism and embryo morphology. Our results lift a constraint on in vitro embryo survival and suggest that lipid metabolism may be a critical metabolic transition relevant for longevity and stem cell function across tissues.